Polyisobutylenes with Controlled Molecular Weight and Chain-End Structure: Synthesis and Actual Applications

The polymerization of isobutylene allows us to obtain a wide spectrum of polyisobutylenes (PIBs) which differ in their molecular weight characteristics and the chemical structure of chain-end groups. The bulk of the PIBs manufactured worldwide are highly reactive polyisobutylenes (HRPIBs) with –C(Me)=CH2 end-groups and low-molecular weights (Mn < 5 kDa). HRPIBs are feedstocks that are in high demand in the manufacturing of additives for fuels and oils, adhesives, detergents, and other fine chemicals. In addition, HRPIBs and CMe2Cl-terminated PIBs are intensively studied with the aim of finding biomedical applications and for the purpose of developing new materials. Both chain control (molecular weight and dispersity) and chemoselectivity (formation of exo-olefinic or –CMe2Cl groups) should be achieved during polymerization. This review highlights the fundamental issues in the mechanisms of isobutylene polymerization and PIB analysis, examines actual catalytic approaches to PIBs, and describes recent studies on the functionalization and applications of HRPIBs and halogen-terminated PIBs.


Introduction
The polymerization of isobutylene (IB) has been used in the production of high-(M n > 100 kDa) and medium-(M n = 40-100 kDa) molecular weight (MW) polyisobutylenes (PIBs) and IB/isoprene copolymers (butyl rubber) [1][2][3]. Due to their remarkable impermeable property, PIBs are extensively used in many applications, such as the inner tubes and layers of tires [4]. However, the low-MW PIB (M n < 5 kDa), possessing the exo-olefinic end-group -C(Me)=CH 2 , holds the major share in the PIB market (more than 70% [2]). The PIB market was valued at USD 2.87 billion in 2021 [5] and is projected to reach USD 3.89 billion by 2028; it is expected to grow at an annual rate of 4.4% from 2021 to 2028. BASF, Braskem S.A., Chevron, Daelim Industrial Petrochemical Division, INEOS, Infineum, Kemat Polybutenes, Kothari Petrochemicals, Lubrizol Corp., SIBUR Holding, and TPC Group are the key players in the global PIB market [4]. Recently, in response to the rising global demand for high-quality medium-molecular weight PIBs, the leading chemical companies have increased their production capacity for these polymers [6,7].
Low-MW PIBs have been widely used in the production of different fine chemicals, the bulk of which are ashless dispersants for motor oils and fuels. PIB-based dispersants represent bis-PIB succinimides (bPIB-SI); their synthesis includes two stages (Scheme 1), viz. the Alder-ene reaction between PIB and maleic anhydride (MA) with the formation of polyisobutylene-succinic anhydrides (PIB-SAs), followed by the reaction of the PIB-SAs with H 2 N(CH 2 CH 2 NH) n CH 2 CH 2 NH 2 (n = 1-4) and other amines [8][9][10]. The rate of the Alder-ene reaction depends significantly on the structure of the chain-end unsaturated fragment; the exo-olefinic group is the most reactive, and low-MW PIB containing >80% methylene fragments are known as highly reactive polyisobutylenes (HRPIBs) (Scheme 1a). In a recent review [2], Kostjuk et al. have suggested that newly developed IB polymerization catalysts may be divided into three generations. The first-generation catalysts are complexes of metal halides with ethers; these complexes are poorly soluble in hydrocarbons. The latter disadvantage was overcome by the development of second-generation catalysts, complexes of alkylaluminum dichlorides with ethers. The third-generation catalysts are heterogeneous systems. In the present paper, we summarize and discuss the fundamental issues of the mechanisms of IB polymerization and PIB characterization, and analyze actual catalytic approaches to HRPIB in comparison with industrial catalysts. Our review is based on articles and patents published since 2010, and particular attention is given to recent articles not included in previous reviews.
However, the negative trend visible in Figure 1 has nothing in common with the applications of HRPIBs and PIB-Cl. Recent studies of HRPIB and PIB-Cl functionalization, as well as applications of the materials obtained, are also the subject of our review.

Analysis of PIBs
The microstructure of the products of IB polymerization is an objective basis for the development of catalysts providing desired molecular weight characteristics and a sufficiently high content of exo-olefinic groups. The Mn requirements of HRPIB (several kDa) greatly simplify the chain-end analysis of the polymers using NMR spectroscopy; however, the possibility of chain scission processes occurring during IB polymerization requires the use of mass spectrometry for the analysis of PIB. The basic methods of the study of PIBs have been developed long ago, but we thought it appropriate to include the results of the earlier studies in our review with recent add-ons.

NMR Spectroscopy
The main types of unsaturated end-groups, related to the desired HRPIB structure and the products of the side processes, can be easily observed using 1 H NMR spectroscopy [31]. As can be seen in Figure 2, the exo-olefinic fragment >C = CH2 can be detected through the presence of singlets at δ = 4.68 and 4.89 ppm; the singlet at 5.19 ppm corresponds to In a recent review [2], Kostjuk et al. have suggested that newly developed IB polymerization catalysts may be divided into three generations. The first-generation catalysts are complexes of metal halides with ethers; these complexes are poorly soluble in hydrocarbons. The latter disadvantage was overcome by the development of second-generation catalysts, complexes of alkylaluminum dichlorides with ethers. The third-generation catalysts are heterogeneous systems. In the present paper, we summarize and discuss the fundamental issues of the mechanisms of IB polymerization and PIB characterization, and analyze actual catalytic approaches to HRPIB in comparison with industrial catalysts. Our review is based on articles and patents published since 2010, and particular attention is given to recent articles not included in previous reviews.
However, the negative trend visible in Figure 1 has nothing in common with the applications of HRPIBs and PIB-Cl. Recent studies of HRPIB and PIB-Cl functionalization, as well as applications of the materials obtained, are also the subject of our review.
The formation of F1 and F2 is a substantial side process during the synthes HRPIB, and corresponding signals f1 and f2 at δ = 4.82 and 4.80 ppm are more visib the 1 H NMR spectra of HRPIB ( Figure 4) [33]. These signals can be attributed to the p ucts of 1,3-methide migration (see Section 2.2) or to the protons of =CH2 in the cou PIBs formed by the addition of a growing PIB carbenium ion to the exo-olefinic end g in another PIB chain. To determine the real reaction pathway, NMR studies shou accompanied by size exclusion chromatography of PIBs (see Section 2.1.3).  (F1, F2). The signals of the end-groups in 1 H NMR spectra are indicated by correspon lowercase letters. Reprinted with permission from [33]. Copyright (2010) Elsevier B. V.
The 13 C NMR spectroscopy can also be used for the study of HRPIBs. 13 C NMR tra of PIB, prepared using H2O/FeCl3/ i PrOH system [34], and commercial GLISSO 1000 are presented in Figure 5.
The ratio of the integral intensities of the end-groups and polymer backbone ca used for the determination of Mn. However, the presence of low-MW products of the c scission can complicate the analysis of PIB spectra.  (F1, F2). The signals of the end-groups in 1 H NMR spectra are indicated by corresponding lowercase letters. Reprinted with permission from [33]. Copyright (2010) Elsevier B. V.
The 13 C NMR spectroscopy can also be used for the study of HRPIBs. 13 C NMR spectra of PIB, prepared using H 2 O/FeCl 3 / i PrOH system [34], and commercial GLISSOPAL 1000 are presented in Figure 5.
The ratio of the integral intensities of the end-groups and polymer backbone can be used for the determination of M n . However, the presence of low-MW products of the chain scission can complicate the analysis of PIB spectra. . 13 C NMR spectra of HRPIB prepared from the mixed C4 fraction feed (top) and commercial HRPIB (GLISSOPAL 1000, bottom). Reprinted with permission from [34]. Copyright (2013) Wiley-VCH Verlag GmbH & Co.

Mass Spectrometry
Mechanistic causes of the formation of tri-substituted olefinic end-groups (see Section 2.2) have been determined using mass spectrometry: appearance of the odd-numbered peaks clearly indicated the possibility of the chain scission during IB polymerization. Dimitrov et al. [31] have analyzed the samples of HRPIB (provided by BASF) and "conventional" PIB with internal double bonds (provided by Infenium) using atmospheric pressure photoionization time-of-flight mass spectrometry (APPI-TOF MS) and have found that only one series of peaks appears in the APPI-TOF mass spectrum of HRPIB ( Figure 6a). In APPI-TOF mass spectrum of "conventional" PIB, meanwhile, three additional series of peaks were observed with a mass difference of 14 Da, which corresponds to the -CH2fragment ( Figure 6b) [31].   13 C NMR spectra of HRPIB prepared from the mixed C4 fraction feed (top) and commercial HRPIB (GLISSOPAL 1000, bottom). Reprinted with permission from [34]. Copyright (2013) Wiley-VCH Verlag GmbH & Co.

Mass Spectrometry
Mechanistic causes of the formation of tri-substituted olefinic end-groups (see Section 2.2) have been determined using mass spectrometry: appearance of the odd-numbered peaks clearly indicated the possibility of the chain scission during IB polymerization. Dimitrov et al. [31] have analyzed the samples of HRPIB (provided by BASF) and "conventional" PIB with internal double bonds (provided by Infenium) using atmospheric pressure photoionization time-of-flight mass spectrometry (APPI-TOF MS) and have found that only one series of peaks appears in the APPI-TOF mass spectrum of HRPIB ( Figure 6a). In APPI-TOF mass spectrum of "conventional" PIB, meanwhile, three additional series of peaks were observed with a mass difference of 14 Da, which corresponds to the -CH 2fragment ( Figure 6b) [31].

Mass Spectrometry
Mechanistic causes of the formation of tri-substituted olefinic end-groups (s tion 2.2) have been determined using mass spectrometry: appearance of the odd bered peaks clearly indicated the possibility of the chain scission during IB polym tion. Dimitrov et al. [31] have analyzed the samples of HRPIB (provided by BAS "conventional" PIB with internal double bonds (provided by Infenium) using pheric pressure photoionization time-of-flight mass spectrometry (APPI-TOF M have found that only one series of peaks appears in the APPI-TOF mass spectr HRPIB (Figure 6a). In APPI-TOF mass spectrum of "conventional" PIB, meanwhile additional series of peaks were observed with a mass difference of 14 Da, which sponds to the -CH2fragment ( Figure 6b) [31].

Size Exclusion Chromatography
Size exclusion chromatography (SEC) is a separation technique which is widely used for the determination of molecular weight characteristics of the polymers. Informative and correct studies of the polymers require calibration using polymer standards with known molecular masses and dispersities. However, in everyday practice, such standards are not readily available. Based on universal calibrating principle [35], the relationship between molecular weight M and intrinsic viscosity [η] is described using the Mark-Houwink Equation (1): [η] = K · M α (1) Reported values of K and α value pairs for PIB were quite different. This issue was clarified by the studies of Puskas et al. [36] using PIB samples with M v 1-1100 kDa. As a result of the measurements, the values K = 2.0·10 −4 dL·g −1 and α = 0.67 have been obtained.
In the follow-up work [37], Puskas et al. referred to the problems of the study of branched PIBs, and proposed the use of hyphenated SEC techniques such as SEC coupled with multi-angle laser light scattering (MALLS), viscometry plus universal calibration principle (VIS/UCP), and viscometry plus right-angle light scattering (VIS/RALS). As a result of this study, empirical Equation (2) where ML is Mooney viscosity, M w is number average molar mass, and M z is Z-average molar mass of the polymer.
In a recent work [38], fractionations of commercial low-MW HRPIB (M n = 902 Da, Ð M = 1.8) and "conventional" PIB (M n = 880 Da and Ð M = 5.1) were performed. "Conventional" PIB contained more low-MW fraction (up to 200 Da), 4.8 vs. 1.3%, and high-MW fraction (>5 kDa), 28.8 vs. 2.4%. Combination of the fractionation with 1 H NMR analysis of PIB fractions allowed to determine chain-end structures of macromolecules and propose the mechanisms of the formation of different unsaturated fragments.

Mechanism of Isobutylene Polymerization
Isobutylene is capable of being polymerized via cationic [20] and free-radical [39] mechanisms. Free-radical polymerization of IB was carried out previously using t BuN=N t Bu initiator in the presence of LiCB 11 Me 12 with a formation of highly branched PIB [40]. Recently, Hero and Kali [41] reported the results of a study on free-radical polymerization of IB, complexed with cyclodextrin and dissolved in water. Linear PIBs with degree of polymerization (DP n ) > 30 were obtained, and the method was proposed for the synthesis of block copolymers with acrylates. However, PIBs with -CH=CMe 2 end-groups were formed during free-radical polymerization.
In this way, it can be concluded that the controlled synthesis of linear PIBs with highly reactive end-groups should be based on cationic polymerization, initiated by tertiary carbenium cation [42]. At the subsequent stage, the cationic species react with IB to yield cationic adducts. The chain propagation involves the repetitive addition of isobutylene to the growing carbenium ion until a chain release via chain transfer to IB or other chain transfer agent occurs [23]. It is important to note that the formation of carbenium ions can be accelerated by the addition of Lewis bases; this phenomenon was first observed and explained by Penczek in 1992 [43].
At strong negative temperatures, IB polymerization proceeds as a "living" process, which opens the way for obtaining HRPIBs or functionalized HRPIBs by quenching with different reagents [44]. At subzero and ambient temperatures, cationic polymerization of IB is often complicated by non-selective proton transfer to monomer or chain scission with a formation of >C=CH-fragments, and by rearrangements of carbenium cations with a formation of >C=C< fragments. The mechanism of IB polymerization was studied in detail more than 10 years ago by Dimitrov et al. [31] on the transformations of model PIB-Cl oligomers in the presence of EtAlCl 2 . Matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectrum of the reaction product, obtained at 0 • C, contained two main series of the peaks. The lower intensity series was identical with that of the starting PIB, while the more intense series was 14 Da (mass of one -CH 2 -group) higher and could only form by a loss of C 4n−3 fragments. To prevent reprotonation, ionization of PIB-Cl by EtAlCl 2 in the presence of 2,6-di-tert-butylpyridine (DTBP) was studied at different temperatures using 1 H NMR spectroscopy (see Section 2.1.1). In the presence of DTBP, exoand endo-olefins were formed, and their amount increased with increasing temperature (Table 1). This suggests that the activation energy for proton elimination is higher than that for isomerization/chain scission. In the absence of DTBP, exoand endoolefins were virtually absent at all temperatures, which can be attributed to a preference for reprotonation, subsequent isomerization, and chain scission. Tri-substituted olefins were not observed at 25 • C, which is important for the identification of the mechanism leading to tetrasubstituted olefins. To develop a comprehensive model of the reaction mechanism, the structures of IB trimers and corresponding cations were optimized using the density functional theory (DFT) method (B3LYP/6-31G level of theory, Scheme 2) [31]. Upon ionization, the least stable tertiary cation 1 is formed. A 2-1 hydride shift to form 2 is energetically unfavorable but is not prohibitive. After formation of 2, the system compensates for the loss of energy by further rearrangement via a methyde shift leading to 3, which is the second most stable tertiary cation. As previously reported, the hydride transfer between carbocations 3 and 4 requires very little activation energy [45]. It should be noted that exo-olefin is more stable in comparison with endo-isomer, i.e., the proton transfer with a formation of exo-olefinic group is favorable both sterically and thermodynamically. oligomers in the presence of EtAlCl2. Matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectrum of the reaction product, obtained at 0 °C, contained two main series of the peaks. The lower intensity series was identical with that of the starting PIB, while the more intense series was 14 Da (mass of one -CH2-group) higher and could only form by a loss of C4n-3 fragments. To prevent reprotonation, ionization of PIB-Cl by EtAlCl2 in the presence of 2,6-di-tert-butylpyridine (DTBP) was studied at different temperatures using 1 H NMR spectroscopy (see Section 2.1.1). In the presence of DTBP, exo-and endo-olefins were formed, and their amount increased with increasing temperature (Table 1). This suggests that the activation energy for proton elimination is higher than that for isomerization/chain scission. In the absence of DTBP, exo-and endo-olefins were virtually absent at all temperatures, which can be attributed to a preference for reprotonation, subsequent isomerization, and chain scission. Tri-substituted olefins were not observed at 25 °C, which is important for the identification of the mechanism leading to tetrasubstituted olefins.

T, °C Olefin End-Group Distribution, % tri-Substituted
exo-endo-tetra- Substituted  -40  77  8  4  11  0  61  18  9  12  20  0  40  20  40 To develop a comprehensive model of the reaction mechanism, the structures of IB trimers and corresponding cations were optimized using the density functional theory (DFT) method (B3LYP/6-31G level of theory, Scheme 2) [31]. Upon ionization, the least stable tertiary cation 1 is formed. A 2-1 hydride shift to form 2 is energetically unfavorable but is not prohibitive. After formation of 2, the system compensates for the loss of energy by further rearrangement via a methyde shift leading to 3, which is the second most stable tertiary cation. As previously reported, the hydride transfer between carbocations 3 and 4 requires very little activation energy [45]. It should be noted that exo-olefin is more stable in comparison with endo-isomer, i.e., the proton transfer with a formation of exo-olefinic group is favorable both sterically and thermodynamically. Scheme 2. Interconversions and relative stability (E scale) of model cations and olefins based on isobutylene trimer [31].
To determine where the chain scission takes place, partially deuterated model polymers with the composition PhCMe2(IB-d8)13(IB)nCl (n = 1-6) were synthesized and subjected to ionization with EtAlCl2 at -40 °C in the presence of DTBP. 1 H NMR spectral studies showed that ~3-4 IB units are cleaved during the formation of tri-substituted olefins. Proposed mechanism of the cleavage was based on the assumption that type 4 carbenium ion may have undergone backbiting via a six-membered transition state [31]. Similar Scheme 2. Interconversions and relative stability (E scale) of model cations and olefins based on isobutylene trimer [31].
To determine where the chain scission takes place, partially deuterated model polymers with the composition PhCMe 2 (IB-d 8 ) 13 (IB) n Cl (n = 1-6) were synthesized and subjected to ionization with EtAlCl 2 at −40 • C in the presence of DTBP. 1 H NMR spectral studies showed that~3-4 IB units are cleaved during the formation of tri-substituted olefins. Proposed mechanism of the cleavage was based on the assumption that type 4 carbenium ion may have undergone backbiting via a six-membered transition state [31]. reaction pathway was proposed earlier for degenerative 1,5-hydride shift in 2,6-dimethyl-2-heptyl cation ( Figure 7); activation barrier for this process was estimated to be ~10 kcal·mol -1 (E scale, MP2/6-311G(d,p) level of theory) [46]. In this way, the reaction mechanism of IB polymerization should be extended to include the chain scission (Scheme 3). In sum, all side products can be correlated with type 3 and 4 cations; at sub-zero temperatures backbiting and chain scission are favored, while at ambient temperatures proton elimination is faster [31]. Scheme 3. Proposed mechanism of chain scission during isobutylene polymerization [31].
Cation rearrangements and chain scission processes, discussed above, are more actual for cationic IB polymerization, catalyzed by AlCl3/H2O and other conventional catalysts unsuitable for the synthesis of HRPIB. During the formation of HRPIB, 1,3-methide shifts become visible (Scheme 4, see also Figure 4). In this way, the reaction mechanism of IB polymerization should be extended to include the chain scission (Scheme 3). In sum, all side products can be correlated with type 3 and 4 cations; at sub-zero temperatures backbiting and chain scission are favored, while at ambient temperatures proton elimination is faster [31]. In this way, the reaction mechanism of IB polymerization should be extended to include the chain scission (Scheme 3). In sum, all side products can be correlated with type 3 and 4 cations; at sub-zero temperatures backbiting and chain scission are favored, while at ambient temperatures proton elimination is faster [31]. Scheme 3. Proposed mechanism of chain scission during isobutylene polymerization [31].
Cation rearrangements and chain scission processes, discussed above, are more actual for cationic IB polymerization, catalyzed by AlCl3/H2O and other conventional catalysts unsuitable for the synthesis of HRPIB. During the formation of HRPIB, 1,3-methide shifts become visible (Scheme 4, see also Figure 4). Cation rearrangements and chain scission processes, discussed above, are more actual for cationic IB polymerization, catalyzed by AlCl 3 /H 2 O and other conventional catalysts unsuitable for the synthesis of HRPIB. During the formation of HRPIB, 1,3-methide shifts become visible (Scheme 4, see also Figure 4). Polymers 2023, 15, x FOR PEER REVIEW 10 of 50 Scheme 4. Formation of "internal" exo-olefins during cationic polymerization of IB [33].
When using toluene as a solvent, for some catalysts (e.g., i BuAlCl2) IB polymerization is complicated by alkylation of the solvent, which leads to the formation of very short PIB chains terminated by a toluene. To clarify the reasons why only very short PIB chains are terminated by alkylate toluene, Kostjuk et al. used the DFT (B3LYP) method to calculate the charges of carbocations formed after protonation of IB ( t Bu + ) and also after the addition of 1, 2, 3 and 5 IB molecules; the calculated values of natural bond orbital (NBO) charges were +0.538, +0.526, +0.514, +0.512 and +0.510, respectively [47]. Reducing the charge entails decrease in the reactivity; however, this effect disappears for DPn > 5, and cannot be used for the chain control. In 2023, Lee et al. tried to create single-state and multi-state statistical models for cationic polymerization of IB using PIBs with narrow and broad molecular weight distribution (MWD), respectively [48]. Verification of the multi-state model with a chain transfer to proton was performed for AlCl3/H2O system at 0, 20 and 40 °C with quite good results, despite the debatable assumption about two-stage process of the formation of initiating species (modern ideas about these species are discussed in Section 3.3).
In conclusion of this section, it is worth considering the process opposite to cationic polymerization, viz. enthalpy-driven PIB depolymerization. In the presence of CF3SO3H, HRPIB depolymerized rapidly at room temperature in aromatic solvents; the process was presumably favored by the formation of stronger Csp2−Csp3 bonds in tert-butylarene in comparison with weaker Csp3−Csp3 bonds in PIB [51]. This reaction is a particular case of reversible polymerization that has attracted the attention of researchers in recent years [52]; its importance for the synthesis of HRPIB is shaped by the understanding that aromatic solvents can not only terminate cationic polymerization of IB, but also depolymerize the reaction products. That is why IB polymerization should be carried out in bulk or in inert solvents (saturated hydrocarbons, MeCl, etc.). When using toluene as a solvent, for some catalysts (e.g., i BuAlCl 2 ) IB polymerization is complicated by alkylation of the solvent, which leads to the formation of very short PIB chains terminated by a toluene. To clarify the reasons why only very short PIB chains are terminated by alkylate toluene, Kostjuk et al. used the DFT (B3LYP) method to calculate the charges of carbocations formed after protonation of IB ( t Bu + ) and also after the addition of 1, 2, 3 and 5 IB molecules; the calculated values of natural bond orbital (NBO) charges were +0.538, +0.526, +0.514, +0.512 and +0.510, respectively [47]. Reducing the charge entails decrease in the reactivity; however, this effect disappears for DP n > 5, and cannot be used for the chain control. In 2023, Lee et al. tried to create single-state and multi-state statistical models for cationic polymerization of IB using PIBs with narrow and broad molecular weight distribution (MWD), respectively [48]. Verification of the multi-state model with a chain transfer to proton was performed for AlCl 3 /H 2 O system at 0, 20 and 40 • C with quite good results, despite the debatable assumption about two-stage process of the formation of initiating species (modern ideas about these species are discussed in Section 3.3).
Another industrially important aspect of the chemistry of PIB cations is their reactivity towards other unsaturated hydrocarbons, especially C4 mixed hydrocarbon feeds. De and Faust showed that when using TiCl 4 catalyst at 0 • C in n-hexane media, isobutylene is 17 times more reactive than buta-1,3-diene,~543 times more reactive than but-1-ene, and 294 times more reactive than (Z)-but-2-ene. [49]. These conclusions were based on 1 H NMR identification of the Cl-containing and unsaturated end-fragments formed during capping reactions with C4 hydrocarbons (e. g. (Z)-but-2-ene formed -CHMeCHMeCl (~40%), -CMeClEt (~50%) and -CH 2 C(Me)=CH 2 (~10%) end-fragments). Note that relative reactivity of C4 hydrocarbons in copolymerization with IB correlated with relative reactivities observed in the interaction of C4 olefins with the p-methoxy-substituted diphenylmethyl cation in CH 2 Cl 2 at −70 • C, reported earlier by Mayr [50].
In conclusion of this section, it is worth considering the process opposite to cationic polymerization, viz. enthalpy-driven PIB depolymerization. In the presence of CF 3 SO 3 H, HRPIB depolymerized rapidly at room temperature in aromatic solvents; the process was presumably favored by the formation of stronger C sp2 −C sp3 bonds in tert-butylarene in comparison with weaker C sp3 −C sp3 bonds in PIB [51]. This reaction is a particular case of reversible polymerization that has attracted the attention of researchers in recent years [52]; its importance for the synthesis of HRPIB is shaped by the understanding that aromatic solvents can not only terminate cationic polymerization of IB, but also depolymerize the reaction products. That is why IB polymerization should be carried out in bulk or in inert solvents (saturated hydrocarbons, MeCl, etc.).

Industrial Production of PIBs
Current technologies of the production of PIBs and their practical applications were considered previously in a number of reviews [1, 3,53]. Butyl rubber [42] and halogenated butyl rubbers [1] occupy a significant share of the PIB market; however, the topic of copolymerization of IB with dienes and subsequent chemical modifications are beyond the subject of our review. From the mechanistic point of view and taking into account the catalysts used, the controlled syntheses of low-MW HRPIBs, as well as medium-and high-MW PIBs, are of practical interest (Scheme 5).

Industrial Production of PIBs
Current technologies of the production of PIBs and their practical applications were considered previously in a number of reviews [1, 3,53]. Butyl rubber [42] and halogenated butyl rubbers [1] occupy a significant share of the PIB market; however, the topic of copolymerization of IB with dienes and subsequent chemical modifications are beyond the subject of our review. From the mechanistic point of view and taking into account the catalysts used, the controlled syntheses of low-MW HRPIBs, as well as medium-and high-MW PIBs, are of practical interest (Scheme 5). Depending on the target MW values and chain-end structure, different catalytic systems and polymerization techniques have been applied in practice. In particular, BF3/ROH catalysts are successfully used for the production of low-MW HRPIBs [13][14][15], whereas AlCl3/ROH (or H2O) and related systems are used for the synthesis of medium-MW polymers [33]. High-MW PIBs dominated the market in 2023 in monetary terms [4], whereas low-MW HRPIBs continue to be the leading volume products, making the study of new efficient catalysts for their synthesis much interesting.

Further Development of BF3/ROH Catalysts
Although BF3/ROH catalysts have been used in bulk production of HRPIB in recent decades, further studies on the optimization of this system is still underway. Outstanding breakthroughs in this field seem unlikely; however, the results of actual studies in this field have been published. As shown in [54], BF3/MeOH can be used as a catalyst for HRPIB production from the technical mixture of C4 hydrocarbons (45 wt% IB; the rest of the components were but-1-ene, but-2-enes and butanes). Polymerization at -20 °C using 0.5 wt% of the catalyst resulted in HRPIB with an Mn of ~1 kDa. Polymerization of C4 hydrocarbon mixtures containing 36-46 wt% of IB using continuous-flow addition of the BF3/MeOH catalyst resulted in HRPIB with Mn = 0.93-1.23 kDa and an exo-olefin content of 71.2-82.3% [55]. Continuous-flow method was also applied for the production of low-MW HRPIB (C8-C28 mixture with 31 wt% of C16) [56].
Polymerization of IB, diluted with an equal amount of butane, with the use of 0.25% BF3 and i PrOH (2:3 ratio) at -27 °C yielded HRPIB with Mn = 2.37 kDa and an exo-olefin content of 87.5%; at -17 °C HRPIB, it yielded HRPIB with lower Mn of 0.96 kDa and an exoolefin content of 93.8% [57]. Further increase in the reaction temperature resulted in a decrease in IB conversion and Mn content of HRPIB. During polymerization of pure IB, continuous-flow addition of the catalyst resulted in increase in the exo-olefin content up to 88% [58].
To produce low-MW HRPIB, the use of IB dimer of the formula t BuCH2C(Me)=CH2 as a chain transfer agent and 2,6-di-tert-butyl-4-methylphenol as a polymerization-retarding agent were proposed in the patent of the TPG Group [59]. Addition of polymerizationretarding agent resulted in a decrease in the ĐM values of PIBs.
For BF3-catalyzed processes, IB conversions are typically 75-85%; however, all the efforts to drive the reaction to higher IB conversions reduced the vinylidene content Depending on the target MW values and chain-end structure, different catalytic systems and polymerization techniques have been applied in practice. In particular, BF 3 /ROH catalysts are successfully used for the production of low-MW HRPIBs [13][14][15], whereas AlCl 3 /ROH (or H 2 O) and related systems are used for the synthesis of medium-MW polymers [33]. High-MW PIBs dominated the market in 2023 in monetary terms [4], whereas low-MW HRPIBs continue to be the leading volume products, making the study of new efficient catalysts for their synthesis much interesting.

Further Development of BF 3 /ROH Catalysts
Although BF 3 /ROH catalysts have been used in bulk production of HRPIB in recent decades, further studies on the optimization of this system is still underway. Outstanding breakthroughs in this field seem unlikely; however, the results of actual studies in this field have been published. As shown in [54], BF 3 /MeOH can be used as a catalyst for HRPIB production from the technical mixture of C4 hydrocarbons (45 wt% IB; the rest of the components were but-1-ene, but-2-enes and butanes). Polymerization at −20 • C using 0.5 wt% of the catalyst resulted in HRPIB with an M n of~1 kDa. Polymerization of C4 hydrocarbon mixtures containing 36-46 wt% of IB using continuous-flow addition of the BF 3 /MeOH catalyst resulted in HRPIB with M n = 0.93-1.23 kDa and an exo-olefin content of 71.2-82.3% [55]. Continuous-flow method was also applied for the production of low-MW HRPIB (C8-C28 mixture with 31 wt% of C16) [56].
Polymerization of IB, diluted with an equal amount of butane, with the use of 0.25% BF 3 and i PrOH (2:3 ratio) at −27 • C yielded HRPIB with M n = 2.37 kDa and an exo-olefin content of 87.5%; at −17 • C HRPIB, it yielded HRPIB with lower M n of 0.96 kDa and an exo-olefin content of 93.8% [57]. Further increase in the reaction temperature resulted in a decrease in IB conversion and M n content of HRPIB. During polymerization of pure IB, continuous-flow addition of the catalyst resulted in increase in the exo-olefin content up to 88% [58].
To produce low-MW HRPIB, the use of IB dimer of the formula t BuCH 2 C(Me)=CH 2 as a chain transfer agent and 2,6-di-tert-butyl-4-methylphenol as a polymerization-retarding agent were proposed in the patent of the TPG Group [59]. Addition of polymerizationretarding agent resulted in a decrease in the Ð M values of PIBs.
For BF 3 -catalyzed processes, IB conversions are typically 75-85%; however, all the efforts to drive the reaction to higher IB conversions reduced the vinylidene content through isomerization. Recycling of unreacted IB requires neutralizing the catalyst and using an impurity adsorption column for removing halogen acid from the distilled material [57]. In this way, monomer recycling implies irreversible consumption of the BF 3 -based catalyst and an accumulation of fluorine-and boron-containing wastewater that later need handling and treatment.
Theoretical study of the reaction of IB with BF 3 -H 2 O complex in n-heptane media was conducted recently by Babkin et al. [60] (ab initio, 6-311G** level of theory). They showed that the presence of n-heptane molecules leads to a decrease in the activation energy of the initiation stage. However, a visible difference between optimized structures of the reaction products in the absence and in the presence of n-heptane (Me 3 C . . . FBF 2 (OH) and Me 2 CCH 2 H . . . O(H)BF 3 , respectively) raises reasonable questions about the reliability of the modeling.
In 2019, mixed catalytic system comprising BF 3 /EtOH and TiCl 4 /H 2 O was studied by Liu et al. [61]. They detected a strong synergistic effect between catalyst components; PIBs with M n = 20-60 kDa were obtained.
In conclusion of this section, successful heterogenization of BF 3 -based catalysts should be mentioned. NTP Tec patent [62] describes IB polymerization with the use of calcined (700 • C) mesoporous γ-Al 2 O 3 or silica-aluminas, treated with BF 3 or BF 3 /MeOH, respectively. In the first case, polymerization of 63 g of IB using 0.4 g of the catalyst (−5 • C for 40 min) afforded HRPIB with (76% conversion, M n = 900 Da, and exo-olefin content of 82%). When using silica-alumina/BF 3 /MeOH catalyst (0.2 g) in 45 g of IB polymerized at −5 • C within 20 min with a conversion of 91%, HRPIB with (M n = 910 Da and exo-olefin content of 80%) was obtained.

AlCl 3 /H 2 O and Related Systems
AlCl 3 -catalyzed polymerization of IB produces conventional PIBs which contain up to 90% of internal double bond end-groups (trisubstituted and tetrasubstituted) [33]. However, since H 2 O can be used as an initiator for the first-generation AlCl 3 -based catalysts, the reaction of H 2 O with AlCl 3 with the formation of catalytically active species is of particular interest and worthy of separate consideration.
The first molecular-level study of the acid-catalyzed IB polymerization was carried out by Johnson's group in 2018 on the H 2 O/AlCl 3 system [63]. Generally accepted view on the nature of the catalytic species as AlCl 3 -OH 2 was not confirmed with DFT calculations (BP86/def2-SVP level of theory; gas phase): the formation of Al 2 species of the formula Cl 3 Al(µ-OH)(µ-H)AlCl 3 (Scheme 6) proved to be preferable. In particular, calculated E a for the initiation of IB polymerization with AlCl 3 -OH 2 was 21.6 kcal·mol −1 . In contrast, the value of E a for Cl 3 Al(µ-OH)(µ-H)AlCl 3 -initiated process was only 5.2 kcal·mol −1 , and subsequent stages were found to be exothermic ( Figure 8). Calculations of the solvent effect (CH2Cl2, conductor-like screening continuum solvation model) resulted in substantial decrease in the activation barriers (E2 = 3.2 kcal·mol -1 for initiation stage), indicating that Cl3Al(µ-OH)(µ-H)AlCl3 is likely to be an active catalyst in polar solvents at strongly negative temperatures. The reaction sequence can be attributed to "living" IB polymerization. However, possible reaction pathways of chain release via proton transfer to monomer of elimination with a formation of Cl3Al(µ-OH)(µ-Calculations of the solvent effect (CH2Cl2, conductor-like screening continuum solvation model) resulted in substantial decrease in the activation barriers (E2 = 3.2 kcal·mol -1 for initiation stage), indicating that Cl3Al(µ-OH)(µ-H)AlCl3 is likely to be an active catalyst in polar solvents at strongly negative temperatures. The reaction sequence can be attributed to "living" IB polymerization. However, possible reaction pathways of chain release via proton transfer to monomer of elimination with a formation of Cl3Al(µ-OH)(µ-H)AlCl3 and HRPIB were not analyzed. An attempt to use the microflow reaction system for AlCl3-catalyzed polymerization of IB [64] resulted in obtaining PIBs with Mn = 20-100 kDa; the content of exo-olefinic endgroups in the reaction products was low. In conclusion, it should be noted that AlCl3 (or HCl/Et2AlCl) was used as a catalyst for low-temperature IB copolymerization with polyfarnesene to obtain highly branched PIBs [65]. The nature of the end-groups was not considered in this patent. Calculations of the solvent effect (CH 2 Cl 2 , conductor-like screening continuum solvation model) resulted in substantial decrease in the activation barriers (E 2 = 3.2 kcal·mol −1 for initiation stage), indicating that Cl 3 Al(µ-OH)(µ-H)AlCl 3 is likely to be an active catalyst in polar solvents at strongly negative temperatures. The reaction sequence can be attributed to "living" IB polymerization. However, possible reaction pathways of chain release via proton transfer to monomer of elimination with a formation of Cl 3 Al(µ-OH)(µ-H)AlCl 3 and HRPIB were not analyzed.
An attempt to use the microflow reaction system for AlCl 3 -catalyzed polymerization of IB [64] resulted in obtaining PIBs with M n = 20-100 kDa; the content of exo-olefinic end-groups in the reaction products was low. In conclusion, it should be noted that AlCl 3 (or HCl/Et 2 AlCl) was used as a catalyst for low-temperature IB copolymerization with polyfarnesene to obtain highly branched PIBs [65]. The nature of the end-groups was not considered in this patent.

Synthesis of PIB-Cl
In the absence of Brønsted acids, initiation of IB polymerization usually requires the use of tert-alkyl chloride (cumyl chloride, t BuCH 2 CMe 2 Cl, t BuCl etc) as an initiator [66]. At the same time, in the first study of living IB polymerization, MeC(O)OCMe 2 Ph/BCl 3 and related systems were used [67]. Commonly, the similar systems invariably consist of two components, namely an initiator (cationogen) and a coinitiator (Lewis acid); their development demonstrates that proper combination of these ingredients is of prime importance in controlled cationic polymerizations [2,23].
TiCl 4 /tert-alkyl chloride have long been successfully used in living low-temperature cationic polymerization of IB. Among recent publications, the study of Wu et al. [68] deserves a separate mention. They have demonstrated that 1,3-bis(1-chloro-1-methyethyl)benzene, a bifunctional initiator commonly used worldwide in carbocationic polymerization, in the case of IB can form indane derivative (Scheme 7a). To avoid this side process, 5-substituted derivative (Scheme 7b), similar to the one previously suggested by Kennedy's group [69], was proposed. Living IB polymerization was terminated by buta-1,3-diene and MeOH with the formation of PIB-Cl containing highly reactive and easily hydrolyzable CH=CHCH 2 Cl end-fragments. serves a separate mention. They have demonstrated that 1,3-bis(1-chloro-1methyethyl)benzene, a bifunctional initiator commonly used worldwide in carbocationic polymerization, in the case of IB can form indane derivative (Scheme 7a). To avoid this side process, 5-substituted derivative (Scheme 7b), similar to the one previously suggested by Kennedy's group [69], was proposed. Living IB polymerization was terminated by buta-1,3-diene and MeOH with the formation of PIB-Cl containing highly reactive and easily hydrolyzable CH=CHCH2Cl end-fragments.

First-Generation Catalysts: Metal Chlorides/Ethers
The very idea of developing new polymerization catalysts entails the use of Lewis base as a reaction component. The main potential advantage of these catalytic systems is the ability to conduct IB polymerization at temperatures above zero Celsius with a retention of high exo-olefin content in the reaction products. The first group of the catalysts, potentially able to replace BF 3 /ROH systems in the synthesis of HRPIBs, were the complexes of metal halides with ethers.

AlCl 3 /R 2 O and Related Systems
In 2010, Kostjuk et al. proposed a simple and prospective initiating system for the synthesis of HRPIB using cumyl alcohol and AlCl 3 /Bu 2 O [70]; similar type of the initiating system was reported independently by Wu et al. [33]. The most valuable finding of the study by Kostjuk et al. was the gradual decrease in IB conversion and MW of PIB when reaction temperature was increased from −60 to −20 • C [70]. On the other hand, increasing the temperature did not significantly influence the content of exo-olefinic end-groups. High activities and regioselectivity were achieved in CH 2 Cl 2 and toluene solvents, whereas a number of limitations (lower activity and exo-olefin content) were observed when n-hexane was used as a reaction media [71].
In [33], Wu et al. have clearly demonstrated the marked difference in catalytic behavior of AlCl 3 /H 2 O and AlCl 3 /Bu 2 O catalytic systems. As can be seen in Figure 9, when using AlCl 3 /H 2 O vinylidene route to produce HRPIB is the minor one; tri-and tetra-substituted olefins are the main reaction products.

AlCl3/R2O and Related Systems
In 2010, Kostjuk et al. proposed a simple and prospective initiating system for the synthesis of HRPIB using cumyl alcohol and AlCl3/Bu2O [70]; similar type of the initiating system was reported independently by Wu et al. [33]. The most valuable finding of the study by Kostjuk et al. was the gradual decrease in IB conversion and MW of PIB when reaction temperature was increased from -60 to -20 °C [70]. On the other hand, increasing the temperature did not significantly influence the content of exo-olefinic end-groups. High activities and regioselectivity were achieved in CH2Cl2 and toluene solvents, whereas a number of limitations (lower activity and exo-olefin content) were observed when n-hexane was used as a reaction media [71].
In [33], Wu et al. have clearly demonstrated the marked difference in catalytic behavior of AlCl3/H2O and AlCl3/Bu2O catalytic systems. As can be seen in Figure 9, when using AlCl3/H2O vinylidene route to produce HRPIB is the minor one; tri-and tetra-substituted olefins are the main reaction products. For AlCl3/Bu2O catalytic system with H2O as an initiator, exo-olefinic fragments have proven to be the major end-groups [33]. Apparently, weakly coordinating anion [Bu2O … AlCl3(OH)] -promotes the rapid β-proton abstraction from -CH3 of the growing chain-ends with the formation of exo-olefinic end-groups and decreases the carbenium ion For AlCl 3 /Bu 2 O catalytic system with H 2 O as an initiator, exo-olefinic fragments have proven to be the major end-groups [33]. Apparently, weakly coordinating anion [Bu 2 O . . . AlCl 3 (OH)] − promotes the rapid β-proton abstraction from -CH 3 of the growing chain-ends with the formation of exo-olefinic end-groups and decreases the carbenium ion rearrangements. In addition, the small amount of excess Bu 2 O probably could act as a Lewis base to suppress isomerization. For the AlCl 3 / i Pr 2 O system, similar results were reported; the main signals in 1 H NMR spectrum corresponded to the signals of exo-olefinic end-groups ( Figure 4). When using the AlCl 3 /Bu 2 O catalytic system, at 0 • C HRPIBs with an M n of 1.5-3.3 kDa and an exo-olefin content of 76.5-90% were obtained (the latter value depended on IB conversion; at high conversions the vinylidene content decreased). At 20 • C and 45% IB conversion, the exo-olefin content was 82%.
Faust et al. [72] have proven that cumyl alcohol is not a true initiator in conjunction with the AlCl 3 /R 2 O catalytic system (this conclusion was drawn based on results of polymerization experiments in the presence of DTP that showed zero activity); the process is initiated with water. Mechanistic studies suggested that the reaction of water with AlCl 3 ·R 2 O yields H + [AlCl 3 OH] − , which initiates the polymerization, and free R 2 O, which abstracts β-proton. In other words, the role of the AlCl 3 ·R 2 O complex is to deliver R 2 O molecules to close proximity of the propagating PIB end. In [71], Kostjuk et al. showed that the initiation with water is preferable; when using AlCl 3 ·Bu 2 O/H 2 O, PIBs with 89-94% content of exo-olefinic end-groups were obtained in high yield (>85%) within 10 min; M n of PIBs obtained was controlled in a range of 1-10 kDa by changing the reaction temperature from −40 to 30 • C.
During the study of IB polymerization, co-initiation by AlCl 3 complexes with different linear (Et 2 O, Bu 2 O, Am 2 O, Hex 2 O, MeOPh) and branched ( i Pr 2 O, t BuOMe) ethers in toluene or CH 2 Cl 2 at 20 • C was investigated [73]; the complexes with ethers of moderate basicity (AlCl 3 ·Bu 2 O and AlCl 3 · i Pr 2 O) afforded PIBs with the highest exo-olefin content (80-95%) and IB conversion, while the use of PhOMe led to the conventional PIBs containing mainly tri-and tetra-substituted olefinic end-groups. The complexes of AlCl 3 with t BuOMe and THF were inactive due to the ether cleavage by AlCl 3 . The polymerization of IB in the presence of AlCl 3 ·Bu 2 O or AlCl 3 · i Pr 2 O as co-initiators in n-hexane at elevated temperatures (−20 • C to 10 • C) and at high monomer concentrations (up to [IB]~7.8 M) afforded low-MW PIBs (M n~1 -8 kDa) with Ð M~2 , containing 70-85% exo-olefinic terminal groups [73]. The 1:1 complexes of AlCl 3 with linear ethers were found to be stable at ambient temperatures; however, in the case of the AlCl 3 / i Pr 2 O system, the formation of less stable and more exchangeable complexes was detected. It is noteworthy that both polymerization rate and chain-end structure of PIB were vulnerable to AlCl 3 /ether ratio: as can be seen in Figure 10, at the ether-starved conditions (AlCl 3 /Bu 2 O = 1:0.8) rapid polymerization of IB took place, and the distribution of end-groups was similar to one obtained with the ether-free initiating system. On the contrary, the use of slight excess of ether (AlCl 3 /Bu 2 O = 1:1.2) resulted in a significant decrease in the reaction rate and exo-olefin content, and endo-olefin was the main side product. Another significant result reported in [73] was the control of M n of PIB within the limits of 3-8 kDa by the length of R in AlCl 3 ·R 2 O.
abstracts β-proton. In other words, the role of the AlCl3·R2O complex is to deliver R2O molecules to close proximity of the propagating PIB end. In [71], Kostjuk et al. showed that the initiation with water is preferable; when using AlCl3·Bu2O/H2O, PIBs with 89-94% content of exo-olefinic end-groups were obtained in high yield (>85%) within 10 min; Mn of PIBs obtained was controlled in a range of 1-10 kDa by changing the reaction temperature from -40 to 30 °C.
During the study of IB polymerization, co-initiation by AlCl3 complexes with different linear (Et2O, Bu2O, Am2O, Hex2O, MeOPh) and branched ( i Pr2O, t BuOMe) ethers in toluene or CH2Cl2 at 20 °C was investigated [73]; the complexes with ethers of moderate basicity (AlCl3·Bu2O and AlCl3· i Pr2O) afforded PIBs with the highest exo-olefin content (80-95%) and IB conversion, while the use of PhOMe led to the conventional PIBs containing mainly tri-and tetra-substituted olefinic end-groups. The complexes of AlCl3 with t BuOMe and THF were inactive due to the ether cleavage by AlCl3. The polymerization of IB in the presence of AlCl3·Bu2O or AlCl3· i Pr2O as co-initiators in n-hexane at elevated temperatures (-20 °C to 10 °C) and at high monomer concentrations (up to [IB] ~ 7.8 M) afforded low-MW PIBs (Mn ~1-8 kDa) with ÐM ~ 2, containing 70-85% exo-olefinic terminal groups [73]. The 1:1 complexes of AlCl3 with linear ethers were found to be stable at ambient temperatures; however, in the case of the AlCl3/ i Pr2O system, the formation of less stable and more exchangeable complexes was detected. It is noteworthy that both polymerization rate and chain-end structure of PIB were vulnerable to AlCl3/ether ratio: as can be seen in Figure 10, at the ether-starved conditions (AlCl3/Bu2O = 1:0.8) rapid polymerization of IB took place, and the distribution of end-groups was similar to one obtained with the ether-free initiating system. On the contrary, the use of slight excess of ether (AlCl3/Bu2O = 1:1.2) resulted in a significant decrease in the reaction rate and exoolefin content, and endo-olefin was the main side product. Another significant result reported in [73] was the control of Mn of PIB within the limits of 3-8 kDa by the length of R in AlCl3·R2O. During further studies of the complexes AlCl 3 ·Bu 2 O and AlCl 3 · i Pr 2 O, IB polymerizations were conducted at −20 to 20 • C in toluene or n-hexane media [74]. In both solvents, AlCl 3 · i Pr 2 O showed higher activity and provided higher exo-olefin content than AlCl 3 ·Bu 2 O; the presence of small excess of i Pr 2 O over AlCl 3 (5-10 mol%) resulted in an increase in vinylidene selectivity in some cases. The marked difference between toluene and n-hexane was in the effect of temperature on the exo-olefin content, which increased in toluene and decreased in n-hexane as the temperature increased.
The use of a microflow reactor system for IB polymerization on AlCl 3 · i Pr 2 O within ≤12 s was suggested by Lu et al. [75]. The temperature window was extended from −20 to 50 • C, and the M n was adjustable between 0.5 and 15 kDa. However, exo-olefin content in the reaction products of the first experiments was 76% or less. Optimization of the reaction conditions in CH 2 Cl 2 was successful: at 20 • C the selectivity of 89% was achieved with 63% conversion of IB (M n = 1.29 kDa, Ð M = 1.7); the excess of i Pr 2 O over AlCl 3 was necessary to avoid the existence of free AlCl 3 [75]. In further studies of the AlCl 3 · i Pr 2 O system [76], the effect of ethyl benzoate (EB) was evaluated. The reactions at −30 • C were also conducted in a microflow reactor, and PIBS with M n = 14.3-47.5 kDa and an exo-olefin content of 29-76% were obtained. In more recent works, some aspects of the interactions in AlCl 3 /R 2 O/H 2 O system were detailed [77]. The value of these results for the synthesis of commercially demanded HRPIBs seems questionable.
The use of AlCl 3 · i Pr 2 O as a co-initiator of the polymerization of C4 hydrocarbon feed was studied by Kostjuk et al. [78]. The reaction proceeded at a considerably lower rate in comparison with the polymerization of pure IB yielding PIBs with higher M n and exoolefinic end-group content. At 0 • C, M n was 4.37 kDa, that is too high value in comparison with M n required for practical applications of low-MW HRPIBs (≤2.3 kDa) [12].
Two-component catalytic system, based on AlCl 3 · i Pr 2 O and AlCl 3 ·Et 2 O, was studied recently in n-hexane by Lu et al. [79]. In this system, AlCl 3 · i Pr 2 O was confirmed to be the key component that stabilized carbenium ions, and the function of AlCl 3 ·Et 2 O was to promote proton elimination. Polymerizations were conducted in a microflow system with [IB] = 0.33-1.30 M in the presence of H 2 O as an initiator. The yields of IBs were up to 89% after 10 min, and the exo-olefin group content was 60-75%.
In recent patents [80,81], the results of the study of AlCl 3 /R 2 O catalyst in IB polymerization (4 M in hexanes) at 0 • C have been presented. As can be seen in Table 2, the use of Bu 2 O instead of i Pr 2 O provided higher exo-olefinic content, but lowered the catalytic activity. However, the use of ether mixtures allowed to optimize the process (Table 2, Entry 8); the replacement of the solvent with toluene, as well as Bu 2 O with THF, had no effect. The further increase in the polymerization temperature to 30 • C for AlCl 3 /R 2 O systems (activated with water in 1:1 [Al]/[H 2 O] ratio) resulted in substantial loss of the selectivity of the formation of exo-olefinic groups (12-37%) [82].
The AlCl 3 ·phenetole/TiCl 4 ·H 2 O system was studied by Liu et al. [83]. They found that mixed catalyst exhibited activities 1.2-3 times higher than those for AlCl 3 /phenetole, and more than an order of magnitude higher than those for TiCl 4 /H 2 O, which indicated a notable synergistic effect. However, relatively high-MW PIBs were obtained, and chain-end analysis was not conducted in this work. The results of the study by Deng et al. [84], that describes catalytic experiments with C4 mixed hydrocarbon feed, AlCl 3 /toluene and additional donors (Et 2 O and ethyl acetate), are difficult to analyze because of the absence of data on chain-end structures of relatively low-MW PIBs obtained.
In conclusion of this section, it is appropriate to mention the recent work of the chemists from Beijing University [85] who studied IB polymerization using the AlCl 3 /Bu 2 O catalytic system in a rotating packed bed reactor. The effects of operating parameters in-  [32]. In the absence of ethers, conventional non-reactive PIB was formed (see Figure 3 in Section 2.1). Among three ethers, i Pr 2 O demonstrated the highest activity, and the maximum exo-olefin content (up to 90% at 0 • C) was observed when using 3 ] ratio resulted in a decrease in polymerization rate and an increase in vinylidene selectivity, which generally confirms the possible role of R 2 O in β-proton abstraction proposed by Faust [72].
Up to 85% exo-olefin content was achieved in IB polymerization when using MCl 3 · i Pr 2 O (M = Fe, Ga) complexes and t BuCl or t BuCH 2 CMe 2 Cl initiators [86]. Di-n-alkyl ethers did not form catalytically active complexes. Also of interest was the inertness of AlCl 3 · i Pr 2 O analog in the ionization of t BuCH 2 CMe 2 Cl. Proposed reaction mechanism is presented in Scheme 8; t BuCH 2 CMe 2 Cl is ionized by Lewis acid/R 2 O complex to yield t BuCH 2 CMe 2 + , which initiates IB polymerization, and this ultimately results in the release of R 2 O from the complex. The propagating PIB + may undergo proton elimination to yield PIB exo-olefin or may be deactivated by ion collapse to yield PIB-Cl, which can be reactivated by Lewis acid. The abstracted proton may then start a new polymer chain after addition to the IB molecule. More detailed kinetic studies of the t BuCl/FeCl 3 · i Pr 2 O system [87] only confirmed the results of the prior research [86], which is that the use of EtOCH 2 CH 2 Cl and O(CH 2 CH 2 Cl) 2 instead of i Pr 2 O resulted in a decrease in catalytic activity and exo-selectivity. The role of the ether molecule in IB polymerization was substantially clarified during the studies of living IB polymerization, initiated by t BuCH2CMe2Cl/TiCl4 and terminated by i Pr2O or s Bu2O with a formation of exo-olefinic end-groups after treatment with MeOH [44]. In this study, the formation of PIB-O i Pr2 + ions was proven using 1 H NMR spectroscopy at -70 °C (Figure 11). When branched ethers were used, 100% exo-selectivity was observed; however, experiments with t BuOMe, Bu2O and Et2O resulted in the formation of PIBs with 88, 82 and 69% of vinylidene PIB, respectively. The role of the ether molecule in IB polymerization was substantially clarified during the studies of living IB polymerization, initiated by t BuCH 2 CMe 2 Cl/TiCl 4 and terminated by i Pr 2 O or s Bu 2 O with a formation of exo-olefinic end-groups after treatment with MeOH [44]. In this study, the formation of PIB-O i Pr 2 + ions was proven using 1 H NMR spectroscopy at −70 • C ( Figure 11). When branched ethers were used, 100% exo-selectivity was observed; however, experiments with t BuOMe, Bu 2 O and Et 2 O resulted in the formation of PIBs with 88, 82 and 69% of vinylidene PIB, respectively.
M.), 81% conversion and 78% exo-selectivity were achieved after 20 min for R = i Pr. However, all experiments with FeCl3·R2O in aliphatic media failed to achieve >80% content of exo-PIB.
The role of the ether molecule in IB polymerization was substantially clarified during the studies of living IB polymerization, initiated by t BuCH2CMe2Cl/TiCl4 and terminated by i Pr2O or s Bu2O with a formation of exo-olefinic end-groups after treatment with MeOH [44]. In this study, the formation of PIB-O i Pr2 + ions was proven using 1 H NMR spectroscopy at -70 °C ( Figure 11). When branched ethers were used, 100% exo-selectivity was observed; however, experiments with t BuOMe, Bu2O and Et2O resulted in the formation of PIBs with 88, 82 and 69% of vinylidene PIB, respectively.

Metal Chlorides/ROH and Related Systems
The possibility of direct synthesis of HRPIB on a H2O/TiCl4/alcohol initiating system was studied by Wu et al. [89]. IB solution in n-hexane and mixed C4 fraction feed ([IB] = 2.9 M) were used in polymerization experiments. H2O/TiCl4/ i AmOH initiating system exhibited high selectivity toward the cationic polymerization of IB from the mixed C4 fractions, resulting in the formation of HRPIBs with Mn = 1.2-1.6 kDa and ĐM = 1.5-1.9; vinylidene selectivity was 80%. A possible reaction mechanism (Scheme 9) included the reaction between ROH and TiCl4 with the formation of TiCl3(OR) and HCl. In the presence of

Metal Chlorides/ROH and Related Systems
The possibility of direct synthesis of HRPIB on a H 2 O/TiCl 4 /alcohol initiating system was studied by Wu et al. [89]. IB solution in n-hexane and mixed C4 fraction feed ([IB] = 2.9 M) were used in polymerization experiments. H 2 O/TiCl 4 / i AmOH initiating system exhibited high selectivity toward the cationic polymerization of IB from the mixed C4 fractions, resulting in the formation of HRPIBs with M n = 1.2-1.6 kDa and Ð M = 1.5-1.9; vinylidene selectivity was 80%. A possible reaction mechanism (Scheme 9) included the reaction between ROH and TiCl 4 with the formation of TiCl 3 (OR) and HCl. In the presence of TiCl 4 , protic initiator H[Ti 2 Cl 9-n (OR) n ] (n = 0-2) formed and reacted with IB leading to tert-butyl cation and PIB. Fast initiation from HCl resulted in higher monomer conversion, lower molecular weight, and narrower MWD of PIBs at higher concentration of ROH. The counter-anion [Ti 2 Cl 9-n (OR) n ] − affected the selective β-proton elimination from -CH 3 groups in the growing PIB chain-ends, and H[Ti 2 Cl 9-n (OR) n ] re-initiated IB polymerization.  Possible mechanism for synthesis of HRPIB via cationic polymerization of isobutylene with H2O/TiCl4/ROH ( i PrOH or i AmOH) initiating system [89].
In addition to etherates, the complex of FeCl3 with i PrOH was studied in IB polymerization [34]. When using water as an initiator, this catalyst have demonstrated higher exoselectivity in non-polar solvents, which is distinct from AlCl3/R2O systems.
Recently, Kostjuk et al. have studied cationic polymerization of IB with dicumyl chloride/FeCl3/ i PrOH in CH2Cl2/n-hexane 40/60 at −80 °C. They showed that this initiating system afforded bifunctional low-MW PIBs (Mn 1.25-21 kDa, ĐM ≤ 1.28) with -CMe2Cl endgroups that can be converted to HRPIBs by end quenching with i Pr2O [90]. In addition to etherates, the complex of FeCl 3 with i PrOH was studied in IB polymerization [34]. When using water as an initiator, this catalyst have demonstrated higher exo-selectivity in non-polar solvents, which is distinct from AlCl 3 /R 2 O systems. ratio and 0 • C, exo-olefin content in PIB was 70%, but its value was increased to 90% with the introduction of additional amounts of O(CH 2 CH 2 Cl) 2 into the reaction. The calculated binding energies of ethers with EtAlCl 2 (DFT, B3LYP/6-31G* level of theory) were −7.5, −7.1 and −5.1 kcal·mol −1 for Et 2 O, EtOCH 2 CH 2 Cl and O(CH 2 CH 2 Cl) 2 , respectively, which correlates with the ability of EtAlCl 2 etherate to the displacement with t BuCl with subsequent ionization and polymerization. Depending on the reaction temperature (from −20 to 10 • C), HRPIBs with an M n of 1-4.5 kDa were obtained. Another study [92] was focused on the experimental procedure and have only demonstrated the importance of all experimental parameters, including the material of the reaction vessels: when chrome stainless steel reactor was used, exo-selectivity of IB polymerization decreased from 90 to~70%. In the studies of the kinetics and mechanism of IB polymerization, initiated by t BuCl and catalyzed by EtAlCl 2 ·O(CH 2 CH 2 Cl) 2 in hexanes at 0 • C, Faust 2 ] ratio by postulating an equilibrium between dormant oxonium and active carbenium ions [93]. An increase in the polymerization rate in the presence of water was explained in this work by the influence of water on oxonium/carbenium ion equilibrium. The effect of impurities in the reaction mixture on IB polymerization (Scheme 10) was studied in [94]. In this work, it was shown that low levels of some organic impurities retard the polymerization of IB initiated by t BuCl and catalyzed by EtAlCl 2 /O(CH 2 CH 2 Cl) 2 in hexanes at 0 • C. For efficient synthesis of HRPIB, the impurity scavenger Lewis acid needs to be insoluble in hexanes and should not polymerize IB; FeCl 3 was found to be efficient scavenger. The EtAlCl2/O(CH2CH2Cl)2 initiating system was further optimized by using micromixing module in the polymerization system [95]. HRPIBs with a 100% conversion were obtained within 5 min in the micromixing enhanced system at 20 °C, in comparison to only 30% conversion after 10  The EtAlCl 2 /O(CH 2 CH 2 Cl) 2 initiating system was further optimized by using micromixing module in the polymerization system [95]. HRPIBs with a 100% conversion were obtained within 5 min in the micromixing enhanced system at 20 • C, in comparison to only Further studies on the EtAlCl 2 /O(CH 2 CH 2 Cl) 2 initiating system at elevated temperatures showed that the reaction rate could be increased by increasing the polymerization temperature with minimal loss of the exo-olefin content [96]. The increase in the polymerization rate was attributed to the increased steady state concentration of carbenium ions; polymerization selectivity depended on the stability of the corresponding tert-alkyloxonium cations (resting states of the process, see Scheme 11). The example of tert-butyloxonium shows that the oxonium ion is stable up to 15 • C, and slowly decomposes at 20 • C (~15% after 6 min) with the formation of ethylene, isobutane and AlCl 3 (Scheme 11). This process was clearly confirmed with 1 H NMR spectroscopy. Scheme 10. Suggested modified mechanism of IB polymerization in the presence of impurities. Reprinted with permission from [94]. Copyright (2017) Wiley-VCH Verlag GmbH & Co.
The EtAlCl2/O(CH2CH2Cl)2 initiating system was further optimized by using micromixing module in the polymerization system [95]. HRPIBs with a 100% conversion were obtained within 5 min in the micromixing enhanced system at 20 °C, in comparison to only 30% conversion after 10 min in a conventional reactor. The Mn and exo-olefin content of HRPIBs were adjusted conveniently to the ratio of EtAlCl2 to O(CH2CH2Cl)2 and monomer concentration. As a result, HRPIBs with Mn = 0.8-2.4 kDa, ĐM = 2.15-2.85 and exoolefin content of 63-94% were obtained.
Further studies on the EtAlCl2/O(CH2CH2Cl)2 initiating system at elevated temperatures showed that the reaction rate could be increased by increasing the polymerization temperature with minimal loss of the exo-olefin content [96]. The increase in the polymerization rate was attributed to the increased steady state concentration of carbenium ions; polymerization selectivity depended on the stability of the corresponding tert-alkyloxonium cations (resting states of the process, see Scheme 11). The example of tert-butyloxonium shows that the oxonium ion is stable up to 15 °C, and slowly decomposes at 20 °C (~15% after 6 min) with the formation of ethylene, isobutane and AlCl3 (Scheme 11). This process was clearly confirmed with 1 H NMR spectroscopy. Scheme 11. Formation of the oxonium ion from EtAlCl2/O(CH2CH2Cl)2 + t BuCl and its thermal decomposition [96].
From the point of view of industrial applications, the use of C4 hydrocarbon feed instead of pure IB is highly attractive. Faust et al. studied the polymerization of IB in the presence of other C4 olefins, but-1-ene, cis-but-2-ene, and buta-1,3-diene using the t BuCl/EtAlCl2/O(CH2CH2Cl)2 initiating system in hexanes at 0 °C [97]. As shown in Figure  12, butenes were only marginally involved in copolymerization with IB; buta-1,3-diene was more active, but its involvement was negligible due to its low content in the C4 feed. Scheme 11. Formation of the oxonium ion from EtAlCl 2 /O(CH 2 CH 2 Cl) 2 + t BuCl and its thermal decomposition [96].
From the point of view of industrial applications, the use of C4 hydrocarbon feed instead of pure IB is highly attractive. Faust et al. studied the polymerization of IB in the presence of other C4 olefins, but-1-ene, cis-but-2-ene, and buta-1,3-diene using the t BuCl/EtAlCl 2 /O(CH 2 CH 2 Cl) 2 initiating system in hexanes at 0 • C [97]. As shown in Figure 12, butenes were only marginally involved in copolymerization with IB; buta-1,3diene was more active, but its involvement was negligible due to its low content in the C4 feed. IB polymerization using H2O and RAlCl2·n i Pr2O (R = Me, Et, i Bu; n = 0.6-1) in n-hexane at 10 °C was studied by Kostjuk et al. in 2014 [98]. EtAlCl2·n i Pr2O and i BuAlCl2·n i Pr2O (n = 0.8-0.9) afforded PIBs with Mn = 1-1.5 kDa and an exo-olefin content of 85-95%. To increase IB conversion and polymerization rate, a "delayed proton abstraction" approach was proposed, i.e., co-initiation by RAlCl2 and separately added ether. The most plausible mechanism of the reaction comprised the formation of alumoxanes. The activating effect of water was studied separately [99], and the best results were achieved when i Bu-AlCl2·0.8 i Pr2O reacted with a suspension of water in n-hexane before IB addition. These studies were continued [100], and the conditions of the IB polymerization were optimized by pre-activation of i BuAlCl2 using wet argon or MgSO4·7H2O in the presence of i Pr2O. The highest regioselectivity of polymerization was observed at the ratio   [98]. EtAlCl 2 ·n i Pr 2 O and i BuAlCl 2 ·n i Pr 2 O (n = 0.8-0.9) afforded PIBs with M n = 1-1.5 kDa and an exo-olefin content of 85-95%. To increase IB conversion and polymerization rate, a "delayed proton abstraction" approach was proposed, i.e., co-initiation by RAlCl 2 and separately added ether. The most plausible mechanism of the reaction comprised the formation of alumoxanes. The activating effect of water was studied separately [99], and the best results were achieved when i BuAlCl 2 ·0.8 i Pr 2 O reacted with a suspension of water in n-hexane before IB addition. These studies were continued [100], and the conditions of the IB polymerization were optimized by pre-activation of i BuAlCl 2 using wet argon or MgSO  In the recent theoretical work of Zaikov et al. [102], qualitatively different view on the initiation of polymerization of IB with the use of Et 2 AlCl/ROH (R = H, Ph) was proposed. The authors assumed that initiation of the IB molecule proceeds involving EtCl 2 Al . . . O(R)H species, and estimated the activation energies of this process at ab initio HF/3-21G level of theory. Regardless of the modeling results, the very idea of the hydrolytic stability of EtAlCl 2 is doubtful.
Addition of (Octyl) 3 Al to EtAlCl 2 /O(CH 2 CH 2 Cl) 2 system resulted in a decrease in the activity with a substantial increase in the exo-olefin functionality [103]. Initiation of IB polymerization using this system required t BuCl or traces the of water; in the latter case, an exo-olefin content of 81% was reached [104].
Promising results were obtained when using AlCl 3 /EtAlCl 2 mixtures [80,81]. As can be seen in Table 3, the low-MW HRPIB with an exo-olefin content of more than 80% was formed with high IP conversion in a number of experiments (Table 3, Entries 3, 8 and 13). The synergistic effect, observed in these experiments, still needs further explanation.  [105]. Tertbutanol was the most effective "exo-enhancer", that reacted with EtAlCl 2 to form t BuOAlCl 2 species (Figure 13). t BuOAlCl 2 is thermally unstable, but can be easily generated in situ. This catalytic system was effective in the polymerization of C4 hydrocarbon feed. Faust et al. suggested that t BuOAlCl 2 suppresses the isomerization of PIB + but does not act as a chain transfer agent. solvent.

Alkoxyaluminum Chloride/R2O
During the study of the EtAlCl2/O(CH2CH2Cl)2 initiating system, Faust et al. have found that an addition of ROH resulted in an increase in vinylidene selectivity [105]. Tertbutanol was the most effective "exo-enhancer", that reacted with EtAlCl2 to form t BuOAlCl2 species (Figure 13). t BuOAlCl2 is thermally unstable, but can be easily generated in situ. This catalytic system was effective in the polymerization of C4 hydrocarbon feed. Faust et al. suggested that t BuOAlCl2 suppresses the isomerization of PIB + but does not act as a chain transfer agent. In [106], Kostjuk et al. described the preparation and catalytic study of a number of alkoxy aluminum chlorides. As shown in Table 4, all the alkoxy aluminum dichlorides afforded fast polymerization of IB. However, the M n and exo-olefin end-group content decreased while polydispersity increased in the series: HexOAlCl 2 < BuOAlCl 2 < i PrOAlCl 2 < PhOAlCl 2 . The decrease in the temperature from 10 to −20 • C (experiments with (BuO) 1.2 AlCl 1.8 ) resulted in a significant increase in the M n value and vinylidene selectivity. Addition of 0.1 eq. i Pr 2 O to BuOAlCl 2 resulted in a significant increase in the exo-olefin end-group content (up to 90%) with the reduction in M n from 5.6 to 2.0 kDa at a high reaction rate (the monomer conversion was completed in 10 min). Possible mechanisms of IB polymerization on alkoxy aluminum chlorides is presented in Scheme 12. During the initiation step, ROAlCl 2 reacts with traces of H 2 O (initiator) to generate a proton, which interacts with a number of IB molecules, highlighting a growing macrocation (I). The growing macrocation can be stabilized via the reversible interaction with oxygen of the same or another co-initiator molecule forming a dormant species (II). The decrease in reaction temperature results in a shift of equilibrium between the active (I) and dormant (II) species toward the latter. Similar interaction of the model Ph 2 CH + cation with t BuOAlCl 2 was experimentally observed by Faust et al. [105]. This stabilization of the growing species leads to the suppression of the isomerization of macrocation/chain scission that allows for the generation of high molecular weight polymers (I ) in contrast to the bicomponent (Lewis acid/ether) initiating systems. Apart from the stabilization of growing species, ROAlCl 2 can also regioselectively abstract protons in the β-position to the macrocation (III) that results in the formation of exo-olefin terminated PIB (IV). Termination of polymerization occurs via ion pair collapse with the formation of chlorineterminated PIBCl. Scheme 12. Possible mechanism of the ROAlCl2 co-initiated cationic polymerization of IB [106].
The key reaction intermediates and products, discussed in the text, are marked with Roman numerals.
In a relatively recent BASF patent [107], polymerization of IB was conducted at 10 °C with the use of (BuO)AlCl2 as an initiator. When 0.1 eq. i Pr2O was added, HRPIB was formed with a high yield (95-99%); Mn was ~2.0 kDa, and the exo-olefin content reached 88-90%. Without i Pr2O, the exo-olefin content was 46%. Replacement of i Pr2O with diethyl ether resulted in a decrease in the catalytic activity.

AlCl3-Based Ionic Liquids
In 2016, Kostjuk et al. [108] suggested the use of ionic liquids (ILs), prepared from AlCl3 and 1-ethyl-3-methylimidazolium chloride (emim-Cl) in combination with i Pr2O, for the synthesis of HRPIBs with more than 90% exo-olefin content. In the absence of donor, ILs with χ(AlCl3) = 0.6 catalyzed the formation of low-MW PIBs (Mn = 1.36 and ÐM = 6.8, Entry 1 in Table 5) containing tri-and tetra-substituted olefinic end-groups. When 0.5 mol equivalents of i Pr2O were introduced, well-defined PIBs with a high exo-olefin content and a narrow MWD was formed (Entry 2 in Table 5). A number of additional experiments were also conducted. Their results are also presented in Table 5. Since IL-based catalyst is heterogeneous, ultrasonification was used to increase the reaction rate and IB conversion. There was no significant influence of the 3 min ultrasonification before polymerization on the content of the exo-olefinic groups; when the conversion reached 95%, Mn increased. Mn SEC , Mn NMR , End-Group Distribution (mol%) Scheme 12. Possible mechanism of the ROAlCl 2 co-initiated cationic polymerization of IB [106]. The key reaction intermediates and products, discussed in the text, are marked with Roman numerals.
In a relatively recent BASF patent [107], polymerization of IB was conducted at 10 • C with the use of (BuO)AlCl 2 as an initiator. When 0.1 eq. i Pr 2 O was added, HRPIB was formed with a high yield (95-99%); M n was~2.0 kDa, and the exo-olefin content reached 88-90%. Without i Pr 2 O, the exo-olefin content was 46%. Replacement of i Pr 2 O with diethyl ether resulted in a decrease in the catalytic activity.

AlCl 3 -Based Ionic Liquids
In 2016, Kostjuk et al. [108] suggested the use of ionic liquids (ILs), prepared from AlCl 3 and 1-ethyl-3-methylimidazolium chloride (emim-Cl) in combination with i Pr 2 O, for the synthesis of HRPIBs with more than 90% exo-olefin content. In the absence of donor, ILs with χ(AlCl 3 ) = 0.6 catalyzed the formation of low-MW PIBs (M n = 1.36 and Ð M = 6.8, Entry 1 in Table 5) containing tri-and tetra-substituted olefinic end-groups. When 0.5 mol equivalents of i Pr 2 O were introduced, well-defined PIBs with a high exo-olefin content and a narrow MWD was formed (Entry 2 in Table 5). A number of additional experiments were also conducted. Their results are also presented in Table 5. Since IL-based catalyst is heterogeneous, ultrasonification was used to increase the reaction rate and IB conversion. There was no significant influence of the 3 min ultrasonification before polymerization on the content of the exo-olefinic groups; when the conversion reached 95%, M n increased. The scientific group of Bahri-Laleh has also conducted studies on ILs in the polymerization of IB [109]. They selected 1-butyl-3-methylimidazolium chloride (bmim-Cl) as a reference system, and prepared a polyionic liquid (PIL) via radical polymerization of 1butyl-3-vinylimidazolium chloride, and a halloysite clay (Hal) supported ionic liquid (S-IL) ( Figure 14). With the use of EtOH as an initiator, a number of polymerization experiments were conducted at −20 and 0 • C. PIBs with M n = 1.5-11.6 kDa were obtained; however, the exo-olefin content in all the samples was less than 19%. Nevertheless, such products are also in demand, and the practical use of ILs without additional donors cannot be ruled out. These studies were continued with the use of boehmite as a support for ILs and the use of C4 hydrocarbon feed as a raw material [110]. The content of isobutylene units in the polymer was 75-85%, with low exo-olefinic content. It should be noted that the use of boehmite-based S-ILs allowed to halve AlCl 3 loading in comparison with the use of neat AlCl 3 , while maintaining the same PIB characteristics, even more so; the exo-olefinic content in the first case was lower. Given that the resulting PIBs were intended for use as oil viscosity improvers, lowering the exo-olefin group content should result in an increase in the thermooxidative stability of these additives (see also Section 5.5).
AlCl 3 /urea/arene ionic liquids were proposed to be used in IB polymerization; the ratio AlCl 3 /urea was 2:1 [111]. Polymerization experiments were conducted at 10 • C using 0.3 wt% of the ionic liquid suspensions; when using C 6 H 6 as an aromatic solvent, the exo-olefinic selectivity amounted to 76%. This patent application contains no data on the molecular weight characteristics of the polymers obtained. viscosity improvers, lowering the exo-olefin group content should result in an increase in the thermooxidative stability of these additives (see also Section 5.5).
AlCl3/urea/arene ionic liquids were proposed to be used in IB polymerization; the ratio AlCl3/urea was 2:1 [111]. Polymerization experiments were conducted at 10 °C using 0.3 wt% of the ionic liquid suspensions; when using C6H6 as an aromatic solvent, the exoolefinic selectivity amounted to 76%. This patent application contains no data on the molecular weight characteristics of the polymers obtained.

Other Ionic Liquids
To solve the problem of slow polymerization with the emimCl-AlCl 3 initiating system, Kostjuk et al. have studied the influence of ionic liquid acidity, and the nature of Lewis acid and imidazolium salt on the reaction rate [112]. When comparing ILs prepared from AlCl 3 , AlBr 3 , i BuAlCl 2 , FeCl 3 , GaCl 3 or BBr 3 and emim-Cl, they showed the highest activity and vinylidene selectivity for the AlCl 3 -based ILs in the presence of 0.5 equivalents of i Pr 2 O. Polymerization was slower with emim-Cl-FeCl 3 and emim-Cl-GaCl 3 , while the content of exo-olefin groups almost does not change with the monomer conversion. The use of emim-Cl-FeCl 3 and emim-Cl-GaCl 3 allowed to synthesize HRPIBs with M n = 1.7-2.0 kDa and Ð M = 1.9-2.2, whereas M n s of the PIBs obtained with emim-Cl-AlCl 3 were slightly higher than required (>3 kDa). Ultrasonification was highly effective for emim-Cl-FeCl 3 , after 3 min of pre-treatment; for [IB] = 5.2 M in hexane, almost full monomer conversion was achieved after 20 min at 0 • C with the formation of an HRPIB with (M n = 1.9 kDa, Ð M = 2.3, and exo-olefin content of 82%). Taking into account all the parameters, emim-Cl-FeCl 3 was considered as the most promising catalyst among the others.
The effect of t BuCl on the activity of the emim-Cl-FeCl 3 / i Pr 2 O (2:1) system in IB polymerization manifested as a decrease in the M n and Ð M values and an increase in vinylidene selectivity [113]. Addition of the minimal amounts of aromatic hydrocarbons (1 vol%) resulted in an increase in the polymerization rate in the following order: benzene < toluene ≈ mesitylene.
The study of the emim-Cl-FeCl 3 / i Pr 2 O (2:1) system in the polymerization of C4 mixed hydrocarbon feed [78] showed a marked difference from pure IB polymerization. Firstly, a high monomer conversion was obtained for IB polymerization at [IL] = 33 mmol·L −1 , while in the case of C4 mixed feed polymerization, a higher concentration of ionic liquid was required. Secondly, relatively high molecular weight polymers were formed during the emim-Cl-FeCl 3 -co-initiated polymerization of C4 mixed feed at high co-initiator concentrations as compared to the polymerization of IB. Also, independent of the nature of the monomer used (IB or C4 mixed feed), PIBs had low polydispersities (Ð M < 2.8). The use of ultrasonification resulted in an increase in the conversion, and HRPIBs with M n = 2.5 kDa, Ð M = 2.5 and an exo-olefin content of 81% was obtained from the C4 feed.
In the recent work [115], Kostjuk et al. reported the results of a study on silicasupported emim-Cl/FeCl 3 ILs of different compositions. They showed that supported ILs with a 30 wt% emimCl/1.5FeCl 3 content possessed a higher specific surface area and demonstrated high activity and regioselectivity in the cationic polymerization of IB in n-hexane, affording HRPIBs with M n < 1.5 kDa (Ð M < 2) and 70-90% content of exo-olefinic end-groups in a range of [IB] = 1-5 M.

Liquid Coordination Complexes
Liquid coordination complexes (LCCs) along with acidic ILs represent a new class of liquid Lewis acids. They usually consist of metal halides (AlCl 3 , GaCl 3 , TiCl 4 , etc.) and donor molecules (e.g., phosphine oxides). In 2021, Kostjuk et al. reported the results of the studies on the preparation of phosphinoxide-based LCCs and their catalytic behavior in IB polymerization [116]. For the Oct 3 PO/AlCl 3 (χ(AlCl 3 ) = 0.60) system in the presence of i Pr 2 O, PIBs with moderate to low exo-olefin content (48, 23 and 2%) were obtained at different temperatures (0, 10 and 20 • C, respectively); M n values were 3.6-6.5 kDa. When using EtOCH 2 CH 2 Cl, M n values were 2.0-5.0 kDa with a vinylidene selectivity of 65% for all three temperatures. In the presence of O(CH 2 CH 2 Cl) 2 at 0 • C, HRPIBs with M n = 6.9 kDa was obtained and the exo-olefin content was 81%; the polymer contained 19% of -CMe 2 Cl end-groups. Further experiments with the latter system allowed to optimize the process conditions, and at 20 • C, an HRPIB with M n = 2.2 kDa, Ð M = 1.9 and an exo-olefin content of 90% was synthesized.
For FeCl 3 -based LCCs, the effect of additional ether was markedly different; i Pr 2 O has been proven to be the best donor component. At 0 • C, HRPIBs with M n = 1.5-2.1 kDa and an exo-olefin content of 91-97% were obtained. The polymerization proceeded smoothly and relatively fast ( Figure 15): about 50% of monomer conversion was reached in 5 min and 100% in the following 15 min. It is interesting to note that at low conversion (17%), a relatively low exo-olefin end-groups content was observed due to the presence of a significant fraction of PIBCl. The PIBCl chains completely disappeared in the course of polymerization (no PIBCl chains at 53% conversion) due to their ionization by chlorophilic Oct 3 PO-FeCl 3 followed by regioselective β-H elimination with the formation of exo-olefinic end-group. CMe2Cl end-groups. Further experiments with the latter system allowed to optimize the process conditions, and at 20 °C, an HRPIB with Mn = 2.2 kDa, ĐM = 1.9 and an exo-olefin content of 90% was synthesized. For FeCl3-based LCCs, the effect of additional ether was markedly different; i Pr2O has been proven to be the best donor component. At 0 °C, HRPIBs with Mn = 1.5-2.1 kDa and an exo-olefin content of 91-97% were obtained. The polymerization proceeded smoothly and relatively fast ( Figure 15): about 50% of monomer conversion was reached in 5 min and 100% in the following 15 min. It is interesting to note that at low conversion (17%), a relatively low exo-olefin end-groups content was observed due to the presence of a significant fraction of PIBCl. The PIBCl chains completely disappeared in the course of polymerization (no PIBCl chains at 53% conversion) due to their ionization by chlorophilic Oct3PO-FeCl3 followed by regioselective β-H elimination with the formation of exo-olefinic end-group. Based on the results of catalytic experiments and UV-Vis spectral data, the mechanism of cationic polymerization of IB catalyzed by liquid coordination complexes (Oct3PO-AlCl3 and Oct3PO-FeCl3) in n-hexane was proposed ( Figure 16). As in the case of ILs, the reactions occurred heterogeneously at the interface of the catalyst particles, with adventitious water acting as an initiator. In contrast to imidazole-based ILs bearing nonreactive imidazolium cation, LLCs contain electron-deficient metal centers stabilized by two ligands ([MCl2L2] + , M = Al or Fe). Therefore, the formation of superacidic protons more likely proceeds via partial hydrolysis of both cationic and anionic parts of LCCs. This assumption correlates well with the much higher activity of LLCs observed in com- Based on the results of catalytic experiments and UV-Vis spectral data, the mechanism of cationic polymerization of IB catalyzed by liquid coordination complexes (Oct 3 PO-AlCl 3 and Oct 3 PO-FeCl 3 ) in n-hexane was proposed ( Figure 16). As in the case of ILs, the reactions occurred heterogeneously at the interface of the catalyst particles, with adventitious water acting as an initiator. In contrast to imidazole-based ILs bearing non-reactive imidazolium cation, LLCs contain electron-deficient metal centers stabilized by two ligands ([MCl 2 L 2 ] + , M = Al or Fe). Therefore, the formation of superacidic protons more likely proceeds via partial hydrolysis of both cationic and anionic parts of LCCs. This assumption correlates well with the much higher activity of LLCs observed in comparison with the corresponding ILs. In addition, partial dissolution of Oct 3 PO-FeCl 3 could not be excluded, and is supported by bimodal MWD of the PIB. In contrast to Oct 3 PO-FeCl 3 , Oct 3 PO-AlCl 3 only heterogeneously catalyzed the polymerization of IB at the catalyst particle interface.

Silica-Supported Catalysts
As disclosed in the Braskem S.A. patent [82], the EtAlCl2/ i Pr2O catalyst can be supported on silica, 3-aminopropyl-functionalized silica, or ZrO2; the obtained catalysts retained high selectivity in the formation of exo-olefinic group up to 82-85% even at 30 °C. The catalytic activities were 28-55 µmolIB·molAl -1 ·s -1 . Evaluation of the prospects of similar catalytic systems is hampered by the lack of data on molecular weight characteristics of the PIBs obtained.

Bis(Arene) Metal Cationic Complexes
The complexes of Ga(I) of the formula [Ga(arene)2] + [Al(OR F )4] -(arene = C6H5F, 1,3,5-Me3C6H3, OR F =OC(CF3)3) catalyzed the polymerization of IB in CH2Cl2 or toluene media with the formation of low-MW HRPIB [117]. At 10 °C in toluene, a polymer with an Mn of ~2 kDa and an exo-olefin content of 93% was obtained using 0.007 mol% of the complex [Ga(C6H5F)2] + [Al(OR F )4] -. Two different mechanisms, a cationic-initiated chain growth polymerization in CH2Cl2 and a coordinative polymerization in toluene, were proposed. In the latter case (toluene media), the Ga(I) salt can be seen as a relatively stable precursor for cyclogalla(III) cations which, being much more electrophilic than Ga(I), initiate and catalyze the polymerization of IB with high reactivity. In further studies [118],

Silica-Supported Catalysts
As disclosed in the Braskem S.A. patent [82], the EtAlCl 2 / i Pr 2 O catalyst can be supported on silica, 3-aminopropyl-functionalized silica, or ZrO 2 ; the obtained catalysts retained high selectivity in the formation of exo-olefinic group up to 82-85% even at 30 • C. The catalytic activities were 28-55 µmol IB ·mol Al −1 ·s −1 . Evaluation of the prospects of similar catalytic systems is hampered by the lack of data on molecular weight characteristics of the PIBs obtained.

Bis(Arene) Metal Cationic Complexes
The complexes of Ga(I) of the formula [Ga(arene) 2 ] + [Al(OR F ) 4 ] − (arene = C 6 H 5 F, 1,3,5-Me 3 C 6 H 3 , OR F =OC(CF 3 ) 3 ) catalyzed the polymerization of IB in CH 2 Cl 2 or toluene media with the formation of low-MW HRPIB [117]. At 10 • C in toluene, a polymer with an M n of~2 kDa and an exo-olefin content of 93% was obtained using 0.007 mol% of the complex [Ga(C 6 H 5 F) 2 ] + [Al(OR F ) 4 ] − . Two different mechanisms, a cationic-initiated chain growth polymerization in CH 2 Cl 2 and a coordinative polymerization in toluene, were proposed. In the latter case (toluene media), the Ga(I) salt can be seen as a relatively stable precursor for cyclogalla(III) cations which, being much more electrophilic than Ga(I), initiate and catalyze the polymerization of IB with high reactivity. In further studies [118], bis(arene) ansa-ligand PhCH 2 CH 2 Ph and 1,3-Ph 2 C 6 H 4 were proposed for Ga(I) complexation. IB 4 ] − salts (Figure 17a) that were found to be active in IB polymerization. However, low-MW HRPIB was obtained using the [EtZn(PhMe) 2 ] + [Al(OR F ) 4 ] − complex (Figure 17b), and only moderate conversions (up to 33%) were achieved. At 15 • C, the exo-olefin content was 84%.  Subsequent studies of the same scientific group were focused on highly acidic dicationic strontium complex, based on the bis(arene) ansa-ligand [(3,5-Me2C6H3)CH2]2 with additional 1,2-difluorobenzene coordination and [Al(OR F )4] -counterions [120]. This complex ( Figure 18) readily polymerized the IB at 0 or -12 °C, forming PIBs with Mn = 70 and 130 kDa, respectively. The data on chain-end structure of the PIBs are not mentioned in this paper.

Perfluorophenyl Metal Derivatives
The initiation of IB polymerization with Zn(C6F5)2 and t BuCl resulted in high-MW PIBs; the catalytic activity of this system was low [121]. However, Zn(C6F5)2 is a convenient starting compound for the synthesis of perfluorophenyl derivatives of other metals, e.g., Al(C6F5)3 and Ga(C6F5)3 [122]. These complexes were studied as catalysts of IB polymerization. In the toluene/saline reaction media, high-MW PIBs with ~1:1 ratio of the exo-and endo-olefinic end-groups were obtained [122]. Experiments in toluene and hexane, as well as bulk polymerization, also resulted in high-MW PIBS containing <50% of vinylidene end-groups [123]. Subsequent studies of the same scientific group were focused on highly acidic dicationic strontium complex, based on the bis(arene) ansa-ligand [(3,5-Me 2 C 6 H 3 )CH 2 ] 2 with additional 1,2-difluorobenzene coordination and [Al(OR F ) 4 ] − counterions [120]. This complex ( Figure 18) readily polymerized the IB at 0 or −12 • C, forming PIBs with M n = 70 and 130 kDa, respectively. The data on chain-end structure of the PIBs are not mentioned in this paper.  Subsequent studies of the same scientific group were focused on highly acidic dicationic strontium complex, based on the bis(arene) ansa-ligand [(3,5-Me2C6H3)CH2]2 with additional 1,2-difluorobenzene coordination and [Al(OR F )4] -counterions [120]. This complex ( Figure 18) readily polymerized the IB at 0 or -12 °C, forming PIBs with Mn = 70 and 130 kDa, respectively. The data on chain-end structure of the PIBs are not mentioned in this paper.

Perfluorophenyl Metal Derivatives
The initiation of IB polymerization with Zn(C6F5)2 and t BuCl resulted in high-MW PIBs; the catalytic activity of this system was low [121]. However, Zn(C6F5)2 is a convenient starting compound for the synthesis of perfluorophenyl derivatives of other metals, e.g., Al(C6F5)3 and Ga(C6F5)3 [122]. These complexes were studied as catalysts of IB polymerization. In the toluene/saline reaction media, high-MW PIBs with ~1:1 ratio of the exo-and endo-olefinic end-groups were obtained [122]. Experiments in toluene and hexane, as well as bulk polymerization, also resulted in high-MW PIBS containing <50% of vinylidene end-groups [123]. Thermal displacement of ellipsoids set at 50% probability. Protons and counterions were omitted for clarity. Reprinted with permission from [120]. Copyright (2020) Wiley-VCH Verlag GmbH & Co.

Perfluorophenyl Metal Derivatives
The initiation of IB polymerization with Zn(C 6 F 5 ) 2 and t BuCl resulted in high-MW PIBs; the catalytic activity of this system was low [121]. However, Zn(C 6 F 5 ) 2 is a convenient starting compound for the synthesis of perfluorophenyl derivatives of other metals, e.g., Al(C 6 F 5 ) 3 and Ga(C 6 F 5 ) 3 [122]. These complexes were studied as catalysts of IB polymerization. In the toluene/saline reaction media, high-MW PIBs with~1:1 ratio of the exoand endo-olefinic end-groups were obtained [122]. Experiments in toluene and hexane, as well as bulk polymerization, also resulted in high-MW PIBS containing <50% of vinylidene end-groups [123].
In a recent study [124], Cai et al. showed that IB polymerization in the temperature interval from −60 to 15 • C in a non-halogenated solvent using Al(C 6 F 5 ) 3 as a Lewis acid and PhCMe 2 Cl as an initiator resulted in PIBs with medium-molecular weights (9-148 kDa). Addition of the donor (Ph 2 O) resulted in the formation of highly active catalysts; however, the M n value of the PIB, synthesized at −40 • C, amounted to 355 kDa. Evidently, further studies of similar complexes with regard to HRPIB synthesis have limited chances of success.

Scheme 13.
Structures of phenols studied in cationic polymerization of IB initiated by phenols/Al(C6F5)3 systems [125]. Molecular structures of the phenols are indicated by the numbers used in Table 6 for comparison of the catalysts.

Visible Light-Induced Polymerization
Photo-induced controlled/living polymerization has been intensively investigated in recent years [127,128]. Interesting results of the first study on photo-induced cationic IB polymerization were presented in 2023 by Kostjuk at the 14th APME conference [129]. Scheme 13. Structures of phenols studied in cationic polymerization of IB initiated by phenols/Al(C 6 F 5 ) 3 systems [125]. Molecular structures of the phenols are indicated by the numbers used in Table 6 for comparison of the catalysts.

Visible Light-Induced Polymerization
Photo-induced controlled/living polymerization has been intensively investigated in recent years [127,128]. Interesting results of the first study on photo-induced cationic IB polymerization were presented in 2023 by Kostjuk at the 14th APME conference [129]. Using the PhCH 2 Br/Mn 2 (CO) 10 /[Ph 2 I] + [PF 6 ] − initiating system, they have demonstrated that during the photoinitiated cationic polymerization of IB, PhCH 2 Br alone is inactive, while initiation occurred via a chlorine radical abstraction from the solvent (CH 2 Cl 2 ), followed by the oxidation of generated radical to the corresponding cation by [Ph 2 I] + (Scheme 14). According to this scheme, the role of PhCH 2 Br is unclear. The reaction proceeded with an induction period at −30 • C in CH 2 Cl 2 /n-hexane, affording an exo-olefin terminated PIB (exo-content >85%) with an M n up to 3 kDa and a Ð M of < 1.

Synthesis of PIBs with Higher Content of Trisubstituted Olefinic End-Groups
High levels of control molecular weight and vinylidene content in modern technologies have the opposite effect. Due to lower photooxidative stability of vin olefins, it became necessary to develop approaches to PIBS with a high content o stituted olefinic end-groups. In the recent BASF patent [130], the results of isome of commercial HRPIB Glissopal 1000 in the presence of Al2O3 and molecular siev presented. The course of the reaction is illustrated through the example prese Scheme 15 and Table 7. This study is of particular interest in terms of isomeriz HRPIBs in the presence of weak acids. It is obvious that such side reactions can during any cationic polymerization of IB. [130]. Molecular structures are indicated by the symbols used in Table 7.

Synthesis of PIBs with Higher Content of Trisubstituted Olefinic End-Groups
High levels of control molecular weight and vinylidene content in modern HRPIB technologies have the opposite effect. Due to lower photooxidative stability of vinylidene olefins, it became necessary to develop approaches to PIBS with a high content of trisubstituted olefinic end-groups. In the recent BASF patent [130], the results of isomerization of commercial HRPIB Glissopal 1000 in the presence of Al 2 O 3 and molecular sieves were presented. The course of the reaction is illustrated through the example presented in Scheme 15 and Table 7. This study is of particular interest in terms of isomerization of HRPIBs in the presence of weak acids. It is obvious that such side reactions can proceed during any cationic polymerization of IB.

Synthesis of PIBs with Higher Content of Trisubstituted Olefinic End-Groups
High levels of control molecular weight and vinylidene content in modern HRPIB technologies have the opposite effect. Due to lower photooxidative stability of vinylidene olefins, it became necessary to develop approaches to PIBS with a high content of trisubstituted olefinic end-groups. In the recent BASF patent [130], the results of isomerization of commercial HRPIB Glissopal 1000 in the presence of Al2O3 and molecular sieves were presented. The course of the reaction is illustrated through the example presented in Scheme 15 and Table 7. This study is of particular interest in terms of isomerization of HRPIBs in the presence of weak acids. It is obvious that such side reactions can proceed during any cationic polymerization of IB. [130]. Molecular structures are indicated by the symbols used in Table 7.

Further Transformations of HRPIB (PIB-Cl) and Applications of the Products
Scheme 15. Structures of PIBs in starting HRPIB (A is the main component) and in isomerization products [130]. Molecular structures are indicated by the symbols used in Table 7. The Alder-ene reaction between HRPIB and MA at~200 • C proceeds mainly with the formation of a vinylidene adduct (Scheme 1a). However, detailed NMR studies have shown that other isomers of PIB-SA can also be identified in the reaction mixture [131] (Figure 19). A very important result reported in [131] was the identification of an exo-olefinic isomer of conventional PIB-SA, presumably formed by the reaction of MA with the PIB isomer containing tri-substituted olefinic end-group (Scheme 16a). Note that in other works, the formation of isomeric PIB-SA was attributed to the free-radical side reaction [132]. It is well known that at higher temperatures PIB-SA can react with an excess of MA with the formation of bis-succinic anhydride derivatives, PIB-BSA (Scheme 16b) [133]. It is quite likely that isomeric PIB-SA is able to react with the second molecule of MA. However, the products of the Alder-ene reaction of isomeric PIB-SA with MA were not detected during thorough analysis of the commercial sample of PIB-SA, whereas both isomers of 'conventional' PIB-BSA were detected [134]. The results of DFT modeling of the mechanism of Alder-ene reaction between HRPI and MA (M06-2X/6-311+G(d,p) level of theory) were reported by Mpourmpakis et al. i 2021 [137]. First, they analyzed the conventional concerted mechanism of this reaction us ing Me3CCH2C(Me)=CH2 as a model of low-ME HRPIB molecule. Calculations showe works, the formation of isomeric PIB-SA was attributed to the free-radical side reaction [132]. It is well known that at higher temperatures PIB-SA can react with an excess of MA with the formation of bis-succinic anhydride derivatives, PIB-BSA (Scheme 16b) [133]. It is quite likely that isomeric PIB-SA is able to react with the second molecule of MA. However, the products of the Alder-ene reaction of isomeric PIB-SA with MA were not detected during thorough analysis of the commercial sample of PIB-SA, whereas both isomers of 'conventional' PIB-BSA were detected [134]. Scheme 16. (a) Presumed reaction mechanism for the formation of the isomeric PIB-SA: the structure of conventional PIB-SA is presented for comparison; (b) reaction of PIB-SA with an excess of MA with a formation of PIB-BSA [131]; and (c) proposed structure of the side product of free-radical reaction [132].
1 H NMR identification of PIB-BSA is a relatively challenging task because of the low content of PIB-BSA, the broad signal resulting from the slowly tumbling PIB-SA chains, and the presence of a forest of peaks in the region corresponding to the SA fragment. In a recent study of Frasca and Duhamel [135], PIB-SA samples were labeled with 1-pyrenemethylamine (180 °C, in n-decane). Detection of PIB-BSA was based on the preference of intramolecular aggregation of pyrene fragments that resulted in higher fluorescence decay for PIB-BSI-(pyrene)2 in comparison with PIB-BSI-pyrene. In further study of the products of the reaction of PIB-SA with different molar ratios of hexamethylene diamine [136], they found that PIBSA samples comprise 0.50, 0.38, and 0.12 molar fractions of PIB-SA, PIB and PIB-BSA, respectively. Scheme 16. (a) Presumed reaction mechanism for the formation of the isomeric PIB-SA: the structure of conventional PIB-SA is presented for comparison; (b) reaction of PIB-SA with an excess of MA with a formation of PIB-BSA [131]; and (c) proposed structure of the side product of free-radical reaction [132].
Lewis acid acceleration and the fact that this is a successful strategy in other Alder-ene reactions [139], the authors of [132] proposed that the minor by-products formed during the reaction between MA and HRPIB are also strong sequestering agents for AlCl3. More recently, Mpourmpakis et al. proposed the use of EtAlCl2 instead of AlCl3, which showed that alkylaluminum chloride retained relatively low activation barriers like AlCl3 but formed less stable complexes with PIB-SA [140]. From a practical point of view, the use of excess MA in an ene reaction with HRPIB, that results in the formation of 'SA-excessive' adducts like PIB-BSA, entails substantial improvement in the characteristics of the corresponding PIB-SI [141,142]. Similar observations prompted the chemists of the Lubrizol Corp. to find alternative synthetic approaches to PIB-BSA analogs. In [143], they suggested to conduct the reaction between HRPIB and MA in the presence of free-radical initiators. For example, ( t BuO)2-initiated interaction of HRPIB with MA was carried out at 170 °C within 2 h, obtaining a colorless product with a higher acid number in comparison with the product of thermal ene reaction. The molecular structure of the products of this free-radical process remains unexplored. In [144], PIB-SA was synthesized from the commercial HRPIB (Mn = 4.17 kDa and ĐM = 1.98) and MA with the addition of benzoyl peroxide. The reaction was conducted at 150-190 °C for 24 h; however, the mechanism of the reaction was not discussed, and the reported structure of the product corresponded to the thermal ene reaction mechanism.
PIB-SA exhibits an amphiphilic molecule with unusual conformational behavior. In 2022, Johnson et al. reported the results of a theoretical study on PIB-SA containing up to nine repeat units [145]. They used a combination of classical potential calculations for generating conformations and DFT (B3LYP/def2-SVP level of theory) for optimizing the structures and computing the dipole moments. The computed average dipole moments decreased from 4.8 ± 0.4 to 3.9 ± 0.6 D with the increase in chain lengths. In a recent study on the kinetics of the reaction of HRPIB with MA [132], the composition of the reaction mixture was studied in a temperature interval of 150-180 • C. Experimental value of the ∆G = for the thermal ene reaction (34-35 kcal·mol −1 ) was in good agreement with the results of DFT modeling (36.6 kcal·mol −1 , [137]). However, conversion of HRPIB and MA in the presence of 3-8 mol% AlCl 3 was essentially unaffected compared to the background reaction, but the proportion of endo-PIB increased compared to the uncatalyzed conditions as AlCl 3 loading increased. Given the clear calculated drivers for Lewis acid acceleration and the fact that this is a successful strategy in other Alder-ene reactions [139], the authors of [132] proposed that the minor by-products formed during the reaction between MA and HRPIB are also strong sequestering agents for AlCl 3 . More recently, Mpourmpakis et al. proposed the use of EtAlCl 2 instead of AlCl 3 , which showed that alkylaluminum chloride retained relatively low activation barriers like AlCl 3 but formed less stable complexes with PIB-SA [140].
From a practical point of view, the use of excess MA in an ene reaction with HRPIB, that results in the formation of 'SA-excessive' adducts like PIB-BSA, entails substantial improvement in the characteristics of the corresponding PIB-SI [141,142]. Similar observations prompted the chemists of the Lubrizol Corp. to find alternative synthetic approaches to PIB-BSA analogs. In [143], they suggested to conduct the reaction between HRPIB and MA in the presence of free-radical initiators. For example, ( t BuO) 2 -initiated interaction of HRPIB with MA was carried out at 170 • C within 2 h, obtaining a colorless product with a higher acid number in comparison with the product of thermal ene reaction. The molecular structure of the products of this free-radical process remains unexplored. In [144], PIB-SA was synthesized from the commercial HRPIB (M n = 4.17 kDa and Ð M = 1.98) and MA with the addition of benzoyl peroxide. The reaction was conducted at 150-190 • C for 24 h; however, the mechanism of the reaction was not discussed, and the reported structure of the product corresponded to the thermal ene reaction mechanism.
PIB-SA exhibits an amphiphilic molecule with unusual conformational behavior. In 2022, Johnson et al. reported the results of a theoretical study on PIB-SA containing up to nine repeat units [145]. They used a combination of classical potential calculations for generating conformations and DFT (B3LYP/def2-SVP level of theory) for optimizing the structures and computing the dipole moments. The computed average dipole moments decreased from 4.8 ± 0.4 to 3.9 ± 0.6 D with the increase in chain lengths.

PIB-SI and Related Compounds
The reaction of PIB-SA with diamines (Scheme 1d) is the basis for the well-operating, large-scale production of ashless dispersants and as such is not of interest in our review. However, a number of works that are devoted to the determination of the molecular structure and spectral characteristics of PIB-SI have been published recently. In particular, as was shown in [144,146], the quantitative NMR study of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine-based PIB-SIs is hindered by hydrogen bonding between the polyamine linker and the succinimide carbonyls; this problem remains critical for the post-modified products of PIB-SI after the reaction with ethylene carbonate [147]. An alternative method of the analysis of PIB-SI was proposed, which was based on fluorescent quenching, and it was found that the fluorescence of the succinimide groups decreased with the increasing number of secondary amine fragments in the polyamine linker [146], whereas the quenching of succinimide moieties by urethane groups was much less efficient [147].
However, the search for prospective PIB-SA-based additives has continued. In particular, the chemists of the Lubrizol Corp. synthesized a series of PIB-SA-based derivatives of sterically hindered diamines (Scheme 17) to reduce deposits and corrosion on the internal parts of engines [10]. The chemists of Innospec, using an example of Me 2 N(CH 2 ) 3 NH 2derived PIB-SI, have demonstrated the higher efficiency of additives prepared from succinic anhydrides with a higher PIB-BSA content [141,142]. The reaction of PIB-SA with diamines (Scheme 1d) is the basis for the well-operating, large-scale production of ashless dispersants and as such is not of interest in our review. However, a number of works that are devoted to the determination of the molecular structure and spectral characteristics of PIB-SI have been published recently. In particular, as was shown in [144,146], the quantitative NMR study of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine-based PIB-SIs is hindered by hydrogen bonding between the polyamine linker and the succinimide carbonyls; this problem remains critical for the post-modified products of PIB-SI after the reaction with ethylene carbonate [147]. An alternative method of the analysis of PIB-SI was proposed, which was based on fluorescent quenching, and it was found that the fluorescence of the succinimide groups decreased with the increasing number of secondary amine fragments in the polyamine linker [146], whereas the quenching of succinimide moieties by urethane groups was much less efficient [147].
However, the search for prospective PIB-SA-based additives has continued. In particular, the chemists of the Lubrizol Corp. synthesized a series of PIB-SA-based derivatives of sterically hindered diamines (Scheme 17) to reduce deposits and corrosion on the internal parts of engines [10]. The chemists of Innospec, using an example of Me2N(CH2)3NH2derived PIB-SI, have demonstrated the higher efficiency of additives prepared from succinic anhydrides with a higher PIB-BSA content [141,142]. In [148], Ponzoni et al. analyzed a series of virtual PIB-SI molecules derived from known reference substrates, and developed a comprehensive model for the search of prospective ashless dispersants using machine learning techniques. In the future, the practical significance of this work will be demonstrated.
The study on the colloidal behavior of carbon black (average particle size 126 ± 3.5 nm) in the presence of commercial diamine-based bis(PIB-SI) dispersant in n-hexane and n-decane [149] showed that the magnitude of effective surface charge of carbon nanoparticles, determined using electrophoretic mobility measurements, decreases with an increase in PIB-SI concentration. Dynamic light scattering (DLS) and UV spectral studies showed that minimal aggregation of the carbon nanoparticles in n-decane is observed at a PIB-SI concentration of 5 wt%, which correlates with the optimized formulations for Group III and IV engine oils. An amphiphilic nature of PIB-SI was used in surface-capping of MoO3; the nanoparticles modified with zinc dialkyldithiophosphate [150] demonstrated a synergistic tribological effect, reduced friction and caused wear in the diamondlike carbon coating under boundary lubrication.

Recent Studies on Other Methods of HRPIB Functionalization
Acid-catalyzed reaction of HRPIB with nitriles, resulting in amide formation (Ritter reaction), was conducted successfully, but only in the case of low-MW oligomers (C12-Scheme 17. Molecular structures of prospective ashless dispersants [10].
In [148], Ponzoni et al. analyzed a series of virtual PIB-SI molecules derived from known reference substrates, and developed a comprehensive model for the search of prospective ashless dispersants using machine learning techniques. In the future, the practical significance of this work will be demonstrated.
The study on the colloidal behavior of carbon black (average particle size 126 ± 3.5 nm) in the presence of commercial diamine-based bis(PIB-SI) dispersant in n-hexane and ndecane [149] showed that the magnitude of effective surface charge of carbon nanoparticles, determined using electrophoretic mobility measurements, decreases with an increase in PIB-SI concentration. Dynamic light scattering (DLS) and UV spectral studies showed that minimal aggregation of the carbon nanoparticles in n-decane is observed at a PIB-SI concentration of 5 wt%, which correlates with the optimized formulations for Group III and IV engine oils. An amphiphilic nature of PIB-SI was used in surface-capping of MoO 3 ; the nanoparticles modified with zinc dialkyldithiophosphate [150] demonstrated a synergistic tribological effect, reduced friction and caused wear in the diamond-like carbon coating under boundary lubrication.
PIB exo OH was obtained in a similar way, using 9-borabicyclo[3.3.1]nonane (9-BBN) as a hydroboration reagent [153]. In addition, termination of quasiliving carbocationic polymerization of IB ( t BuCH 2 CMe 2 Cl/TiCl 4 /N,N,N ,N -tetramethylethylenediamine) by Me 3 SiCH 2 CH=CH 2 , followed by hydroboration/oxidation afforded PIB all OH. PIB exo OH and PIB all OH was further converted into PIB-OSO 2 Ar (Ar = p-tolyl and 4-nitrophenyl, respectively) (Scheme 19) [153]. A significant difference in the reactivity was found, i.e., PIB all OH reacted faster than PIB exo OH, presumably because of the steric reasons. While quantitative tosylation was achieved with PIB exo OH, nosylation led to PIB exo ONs with a functionality of~90%. It was also found that PIB all OTs react further with the chloride anion formed during tosylation, leading to the yield of a chlorine-ended PIB (PIB all Cl). The latter reaction illustrates high synthetic potential of PIB sulfonates. Due to high lipophilicity, functionalized PIBs are prospective candidates for use in the two-phase processes. Transformation of PIBexoOH (Mn = 1 kDa) to corresponding iodides (I2/PPh3, imidazole in CH2Cl2), followed by the treatment with Et2NH and quaternization via MeI afforded PIB-ammonium salts (Scheme 20a) that have demonstrated high efficiency as phase-transfer agents for SN2 reactions [154]. In 2020 [155], Fu and Bergbreiter described the synthesis of a PIB-bound hexamethylphosphoramide (HMPA) analog starting from PIBexoOH (Scheme 20b), and its applications as a recyclable catalyst in allylation of aldehydes (ArCHO + Cl3SiCH2CH=CH2) and reduction in enones with SiHCl3 in a polyα-olefin (PAO) solvent (hydrogenated dec-1-ene trimer was used as a PAO). These studies have shown that PIB-bound phosphoramide/PAO is a competent alternative to HMPA in both the processes as a recyclable catalyst. Scheme 20. Synthesis of (a) PIB-bound quaternary ammonium salt [154] and (b) PIB-bound phosphoramide [155].
Interaction of PIBexoI, obtained from HRPIB (Mn = 450 Da) with different N-nucleophiles (secondary amines, imidazole, and substituted imidazoles), led to the products of nucleophilic substitution [156]. These functionalized PIBs in combination with PAO (hydrogenated dec-1-ene trimer) were studied in the extraction of phenols from water. For example, using an imidazole-functionalized PIB at a concentration of 0.1 M in PAO, >99% of the bisphenol A (BPA) was extracted from the aqueous solution with a concentration of 200 mg·L -1 . Other chlorinated, brominated, and alkylated phenols (200-500 mg·L -1 ) were also extracted with >95% efficiency. Recycling of the PB-imidazole/PAO phase containing sequestered phenol was easily performed by subsequent mixing with solid NaOH. These Scheme 19. Synthesis of arenesulfonyl-ended PIBs derived from exo-olefin-ended and allylterminated polymers [153].
Due to high lipophilicity, functionalized PIBs are prospective candidates for use in the two-phase processes. Transformation of PIB exo OH (M n = 1 kDa) to corresponding iodides (I 2 /PPh 3 , imidazole in CH 2 Cl 2 ), followed by the treatment with Et 2 NH and quaternization via MeI afforded PIB-ammonium salts (Scheme 20a) that have demonstrated high efficiency as phase-transfer agents for S N 2 reactions [154]. In 2020 [155], Fu and Bergbreiter described the synthesis of a PIB-bound hexamethylphosphoramide (HMPA) analog starting from PIB exo OH (Scheme 20b), and its applications as a recyclable catalyst in allylation of aldehydes (ArCHO + Cl 3 SiCH 2 CH=CH 2 ) and reduction in enones with SiHCl 3 in a poly-α-olefin (PAO) solvent (hydrogenated dec-1-ene trimer was used as a PAO). These studies have shown that PIB-bound phosphoramide/PAO is a competent alternative to HMPA in both the processes as a recyclable catalyst. Due to high lipophilicity, functionalized PIBs are prospective candidates fo the two-phase processes. Transformation of PIBexoOH (Mn = 1 kDa) to correspon dides (I2/PPh3, imidazole in CH2Cl2), followed by the treatment with Et2NH and q zation via MeI afforded PIB-ammonium salts (Scheme 20a) that have demonstrat efficiency as phase-transfer agents for SN2 reactions [154]. In 2020 [155], Fu and Ber described the synthesis of a PIB-bound hexamethylphosphoramide (HMPA) analo ing from PIBexoOH (Scheme 20b), and its applications as a recyclable catalyst in al of aldehydes (ArCHO + Cl3SiCH2CH=CH2) and reduction in enones with SiHCl3 in α-olefin (PAO) solvent (hydrogenated dec-1-ene trimer was used as a PAO). These have shown that PIB-bound phosphoramide/PAO is a competent alternative to H both the processes as a recyclable catalyst. Scheme 20. Synthesis of (a) PIB-bound quaternary ammonium salt [154] and (b) PIB-bou phoramide [155].
Interaction of PIBexoI, obtained from HRPIB (Mn = 450 Da) with different N philes (secondary amines, imidazole, and substituted imidazoles), led to the pro nucleophilic substitution [156]. These functionalized PIBs in combination with PA drogenated dec-1-ene trimer) were studied in the extraction of phenols from wa example, using an imidazole-functionalized PIB at a concentration of 0.1 M in PA Scheme 20. Synthesis of (a) PIB-bound quaternary ammonium salt [154] and (b) PIB-bound phosphoramide [155].
Interaction of PIB exo I, obtained from HRPIB (M n = 450 Da) with different N-nucleophiles (secondary amines, imidazole, and substituted imidazoles), led to the products of nucleophilic substitution [156]. These functionalized PIBs in combination with PAO (hydro-genated dec-1-ene trimer) were studied in the extraction of phenols from water. For example, using an imidazole-functionalized PIB at a concentration of 0.1 M in PAO, >99% of the bisphenol A (BPA) was extracted from the aqueous solution with a concentration of 200 mg·L −1 . Other chlorinated, brominated, and alkylated phenols (200-500 mg·L −1 ) were also extracted with >95% efficiency. Recycling of the PB-imidazole/PAO phase containing sequestered phenol was easily performed by subsequent mixing with solid NaOH. These results are of great importance due to the high toxicity of BPA, which is widely used in the production of polycarbonates [157,158].
In 2021, Bergbreiter et al. proposed the use of fully recyclable PAO and soluble PAOanchored PIB-bound sulfonic acid catalysts [159]. HRPIB with M n = 432 was subjected to a thiol-ene reaction followed by oxidation affording PIB-SO 3 H with a 47% yield (Scheme 21). PIB analog of arenesulfonic acids, PIB-ArSO 3 H, was synthesized in two stages in a 62% yield via alkylation of anisole, followed by sulfonation (Scheme 21). Catalytic potential of sulfonic acids was studied during the reaction of PhCH 2 OH with 3,4-dihydro-2H-pyran (5 mol% of the catalysts and >90% yields over 10 cycles); comparison showed that the p-dodecylbenzenesulfonic acid/PAO system lost activity after the third cycle. Catalytic potentials of PIB-SO 3 H and PIB-ArSO 3 H were also estimated for esterification and threecomponent condensation between PhCOH, urea and acetoacetic ester; at least 80% yields of the products were achieved after recycling seven times and more [159].
ymers 2023, 15, x FOR PEER REVIEW potential of sulfonic acids was studied during the reaction of PhCH2O 2H-pyran (5 mol% of the catalysts and >90% yields over 10 cycles); c that the p-dodecylbenzenesulfonic acid/PAO system lost activity after alytic potentials of PIB-SO3H and PIB-ArSO3H were also estimated fo three-component condensation between PhCOH, urea and acetoaceti yields of the products were achieved after recycling seven times and m Scheme 21. Synthesis of PIB-bound (a) alkane and (b) arenesulfonic acids [15 It is worth pointing out here that alkylation of the active aromat stituted anilines) by HRPIB (Mn = 1 kDa) was efficiently used in the sy azo dyes [160] (Scheme 22), that are the colored probes used in the st non-toxic solvents, including PAOs. Alkylation of 6-phenoxyhexan-1sulted in an amino-functionalized PIB that was used for the stabiliza line semiconductors (CdSe and PbS) [161]. It is worth pointing out here that alkylation of the active aromatic compounds (substituted anilines) by HRPIB (M n = 1 kDa) was efficiently used in the synthesis of lipophilic azo dyes [160] (Scheme 22), that are the colored probes used in the study of high-boiling non-toxic solvents, including PAOs. Alkylation of 6-phenoxyhexan-1-amine by PIB-Cl resulted in an amino-functionalized PIB that was used for the stabilization of nanocrystalline semiconductors (CdSe and PbS) [161].
PIB-substituted phenols were synthesized using alkylation of phenol by HRPIBs, and catalyzed by PhOH·BF 3 in n-hexane/toluene media [162]. The structure of the reaction products is not entirely clear; 1 H NMR data, presented in this patent, raise questions about the assignment of the signals. On the other hand, in [163], the structure of the alkylation product was clearly proven. In this work, bifunctional HRPIB and the product of its reaction with phenol were used as starting compounds for the synthesis of PIBs with oxirane functionality; the reactions on one chain-end fragment are presented in Scheme 23. Alkylation of phenol by three-arm star HRPIB (prepared using 1,3,5-C 6 H 3 (CMe 2 Cl) 3 initiator and FeCl 4 / i PrOH catalysts), followed by acylation with methacryloyl chloride, resulted in a highly reactive photo-crosslincable polymer that was used for matrix stabilization of quantum dots [164]. Scheme 21. Synthesis of PIB-bound (a) alkane and (b) arenesulfonic acids [159].
It is worth pointing out here that alkylation of the active aromatic compounds (substituted anilines) by HRPIB (Mn = 1 kDa) was efficiently used in the synthesis of lipophilic azo dyes [160] (Scheme 22), that are the colored probes used in the study of high-boiling non-toxic solvents, including PAOs. Alkylation of 6-phenoxyhexan-1-amine by PIB-Cl resulted in an amino-functionalized PIB that was used for the stabilization of nanocrystalline semiconductors (CdSe and PbS) [161].
PIB-substituted phenols were synthesized using alkylation of phenol by HRPIBs, and catalyzed by PhOH·BF3 in n-hexane/toluene media [162]. The structure of the reaction products is not entirely clear; 1 H NMR data, presented in this patent, raise questions about the assignment of the signals. On the other hand, in [163], the structure of the alkylation Scheme 22. Synthesis of PIB-bound azo dyes [160]. product was clearly proven. In this work, bifunctional HRPIB and the product of its reaction with phenol were used as starting compounds for the synthesis of PIBs with oxirane functionality; the reactions on one chain-end fragment are presented in Scheme 23. Alkylation of phenol by three-arm star HRPIB (prepared using 1,3,5-C6H3(CMe2Cl)3 initiator and FeCl4/ i PrOH catalysts), followed by acylation with methacryloyl chloride, resulted in a highly reactive photo-crosslincable polymer that was used for matrix stabilization of quantum dots [164]. In 2021 [165], Pebriani et al. published a controversial report on the synthesis of diamine derivatives of PIB based on direct acid-catalyzed interaction between HRPIB and pentaethylenehexamine. The structure of the reaction product (Scheme 24), proposed by the authors, was confirmed via FT-IR spectra. In the absence of NMR data, this structure (as a result of nucleophilic addition to =CH2) raises reasonable doubts from a mechanistic In 2021 [165], Pebriani et al. published a controversial report on the synthesis of diamine derivatives of PIB based on direct acid-catalyzed interaction between HRPIB and pentaethylenehexamine. The structure of the reaction product (Scheme 24), proposed by the authors, was confirmed via FT-IR spectra. In the absence of NMR data, this structure (as a result of nucleophilic addition to =CH 2 ) raises reasonable doubts from a mechanistic point of view. Note that in another study [166], this process was called 'electrophilic substitution' (no NMR data). Some doubt is also expressed regarding the choice of the solvent (o-xylene) that can react with HRPIB.

PIB-Based Copolymers
High reactivity of exo-olefin-and -CMe2Cl-terminated PIBs have been successfully used for the synthesis of amphiphilic block copolymers with excellent prospects for bio medical application and materials' design [167]. In this section, we provide some practica examples of the preparation and applications of PIB copolymers.
Highly reactive 'Cl-allyl-PIB-allyl-Cl' copolymers (see Scheme 7) were hydrolyzed to the corresponding diols using aqueous tetrabutylammonium hydroxide in THF (reflux 22 h, and 98.2% conversion); these diols in a mixture with butane-1,4-diol were further used for polyuerthane preparation through the reaction with 4,4′-methylenebis(pheny isocyanate) in the presence of Sn(Oct)2 [68]. In subsequent studies [168], the synthesis of a copolymer was optimized to achieve mechanical characteristics and biocompatibility suit able for a fully synthetic heart valve. With the use of freshly distilled 4,4′-meth ylenebis(phenyl isocyanate) and increasing the PIB diol concentration in the reaction mix ture, a copolymer with an increased MW (>100 kDa), a tensile strength of ≈32 MPa, an elongation of ≈630%, and a toughness of >4.0 J was achieved. This material had an impres sive fatigue life, >1 million cycles around 1 MPa SED and >0.5 million at 2-3 MPa SED; i exhibited no calcification, and had less protein adsorption and cell attachment in compar ison with silicone-rubber materials. A similar copolymer, synthesized from (4-HOC6H4) PIB-(4-C6H4OH), was proposed for medical applications in a recent patent of Medtronics Inc. [169].
PIB-OH was also used as a starting compound for the synthesis of chain transfer agent PIB-OC(O)CH2CH2C(Me)(CN)SC(S)SC12H25 for reversible addition-fragmentation chain transfer (RAFT) polymerization of fatty ester methacrylates [171]. The block copol ymers showed varied crystalline behavior depending on the chain length of the fatty acids PIB-Br-initiated alternating RAFT polymerization was also used in the synthesis of the copolymer of tert-butoxy carbonyl (Boc)-leucine containing a maleimide unit and Boc-al anine containing a styrene unit. Its deprotection resulted in an amphiphilic copolymer with free -NH3 + and -COOfunctionalities in an alternating fashion that demonstrated pH-responsive self-assembly [172]. A similar approach was used in the synthesis of PIB block copolymers containing aninonic (acrylic acid monomer), neutral (MeO(CH2CH2O)4 actylate monomer) and cationic ((Boc)-leucine-OCH2CH2O-acrylate monomer) frag ments, designed for ion transport through the liposomal membrane [173]. RAFT copoly

PIB-Based Copolymers
High reactivity of exo-olefinand -CMe 2 Cl-terminated PIBs have been successfully used for the synthesis of amphiphilic block copolymers with excellent prospects for biomedical application and materials' design [167]. In this section, we provide some practical examples of the preparation and applications of PIB copolymers.
Highly reactive 'Cl-allyl-PIB-allyl-Cl' copolymers (see Scheme 7) were hydrolyzed to the corresponding diols using aqueous tetrabutylammonium hydroxide in THF (reflux, 22 h, and 98.2% conversion); these diols in a mixture with butane-1,4-diol were further used for polyuerthane preparation through the reaction with 4,4 -methylenebis(phenyl isocyanate) in the presence of Sn(Oct) 2 [68]. In subsequent studies [168], the synthesis of a copolymer was optimized to achieve mechanical characteristics and biocompatibility suitable for a fully synthetic heart valve. With the use of freshly distilled 4,4methylenebis(phenyl isocyanate) and increasing the PIB diol concentration in the reaction mixture, a copolymer with an increased MW (>100 kDa), a tensile strength of ≈32 MPa, an elongation of ≈630%, and a toughness of >4.0 J was achieved. This material had an impressive fatigue life, >1 million cycles around 1 MPa SED and >0.5 million at 2-3 MPa SED; it exhibited no calcification, and had less protein adsorption and cell attachment in comparison with silicone-rubber materials. A similar copolymer, synthesized from (4-HOC 6 H 4 )-PIB-(4-C 6 H 4 OH), was proposed for medical applications in a recent patent of Medtronics Inc. [169].
PIB-OH was also used as a starting compound for the synthesis of chain transfer agent PIB-OC(O)CH 2 CH 2 C(Me)(CN)SC(S)SC 12 H 25 for reversible addition-fragmentation chain transfer (RAFT) polymerization of fatty ester methacrylates [171]. The block copolymers showed varied crystalline behavior depending on the chain length of the fatty acids. PIB-Brinitiated alternating RAFT polymerization was also used in the synthesis of the copolymer of tert-butoxy carbonyl (Boc)-leucine containing a maleimide unit and Boc-alanine containing a styrene unit. Its deprotection resulted in an amphiphilic copolymer with free -NH 3 + and -COO − functionalities in an alternating fashion that demonstrated pH-responsive self-assembly [172]. A similar approach was used in the synthesis of PIB block copolymers containing aninonic (acrylic acid monomer), neutral (MeO(CH 2 CH 2 O) 4 -actylate monomer) and cationic ((Boc)-leucine-OCH 2 CH 2 O-acrylate monomer) fragments, designed for ion transport through the liposomal membrane [173]. RAFT copolymerization was also used for the synthesis of PIB-containing copolymer with glucose-decorated hydrophilic segment that showed high efficiency in the protection of the insulin aggregation [174].
The prospects of using PIB blocks in the design of stretchable semiconducting polymers was demonstrated on the example of ABA-type architecture comprising PIB and poly(naphthalenediimide-bithiophene) with various compositions [175].
Commercial PIB-SA was introduced to polycondensation with glycerol oligomers with a formation of 1:1 adducts and relatively high-MW polymers [176]. The optimal molar ratio of PIB-SA to polyglycerol was found to be 1:1.

Conclusions
The development of new, cost-efficient and environmentally friendly catalysts for replacing commercially used BF 3 in the synthesis of HRPIBs is still relevant. Since 2010, a number of alternative catalytic systems have been studied. The most prospective initiators of the IB polymerization for the production of HRPIBs are the complexes of metal chlorides with ethers, the complexes of alkylmetal chlorides with ethers, and heterogeneous systems mainly based on the metal chloride/ether adducts-the first-, second-, and third-generation catalysts-according to the classification proposed by S. Kostjuk in 2021 [2].
Among the metal chloride/R 2 O catalytic systems, AlCl 3 /R 2 O could be a good alternative to BF 3 -based catalysts; however, the problem of low activity and poor solubility of the catalytic complexes in aliphatic media should be solved. Further improvement in similar catalysts in terms of the selectivity of β-H elimination (preferably in n-hexane or in C4 feed), via both the design of new Lewis acids and new electron donors, can be complicated by the formation of relatively high-MW PIBs when highly active catalyst could be found [22]. Also note that aromatic solvents are still proposed to be used in IB polymerization [80,81,84,126], even though aromatic hydrocarbons can undergo alkylation [47] and depolymerize PIBs in the presence of acids [51].
Using C4 hydrocarbon feeds instead of pure IB is a growing trend in modern technologies of HRPIB manufacturing [55,56]. Therefore, new-generation catalysts should be active in a non-polar reaction medium and inert towards but-1-ene and but-2-enes, that hardly fits with a high catalytic activity in cationic processes. The first results in this field have already been published [34,78,84,89,97], but further studies are required on the use of hydrocarbon mixtures with comprehensive analyses of the microstructure of polymers.
In the development of heterogeneous catalysts for the production of HRPIBs, certain analogies with the development of supported α-olefin polymerization catalysts arise. Except for the problem of the resulting polymer particle morphology (irrelevant for more soluble PIBs), the problems associated with the efficient and stable supporting of the active species, maximizing the surface area, and selecting the donor additives seem common for both processes. These problems have been solved successfully for Ziegler-Natta catalysts, and we can also expect this progress in the area of heterogeneous catalysis for IB cationic polymerization in the near future.
Finally, one cannot fail to note the structural similarity of HRPIBs and methylenealkanes, including vinylidene dimers and oligomers of α-olefins [180,181]. Apparently, the methods of HRPIB functionalization can be successfully applied in methylenealkane chemistry, and vice versa, especially in view of the progress achieved recently in catalytic transformations of methylenealkanes [182][183][184][185][186][187].