A Bibliometric Survey on Polyisobutylene Manufacture

: Polyisobutylenes (PIB) constitute a versatile family of polymer materials that have been used mainly as fuel and lubricant additives. Particularly, the current commercial demand for highly reactive polyisobutylene (HR-PIB) products motivates the development of new processes and procedures to produce PIBs with high polymer yields, narrow molar mass distributions and high vinyl contents. For this reason, a bibliometric survey is presented here to map and discuss important technical aspects and technological trends in the ﬁeld of solution cationic polymerization of isobutylenes. It is shown that investigations in this ﬁeld are concentrated mainly on developed countries and that industrial initiatives indicate high commercial interest and signiﬁcant investments in the ﬁeld. It is also shown that use of catalyst systems based on AlCl 3 and ether cocatalysts can be very beneﬁcial for PIB and HR-PIB manufacture. Finally, it is shown that investigations search for cheaper and environmentally friendly catalysts and solvents that can be employed at moderate temperatures, particularly for the production of HR-PIB.


Introduction
Polyisobutylenes (PIB) constitute a highly versatile family of saturated aliphatic polymer materials that are produced as commodity products, although in much lower volumes than other typical polymer commodities. PIBs were originally developed by BASF SE in 1931, presenting several important and characteristic properties, including low glass and crystallization temperatures (−60 and 5 • C, respectively), relatively high densities (0.9 g/mL), and low permeabilities for common small molecules [1][2][3]. PIBs are also characterized by good chemical resistance against strong oxidizers (such as ammonium hydroxide, peroxides and hydrochloric acid), corrosives (such as sulfuric acid and dilute hydrofluoric acid) and harsh chemicals (such as n-methylpyrollidone), due to the inherent low reactive character of PIB materials [4]. Particularly, the weak dependence between the structural relaxation temperature and the viscosity, the wide mechanical segmental dispersion and the high degree of inelastic spreading are unique and very unusual when compared to other polymer materials, and can be associated with the high rates of structural relaxation [3]. Consequently, PIBs find numerous important commercial applications as food additives, chewing gums, additives for cosmetic products and raw materials for manufacture of lube additives, metalworking fluids, adhesives and a number of different materials used in Table 1. Reaction Mechanism Usually Adopted to Describe Isobutylene Polymerizations [19].

Reaction Steps Kinetic Constants
Initiation Chain Transfer I is the Lewis acid, X is the Lewis base, I * is the catalytic complex, M is the monomer molecule, P i is the dormant polymer species of size i, P * i is the active polymer species of size i, Λ i is the dead polymer chain and X * is the counterion or a chain transfer agent [18].
After the polymerization step, many applications (such as lubricating oils additives, adhesives, putty glue and electrical insulating) demand the posterior functionalization of PIB chains, so that PIB chains must be reactive to some extent to make these applications feasible. This explains the current commercial demand for development of highly reactive polyisobutylene (HR-PIB) products and respective production processes [19]. Conventional PIB (C-PIB) and HR-PIB grades present distinct chemical structures, particularly regarding the position of the unsaturation bonds formed after the chain transfer and chain termination steps [11,15,19]. For C-PIB, most unsaturated double bonds are located internally in the chain (endo groups, sometimes more than 90% of the double bonds), whereas for HR-PIB, most unsaturated double bonds are located at the terminal chain end (exo groups, more than 70% of the double bonds) [20], which facilitates the subsequent functionalization reactions. Figure 1 presents a simplified mechanism that explains the formation of C-PIB and HR-PIB chains through cationic polymerization of isobutylene. As already described, the first step comprises the initiation reactions, when the active carbocationic species are generated to allow the polymer chain growth. The addition of monomer molecules to the growing chain takes place at the ionic end group during the propagation step. Chain transfer can also take place, being characterized by hydrogen abstraction from the active chain end by monomer molecules. This ensures that the newly formed active sites will continue to propagate, keeping the characteristic high rates of the reaction system. Lastly, termination Processes 2021, 9, 1315 4 of 38 or spontaneous chain transfer can take place through rearrangement of the carbon chain end or reaction of the reactive group with the counterion. Generally, PIB is produced through a living polymerization mechanism, so that the rate of chain termination is usually negligible and the growth of the cationic chain is limited by monomer consumption [19,21]. During termination and chain transfer steps, the nature of the produced unsaturated double bond is related to the charge distribution between the tertiary (leading to exo double bonds) and the secondary (leading to endo double bonds) carbon atoms [19,21,22]. In order to describe this very important kinetic effect, which explains the formation of C-PIB and HR-PIB chains, according to the current kinetic interpretations, the transfer of the cationic positive charge between neighboring carbon atoms must be described by an independent "charge walking step", which is somewhat analogous to the well-known "chain walking step" of coordination polymerization mechanisms [23].
growing chain takes place at the ionic end group during the propagation step. Chain transfer can also take place, being characterized by hydrogen abstraction from the active chain end by monomer molecules. This ensures that the newly formed active sites will continue to propagate, keeping the characteristic high rates of the reaction system. Lastly, termination or spontaneous chain transfer can take place through rearrangement of the carbon chain end or reaction of the reactive group with the counterion. Generally, PIB is produced through a living polymerization mechanism, so that the rate of chain termination is usually negligible and the growth of the cationic chain is limited by monomer consumption [19,21]. During termination and chain transfer steps, the nature of the produced unsaturated double bond is related to the charge distribution between the tertiary (leading to exo double bonds) and the secondary (leading to endo double bonds) carbon atoms [19,21,22]. In order to describe this very important kinetic effect, which explains the formation of C-PIB and HR-PIB chains, according to the current kinetic interpretations, the transfer of the cationic positive charge between neighboring carbon atoms must be described by an independent "charge walking step", which is somewhat analogous to the well-known "chain walking step" of coordination polymerization mechanisms [23].  [19,21]. "I" stands for the initiator and "X" for the counterion Lewis base.
Commercially, both HR-PIB and C-PIB are produced through similar cationic solution polymerization processes, although using different catalyst systems and reaction conditions [11,15,19,22]. As a matter of fact, the HR-PIB market is consolidated worldwide and involves applications in the packaging, automotive and industrial sectors, with an expected market value of USD 550 million per year by the end of 2030 [24].
Recently, Rajasekhar et al., 2020 presented a review on academic PIB research, with emphasis on HR-PIB, presenting recent advances on polymerization mechanisms and industrial practices, including the use of nonpolar solvents (usually cheaper and more  [19,21]. "I" stands for the initiator and "X" for the counterion Lewis base.
Commercially, both HR-PIB and C-PIB are produced through similar cationic solution polymerization processes, although using different catalyst systems and reaction conditions [11,15,19,22]. As a matter of fact, the HR-PIB market is consolidated worldwide and involves applications in the packaging, automotive and industrial sectors, with an expected market value of USD 550 million per year by the end of 2030 [24].
Recently, Rajasekhar et al., 2020 presented a review on academic PIB research, with emphasis on HR-PIB, presenting recent advances on polymerization mechanisms and industrial practices, including the use of nonpolar solvents (usually cheaper and more easily handled), higher temperatures (which leads to less expensive and more convenient industrial processes) and industrial raffinate (C4 raffinate) as feed stream (usually available at oil-based chemical plants and avoiding purification of feed streams). Moreover, multiple strategies were suggested for development of new catalyst systems that can eventually replace the traditional and toxic catalyst BF 3 [22]. In fact, other studies have already reported that alternative Lewis acid catalytic complexes (based on the AlCl 3 catalyst and distinct ether co-catalysts) are already being applied industrially [19].
Based on the previous paragraphs, Table 2 summarizes the main properties, industrial players, and applications of commercially available PIB materials. As one can see, PIB products are employed in multiple economic sectors, reflecting their commercial and economic importance. C-HMPIB >20 kDa Elastomers with low density, low permeability and excellent electrical properties [25].
Intermediates for preparation of lubricant additives and fuels [31].
Despite the commercial relevance, well-established industrial production processes and worldwide applicability of PIB products, a comprehensive bibliometric survey of the literature available in this field is not available. For this reason, the following sections of the present work map, measure, describe and discuss important technical aspects and technological trends in the field of solution cationic polymerization of isobutylenes, aiming at the production of both C-PIB and HR-PIB products. Among the technical aspects, the most important process operational parameters, including the selected Lewis acids, Lewis bases, solvents and temperatures are highlighted, as these parameters exert significant influence on the course of the polymerization and, consequently, on the final polymer properties.

Papers
The conducted bibliometric study was based on electronic search tools. Web of Science and Scopus were used to explore academic papers. The ISI Web of Science indexes 9370 science journals with worldwide journal coverage [34], while Scopus counts with 23,452 peer-reviewed journals and more than 5000 international publishers [35]. Thus, both selected electronic databases can provide extensive and reliable references to conduct a comprehensive analysis of academic papers in our field of study. The Scopus database was used to collect academic papers written in English in the field of isobutylene polymerization. The initial search was carried out on 7 January 2020, and has been complemented since then, based on the references of downloaded papers and information provided by published journals. At first, the used search terms were "isobutylene polymerization" OR "isobutene polymerization". In order to refine the results, the following subject areas were excluded from the initial search: "PHYS", "BIOC", "PHAR", "ENER", "ENVI", "COMP" and "ENGI". Only articles already published in scientific journals were considered to perform the bibliometric analysis, which considered papers published from 1960 to 2020.
Although comprehensive reviews of papers published in languages other than English has not been performed, it is important to emphasize that relatively few papers regarding PIB manufacture were published in other languages, most of them more than 30 years ago, and that the respective scopes were very similar to the ones presented in the papers written in English. Consequently, it can be assumed that the English language does not constitute a relevant drawback for the present study.
The Web of Science database was also used to analyze academic publications written in English in the field of isobutylene polymerization. The initial search was carried out on 4 April 2020, and has been complemented since then, based on the references of downloaded papers and information provided by published journals. In this case, the used search terms were "isobutylene AND (polymerization or polymerisation)" OR "isobutene AND (polymerization or polymerisation)" in the topic field. Only articles already published in scientific journals were considered to perform the bibliometric analysis, which considered papers published from 1960 to 2020. The following subject areas were excluded from the initial search: " Electrochemistry  For the purposes of the present study, papers obtained from both databases were combined and downloaded. As informed previously, this combined list of publications has been complemented since then, based on the references of downloaded papers and information provided by published journals. Initially, the manuscripts were grouped in terms of the categories described in Table 3 to conduct the bibliometric analyses. Categories #3 and #4, Institution and Country, respectively, were reported in accordance with the affiliation of the last author. The Number of Citations, category #5, was obtained from the database at the moment of paper downloading and data acquisition and, therefore, are not necessarily synchronized. Later, the Number of Citations was used to determine the five most influential works in the field, assuming that this index can be used to measure the impact of the article on the technical and scientific communities. Cate-gories #6 to #9 are related to process operation conditions, which are needed to evaluate the conditions used most often to perform isobutylene polymerizations in the academic environment. In particular, category #10 was added in order to identify papers involved with the production of HR-PIB (more than 70% of exo groups). It is worth mentioning that papers focused on the copolymerizations of isobutylene with other monomers and the synthesis of new catalyst systems were only considered in the bibliometric analysis when the authors provided information about the operation and characterization procedures used to produce and characterize the properties of the obtained isobutylene homopolymer.

Patents
The conducted bibliometric study was also based on electronic search tools, and ESPACENET was used to explore published patents. This patent database indexes patents from most countries with worldwide coverage of innovation and development. Thus, these references are presented as reliable to conduct a comprehensive analysis of academic articles in our field of study.
Patents written in English, German and French were collected in the area of isobutylene polymerization. The initial search was carried out on 7 January 2020, and has been complemented since then, based on information provided by published journals and patents, country or region holding the technology and patent applicants and players. Initially, the search terms used were "isobutylene AND polymerization" OR "isobutene AND polymerization". Only patents already published, regardless of their status, were considered for the bibliometric analyses, which considered papers published between 1960 and 2020. For the purposes of the present study, the obtained patents were grouped according to the categories described in Table 4 to carry out the bibliometric analyses. Category #3 was classified in terms of three possible outcomes: active, pending and not active. Active status includes the following terms exposed as status in the "active", "application granted" and "ip right grant" patents. Pending status includes the following terms displayed as status in the "pending", "application filling" and "searching and examination" patents. Not Active status includes the following terms exposed as status in the patents: "withdraw", "expired", "ip right cessation", "revoked", "application discontinuation". Category #4 was reported considering the country of origin where the considered process was developed. Categories #5 to #8 are related to process operation conditions, which are needed to evaluate the conditions used to perform isobutylene polymerizations in the academic and industrial environments. In particular, Category #9 was added in order to identify patents involved with the production of HR-PIB (more than 70% of exo groups). These process operation conditions were based on the claims that form the patent description.

Statistical Analyses
Standard statistical ad correlation analyses were performed with help of the Statistica 7.0 software (TIBCO Software Inc., Palo Alto, CA, USA). Unless stated otherwise, all statistical evaluations were performed within the confidence level of 95%.

Papers
After the data mining phase, the final set of downloaded documents comprised 341 published papers, including 86% of the papers related to C-PIB technologies and 14% of the papers related to HR-PIB technologies. This scenario illustrates the more traditional use of C-PIB products and the largest commercial market of C-PIB materials, despite the growing market share of HR-PIB grades.
3.1.1. Yearly Distribution Figure 2 shows the evolution of the annual number of published articles involving PIB materials in the past decades. As a whole, the number of published papers per year is not very impressive, which reflects the relatively small number of researchers involved with investigations in this field, when compared with other areas of knowledge. Particularly, in the 1990s there was an apparent increase in academic interest in this field, which can be related to the appeal of the newly developed HR-PIB products and to implementation of worldwide policies, encouraging the development of environmentally safe practices and imposing restrictions on vehicular polluting emissions [8]. As a matter of fact, functionalized HR-PIB materials can be used as fuel and lubricant additives for enhancement of engine performance, diminishing the production of particle residues and reducing the emissions of hydrocarbons and particles [22]. For this reason, laws are enforcing the use of these HR-PIB based materials in many countries and providing an additional push for development of HR-PIB technologies. Nevertheless, the reduction in the number of scientific publications in the field over the last decade indicates that the available technologies are mature and that new breakthroughs are needed to encourage investigations and scientific development in this area.  Table 5 presents the 10 journals used most frequently by researchers to publish the results of PIB investigations. As shown in Table 5, papers related to PIB are concentrated in scientific journals connected to materials science, chemistry and engineering, although journals connected to physics and catalysis have also been used less frequently for communication of catalyst developments and advances in the interpretation of reaction mechanisms. Nevertheless, significant concentration of papers was observed in the journals "Macromolecules" and "Journal of Polymer Science (Part A: Polymer Chemistry)", with 76 and 49 papers, respectively. When the quantitative (CiteScore and JIF-Journal Impact Factor) [36] and qualitative (SNIP-Source Normalized Impact per Paper) [36] metric indicators are used to evaluate the impact of the research done in the field of interest, one can conclude that the selected journals can be regarded as influential (high citation scores) but that investigations published in the field of interest are much less influential than the average papers published in those journals (much lower SNIP scores). This reflects the much smaller size of the PIB investigation communities, when compared to other areas of  Table 5 presents the 10 journals used most frequently by researchers to publish the results of PIB investigations. As shown in Table 5, papers related to PIB are concentrated in scientific journals connected to materials science, chemistry and engineering, although journals connected to physics and catalysis have also been used less frequently for communication of catalyst developments and advances in the interpretation of reaction mechanisms. Nevertheless, significant concentration of papers was observed in the journals "Macromolecules" and "Journal of Polymer Science (Part A: Polymer Chemistry)", with 76 and 49 papers, respectively. When the quantitative (CiteScore and JIF-Journal Impact Factor) [36] and qualitative (SNIP-Source Normalized Impact per Paper) [36] metric indicators are used to evaluate the impact of the research done in the field of interest, one can conclude that the selected journals can be regarded as influential (high citation scores) but that investigations published in the field of interest are much less influential than the average papers published in those journals (much lower SNIP scores). This reflects the much smaller size of the PIB investigation communities, when compared to other areas of research, as stated previously. Table 5. Journals used most frequently to publish investigation results in the field of isobutylene polymerization between 1960 and 2020.  Figure 3 displays the countries of research institutions that published scientific papers regarding the PIB manufacture. As one can see, American institutions published approximately 42% of the articles in the field, followed by Germany and China, with approximately 10% each. Not surprisingly, this is related to the most important commercial players in the field (as shown in Table 2) and to the commercial demand for this material in these countries. Table 6 shows the research institutions that published scientific papers regarding PIB manufacture most frequently. As expected, the research institutions with most of the publications in the area of PIB are American universities (The University of Akron, University of Massachusetts Lowell and The University of Southern Mississippi). Although they do not appear among the main countries in publications, a Czech institution (Czech Academy of Sciences) and a Belarusian institution (Belarusian State University) appear among the most productive institutions in the analyzed field. In addition to these ones, other relevant Chinese, Canadian, German and Russian institutions could be detected in the area. Figure 3 displays the countries of research institutions that published scientific papers regarding the PIB manufacture. As one can see, American institutions published approximately 42% of the articles in the field, followed by Germany and China, with approximately 10% each. Not surprisingly, this is related to the most important commercial players in the field (as shown in Table 2) and to the commercial demand for this material in these countries.  Table 6 shows the research institutions that published scientific papers regarding PIB manufacture most frequently. As expected, the research institutions with most of the publications in the area of PIB are American universities (The University of Akron, University of Massachusetts Lowell and The University of Southern Mississippi). Although they do not appear among the main countries in publications, a Czech institution (Czech Academy of Sciences) and a Belarusian institution (Belarusian State University) appear among the most productive institutions in the analyzed field. In addition to these ones, other relevant Chinese, Canadian, German and Russian institutions could be detected in the area.

Operation Parameters for Isobutylene Polymerizations
PIB manufacture technologies may depend on numerous distinct operation variables, although the most important ones are the Lewis acid catalyst, the Lewis base co-catalyst, the solvent and the temperature, as described in the following subsections.
Lewis Acid Catalysts Figure 4 shows the citation frequencies of catalysts employed most often for manufacture of both C-PIB and HR-PIB grades. As one can see, most analyzed catalysts have been used for manufacture of both C-PIB and HR-PIB products, although with different frequencies. The catalysts reported most usually for isobutylene polymerizations are TiCl 4 , BCl 3 , AlCl 3 and SnCl 4 because of their well-known high catalytic activities [25,37]. Among these catalysts, TiCl 4 was cited in almost 35% of the analyzed papers for manufacture of C-PIB. Surprisingly, although BF 3 has been widely used for production of HR-PIB in industrial plants, this catalyst has not been used very frequently in scientific publications, possibly because of safety issues and difficult handling [19,22]. have been used for manufacture of both C-PIB and HR-PIB products, although with different frequencies. The catalysts reported most usually for isobutylene polymerizations are TiCl4, BCl3, AlCl3 and SnCl4 because of their well-known high catalytic activities [25,37]. Among these catalysts, TiCl4 was cited in almost 35% of the analyzed papers for manufacture of C-PIB. Surprisingly, although BF3 has been widely used for production of HR-PIB in industrial plants, this catalyst has not been used very frequently in scientific publications, possibly because of safety issues and difficult handling [19,22].  Although TiCl 4 has been the most cited Lewis acid catalyst since the 1950s, the steady decrease in the number of citations has also been observed in the last 10 years, indicating maturation of this technology and replacement by other catalysts. TiCl 4 -based catalyst systems are efficient to produce C-HMPIBs, although requiring cryogenic temperatures to control the catalyst ionization more effectively [38][39][40][41][42] Additionally, its toxic and corrosive nature constitutes a drawback for industrial applications, demanding expensive procedures to improve safety handling requirements and waste disposal [42]. For this reason, the declining interest in this catalyst can possibly be associated with the search for more environmentally safe practices. Besides, the use of TiCl 4 as co-initiator makes the control of the average molar masses difficult, as the isomerization of polyisobutylene chains can be significant during prolonged polymerization times [31,43,44]. Consequently, the production of TiCl 4 -based HR-PIBs with vinyl contents above 80% has been seldomly reported in the literature [45]. In fact, only 10% of the papers regarding HR-PIBs reported the use of TiCl 4 for synthesis of the polymer.
As in the case of TiCl 4 , BCl 3 -based catalyst systems also exerted influence on academic research related to C-PIB production, especially in the 1990s [15,32,33,46]. The interest was focused mainly on the living polymerization of isobutylene and production of low molar mass telechelic PIB products [26,28,33,47]. This can be associated with the low stability of the intermediate BCl 4 that leads to rapid termination of active PIB chains, forming a chlorinated end group [27,29,30,32].
Catalyst systems involving organic boron complexes were cited in almost 9% of the papers that investigated the production of C-PIB grades. Weak Lewis acids, such as B(C 6 F 5 ) 3 [48][49][50][51][52][53][54] 56], have been described as efficient catalysts for C-PIB production because of their excellent single-site catalyst performances and ability to produce materials with high average molar masses (M n of 10 5 g.mol −1 ) under mild reaction conditions and short reaction times (less than 60 min).
Currently, there is significant demand for replacement of the BF 3 catalyst for commercial production of HR-PIB [57] due to safety issues (BF 3 is very toxic) and difficulties to handle this halogenated gas, which leads to high operation and maintenance costs and expensive and dangerous treatment of effluents [58]. As an alternative, many investigations have been reporting the use of the AlCl 3 catalyst for production of HR-PIB (almost 21% of the articles related to manufacture of HR-PIB), especially because this catalyst has also been used for long time to prepare C-PIB commercially [59], although only 10% of the papers related to production of C-PIB cited the use of the AlCl 3 catalyst.
The use of the AlCl 3 catalyst has been reported for manufacture of HR-PIB since 2010, which indicates that this indeed constitutes a new trend in the PIB technology [25,59]. Particularly, different researchers have reported contents of exo groups between 80 and 95%, for polymer materials produced in presence of oxygenated co-catalysts (as described in the next section) at mild conditions, short reaction times and with narrow molar mass distributions [25,59]. Consequently, AlCl 3 constitutes a promising alternative to replace the BF 3 catalyst in the near future. It is important to recall that the isobutylene polymerization catalyzed by AlCl 3 is usually very fast, and consequently highly exothermic and difficult to control, requiring the association with weak bases for selective electron abstraction and production of HR-PIB [60][61][62][63]. On the other hand, it offers convenient operation at relatively mild temperatures [43,60].
The use of solvent-ligated complex catalysts also constitutes a promising strategy to prepare HR-PIB through cationic polymerizations performed under mild conditions, in particular when the metal complexes (normally based on Mn, Ni, Cr, Co, Cu, Zn, Mo, Ga and Fe) are associated with weakly coordinating counteranions (WACs) [64][65][66][67][68][69][70][71][72]. The use of these compounds was cited in 13% of the papers related to the production of HR-PIB grades, where the high reactivity of the metal complexes was always highlighted. Figure 4 also shows that about 10% of the analyzed papers related to HR-PIB manufacture cited the use of tert-butyl chloride (t-BuCl) or iron chloride (FeCl 3 ) as catalysts. In these cases, various authors reported the simultaneous combination of high vinyl contents (higher than 70%) and high monomer yields (monomer conversions between 80 and 100%) for short reaction times, when n-hexane was used as solvent with these catalysts [73][74][75][76].
The uses of ethylaluminum dichloride (EADC), GaCl 3 and other Lewis acid catalyst systems have also been reported by considerable fractions of the analyzed papers, as shown in Figure 4 [74,77]. The many Lewis acid compounds that have been tested to produce PIB materials, especially in the last two decades, illustrate the efforts that are still being made to select more efficient and viable alternatives to replace the more conventional catalysts in these processes [68,71,72,[78][79][80]. Generically, the development of new catalysts pursues the combination of high activity and selectivity, cost reduction, easy handling and possible operation at mild conditions [59,65,76,81].

Lewis Base Co-Catalysts
In order to facilitate the quantitative analyses, the Lewis base co-catalysts described in the analyzed papers were classified in terms of their active functional groups. Figure 5 presents the relative frequencies of the main chemical functions used in the Lewis base co-catalysts. Organic halides (21%) and pyridine (22%) represent the most frequent Lewis base co-catalysts reported in papers related to production of C-PIB, while ethers (38%), alcohols (16%), pyridine (10%), and other inorganic compounds (10%) represent the most frequent Lewis base co-catalysts reported in papers related to production of HR-PIB. Thus, ether compounds have been the commonest Lewis base co-catalysts used in HR-PIB studies. On the other hand, hydrocarbons, esters, organometallic compounds and phenol have been used as co-catalysts for both technologies, while phthalates have been used exclusively for C-PIB reactions and benzene and nitriles have been used as co-catalysts only for manufacture of HR-PIB products.
For TiCl 4 -based systems, 2,6-di-tert-butylpyridine (DtBP) is the most cited co-catalyst, regarded as a strong Lewis base in the gas phase. DtBP acts as a proton trap in isobutylene polymerizations to avoid chain transfer and reversible termination steps, being commonly used to confer controlled/living characteristics to TiCl 4 -based C-PIB technologies [82,83]. However, Bae et al., 1998, [84] and Simison et al., 2006, [85] also reported the successful use of DtBP as Lewis base for HR-PIB production. The use of strong bases with the AlCl 3 catalyst was also evaluated for production of HR-PIB grades, but the researchers observed the decrease in monomer conversion and the exo vinyl content with these catalyst systems [43,86]. This certainly illustrates the importance of evaluating synergetic effects among the different constituents of the initiation system. presents the relative frequencies of the main chemical functions used in the Lewis base co-catalysts. Organic halides (21%) and pyridine (22%) represent the most frequent Lewis base co-catalysts reported in papers related to production of C-PIB, while ethers (38%), alcohols (16%), pyridine (10%), and other inorganic compounds (10%) represent the most frequent Lewis base co-catalysts reported in papers related to production of HR-PIB. Thus, ether compounds have been the commonest Lewis base co-catalysts used in HR-PIB studies. On the other hand, hydrocarbons, esters, organometallic compounds and phenol have been used as co-catalysts for both technologies, while phthalates have been used exclusively for C-PIB reactions and benzene and nitriles have been used as co-catalysts only for manufacture of HR-PIB products. For TiCl4-based systems, 2,6-di-tert-butylpyridine (DtBP) is the most cited cocatalyst, regarded as a strong Lewis base in the gas phase. DtBP acts as a proton trap in isobutylene polymerizations to avoid chain transfer and reversible termination steps, being commonly used to confer controlled/living characteristics to TiCl4-based C-PIB technologies [82,83]. However, Bae et al., 1998, [84] and Simison et al., 2006, [85] also reported the successful use of DtBP as Lewis base for HR-PIB production. The use of strong bases with the AlCl3 catalyst was also evaluated for production of HR-PIB grades, Organic halides accounted for 21% of the cited co-catalysts in papers related to C-PIB, although the best performances can be assigned to 2-chloro-2,4,4-trimethylpentane (TMPCl), which has been used mainly to perform isobutylene polymerizations in association with the TiCl 4 catalyst at cryogenic conditions [87][88][89][90][91]. TMPCl has also been used to study fundamental kinetic aspects of polymerization mechanisms because it allows the adequate control of the polymerization rates and the production of more uniform polymers [90,91]. Similarly, studies reporting the use of DtBP as co-catalyst and slow rates of polymerization to investigate aspects of the reaction kinetics can also be found in the field of carbocationic polymerization [38,84,87,92,93]. For all these reasons, the use of both DtBP and TMPCl as co-catalysts can be regarded as consolidated in the field of isobutylene cationic polymerizations.
Ether compounds (38%) have been largely used as Lewis base co-catalysts for synthesis of HR-PIB, although diisopropyl ether (Pr 2 O) [58][59][60]94,95], bis(2-chloroethyl) ether (CEE) [96] and dibutyl ether (Bu2O) [59,60] are the ones cited most often, usually in association with the AlCl 3 co-catalyst. This is probably due to the fact that ethers are known to form well-defined complexes with AlCl 3 [97]. The use of ether compounds leads to increase in the exo vinyl contents of the PIB products, as complexes formed with AlCl 3 are efficient to stabilize the carbenium ions, leading to high exo vinyl contents (>90%) at mild temperatures [98]. It is important to emphasize that the chain transfer step is extremely relevant in HR-PIB polymerizations, as the exo double-bond and a new protonated monomer result from the chain transfer reaction promoted by the ether [85]. Investigations carried with ethers of different basicities and with distinct ether/catalyst ratios indicated the enormous influence of basicity (and β-protons abstraction) on the average molar masses, exo vinyl contents, reaction rates and monomer conversions due to the modification of the reactivity and selectivity of the growing cations [37,60,86].
Water is a peculiar additive that has been used as co-catalyst in isobutylene polymerizations because of its many known advantages, such as low cost and non-toxicity [81,86,95,[103][104][105]. In the field of HR-PIB, the use of water has been extensively investigated [43,45,71,106]. Particularly, many authors observed that low concentrations of water did not affect the monomer conversions significantly [43,75,95,104,107], although at least one paper reported that addition of water might indeed affect the rates of polymerization [96]. Additionally, some authors reported that polymer materials with high exo vinyl contents (>80%) and narrow molar mass distributions can be produced at mild reaction temperatures (from −40 to 30 • C) using water as one of the key components of the catalyst system [43,104,107]. As a matter of fact, water can exert different and contradictory effects on the analyzed carbocationic reactions, depending on the analyzed catalyst system, due to participation in isomerization and chain termination reaction steps [43,62,74,77,95].
It is important to emphasize that many of the analyzed articles investigated the concomitant use of more than one co-catalyst in the initiation complex, although the observed results and trends have been essentially the ones already reported. Most of these investigations have been carried out as attempts to improve the selectivity of β-protons abstraction for catalytic chain transfer steps, and therefore to improve the control over the exo vinyl content of the obtained products [22,61,93,96].

Solvents
The 10 most frequently used solvents for each technology (C-PIB and HR-PIB), as reported in scientific publications, are presented in Figure 6. It must be highlighted that some solvents have been used in both technologies, which is not so common regarding Lewis acid catalysts and Lewis base co-catalysts. Particularly, both polar (55% of the citations) and nonpolar (45% of the citations) solvents have been used almost equally for production of C-PIB, although nonpolar solvents (64% of the citations) have been used more frequently for HR-PIB syntheses than polar solvents (36% of the citations). Among the reported nonpolar solvents, hexane and toluene are the ones cited most often, while dichloromethane (DCM) and chloromethane (CM) were the commonest ones among the polar solvents. The selection of the most appropriate solvent is strongly associated with the solubility of the Lewis acid catalyst and Lewis base co-catalyst in the reaction medium, as the solvent plays significant roles on solvation and stabilization of the carbocations [97]. Additionally, environmental effects, adequacy of safety procedures and practices affect the selection of the best suited solvent to conduct the isobutylene polymerization reaction [11]. The 10 most frequently used solvents for each technology (C-PIB and HR-PIB), as reported in scientific publications, are presented in Figure 6. It must be highlighted that some solvents have been used in both technologies, which is not so common regarding Lewis acid catalysts and Lewis base co-catalysts. Particularly, both polar (55% of the citations) and nonpolar (45% of the citations) solvents have been used almost equally for production of C-PIB, although nonpolar solvents (64% of the citations) have been used more frequently for HR-PIB syntheses than polar solvents (36% of the citations). Among the reported nonpolar solvents, hexane and toluene are the ones cited most often, while dichloromethane (DCM) and chloromethane (CM) were the commonest ones among the polar solvents. The selection of the most appropriate solvent is strongly associated with the solubility of the Lewis acid catalyst and Lewis base co-catalyst in the reaction medium, as the solvent plays significant roles on solvation and stabilization of the carbocations [97]. Additionally, environmental effects, adequacy of safety procedures and practices affect the selection of the best suited solvent to conduct the isobutylene polymerization reaction [11].   The use of polar solvents favors the dissociation of the ionic polymer chains and increases the solubility of Lewis complexes and active species in the reaction medium, facilitating chain propagation reactions and increasing the reaction rates [15,64,108,109]. Consequently, polar solvents often boost the catalyst activities, also leading to faster rates of ionization and slower rates of chain termination, resulting not only to higher rates of polymerization but also to products with higher average molar masses [15,40,110]. This behavior has been characterized in detail in C-PIB polymerizations [15].
Nonpolar solvents, such as hexane and toluene, promote slower reaction rates as active species tend to present poor solubility in these solvents, consequently reducing the catalyst activities [13,65,111]. This fact favors the enhanced control of the reaction, as the lower rates of polymerization also lead to lower rates of heat release and better temperature control [13,65,111]. Besides, the use of nonpolar solvents can also be advantageous for industrial operation because it prevents the undesired production of toxic chlorinated residues.
It is important to note that most reported strategies that seek to balance the appropriate rates of polymerization and the desired set of polymer properties make use of (i) a single nonpolar solvent with highly efficient initiator complexes or (ii) mixtures of polar and nonpolar solvents [108,[112][113][114], especially regarding the production of HR-PIB [39,43,74,108,115].
The use of TiCl 4 , the most reported Lewis acid catalyst, has been regularly reported in association with polar solvents, which can increase the concentration of active centers, resulting in a dormant and an active species connected by a mobile equilibrium [15]. As AlCl 3 complexes usually present poor solubility in nonpolar solvents due to the strong acidity, the use of these catalysts usually demands the use of solvent mixtures, higher initiator concentrations and high monomer concentrations [62,86,116]. However, GaCl 3 or FeCl 3 complexes can be used to produce HR-PIB grades in nonpolar solvents [80,81,103,117].
Temperatures A wide range of reaction temperatures has been identified during the bibliometric analyses of the scientific articles. In order to facilitate the interpretation of the data, the temperature range was divided into intervals of 10 • C. Figure 7 shows the temperatures reported for C-PIB and HR-PIB syntheses with the respective frequencies. C-PIB reactions have been performed mostly at exceptionally low temperatures (from −80 to −30 • C), whereas HR-PIB reactions have been investigated at relatively higher temperatures (from −20 to 30 • C). As HR-PIB products have been investigated more lately, this also reflects the continuous trend pursued by commercial operations to shift the reaction temperatures towards more convenient and milder reaction conditions, as a strategy to reduce the operation costs.
As one might already expect, the reaction temperature constitutes a critical factor for chain initiation, propagation and termination in cationic polymerizations [117]. For isobutylene polymerizations, reaction rates tend to increase at lower temperatures [39,40,118], due to combination of thermodynamic (mainly solubilization and ionization effects) and kinetic reasons, turning the industrial process expensive and environmentally unsustainable due to the high demand for energy [22]. Besides, the low reaction temperatures can be needed to control the properties of the reaction product, given the characteristic extremely high rates of polymerization in these systems. This explains why C-PIB reactions have been mostly performed at very low temperatures (Figure 7) in scientific papers [31,96,108]. As a matter of fact, the lower temperatures favor the solvation of the active ions, causing the decrease in the rate of ion collapse, which impacts the final average molar masses of the products [37,75,96]. On the other hand, HR-PIB syntheses have been preferably conducted at relatively higher temperatures. Although higher temperatures contribute to production of materials with lower average molar masses [37,80], higher temperatures also lead to higher rates of β-proton abstraction, which leads to creation of exo vinyl bonds [37,116,118].
reported for C-PIB and HR-PIB syntheses with the respective frequencies. C-PIB reactions have been performed mostly at exceptionally low temperatures (from −80 to −30 °C), whereas HR-PIB reactions have been investigated at relatively higher temperatures (from −20 to 30 °C). As HR-PIB products have been investigated more lately, this also reflects the continuous trend pursued by commercial operations to shift the reaction temperatures towards more convenient and milder reaction conditions, as a strategy to reduce the operation costs. As one might already expect, the reaction temperature constitutes a critical factor for chain initiation, propagation and termination in cationic polymerizations [117]. For isobutylene polymerizations, reaction rates tend to increase at lower temperatures [39,40,118], due to combination of thermodynamic (mainly solubilization and ionization effects) and kinetic reasons, turning the industrial process expensive and environmentally unsustainable due to the high demand for energy [22]. Besides, the low reaction temperatures can be needed to control the properties of the reaction product, given the characteristic extremely high rates of polymerization in these systems. This explains why C-PIB reactions have been mostly performed at very low temperatures (Figure 7) in scientific papers [31,96,108]. As a matter of fact, the lower temperatures favor the solvation As an important technological trend, studies are being developed to search for reaction systems that can be used to conduct isobutylene carbocationic reactions at temperatures that are close to room temperature in order to reduce the operation costs [42,62,71]. Particularly, some studies reported that manufacture of HR-PIB with AlCl 3 /ether using polar and non-polar solvents can be performed at temperature conditions that are close to room temperature (0 to 40 • C) and at short reaction times (below 30 min), leading to materials with high exo vinyl contents and better functionalization performances than industrially available HR-PIB grades [58,59,62,65].

Most Cited Papers
The downloaded papers were ranked in accordance with the total number of citations by other scientific publications, as shown in Table 7 for both C-PIB and HR-PIB products. As displayed in Table 7, the high citation frequencies of these papers in the last 2 decades indicate the persistent interest in these polymerization technologies. Table 7. The 5 most cited papers in the field of cationic isobutylene polymerizations for production of C-PIB and HR-PIB.  For the C-PIB technology, the most influential papers were published in the late 1980s and early 1990s. Kennedy (1986 and presented comprehensive studies [15,119] about the effects of operation parameters on isobutylene carbocationic polymerizations with BCl 3 as catalyst and polar and non-polar solvents at low temperatures (−30 • C), motivating discussions about the reaction mechanism and the modification of polymer chains of materials of commercial interest, which explains the interest arisen by these papers. Later, Gyor et al., 1992 (TiCl 4 /ether/mixture of polar and nonpolar solvents at extreme low temperatures) [83], Jacob et al., 1996 (TiCl 4 or BCl 3 /DtBP/polar or nonpolar solvents at extreme low temperatures) [121] and Bochmann and Dawson, 1996 (AlCl 3 /Tris(pentafluorophenyl)borane/polar solvents at extreme low temperatures) [120] discussed operation procedures to control the molar mass distributions, to reduce the dispersity and to enhance the functionalization efficiency the obtained polymer materials. As one can see, these papers provide information about the process operation conditions required to produce C-PIB materials with desired controlled properties.

Year of Publication
The more contemporary literature regarding HR-PIB technologies focuses on improving the highly reactive behavior of the material through modification of the polymerization reaction conditions [25,43,59,66,85,107]. The complexity and high sensitivity of isobutylene polymerizations to changes of operation conditions challenge the production of HR-PIBs with specified properties. The success of these papers can be explained by the extremely promising results regarding the obtainment of high polymer yields, high exo vinyl contents and use of more convenient higher temperatures (above 0 • C) for manufacture of HR-PIB, especially with AlCl 3 catalyst and hexane or chlorinated solvents, employing environmentally safer procedures and reagents and materials of lower costs.

Patents
After the data mining phase, the final set of downloaded documents comprised 203 published patents, including 66% patents related to C-PIB technologies and 34% papers related to HR-PIB technologies. As described for scientific papers, this scenario illustrates the more traditional use of C-PIB products and the largest commercial market of C-PIB materials, despite the growing market share of HR-PIB grades. The final set of documents included patents filed since the 1930s, which were analyzed in accordance with the categories described in Table 4 [57,66,. Figure 8 presents the annual distribution of patents in the field of cationic isobutylene polymerizations, which were grouped into three categories (active, pending and not active), based on their current status at data acquisition date.

Yearly Distribution
included patents filed since the 1930s, which were analyzed in accordance with the categories described in Table 4 [57,66,.
3.2.1. Yearly Distribution Figure 8 presents the annual distribution of patents in the field of cationic isobutylene polymerizations, which were grouped into three categories (active, pending and not active), based on their current status at data acquisition date. As displayed in Figure 8, the number of patents published annually has grown since the 1990s, although the annual frequencies are relatively small. The oldest patent found in the search was published in 1933 by BASF SE (former IG Farben), regarding the polymerization of isobutylene using BF3 and AlCl3 as co-initiator and water as initiator As displayed in Figure 8, the number of patents published annually has grown since the 1990s, although the annual frequencies are relatively small. The oldest patent found in the search was published in 1933 by BASF SE (former IG Farben), regarding the polymerization of isobutylene using BF 3 and AlCl 3 as co-initiator and water as initiator [292]. Compared to the first patent related to the production of HR-PIB, which was published in 1990 by Zeon Co [290], there is a gap of almost 60 years between the first accepted C-PIB and HR-PIB patents. Quantitatively, among the 203 analyzed patents, 35% were withdrawn or expired, 46% were still active and 19% were under analysis, which can be regarded as a very significant number because of the analyzed time period and highlights the continuing interest in the technological development in this field. Nevertheless, comparing Figure 2 to Figure 8, similar trends can be observed, with fast growth of the number of documents after publication of the first patent about HR-PIB in 1990, and slow decrease in the number of documents in the 2010-2020 decade, indicating that the available technologies are mature and that new breakthroughs are needed to encourage innovative developments in this area.
It must be noted that the discussions presented in the next sections consider only active and pending patents, since the technologies proposed by expired patents are no longer protected and may be of no significant commercial interest anymore. Figure 9 and Table 8 show the distributions of active, pending and priority patents related to C-PIB and HR-PIB products in terms of the country of origin. In patents that are active in more than one country, the first filing was classified as the priority patent. Patents that have priority represent countries where the technology was developed and consequently companies that perform priority operations in the field. Patents regarding C-PIB manufacture have been filed primarily in 10 countries, mostly individually, although some of them have been deposited at European and worldwide offices. On the other hand, patents regarding HR-PIB manufacture have been filed in only 8 countries individually, although some of them have also been deposited at European and worldwide offices. It is important to note that the most recent patents have been deposited primarily in offices that represent several countries simultaneously (as EPO and WIPO), instead of one country at a time. This could indicate the strategy to initially protect the technology in a globalized context and postpone the selection of the countries where the patent will remain active for longer periods of time.

Distribution of Countries
other hand, patents regarding HR-PIB manufacture have been filed in only 8 countries individually, although some of them have also been deposited at European and worldwide offices. It is important to note that the most recent patents have been deposited primarily in offices that represent several countries simultaneously (as EPO and WIPO), instead of one country at a time. This could indicate the strategy to initially protect the technology in a globalized context and postpone the selection of the countries where the patent will remain active for longer periods of time. Figure 9. Distribution of patents regarding C-PIB and HR-PIB in terms of the country of origin, as a function of the current status [57,66,. Table 8. Distribution of patents regarding C-PIB and HR-PIB in terms of the country of origin, as a function of the current status [57,66,.

C-PIB Active
Pending Priority  The development of PIB technologies is highly concentrated in the USA, China, Japan, Russia, and Germany, which represent almost 80% of all the active and pending filed patents (Table 8). This pattern is similar to the one observed for academic articles, although it is possible to note a growing incidence of active and pending patents deposited in China and through the World Intellectual Property Organization (WIPO), reinforcing the previously discussed trend for development of PIB technologies. Table 9 shows the companies and research centers that filed the analyzed patents. The patents regarding C-PIB technologies involved 25 institutions of 10 countries, while the patents regarding HR-PIB technologies involved 16 institutions of 8 countries. Kaneka Co. (Japan), BASF SE (Germany) and Otkrytoe Aktsionernoe Obshches (Russia) are the three top applicants for C-PIB patents, while BASF SE (Germany), Beijing University of Chemical Technology (China) and TPC Group (USA) are the top applicants for HR-PIB patents. It must be observed the main players in the PIB market are the TPC Group, Infineum, Lubrizol [246], BASF SE, INEOS (British Petroleum sold petrochemical business to INEOS in 2020) [302], Kaneka and Daelim. Based on this list, it can be observed that most of the main players are also among the main technology developers, as shown in Table 9 [219]. Table 9. Distribution of patents regarding C-PIB and HR-PIB in terms of the patent applicants [57,66,.

Ranking
Applicants Country Total Publications (%)

Operation Parameters for Isobutylene Polymerizations
The main operation parameters reported in the published patents were analyzed and the most significant ones were grouped for each technology (C-PIB or HR-PIB), as performed previously for scientific papers.
Lewis Acid Catalysts Figure 10 shows the frequencies of catalyst citations in published patents. Among the active patents, some catalysts were more widely used for manufacture of C-PIB, such as TiCl 4 and SnCl 4 , some catalysts were related more specifically to production of HR-PIB, such as AlBr 3  It is also worth mentioning the relatively high number of patents related to the use of the AlCl3 catalyst for manufacture of HR-PIB, which reflects the continuous search for environmentally safer catalysts and operation at higher temperatures, as BF3 processes are normally operated at exceptionally low reaction temperatures. It is also interesting to observe the very high relative number of patents involved with "Others" catalysts, indicating that the PIB business is still searching for new less expensive and environmentally safer catalysts for manufacture of both C-PIB and HR-PIB grades.
Lewis Base Co-Catalysts Figure 11 presents the chemical functions used most often as co-catalysts for production of PIB grades. It is interesting to observe that 20% of the patents regarding C-PIB did not use any sort of co-catalyst to produce the polymer. When production of HR-PIB is concerned, the absence of the co-catalysts was less frequent (10%), indicating the importance of the co-catalyst to produce the exo vinyl double bond. As explained previously, this is supported by mechanistic interpretation of the chain transfer step, which must involve elimination of -proton to generate the exo vinyl group [22]. Amines and silanes were reported as co-catalysts for C-PIB reactions only, while thiocompounds were used as cocatalysts to produce HR-PIB exclusively. Regarding HR-PIB, BF 3 was the catalyst reported most frequently (27.5%), confirming the relevance of the BF 3 catalyst in the industrial environment and reinforcing the interpretation that the relatively small number of academic investigations related to BF 3 is due to safety issues and difficult handling of this chemical compound. This result also agrees well with the evolution of the HR-PIB technology, as BASF SE patented the process to produce HR-PIB with high monomer conversions (around 95%), average molar masses between 500 and 5000 Da and more than 90% of exo vinyl groups with the BF 3 catalyst [22,95].
It is also worth mentioning the relatively high number of patents related to the use of the AlCl 3 catalyst for manufacture of HR-PIB, which reflects the continuous search for environmentally safer catalysts and operation at higher temperatures, as BF 3 processes are normally operated at exceptionally low reaction temperatures. It is also interesting to observe the very high relative number of patents involved with "Others" catalysts, indicating that the PIB business is still searching for new less expensive and environmentally safer catalysts for manufacture of both C-PIB and HR-PIB grades.
Lewis Base Co-Catalysts Figure 11 presents the chemical functions used most often as co-catalysts for production of PIB grades. It is interesting to observe that 20% of the patents regarding C-PIB did not use any sort of co-catalyst to produce the polymer. When production of HR-PIB is concerned, the absence of the co-catalysts was less frequent (10%), indicating the importance of the co-catalyst to produce the exo vinyl double bond. As explained previously, this is supported by mechanistic interpretation of the chain transfer step, which must involve elimination of β-proton to generate the exo vinyl group [22]. Amines and silanes were reported as co-catalysts for C-PIB reactions only, while thiocompounds were used as cocatalysts to produce HR-PIB exclusively. Processes 2021, 9, x FOR PEER REVIEW 23 of 40 Figure 11. Frequencies of co-catalyst citations in patents related to C-PIB and HR-PIB manufacture.
Regarding the use of co-catalysts, data extracted from academic articles and patents differ significantly. For instance, academic studies investigated the use of different nitrogen compounds as co-catalysts, such as pyridine (22%), while only 5% of the patents reported the use of amines for this function. Besides, 30% of the co-catalysts reported by patents were classified as "Others" for C-PIB and 27% for HR-PIB, illustrating the intense search for new co-catalysts for PIB manufacture. Additionally, scientific papers described the use of ethers as co-catalysts in 40% of the publications regarding HR-PIB production (as one can see in Figure 5), while the preference among patents is for alcohols (22%, as shown in Figure 11). Once more, this reinforces the continuous efforts to use less hazardous and less expensive compounds in industrial environments, such as water and simple alcohols, over halides and nitrogen compounds. Figure 12 presents the solvents reported most often in patents related to cationic isobutylene polymerizations. As reported in the patents, numerous solvents have been used to produce C-PIB and HR-PIB. Hexane, toluene, dichloromethane, and chloromethane have been preferred for production of C-PIB, while hexane, chloromethane and the C4 mixture have been used most often for production of HR-PIB. The use of C4 mixtures illustrates the efforts to obtain a better added value product from a feedstock that is available in most chemical centers. However, academic articles and patents converge about the commonest solvents, as in both cases hexane and toluene (among nonpolar solvents) and dichloromethane and chloromethane (among the polar solvents) are the solvents used most frequently. Nevertheless, the group "Others" contributes significantly to the analyzed solvent scenario, illustrating once more in this case the intense search for new solvents for PIB manufacture. Regarding the use of co-catalysts, data extracted from academic articles and patents differ significantly. For instance, academic studies investigated the use of different nitrogen compounds as co-catalysts, such as pyridine (22%), while only 5% of the patents reported the use of amines for this function. Besides, 30% of the co-catalysts reported by patents were classified as "Others" for C-PIB and 27% for HR-PIB, illustrating the intense search for new co-catalysts for PIB manufacture. Additionally, scientific papers described the use of ethers as co-catalysts in 40% of the publications regarding HR-PIB production (as one can see in Figure 5), while the preference among patents is for alcohols (22%, as shown in Figure 11). Once more, this reinforces the continuous efforts to use less hazardous and less expensive compounds in industrial environments, such as water and simple alcohols, over halides and nitrogen compounds. Figure 12 presents the solvents reported most often in patents related to cationic isobutylene polymerizations. As reported in the patents, numerous solvents have been used to produce C-PIB and HR-PIB. Hexane, toluene, dichloromethane, and chloromethane have been preferred for production of C-PIB, while hexane, chloromethane and the C4 mixture have been used most often for production of HR-PIB. The use of C4 mixtures illustrates the efforts to obtain a better added value product from a feedstock that is available in most chemical centers. However, academic articles and patents converge about the commonest solvents, as in both cases hexane and toluene (among nonpolar solvents) and dichloromethane and chloromethane (among the polar solvents) are the solvents used most frequently. Nevertheless, the group "Others" contributes significantly to the analyzed solvent scenario, illustrating once more in this case the intense search for new solvents for PIB manufacture. Temperatures Figure 13 presents the reaction temperatures claimed in patent descriptions. As performed previously, the claimed temperature range was divided into intervals of 10 °C. It is important to mention that the range of temperatures that each patent aimed to protect was not necessarily the same used to perform the reaction trials in the examples. The most frequent range of temperatures used in C-PIB technologies was placed between −70 °C and +50 °C, which is an overly broad range. For HR-PIB, the claimed temperature range was narrower: between −20 °C and +30 °C. This extensive range of temperatures, 80 °C for C-PIB and 50 °C for HR-PIB, could be part of the intellectual protection strategy, once potential competitors would not be able to overcome the claimed process. When used for manufacture of engine oil/lubricant dispersants, PIBs with low average molar masses perform better. For this type of application, polymerizations performed at higher temperatures are essential for production of short polymer chains through chain transfer reactions [22]. Temperatures Figure 13 presents the reaction temperatures claimed in patent descriptions. As performed previously, the claimed temperature range was divided into intervals of 10 • C. It is important to mention that the range of temperatures that each patent aimed to protect was not necessarily the same used to perform the reaction trials in the examples. The most frequent range of temperatures used in C-PIB technologies was placed between −70 • C and +50 • C, which is an overly broad range. For HR-PIB, the claimed temperature range was narrower: between −20 • C and +30 • C. This extensive range of temperatures, 80 • C for C-PIB and 50 • C for HR-PIB, could be part of the intellectual protection strategy, once potential competitors would not be able to overcome the claimed process. When used for manufacture of engine oil/lubricant dispersants, PIBs with low average molar masses perform better. For this type of application, polymerizations performed at higher temperatures are essential for production of short polymer chains through chain transfer reactions [22]. Temperatures Figure 13 presents the reaction temperatures claimed in patent descriptions. As performed previously, the claimed temperature range was divided into intervals of 10 °C. It is important to mention that the range of temperatures that each patent aimed to protect was not necessarily the same used to perform the reaction trials in the examples. The most frequent range of temperatures used in C-PIB technologies was placed between −70 °C and +50 °C, which is an overly broad range. For HR-PIB, the claimed temperature range was narrower: between −20 °C and +30 °C. This extensive range of temperatures, 80 °C for C-PIB and 50 °C for HR-PIB, could be part of the intellectual protection strategy, once potential competitors would not be able to overcome the claimed process. When used for manufacture of engine oil/lubricant dispersants, PIBs with low average molar masses perform better. For this type of application, polymerizations performed at higher temperatures are essential for production of short polymer chains through chain transfer reactions [22]. Although patent claims tend to protect very wide temperature ranges, scientific articles and patents converge to similar polymerization temperature ranges for C-PIB (−70 • C to −20 • C) and for HR-PIB (−20 • C to 30 • C). This once more indicates the efforts to produce PIBs with high exo vinyl contents at milder temperatures [22].

Correlation Analysis
To understand the effects of certain operating parameters on important properties of the final polymer material, standard principal component analysis (PCA) was conducted with operating parameters extracted from the bibliometric review [303]. The selected parameters were catalyst, cocatalyst, solvent, temperature, yield and vinyl content. Nonnumeric variables were digitally normalized as shown in Table 10, considering the acidity (catalyst) and polarity (solvent and cocatalyst) of the analyzed species. PCA analyses were performed using STATISTICA ® 10 software [303]. The data were initially normalized in accordance with Equation (1) and used as inputs for the analyses. Tables 11 and 12 show the eigenvalues, the eigenvectors (factors), the total variances (%), the cumulative eigenvalues and the cumulative variances (%).  Table 11 shows that Factor 1, the largest one, explains only 32.7% of the data variability, while Factor 2, the second largest one, explains only 18.8% of the variability. The cumulative variance for the first two factors is therefore equal to only 51.6%. As a matter of fact, it is necessary to consider five factors simultaneously to reach the cumulative variability of 95%, indicating that the analyzed conditions present high variability and that the analyzed variables tend to vary independently, so that very heterogeneous combinations of all analyzed variables should be expected in the data set. Table 12 presents the values of the individual components of the eigenvectors, which indicate the observed correlations among the analyzed variables along the principal directions. One can see that the first direction describes variations of the catalyst systems, which tend to combine AlCl 3 with ethers and BF 3 with alcohols (explaining the opposite signs). This probably reflects the continuous search for new catalyst systems and the current industrial activity in the field. The second factor concentrates solvent variations, while factor 3 describes polymer product changes and factor 4 describes temperature changes. Although the observed component values of vinyl contents are generally small and not significant, they seem to be inversely correlated with the yield values in factor 3 (the one that describes product changes), which should be considered during mechanistic studies.
The covariance matrices of available data were calculated with help of different factor rotation functions, which are used to maximize the correlations among the measured variables and the factors calculated by the PCA [303]. However, in all cases the covariance values were not significant. This result indicates that quantitative analyses of variable fluctuations depend on multiple disturbances and possibly respond nonlinearly to modification of process operation conditions. From a practical point of view, this also indicates that it is not possible to determine how the exo vinyl content of obtained PIB products depend on observed process variables through standard multivariate linear analyses. This might already be expected, as HR-PIB products can be obtained with distinct catalyst systems, solvents, and operation conditions, as described throughout this manuscript, imposing the careful optimization of the particularly analyzed reaction system whenever maximization of the exo vinyl content of the final resin is sought.
Finally, it is important to note that during the writing of the present document no significant change of the article publication and of the patent filing scenarios was observed in the field of polyisobutylene manufacture. For this reason, the main trends and perspectives presented here and discussed in the previous sections are not constrained by the dates of publication of the utilized references and can be regarded as synchronized with the disclosure of the present study.

Conclusions
Polyisobutylenes (PIBs) constitute a very versatile family of polymer materials that has arisen growing industrial interest and stimulated the continuous growth of the PIB market size. Particularly, PIBs are produced commercially through cationic polymerization mechanisms, which are characterized by high reactivity and high sensitivity to changes of operation conditions, demanding operation in controlled environments and very peculiar sets of reaction conditions. Consequently, there has been continuous search for more efficient, less expensive, and environmentally friendly and safer production methodologies both in academia and in industrial environments.
Broadly speaking, PIBs are grouped commercially into two large families of products: conventional PIBs (or C-PIBs) and highly reactive PIBs (or HR-PIBs), depending on the contents of unsaturated terminal double bonds of polymer chains (exo vinyl content). HR-PIB materials contain high amounts of exo vinyl groups (>70%) and allow much easier functionalization for manufacture of oil and lubricant additives.
The proposed bibliometric survey on isobutylene cationic polymerization allowed for the first time the investigation of extensive scientific data published in the form of research papers (dating from 1949 to 2020) and in the form of deposited patents (dating from 1933 to 2020), which provided an updated and comprehensive literature review on the theme and quantitative measures of technological trends.
Academic investigations focused mostly on C-PIB materials, although the number of investigations regarding HR-PIB products has increased steadily, revealing the growing commercial importance and ever-increasing maturity of this technology. Consequently, the vast majority of scientific publications in this field is concentrated in developed countries, where the main commercial players of PIB materials are located. In particular, the main focus of academic works has been the interpretation of the mechanism of polymerization and the adjustment of operation conditions for manufacture of C-PIB and HR-PIB grades with controllable properties, using mild reaction temperatures and alternative catalyst systems. The development of more appropriate catalyst systems still constitutes a significant challenge in the field, since the successful stabilization of the carbocation, and consequently the success of the polymerization, is the result of complex synergetic interactions that involve thermodynamic and kinetic effects, reflected on the characteristics of the solvent, of the Lewis acid catalyst, of the Lewis base co-catalyst and on the reaction temperature.
When patents are analyzed and compared to scientific publications, similar trends can be observed, although the variabilities of reported solvents, Lewis acid catalysts and Lewis base co-catalysts are higher (reflecting the continuous search for more appropriate catalysts and solvents); the reported use of the BF 3 catalyst is more frequent (reflecting the fact that safety and handling issues prevent the use of the BF 3 catalyst in academic environments); and the use of less expensive co-catalysts (such as organic alcohols and water) is commoner.
Finally, based on the performed survey, it seems appropriate to say that efforts are being made to develop operation strategies to obtain PIB grades with low dispersities, controlled average molar masses and high exo vinyl contents with high monomer conversions, using mild reaction temperatures (0 to 40 • C) and environmentally friendly and safer solvents and catalyst systems (involving more frequently the AlCl 3 catalyst and ethers as co-catalysts).