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Review

Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for Syndiospecific Polymerization of Styrene and Its Derivatives

by
Junsong Wang
1,2,
Mingming Bai
3,
Wenyan Wang
4,
Handou Zheng
2,
Chunyu Feng
2,
Jiayue Gu
2,
Guoliang Mao
1,* and
Haiyang Gao
2,*
1
Provincial Key Laboratory of Polyolefin New Materials, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, China
2
PCFM Lab, GD HPPC Lab, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
3
Blue Ocean New Materials (Tongzhou Bay) Co., Ltd., Nantong 226000, China
4
Daqing Petrochemical Research Center, Petrochemical Research Institute of PetroChina, Daqing 163714, China
*
Authors to whom correspondence should be addressed.
Inorganics 2025, 13(8), 274; https://doi.org/10.3390/inorganics13080274
Submission received: 30 June 2025 / Revised: 14 August 2025 / Accepted: 18 August 2025 / Published: 20 August 2025
(This article belongs to the Section Organometallic Chemistry)
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Abstract

Syndiotactic polystyrene (sPS) is an important class of engineering plastics, primarily produced through metal-catalyzed highly stereoselective polymerization of styrene monomer. This paper summarizes the advances in metal catalysts for syndiospecific polymerization of styrene and its derivatives including mono-cyclopentadienyl titanium and rare-earth metal catalysts. The effects of the cyclopentadienyl, the metal center, and the ancillary ligand on styrene polymerization are emphasized. It provides a practical reference for polymer and organometallic chemists who are interested in developing and designing highly efficient mono-cyclopentadienyl metal catalysts for the synthesis of sPS and functionalized sPS.

1. Introduction

The precise control of polymer stereoregularity by metal-catalyzed olefin polymerization is a cornerstone of modern polymer science and engineering, profoundly impacting material properties and enabling advanced applications [1,2]. Stereoregular polymers exhibit significantly enhanced crystallinity, thermal stability, mechanical strength, and chemical resistance compared to their atactic analogues [3,4]. Controlling tacticity results in macromolecular architecture and serves as a crucial technological lever for designing polymers with tailored performance [5,6].
Polystyrene (PS) is a significant class of polymeric materials that has found extensive applications across various fields due to its excellent properties [7,8]. The physicochemical properties and application performance of polystyrene are significantly influenced by the microstructure and stereoregularity of the polymer chain [9,10]. Based on the spatial arrangement of the substituents on the phenyl rings along the main chain, polystyrene isomers are primarily categorized into three types (Figure 1) including atactic polystyrene (aPS), isotactic polystyrene (iPS), and syndiotactic polystyrene (sPS) [11,12]. Regulating the stereoregularity of styrene polymerization using metal catalysis has consistently been a pivotal research topic in polystyrene materials.
Early research on polystyrene primarily focused on atactic polystyrene (aPS), which was synthesized via free radical polymerization and cross-linking reactions [13]. Commercial production of aPS via free radical polymerization was achieved as early as the 1930s [14]. A significant advancement occurred in the 1950s when Natta first synthesized isotactic polystyrene (iPS) with 89% isotacticity using the AlEt3-TiCl4 catalytic system [12,15]. The iPS exhibited a high melting point and semi-crystalline properties but had a slow crystallization rate, which severely constrained its practical application [16].
In 1986, Ishihara and coworkers reported the first successful synthesis of highly stereoregular syndiotactic polystyrene (sPS) using a mono-cyclopentadienyl (mono-Cp) titanium complex [17]. This specific microstructure imparts exceptional properties that distinguish it from aPS and iPS, including a higher melting point (~270 °C), rapid crystallization kinetics (40–80 times faster than iPS at comparable undercooling), excellent chemical resistance, a low dielectric constant (2.6), low moisture absorption, and high dimensional stability [18]. Currently, sPS is primarily produced industrially by Idemitsu Kosan Co., Ltd. [19]. Pure sPS shows inherent brittleness; sPS is often used as a polymeric material by blending it with glass fibers in modern industry. This composite material has found extensive applications in electronic components requiring high-frequency stability, automotive heat-resistant parts, and high-load structural components, owing to its enhanced mechanical properties, thermal resistance, and dimensional precision [20,21].
Metal-catalyzed styrene polymerization remains the exclusive method for producing syndiotactic polystyrene (sPS) [22]. Recent research has focused on two classes of highly efficient catalyst systems: cyclopentadienyl (Cp) titanium and rare-earth (RE) metal catalysts (Figure 2) [23]. Mono-cyclopentadienyl titanium (mono-Cp Ti) catalysts are renowned for their high activity and syndiospecificity (with >99% rrrr pentads) [24]. In addition, mono-cyclopentadienyl rare-earth metal catalysts (particularly Sc and Y complexes) also offer exceptional stereocontrol [25,26]. Modifications to the Cp ring structure, the identity of the central metal, and the choice of ancillary ligands are critical factors for optimizing activity, enhancing thermal stability, and improving stereoselectivity [27,28].
This paper summarizes the major research advances in mono-Cp titanium and rare-earth metal catalysts for syndiospecific styrene polymerization. The effects of Cp ring structure and ancillary ligands are discussed, with an emphasis on their influences on the (co)polymerization of styrene. Additionally, the syndiospecific polymerization of styrene derivatives including polar group-substituted styrene to produce functional sPS is also summarized. This feature paper aims to provide strategic insights into the design of sPS catalysts and functional sPS materials.

2. Mono-Cyclopentadienyl Ti Catalysts

Since Ishihara and coworkers pioneered catalytic systems based on group 4 metals in 1986 [17], mono-Cp Ti catalysts (Figure 3) have demonstrated significant catalytic advantages. Polymerization of styrene using various group 4 catalysts, CpMtCl3 (Mt = Ti, Zr), revealed that the Ti-based catalyst (CpTiCl3) exhibits higher catalytic activity and stereoselectivity in styrene polymerization compared to the Zr-based catalyst (CpZrCl3) [29]. The latter typically displays low activity and stereoselectivity. This can be attributed to the fact that Ti centers can be readily reduced to active Ti(III) species, whereas Zr centers tend to remain in their tetravalent state, resulting in virtually no activity or stereocontrol [30]. Moreover, the formation of active species critically depends on the involvement of alkylaluminium compounds. The presence of triisobutylaluminum (Al(iBu3)) serves as both a reducing agent and an alkylating agent, facilitating the conversion of metal complexes into catalytically active species (Figure 3) [31,32]. For CpTiCl3 complexes, the reduction and alkylation of CpTiCl3 by Al(iBu3) afford the cationic Ti(III) active species for syndiotactic styrene polymerization. For CpTiX2L (L: ancillary ligand) complexes, Nomura and coworkers, employing characterization techniques such as XAS (X-ray Absorption Near Edge Structure), identified the alkylated neutral CpTi(III)RL species as the active catalytic entity [33,34].
Subsequently, research on syndiotactic polymerization of styrene has predominantly focused on mono-Cp Ti catalyst systems. Modifications of these catalysts primarily focus on two approaches: (1) modifying the cyclopentadienyl moiety, including alterations to the ring structure [35], and (2) substituting the ancillary ligands bound to the metal center [36]. Both strategies modulate the steric and electronic properties of the complexes, allowing for precise control over syndiospecific styrene polymerization.

2.1. Substituted Cyclopentadienyl Ti Complexes

The substituents on the Cp ring significantly influence both the activity of the catalyst and the properties of the resulting polymer. Compared to mono-Cp Ti complex 1 (CpTiCl3) (Figure 4), pentamethyl-substituted cyclopentadienyl complex 2 (Cp*TiCl3) (Figure 4) exhibits multiple advantages through synergistic electronic and steric effects. Firstly, complex 2 possesses a higher concentration of active centers. The electron-donating methyl groups enhance the electron density at the Ti center, optimizing the symmetry matching between the metal’s d-orbitals and the styrene monomer’s π-orbitals, thereby increasing polymerization activity [37]. Secondly, the pentamethyl-substituted cyclopentadienyl (Cp*) moiety provides superior stabilization to the cationic active center. This stabilization retards β-H elimination, enabling more sustained chain growth and resulting in sPS with higher molecular weight. Finally, the substantial steric bulk of the Cp* ring further restricts the rotational freedom around the Ti–C bond. This constraint promotes regioselective insertion of the monomer in a syndiotactic fashion, achieving high syndiotacticity in the final product [37]. Table 1 summarizes the catalytic performance of Ti complexes with substituted Cp rings in styrene polymerization.
Ishihara also reported the 1/methylaluminoxane (MAO) catalyst system for homopolymerization of para-alkyl- and para-halogen-substituted styrene monomers [24]. This approach successfully produced a series of substituted syndiotactic polystyrenes (sPS). Among these, the para-methylstyrene system exhibited the highest catalytic conversion (>90%), while halogen-substituted styrene systems showed significantly lower conversion (<10%). Furthermore, the poly (para-methylstyrene) displayed a melting point of 173 °C, which is notably lower than that of unsubstituted sPS. Similarly, Nakatani reported the 2/MAO catalyst system for the syndiotactic polymerization of alkyl-substituent styrenes [38], and successfully obtained syndiotactic poly (para-methylstyrene) with syndiotacticity > 99%.
Building on the exploration of alkyl-substituents on the Cp ring, Duncalf introduced an electron-donating t-butyl substituent into Cp ring 3 (Figure 4) for styrene polymerization [39]. This modification altered the ligand’s electron density and steric hindrance, resulting in 54% monomer conversion at 0 °C, and the polystyrene produced exhibited higher molecular weights (3.89 × 104 g/mol), consistent with high syndiotacticity (>99%). While this work demonstrated the impact of an electron-donating alkyl-group, substituent effects encompass concurrent electronic and steric influences.
Pinkas and coworkers investigated the influence of introducing different substituents into the tetramethyl-substituted Cp ring on the electronic properties and catalytic performance of Ti complexes 4 and 5 (Figure 4) [40]. The results showed that the electron-withdrawing phenyl substitution on complex 4 led to a significant red shift in the absorption spectrum (λmax = 445 nm) due to conjugation between the phenyl ring and the Cp ring, which reduced the HOMO–LUMO energy gap. Thus, it can be concluded that the conjugation effect predominantly influences the modification of the electronic structure. In contrast, the electron-donating trimethylsilyl group, possessing a slightly weaker electron-donating ability than methyl, induced a blue shift in absorption (λmax = 432 nm). The influence of this substituent on catalytic activity was found to be negligible. Under MAO activation, the catalyst system 4/MAO exhibited a slight increase in activity but produced polymers with substantially higher molecular weights (Mw = 3.0 × 105 g/mol).
Further expanding the structural scope, Lee and coworkers reported a series of 1,2,3-trisubstituted cyclopentadienyl Ti complexes 6–10 (Figure 4) for styrene polymerization [41]. All trisubstituted catalyst systems demonstrated higher activity than the benchmark 2 (Table 1). Notably, the phenyl-substituted complexes 8 and 9 exhibited significantly enhanced activity compared to their alkyl-substituted analogues 6 and 7. This activity boost aligns with the aforementioned studies, which indicate that resonance stabilization through phenyl introduction enhances catalytic performance. However, the triphenyl-substituted complex 10 showed a dramatic activity decrease, possibly due to excessive steric hindrance, and was completely deactivated at 100 °C.
Table 1. Styrene polymerization results using Ti complexes with substituted Cp rings.
Table 1. Styrene polymerization results using Ti complexes with substituted Cp rings.
EntryComplexActivity aMw
×104 g/mol		ĐMTm
(°C)		Ref.
1 b36704.32.2272[39]
2 c441313.01.9268[42]
3 c5158--269[40]
4 b628003.12.2264[41]
5 b721002.12.0265[41]
6 b829003.72.1269[41]
7 b939002.02.0262[41]
8 b1017001.01.7255[41]
a Activity in kg sPS/(mol Mt·h). b Polymerization conditions: [Ti] = 50 μmol, [Al]/[Ti] = 4000, styrene = 5 mL, time = 10 min. c Polymerization conditions: [Ti] = 10 μmol, [Al]/[Ti] = 1500, styrene = 4.36 × 10−2 mol/L, time = 1 h.

2.2. Fused Cyclopentadienyl Ti Complexes

In addition to substituent modification of the cyclopentadienyl, fused cyclopentadienyl is an alternative approach. Chien and Knjazhanski extended the principal ligand from cyclopentadienyl (Cp) to indenyl (Ind), fluorenyl (Flu), and phenanthrene (Phe), resulting in the corresponding complexes 11 (IndTiCl3), 12 (FluTiCl3), and 13 (PheTiCl3), respectively (Figure 5) [43,44,45]. Similarly to the effects observed with methyl substitution on the Cp ring, the synergistic interplay of electron-donating properties and steric hindrance in these extended ligands significantly enhanced the catalytic activity for styrene polymerization as well as syndiotactic selectivity. Notably, complex 11 achieved exceptional activity of 400 kg sPS/(mol Mt·h), while delivering polystyrene with 86% syndiotacticity. Complex 13 exhibited high activity at 100 °C (3.6 × 103 kg sPS/(mol Mt·h)). The comparison between complex 11 and complex 13 indicates that appropriate steric bulk and electronic effects facilitate monomer coordination, thereby enhancing catalytic efficiency [43,44]. Table 2 summarizes the catalytic performance of Ti complexes with fused Cp rings in styrene polymerization.
The substituent effects of the indenyl ring on styrene polymerization were also studied. Tiao and coworkers reported that the siloxy-substituted tetramethylindenyl Ti complex 14 (Figure 5) exhibited significantly reduced performance in styrene polymerization. Specifically, complex 14 showed lower catalytic activity (24 kg sPS/(mol Mt·h)) and reduced syndiotacticity (~70%) [46]. Foster and coworkers systematically introduced various substituents at different positions on the indenyl framework, resulting in the corresponding complexes 15–19 (Figure 5) for styrene polymerization [47]. The observed polymerization activity trend (15 > 16 > 17 > 19 > 18) revealed that phenyl substitution significantly enhanced catalytic activity due to conjugation effects. Complexes 15 and 16 exhibited moderate activities (3388 and 4.6 kg sPS/(mol Mt·h), respectively). In contrast, the diphenyl-substituted complex 17 showed reduced activity (3.6 kg sPS/(mol Mt·h)), due to the excessive steric hindrance. Nevertheless, complex 17 afforded polystyrenes with the highest syndiotacticity (94.8%) and superior thermal stability.
Table 2. Styrene polymerization results using Ti complexes with fused Cp rings.
Table 2. Styrene polymerization results using Ti complexes with fused Cp rings.
EntryComplexActivity aMw
×104 g/mol		ĐMTm
(°C)		Ref.
1 b11400102.0268[43]
2 c1336002.9--[45]
3 d1424---[46]
4 c15338842.4-261[47]
5 c16228832.3-262[47]
6 c17158449.6-262[47]
7 c188832.3-268[47]
8 c1948440.1-268[47]
a Activity in kg sPS/(mol Mt·h). b Polymerization conditions: [Ti] = 10 μmol, [Al]/[Ti] = 1500, styrene = 4.36 × 10−2 mol/L, time = 1 h. c Polymerization conditions: [Ti] = 50 μmol, [Al]/[Ti] = 4000, styrene = 0.88 mol/L, time = 10–20 min. d Polymerization conditions: [Ti] = 25 μmol, [Al]/[Ti] = 400, styrene = 4.3 × 10−2 mol, time = 30 min.

2.3. Substitution of the Ancillary Ligands

Ancillary ligands exert significant effects on the polymerization process by modulating the metal center through distinct electronic and steric effects. Kaminsky and coworkers designed Ti complexes 20 and 22 (Figure 6) by incorporating electron-withdrawing fluorine atoms [37]. The polymerization results also show that complexes featuring fluorine ancillary ligands exhibit significantly enhanced activity in styrene polymerization (3 × 103 kg sPS/(mol Mt·h)), but the resulting polymers show lower molecular weights (1 × 105 g/mol). Systematic comparisons between chlorine- and fluorine-containing systems demonstrated a substantial increase in polymerization rates for syndiospecific styrene polymerization when utilizing fluorine ligands. Table 3 summarizes the catalytic performance of Ti complexes with substitution of the ancillary ligands in styrene polymerization.
Building on these findings, Kucht and coworkers modified the catalyst system by introducing an isopropoxy group (-OC3H7) as an ancillary ligand, resulting in the development of complex 21 (Figure 6) [48]. This system achieved high activity for syndiospecific styrene polymerization at 75 °C, though the resulting polymers exhibited relatively low molecular weights (1.2 × 105 g/mol). Subsequently, the replacement of ancillary ligands with electron-donating groups in complex 23 (Figure 6) facilitated highly efficient styrene polymerization within five minutes when activated by [Ph3C][B(C6F5)4]/Al(iBu)3, yielding polymers with >99% syndiotacticity [49].
These two seemingly paradoxical electronic effects optimize the catalytic process through distinct mechanisms: Fluorine atoms, as strong electron-withdrawing substituents, reduce the electron density in d-orbitals of Ti center [50], thereby facilitating π-electron acceptance from styrene monomers to promote monomer coordination [51]. Conversely, methoxy groups, through lone-pair conjugation, increase the electron density at the titanium center, optimizing the d-π orbital overlap for enhanced stereocontrol.
Xu and coworkers also utilized the 23/MAO catalyst system for polar styrene polymerization, employing trimethylsilyl groups (-SiMe3) to protect amino-substituted styrene monomers [52], which include para-aminostyrene, para-(aminomethyl)styrene, and para-(aminoethyl)styrene. Upon activation with B(C6F5)3, the catalyst system achieved high conversions (>45%) for all three polar substituted styrenes within 60 min, yielding syndiotactic amino-functionalized polystyrenes with syndiotacticities greater than 76%.
Grassi introduced a sterically demanding benzyl group as an ancillary ligand to yield complex 24 (Figure 6) [53]. Complex 24 achieved 60% monomer conversion within 3 min, producing syndiotactic polystyrene with high molecular weight (6.8 × 105 g/mol) and exceptional stereoregularity (rrrr > 95%). Newman and coworkers reported complex 25 (Figure 6) with varying oxidation states to investigate the valence effects on syndiospecific polymerization [54], and achieved 62% styrene conversion when MAO/Al(iBu)3 was used as the cocatalyst system. Subsequently, Chung’s group reported the syndiospecific homopolymerization of 4-[B-(n-butylene)-9-BBN]styrene and its copolymerization with styrene monomer using the 25/MAO catalyst system, yielding syndiotactic polymers with high stereoregularity [55].
Lin’s group synthesized a series of mono-Cp Ti complexes 26–33 (Figure 6) with diverse ancillary ligands for styrene polymerization [56,57,58,59,60]. Complexes 28 and 29 exhibited high activity (8.1 × 104 kg and 7.6 × 104 kg sPS/(mol Mt·h), respectively). Furthermore, the influence of para-halogen-substituted phenoxy ancillary ligands on catalytic performance was investigated. The observed activity trend for this catalyst series was 30 > 31 > 32 > 33. This trend can be attributed to the electron-withdrawing capacity of the halogens (F > Cl > Br > I), which modulates the electronic properties of the Ti’s active center. Such electron-withdrawing effects likely facilitate both the coordination of St monomers to the Ti and the subsequent insertion step during chain propagation in coordination polymerization [61].
In addition to X of halogen groups and alkoxides, mono-Cp Ti complexes with two X groups and an ancillary ligand L were also developed. The ancillary ligand L can modulate the formation rate of active species by regulating the Ti reduction and alkylation processes.
Liu and coworkers developed a series of monoalkoxy-substituted mono-Cp Ti complexes 34–39 (CpTiCl2)(L) (Figure 7) for styrene polymerization [62]. The influence of various alkoxy substituents (L groups) on the catalytic performance for the syndiotactic polymerization of styrene was investigated. The results revealed a significant dependence of catalyst activity on the steric bulk of the L group, with activity increasing significantly as steric hindrance increased (1139 kg sPS/(mol Mt·h)). Catalysts featuring bulky aliphatic substituents, such as cyclohexyl, demonstrated the highest efficacy, consistently yielding syndiotactic polystyrene (sPS) with a syndiotacticity > 98%. The trend of activity followed the order 34 > 37 > 36, which is consistent with the increasing steric hindrance of the L group. This enhancement is attributed to the sterically demanding L groups, which reduce the coordination tendency of the alkoxy oxygen atom with aluminum in the MAO cocatalyst, thereby preventing deactivation of the active Ti species [62].
Table 3. Styrene polymerization results using Ti complexes with different ancillary ligands.
Table 3. Styrene polymerization results using Ti complexes with different ancillary ligands.
EntryComplexActivity aMw
×104 g/mol		ĐMTm
(°C)		Ref.
1 b203000102.0265[37]
2 c2126812--[48]
3 b22690662.0275[37]
4 d23510---[52]
5 e241200682.4270[53]
6 f2659,200182.1269[58]
7 f2777,300331.9272[58]
8 f2881,200362.0273[58]
9 f2975,700362.2273[58]
10 g30206044-272[61]
11 g31198036-270[61]
12 g32186039-272[47]
13 g33154015-270[47]
14 h341139--258[62]
15 h35261--256[62]
16 h3683--259[62]
17 h37456--259[62]
18 h3870--258[62]
19 i406390482.1-[63]
20 i416020242.3-[63]
21 i4212,400212.1-[63]
22 i434290322.2-[63]
23 i445690232.2-[63]
24 i45568052.2-[64]
25 i46489062.8-[64]
a Activity in kg sPS/(mol Mt·h). b Polymerization conditions: [Ti] = 6.25 × 10−5 mol/L, [Al]/[Ti] = 300, styrene = 4.3 mol/L. c Polymerization conditions: [Ti] = 50 μmol, [Al]/[Ti] = 2000, styrene = 0.044 mol. d Polymerization conditions: [Ti] = 25 μmol, [Al]/[Ti] = 400, styrene = 4.3 × 10−2 mol, time = 30 min. e Polymerization conditions: [Ti] = 44 μmol, [B(C6F5)3]/[Ti] = 1, styrene = 4.3 × 10−2 mol, time = 30 min. f Polymerization conditions: [Ti] = 83 μmol, [Al]/[Ti] = 600, styrene = 0.173 mol, time = 20 min. g Polymerization conditions: [Ti] = 100 μmol/L, [Al]/[Ti] = 600, styrene = 10 mL, time = 30 min. h Polymerization conditions: [Ti] = 420 μmol/L, [Al]/[Ti] = 2000, styrene = 2 mL, time = 20 min. i Polymerization conditions: [Ti] = 2 μmol, [Al]/[Ti] = 1500, styrene = 10 mL, time = 10 min.
Following studies on alkoxy ligands, Nomura investigated mono-Cp Ti complexes 40–44 (Figure 7) bearing aryloxo ancillary ligands for syndiotactic styrene polymerization [63]. The results showed that complex 42 exhibited the highest activity at 85 °C (1.24 × 104 kg sPS/(mol Mt·h)), and the activity decreased in the order 42 > 40 > 41 > 44 > 43. These results demonstrate that substituents on the aryloxo ligand directly govern catalytic performance. Regarding this result, the authors attributed the enhanced activity of the methyl-substituted complex 42 primarily to steric effects that promote 2,1-insertion of styrene, rather than to electronic contributions [57]. In addition to ancillary ligands with the oxygen donor atoms, other ancillary ligands with the nitrogen donor atoms were also ligated to the Ti center. Mono-Cp Ti complexes 45 and 46 (Figure 7), which bear amide and aniline ancillary ligands, respectively, exhibited lower catalytic activities (5.6 × 103 and 4.9 × 103 kg sPS/(mol Mt·h)) compared to their aryloxide-based analogues [64].

3. Mono-Cyclopentadienyl Rare-Earth Metal Complexes

Early studies reported that dicyclopentadienyl rare-earth catalysts yielded only atactic polystyrene with moderate polymerization activity during styrene polymerization [65]. A major breakthrough occurred in 2004 when Carpentier and coworkers reported the first single-component, high-activity lanthanide (Ln) rare-earth catalyst 47 (Figure 8), capable of syndiospecific styrene polymerization without the need for any cocatalyst [66]. This Ln catalyst produced polystyrene with exceptional syndiotacticity (rrrr > 99%).
Mechanistically, dicyclopentadienyl Ln catalyst 47 derives its exceptional syndiospecificity from fundamental physicochemical properties: the stable +3 oxidation state of Ln metals and their pronounced Lewis acidity, which facilitate the coordination of monomers [67]. Furthermore, the highly polar nature of Ln–H bonds and Ln–C bonds promotes the insertion of the monomer [68]. Almost at the same time, Hou also reported mono-cyclopentadienyl lanthanide (mono-Cp Ln) complexes 48–51 for syndiospecific polymerization of styrene, and mono-Cp Ln complexes subsequently received more attention [69]. Table 4 summarizes the catalytic performance of rare-earth metal (Ln) complexes in styrene polymerization.

3.1. Substituted Mono-Cyclopentadienyl Ln Complexes

Hou and coworkers reported a series of mono-cyclopentadienyl lanthanide (mono-Cp Ln) complexes 48–51 (Figure 9) for the polymerization of styrene [69]. Upon treatment with 1 equivalent of [Ph3C][B(C6F5)4], these catalysts all efficiently catalyzed the syndiospecific polymerization of styrene. The resulting polymers displayed exceptionally high stereoregularity (rrrr > 99%). Among these, the scandium (Sc) complex 48 demonstrated the highest activity (≥1.3 × 104 kg sPS/(mol Mt·h)) and produced sPS with a narrow molecular weight distribution (ĐM = 1.29–1.55).
The same research group also developed nitrogen-containing mono-Cp Ln complexes 52–55 (Figure 9) for the polymerization of styrene [77]. Substituents with tailored steric and electronic properties were introduced onto the Cp rings to evaluate their influence on the catalytic performance in styrene polymerization. These complexes efficiently catalyzed the syndiospecific polymerization of styrene using only [Ph3C][B(C6F5)4] as a cocatalyst without alkylaluminium cocatalysts, to afford polystyrene with exceptionally high syndiotacticity (rrrr > 99%) and molecular weight (Mn = 1.27 × 105 g/mol).
Further studies extended complex 52 to a series of amino-substituted styrene monomers. The complex exhibited exceptional tolerance toward monomers para-(dimethylamino)styrene (para-DMAS), directly enabling syndiospecific polymerization without amino-group protection [78]. High catalytic activities were achieved with high monomer conversions (>90%) and high stereoselectivity (rrrr > 99%). Notably, DSC analysis of the obtained functionalized polystyrene revealed a distinctive single melting peak behavior (Tm = 236 °C). Analogous complexes 53–55 (Figure 9) enabled the simultaneous chain-growth and step-growth polymerization of methoxy-substituted styrenes when activated by [Ph3C][B(C6F5)4] and Al(iBu)3 [79]. These catalytic systems facilitated the chain-growth polymerization of ortho-methoxystyrene (ortho-MOS) (Figure 10), resulting in a highly syndiotactic polymer with a syndiotacticity of rrrr > 99%. The molecular weight (Mn) increased linearly with the monomer-to-catalyst ratio, consistently achieving mass conversions exceeding 90% to syndiotactic poly (ortho-methoxystyrene). Subsequently, Cui proposed a self-activation mechanism to rationalize the high reactivity of ortho-methoxy groups. This mechanism involves σ-π chelation, in which the oxygen atom of the methoxy group coordinates to the yttrium center via a σ-bond, while the vinyl group (C=C) simultaneously engages in π-coordination. DFT calculations confirmed that such σ-π chelation significantly reduces the transition-state energy barrier for monomer coordination [80].
Recently, Cui and coworkers developed complex 56 (Figure 9) with phenyl-substituted cyclopentadienyl for the polymerization of styrene [70]. Activated by DMAO (dry methylaluminoxane), the 56/DMAO catalyst system exhibited low styrene conversion (18%) and an activity of only 1 kg sPS/(mol Mt·h). Nevertheless, the resulting polystyrene possessed a remarkably high syndiotacticity (rrrr > 99%).
To further investigate the effects of bulky substituents on cyclopentadienyl, Li and coworkers reported a series of chiral dialkyl complexes 57–60 (Figure 9) bearing sterically bulky cyclohexane-derived cyclopentadienyl ligands [71]. When activated by [Ph3C][B(C6F5)4] and AliBu3, these catalyst systems exhibited high activity for the polymerization of styrene. Complex 57 demonstrated the highest activity, achieving complete consumption of styrene within 3 min with an activity of 3100 kg sPS/(mol Mt·h). Notably, when using [PhMe2NH][B(C6F5)4] as the cocatalyst, the resulting polymers from the polymerization of styrene displayed broad molecular weight distributions (ĐM = 1.49–5.03). In contrast, the complexes with lutetium (Lu), yttrium (Y), and dysprosium (Dy) metal centers showed significantly lower activities (activity trend: Sc > Y > Lu > Dy). The Lu- and Y-based catalysts produced only atactic polystyrene.

3.2. Fused Mono-Cyclopentadienyl Ln Complexes

Due to their enhanced electron-donating effects and pronounced steric constraints, fused cyclopentadienyl catalysts have been extensively studied in rare-earth metal catalysts. Cui and coworkers pioneered the incorporation of sulfur-containing annulated thiophene rings onto the cyclopentadienyl to facilitate ethylene copolymerization with dicyclopentadienyl precursors [77]. The styrene polymerization results demonstrated that complex 61 (Figure 11) showed moderate activity (35 kg sPS/(mol Mt·h)) [72], and complex 62 (Figure 11), bearing a phenyl-substituted thiophene moiety, exhibited marginally higher activity (54 kg sPS/(mol Mt·h)) [81].
To investigate the effect of different substituents on the fluorenyl group in styrene polymerization, Hou and Cui introduced different substituents at the fluorenyl position to develop complexes 63–67 (Figure 11) [73]. Catalyst system 63/[Ph3C][B(C6F5)4] with unsubstituted fluorenyl for the polymerization of styrene exhibited relatively low activity (208 kg sPS/(mol Mt·h)). The introduction of substituents onto the fluorenyl ring, which increased the ligand electron density, accelerated the styrene polymerization rate. Complex 65 with a SiMe3 substituent for styrene polymerization showed the highest activity (up to 3.3 × 104 kg sPS/(mol Mt·h)). However, complex 66 with both tBu and SiMe3 substituents showed slightly lower activity (3.0 × 104 kg sPS/(mol Mt·h)) than complex 65. Significantly, complex 67 became completely inactive towards styrene polymerization upon the introduction of a benzal substituent onto the fluorenyl group. DFT calculations suggested that the heterobimetallic Sc-iBu-Al catalyst is the true active species.

3.3. Bridged Mono-Cyclopentadienyl Ln Complexes

Bridged mono-cyclopentadienyl Ln catalysts have also been extensively investigated due to their distinctive electronic effects. Cui and coworkers reported a series of bridged pyridyl-methylene-fluorenyl Ln complexes 68–70 (Figure 12) for styrene polymerization [82]. These catalysts demonstrated high catalytic activity (1.6 × 104 kg sPS/(mol Mt·h)), with the central metal identity exerting negligible influence on activity.
Moreover, these catalysts’ tolerance toward substituted styrene with polar groups represents a critical research focus. When [Ph3C][B(C6F5)4] and Al(iBu)3 were used as cocatalysts for complex 68, the catalyst systems efficiently achieved syndioselective coordination polymerization of unmasked ortho-methoxystyrene (ortho-MOS), meta-methoxystyrene (meta-MOS), and para-methoxystyrene (para-MOS) [83]. Notably, the polymerization of ortho-MOS exhibited an activity of 30 kg sPS/(mol Mt·h). This catalyst system also successfully catalyzed the homopolymerization of para-methylthiostyrene (para-MTS) (Figure 13), exhibiting exceptionally high activity at 25 °C to yield sulfur-functionalized syndiotactic polystyrene (451 kg sPS/(mol Mt·h)) with a high molecular weight (1.7 × 105 g/mol) [84]. However, copolymerization of para-MTS with styrene yielded the ideal random copolymers, while both monomer sequences remained highly syndiotactic.
Recently, complex 68 was reported to facilitate the highly efficient polymerization of para-chlorostyrene (para-CS) at a low temperature of −25 °C (Figure 14) [85]. This system exhibited a remarkable activity of 62.2 kg·sPS/(mol Mt·h). In copolymerization with St, the conversion of the monomer increased as the feed ratio of St was raised. The resulting copolymers exhibited a narrow molecular weight distribution (PDI = 1.19), with the monomer sequences being randomly distributed. Additionally, both monomer sequences preserved a highly syndiotactic microstructure.
To address the challenge of catalyst deactivation caused by coordination of polar groups in styrenic monomers to metal centers, Chung’s group developed a “reactive” polymerization strategy. This approach involves introducing functional groups into the polymer chain that serve as reactive sites for subsequent modification [86]. Based on this strategy, Cui reported the highly efficient syndioselective coordination polymerization of para-(1-hexynyl)styrene (para-HES), para-(phenylethynyl)styrene (para-PES), and para-(trimethylsilylethynyl)styrene (para-TES) (Figure 15) using complex 68 [87]. The resulting polymers all exhibited exceptionally high syndiotacticity (rrrr > 99%).
Divinylstyrene-based polymers, which contain unreacted olefins in their side chains, represent a class of potentially post-functionalizable polymers [88]. Complex 69 produced highly syndiotactic polymers (rrrr > 95%) from long-chain monomers 1-(pent-4-en-1-yloxy)-4-vinylbenzene (POS) and 1-(hex-5-en-1-yloxy)-4-vinylbenzene (HOS), while only atactic products were obtained from short-chain monomers 1-(allyloxy)-4-vinylbenzene (AOS) and 1-(but-3-en-1-yloxy)-4-vinylbenzene (BOS) (Figure 16).
To investigate the electronic effects of substituents on the bridged pyridyl-methylene-Cp Ln catalysts, Cui’s group synthesized a series of corresponding complexes 71–76 (Figure 12) for styrene polymerization [68,75], and these catalytic systems employed AliBu3/[Ph3C][B(C6F5)4] as the cocatalyst. Complex 71 exhibited negligible activity during the polymerization process. Additionally, the introduction of tetramethyl substituents on the cyclopentadienyl ring in complex 72 resulted in only a marginal enhancement of activity (15 kg sPS/(mol Mt·h)), yielding polystyrene with low syndiotacticity (<60%). The substitution of the cyclopentadienyl group with a fluorenyl group in complex 73 resulted in a slight enhancement of electron density, which corresponded to a marginal increase in activity (17 kg sPS/(mol Mt·h)).
In contrast, complexes bearing SiMe3 substituents on the fluorenyl group, specifically complexes 74–76, exhibited significantly enhanced activity, achieving a production rate of 143 kg sPS/(mol Mt·h) and resulting in highly syndiotactic polystyrene (rrrr > 99%). DFT calculations revealed the critical role of pyridine ligands in catalysis, indicating that complex activity correlates positively with the participation of the Lowest Unoccupied Molecular Orbital (LUMO). The bridging of pyridine effectively lowers the LUMO energy of the active species, thereby facilitating the coordination of styrene [25]. To validate this mechanism, complexes 77 and 78 (Figure 12) were employed for styrene polymerization and exhibited substantially reduced activity. Furthermore, the resulting polymers displayed a bimodal molecular weight distribution (ĐM = 1.81 and 1.39), indicating the presence of two distinct active species [73].
Complexes 79–81 (Figure 12) with bulky tert-butyl substituents for styrene polymerization exhibited significantly enhanced monomer conversion [87]. These complexes achieved complete conversion of the styrene monomer within one minute at a molar ratio of [St]:[Ln] = 1000:1. Additionally, they also demonstrated high activity (compared to complex 68) for the polymerization of the aforementioned alkyne-substituted monomers (≥245 kg sPS/(mol Mt·h)). This effect is attributed to the increased steric hindrance at the fluorenyl group, which reduces the Lewis acidity of the rare-earth metal center. Consequently, the interaction between the metal and the alkyne functionality is weakened, leading to improved polymerization activity.
Building upon the fluorenyl scaffold, Luo reported a series of analogous complexes 82 and 83 (Figure 12) with substituted fluorenyl complexes for styrene polymerization [76]. The Sc-based complex 82 exhibited remarkably high polymerization activity (6.2 × 103 kg sPS/(mol Mt·h)) in syndiospecific homopolymerization. Notably, the resulting syndiotactic polystyrene exhibited a linear increase in molecular weight as the ratio of monomer to catalyst increased, while maintaining a narrow molecular weight distribution.
Furthermore, complex 83 effectively mediated the syndiospecific polymerization of para-methoxystyrene, achieving an 84% monomer conversion within 30 min. This process yielded syndiotactic poly (para-methoxystyrene) characterized by high stereoregularity (rrrr > 99%) [74]. The copolymerization of styrene (St) and para-MOS using complex 83 yielded copolymers characterized by high syndiotacticity in both the para-MOS and St sequences (Figure 17). Furthermore, these copolymers exhibited high molecular weights (6.34 × 104 g/mol) and narrow molecular weight distributions.

3.4. Dinuclear Cyclopentadienyl Ln Complexes

Cui and coworkers developed dinuclear pyridyl-methylene-fluorenyl Sc complexes 84 and 85 (Figure 18) for styrene polymerization [74]. The results revealed that upon activation with [Ph3C][B(C6F5)4], these dinuclear complexes only exhibited relatively low catalytic activity (84: 240 kg sPS/(mol Mt·h) and 85: 110 kg sPS/(mol Mt·h)). Notably, complex 85, featuring the largest buried volume, produced sPS with an ultra-high molecular weight (7.82 × 105 g/mol). This significant increase in molecular weight is attributed to the steric bulk that hinders both β-hydride elimination and chain transfer to alkylaluminum.

4. Conclusions and Outlook

Syndiotactic polystyrene (sPS) is a pivotal engineering thermoplastic, recognized for its exceptional thermal stability, rapid crystallization, and chemical resistance. Mono-cyclopentadienyl titanium and rare-earth metal catalysts are two types of highly efficient catalyst systems for the syndiospecific polymerization of styrene. Strategic modifications of the cyclopentadienyl, the metal center, and the ancillary ligand on styrene polymerization represent effective approaches to enhance catalytic activity and achieve precise control over sPS. Strikingly, rare-earth catalysts show good catalytic activity toward polar styrene monomers by a self-activation mechanism, enabling the direct synthesis of advanced functional sPS materials. Looking forward, several key challenges and pressing issues must be addressed for the advancement of this research area:
(1)
The cooperative action of steric/electronic modifications on the Cp ring and ancillary ligand is well known in the metal-catalyzed polymerization of styrene. How do we use the combination of steric/electronic modifications on the Cp and ancillary ligand moieties to achieve positive effects on styrene polymerization? We believe that exploiting this synergy more deliberately through advanced ligand design and computational modeling holds immense promise. In addition, AI-assisted catalyst design would provide a promising future.
(2)
Mono-cyclopentadienyl titanium catalysts typically possess a Ti(IV) state, necessitating reduction to the active Ti(III) species through the use of an excess of alkylaluminum compounds. However, the requirement for large quantities of alkylaluminum and MAO reagents raises safety and cost concerns in practical industrial applications. Therefore, developing novel activation strategies that minimize or eliminate the need for large quantities of alkylaluminum and MAO cocatalysts represents a critical research priority. Additionally, due to the high electrophilicity of titanium, these catalysts demonstrate significantly low polymerization activity toward polar styrenic monomers.
(3)
Pure sPS is inherently brittle; its current industrial applications typically depend on the incorporation of glass fibers to enhance performance. However, copolymerizing styrene with alkyl-substituted styrenes or specific olefin monomers presents a promising route to modify the polymer chain structure and improve impact resistance. Importantly, the direct incorporation of functional polar comonomers facilitated by advanced rare-earth catalysts could simultaneously impart desired functionality and potentially modulate mechanical properties.

Author Contributions

Writing—original draft preparation, J.W.; writing—review, M.B., W.W., H.Z., C.F., and J.G.; review and editing, H.Z. and H.G.; funding acquisition, H.Z. and H.G. supervision, G.M. and H.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by National Natural Science Foundation of China (NSFC) (52173016), the State Key Research Development Program of China (Grant No. 2021YFB3800701), Guangdong Basic and Applied Basic Research Foundation (2024A1515012784, 2024A1515011102, and 2023A1515110549), and PetroChina Projects (2022DJ6308).

Data Availability Statement

This concise review has no database.

Conflicts of Interest

Author Mingming Bai was employed by Blue Ocean New Materials (Tongzhou Bay) Co., Ltd., Wenyan Wang was employed by Petrochemical Research Institute of PetroChina. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Inorganics 13 00274 g001
Figure 1. Structural schematics of three types of PS chains.
Figure 1. Structural schematics of three types of PS chains.
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Figure 2. Mono-Cp Ti and rare-earth metal (Ln) complexes with substituted and fused Cp rings and substituted ancillary ligands.
Figure 2. Mono-Cp Ti and rare-earth metal (Ln) complexes with substituted and fused Cp rings and substituted ancillary ligands.
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Inorganics 13 00274 g003
Figure 3. Reduction and alkylation of mono-cyclopentadienyl Ti complexes by Al(iBu3) in syndiotactic styrene polymerization.
Figure 3. Reduction and alkylation of mono-cyclopentadienyl Ti complexes by Al(iBu3) in syndiotactic styrene polymerization.
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Inorganics 13 00274 g004
Figure 4. Mono-Cp Ti complexes with substituted Cp ring.
Figure 4. Mono-Cp Ti complexes with substituted Cp ring.
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Figure 5. Mono-Cp Ti complexes with indenyl, fluorenyl, and phenanthrene rings.
Figure 5. Mono-Cp Ti complexes with indenyl, fluorenyl, and phenanthrene rings.
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Figure 6. Mono-Cp Ti complexes with various ancillary ligands.
Figure 6. Mono-Cp Ti complexes with various ancillary ligands.
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Figure 7. Mono-Cp Ti complexes with ancillary ligands containing oxygen and nitrogen donors.
Figure 7. Mono-Cp Ti complexes with ancillary ligands containing oxygen and nitrogen donors.
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Inorganics 13 00274 g008
Figure 8. Dicyclopentadienyl Ln catalyst for styrene polymerization.
Figure 8. Dicyclopentadienyl Ln catalyst for styrene polymerization.
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Inorganics 13 00274 g009
Figure 9. Mono-Cp Ln complexes for syndiospecific polymerization of styrene.
Figure 9. Mono-Cp Ln complexes for syndiospecific polymerization of styrene.
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Inorganics 13 00274 g010
Figure 10. Homopolymerization of ortho-methoxystyrene.
Figure 10. Homopolymerization of ortho-methoxystyrene.
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Figure 11. Fused mono-Cp Ln complexes for syndiospecific polymerization of styrene.
Figure 11. Fused mono-Cp Ln complexes for syndiospecific polymerization of styrene.
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Figure 12. Bridged pyridyl-methylene-Cp Ln complexes.
Figure 12. Bridged pyridyl-methylene-Cp Ln complexes.
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Figure 13. (Co)polymerization of para-MTS monomer using complex 68.
Figure 13. (Co)polymerization of para-MTS monomer using complex 68.
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Figure 14. (Co)polymerization of para-CS monomer using complex 68.
Figure 14. (Co)polymerization of para-CS monomer using complex 68.
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Figure 15. Homopolymerization of alkyne-substituted styrene using complex 68.
Figure 15. Homopolymerization of alkyne-substituted styrene using complex 68.
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Figure 16. Homopolymerization of polar divinyl styrene using complex 69.
Figure 16. Homopolymerization of polar divinyl styrene using complex 69.
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Figure 17. (Co)polymerization of para-methoxystyrene monomer using complex 75.
Figure 17. (Co)polymerization of para-methoxystyrene monomer using complex 75.
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Figure 18. Binuclear Sc complex catalysts for syndiospecific polymerization of styrene.
Figure 18. Binuclear Sc complex catalysts for syndiospecific polymerization of styrene.
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Table 4. Styrene polymerization results using rare-earth metal (Ln) complexes.
Table 4. Styrene polymerization results using rare-earth metal (Ln) complexes.
EntryComplex[St]/[Ln]t
(min)		Activity aMn
×104 g/mol		ĐMTm
(°C)		Ref.
1 b482500113,61837.91.4273[69]
2 b4910030131.11.4269[69]
3 b5010030150.91.4269[69]
4 b511003060.51.4268[69]
5 c5650012013.22.3272[70]
6 d571500331260.65.7266[71]
7 d58500900.7--266[71]
8 e595007200.8--267[72]
9 d60500900.3--268[71]
10 f63250060208433.8268[73]
11 f6455001≥34,370124.6267[73]
12 f6555001≥34,370672.3272[73]
13 f6655001≥34,370644.1274[73]
14 g6750021360371.9268[74]
15 h685000215,60048.52.4270[68]
16 h695000215,60049.52.6270[68]
17 h705000215,60048.72.4270[68]
18 h721000240153.01.4228[68]
19 h731000240173.61.6234[68]
20 h7410001801433.52.1269[75]
21 f77250030013771.8, 1.4265[73]
22 f78250030071222.0, 1.6267[73]
23 h821000162403.51.9270[76]
24 h8350054742.21.5270[76]
25 g845002240442.1269[74]
26 g855005180782.0269[74]
a Activity in kg sPS/(mol Mt·h). b Polymerization conditions: [Ln] = 21 μmol, [Ln]/[Ph3C][B(C6F5)4] = 1, T = 25 °C. c Polymerization conditions: [Ln] = 10 μmol, [Ln]/[DMAO] = 60, T = 25 °C. d Polymerization conditions: [Ln] = 10.5 μmol, [Ln]/[AliBu3] = 10, [Ln]/[PhMe2NH][B(C6F5)4] = 1, T = 25 °C. e Polymerization conditions: [Ln] = 10.5 μmol, [Ln]/[AliBu3] = 5, [Ln]/[PhMe2NH][B(C6F5)4] = 1, T = 25 °C. f Polymerization conditions: [Ln] = 21 μmol, [Ln]/[AliBu3] = 15, [Ln]/[Ph3C][B(C6F5)4] = 1, T = 25 °C. g Polymerization conditions: [Ln] = 20 μmol, [Ln]/[AliBu3] = 5, [Ln]/[Ph3C][B(C6F5)4] = 2, T = 25 °C. h Polymerization conditions: [Ln] = 10 μmol, [Ln]/[AliBu3] = 10, [Ln]/[Ph3C][B(C6F5)4] = 1, T = 20 °C.
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Wang, J.; Bai, M.; Wang, W.; Zheng, H.; Feng, C.; Gu, J.; Mao, G.; Gao, H. Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for Syndiospecific Polymerization of Styrene and Its Derivatives. Inorganics 2025, 13, 274. https://doi.org/10.3390/inorganics13080274

AMA Style

Wang J, Bai M, Wang W, Zheng H, Feng C, Gu J, Mao G, Gao H. Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for Syndiospecific Polymerization of Styrene and Its Derivatives. Inorganics. 2025; 13(8):274. https://doi.org/10.3390/inorganics13080274

Chicago/Turabian Style

Wang, Junsong, Mingming Bai, Wenyan Wang, Handou Zheng, Chunyu Feng, Jiayue Gu, Guoliang Mao, and Haiyang Gao. 2025. "Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for Syndiospecific Polymerization of Styrene and Its Derivatives" Inorganics 13, no. 8: 274. https://doi.org/10.3390/inorganics13080274

APA Style

Wang, J., Bai, M., Wang, W., Zheng, H., Feng, C., Gu, J., Mao, G., & Gao, H. (2025). Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for Syndiospecific Polymerization of Styrene and Its Derivatives. Inorganics, 13(8), 274. https://doi.org/10.3390/inorganics13080274

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