DFT Modeling of Coordination Polymerization of Polar Olefin Monomers by Molecular Metal Complexes

Abstract

Introducing polar functional groups into polyolefin chains through polar olefin monomer coordination (co)polymerization can directly and significantly improve the surface properties of polymer materials and expand their application range. Therefore, the related research has always received considerable attention from both academia and industry. Many experimental studies have been reported in this field, and molecular metal complexes have shown high catalytic activity and selectivity in polar olefin monomer polymerizations. Although considerable DFT calculations have also been conducted for better understanding of the (co)polymerization performance, the factors governing the activity, selectivity, and molecular weight of resulting polymers are still ambiguous. This review mainly focuses on the DFT studies of polar olefin monomer coordination (co)polymerization catalyzed by molecular metal complexes in recent years, discussing the chain initiation and propagation, the origin of polymerization activity and selectivity, and the specific role of additives in the (co)polymerization reactions.

1. Introduction

More than half a century ago, Karl Ziegler and Giulio Natta won the Nobel Prize for their great achievements in olefin coordination polymerization, marking the beginning of rapid development within the realm of polyolefin materials. So far, tremendous success in olefin polymerization has been achieved in both academic and industrial fields [1,2,3,4,5,6]. Polyolefin is currently the most widely used among polymer materials, and the global demand for polyolefin accounts for more than half of the total demand in the plastic market [7,8,9,10,11,12,13]. However, the non-polarity of conventional polyolefins limits their performance improvement and the expansion of their application range. In order to further improve their properties of printing, dyeing, adhesion, compatibility and blending, polyolefins can be functionalized by introducing highly reactive polar functional groups into the polymer chain (Scheme 1) [14,15].

There are four common methods for preparing functionalized polyolefins [16,17]: (a) post-functionalization method. This method is currently commercially used. However, due to the high inertness of the C-H bond in the polymer chain, the reaction conditions are relatively harsh (high temperature and pressure), and side reactions often occur. (b) Free radical polymerization. Through free radical polymerization, copolymerization of olefins with a small number of polar olefin monomers can be achieved, and the prepared functionalized polyolefins have been commercialized. However, the disadvantage of this method is the harsh polymerization reaction conditions (high temperature and high pressure), high product branching, poor microstructure controllability, and high content of polar olefin monomers. (c) Ring-opening metathesis polymerization. This method can provide functionalized polyolefins with precise and controllable chain structures, but the monomers required for this method are not easily obtainable and are relatively expensive. (d) Coordination (co)polymerization catalyzed by transition metals. Compared to the other three methods, the coordination (co)polymerization catalyzed by transition metal complexes has the advantages of mild conditions, controllability (including sequence distribution and structure, stereo and region regularity, geometry structure, and branching index), etc. [18,19,20]. Therefore, it is a more efficient and economical method.

At present, the system of polar monomer coordination polymerization catalyzed by transition metal complexes is under development [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. Experimentally, the polymerization activity and the microstructure of the product can be regulated by changing the catalyst ligand, metal center, monomer substituent, and additive. However, the extreme sensitivity of most active species to air and water and the inability to capture key transition states (TS) in the reaction hinder the in-depth study of a series of basic but important chemical issues regarding the factors governing regio- and stereo-selectivity as well as activity. With the gradual development of quantum chemistry and the continuous improvement of computer hardware, computational chemistry has become an effective way to study polymerization reaction mechanisms at the molecular level. Up to now, many research groups have widely applied computational chemistry to the study of the reaction mechanism of olefin polymerization catalyzed by transition metal complexes, which has promoted the design and development of catalysts [40,41,42,43,44,45]. Therefore, this review introduces the important theoretical progress of copolymerization of olefins with polar olefin monomers catalyzed by molecular metal complexes, in order to help experimental and theoretical chemists further comprehensively understand the mechanism of polar olefin monomer coordination copolymerization. According to different types of monomers, this review is divided into four parts, namely the DFT theoretical calculations related to the polymerizations of conjugated polar olefin monomer, non-conjugated polar olefin monomer, heteroatom functionalized styrene and vinylpyridine monomer (Scheme 2). In addition to early transition metal complexes and late-transition metal complexes, rare earth metal complexes (IIIB) are also included in the scope of this review.

2. Coordination Polymerization of Conjugated Vinyl Polar Monomer

Substantial progress has been made in the coordination polymerization of conjugated polar olefin monomers by early and late metal catalysts over the past two decades, particularly in terms of enhancing polymerization activity and stereochemical regulation. A large number of publications have been published on this important topic. The experimental and computational works on the polymerization of conjugated polar monomer have been reviewed in 2009 by Eugene Y.-X. Chen [41]. This article provides a detailed and comprehensive description of the important experimental and theoretical progress in the coordination polymerization of conjugated vinyl polar monomers by transition metal and rare-earth metal complexes. In addition, some reviews have covered the important experimental findings on the polymerization of conjugated vinyl polar monomer in the past fifteen years [40,41,42,43,44,45]. However, there is currently no comprehensive review on related theoretical calculations. Therefore, to add better understanding to this topic, it is necessary to comprehensively review the main progress of theoretical calculation in the field of coordination polymerization of conjugated polar olefin monomer by transition metal and rare-earth metal complexes (Scheme 3).

In 2010, Luigi Cavallo and Eugene Y.-X. Chen et al. conducted a combination of synthesis, kinetics, and theoretical calculations to explore several unique features of the polymerization of conjugated vinyl polar monomer catalyzed by Cs-ligated cationic ansa-metallocene ester enolate complexes (Figure 1) [46]. DFT calculations have revealed that installing the bulky Ph₂C < bridge can decrease the Flu-C-Cp angle, effectively bringing the η³-bound Flu ligand in closer proximity to both the growing chain and the monomer. This could increase energy difference ∆E_stereo between the competing TS for the addition of an MMA molecule to the opposite (correct and wrong) enantiofaces of the growing enolate chain; thus catalyst 6 (Ph₂C < bridged) exhibits higher stereoselectivity than 2 (the Me₂C < bridge) or 11 (the Me₂Si < bridge) (Figure 2). Moreover, computational results also offer theoretical insights into the monomer-assisted catalyst-site epimerization, occurring after an enantiofacial error, leading to a transition to a thermodynamically more stable resting state (Figure 3). The dissociation energy of the C=O group in the last inserted unit within the three representative systems 2, 3, and 6 exhibits a decreasing trend that qualitatively aligns with the observed experimental activity. Moreover, the enhanced activity observed in these closely related catalysts can be reasonably attributed to the metallocene ligand’s capacity to hinder the tight binding of the bulky counterion.

In the same year, Luigi Cavallo and Stefan Mecking et al. also offered detailed insights into the pathways of multiple consecutive acrylate insertions through meticulous DFT calculations (Figure 4) [47]. The theoretical results indicate that the π-coordination of acrylate to the products resulting from multiple acrylate insertions is linked to a relatively modest energy gain. On a free energy basis, this process is likely endergonic, owing to the relatively favorable chelating coordination. Acrylate needs to overcome a 100 kJ mol⁻¹ energy barrier to insert into 3-meso, 3-rac. Additionally, the authors also meticulously compared the relative energies (in kJ mol⁻¹) of various isomers resulting from the coordination of methyl acrylate or ethylene to 3-meso.

In 2011, Bas de Bruin et al. explored the significance of β-H elimination as a possible mechanism to induce chain termination/transfer and/or the formation of stereodefects in the Rh(diene)-mediated oligomerization and polymerization of carbenes [48]. They found through theoretical calculations that the energy barrier of chain propagation is considerably higher than that of β-H elimination and complete dissociation of the elimination product at the beginning of the reaction, thereby elucidating the experimental formation of atactic oligomers at the beginning of the reaction. However, under the catalytic conditions, the Rh(diene) pre-catalyst undergoes modifications to generate new active species. This process appears to decelerate β-H elimination, resulting in nonstereospecific oligomerization activity, and ultimately, it may cease entirely, leading to stereoselective polymerization activity (Figure 5).

Laurent Maron and Peter W. Roesky et al. investigated the ability of rare earth metal complex [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln = Y, Lu) to polymerize methyl methacrylate (MMA) both experimentally and computationally [49]. DFT investigations into the insertion of the first two MMA molecules unveiled that the arrival of the initial MMA molecule was the pivotal step, irrespective of the metal center’s nature. The nucleophilic attack of MMA triggers the formation of the initial adduct B, succeeded by the remarkable trapping of the liberated BH₃ group by the nitrogen of the phosphiniminomethanide ligand, yielding the active enolate species C. The distinctive and crucial function of the phosphiniminomethanide ligand has been distinctly uncovered and supported computationally (Figure 6). In addition, the chain propagation energy barrier of MMA polymerization catalyzed by yttrium catalyst was significantly higher (19.5 kcal mol⁻¹) than that of lanthanum (steric considerations), indicating a lower reaction activity.

Luigi Cavallo and Eugene Y.-X. Chen et al. proposed using chiral C₂-symmetric zirconocene catalysts to catalyze coordination polymerization of renewable α-methylene-γ-(methyl)butyrolactones [50]. Through the analysis of the key transition state, it can be concluded that the stereoselectivity of the product is mainly governed by steric interactions involving the monomer, the chain, and the catalyst ligand (Figure 7).

Luigi Cavallo and Stefan Mecking et al. explored the steric effects of the phosphinesulfonato ligand framework (Figure 8) on monomer (ethylene and MA) coordination and insertion through DFT methods [51]. According to the calculation results, monomers tend to coordinate at the cis position and then insert at the trans position in this polymerization system (Figure 9). In addition, the calculation results also show that the steric shielding leads to a pronounced increase in the molecular weight of resulting copolymer. They also discovered that the behavior of the catalysts cannot be comprehensively explained solely by considering the energetic profiles of the main steps of the insertion reaction. Additional factors such as deactivation and compound stability must also be taken into account; for instance, the deactivation of the compounds ^Ph1.

The stereoselectivity regulation mechanism of β-methyl-α-methylene-γ-butyrolactone (_βMMBL) polymerization catalyzed by half-sandwich rare earth metal complexes has also been systematically explored by Luigi Cavallo et al. [52]. The formation of an isotactic polymer primarily results from interactions involving the methyl groups on the chiral β-C atom of the five-membered ring in both the monomer and the last inserted _βMMBL unit of the chain. Contrary to the stereo-control exhibited by the current half-sandwich complexes, the auxiliary ligand on the metal minimally contributes to this process (Figure 10).

A new strategy proposed by Luigi Cavallo and Eugene Y.-X. Chen et al. can achieve complete chemoselective and highly syndiospecific coordination polymerization of divinyl polar monomers (AMA and VMA), facilitating the synthesis of highly syndiotactic polar vinyl polymers containing a pendant reactive C=C bond on every repeat unit [53]. At the same time, the stereoselectivity of these reactions was studied in detail by DFT. The detailed analysis of the energy and geometric structure of the key transition state shows that the main source of stereoselectivity is the steric interaction between monomer and metallocene skeleton (Figure 11).

The origin of different catalytic performance of diphosphazanemonoxide and phosphine-sulfonate palladium complexes toward copolymerization of ethylene and vinyl polar monomers (methyl acrylate MA or methyl methacrylate MMA) has been disclosed through DFT calculations by Luo et al. [54] It has been discovered that in the five-membered cationic system based on diphosphazanemonoxide, the coordination of monomers preferably occurs at the trans site (monomer trans to the P atom), while their insertions occur at the cis site (monomer cis to the P atom) (Figure 12). The geometric deformation caused by monomer insertion is the main reason for the occurrence of 2,1-insertion in the diphosphazane-monoxide palladium system. Furthermore, in the 2,1-insertion TS, the stronger MeO···H interaction between the ancillary ligand and the Me group of the inserting MMA moiety makes the TS more stable. The rate-determining step is the ethylene insertion into the MA pre-enchained species in the copolymerization reaction, rather than the MA insertion itself. Moreover, β-H elimination reaction is more likely to occur to result in the MMA chain-end, owing to the steric repulsion between the methyl group of preinserted MMA and the subsequent inserting ethylene moiety.

Bimal Pudasaini has provided an in-depth computational mechanistic analysis of the Y-catalyzed precision polymerization of vinyl phosphonates [55]. First of all, two different initiation mechanisms of dimethylvinyl-phosphonate (DMVP) catalyzed by rare earth metal alkyl complexes (with –CH₃ ligand) have been compared, and the calculation results show that the methyl addition mechanism is more favorable than the deprotonation mechanism (Figure 13). Then, a series of various ligands were compared by DFT calculation, and the results showed that most of the other ligands studied did not show preference for the deprotonation mechanism. It is worth noting that catalysts featuring a dimethylamido (–NMe2) ligand exhibit the lowest initiation barriers and may serve as a promising experimental option. In addition, when the catalyst with ligand –CH₂TMS is calculated, barrier tunneling needs to be considered, because transmission coefficients (κ) of –CH₂TMS increase very quickly as the temperature decreases.

In 2020, the polymerization mechanism of methyl methacrylate (MMA) catalyzed by rare-earth/phosphorus (RE/P) Lewis pairs has been comprehensively examined through DFT calculations by Luo et al. [56]. They found that the polymerization of MMA mediated by intermolecular RE/P (RE = Sc and La) Lewis pairs mainly follow the bimetallic mechanism (Figure 14), while the possibility of the monometallic pathway cannot be disregarded in the case of La analogue. By calculating and comparing several different Lewis bases, it can be found that the enhanced activity of cyclohexyl phosphorus in MMA polymerization may be attributed to its electron-donating capabilities, facilitating increased electron transfer in the addition reaction.

In order to obtain an ideal catalyst with high selectivity, high activity, and high polar monomer incorporation rate, Barbara Milani and colleagues altered the conventional Brookhart’s catalyst by incorporating a hemilabile, potentially bidentate ligand, such as a thiophenimine (N–S), into the fourth coordination site of palladium [57]. In addition to verifying the activity and selectivity of the new catalyst through various experimental methods, the author also used DFT theoretical calculations to explore the ethylene/MA copolymerization mechanism of the new catalyst 1S^Ph (Figure 15). The calculation results indicate that the N–S ligand maintains proximity to the palladium ion, promoting the formation of an open-chain intermediate over the anticipated six-membered metallacycle. This open-chain intermediate plays a crucial role in capturing the polar monomer into the main chain.

In 2022, a new type of α-sulfonato-β-diimine nickel catalyst has been proposed by Gao Haiyang et al., demonstrating notable thermal stability and control, along with exceptional tolerance towards polar alkyl acrylates [58]. To better understand the mechanism of α-sulfonato-β-diimine nickel-catalyzed copolymerization of ethylene and MA, DFT study was also carried out (Figure 16). The calculation results reveal that the 2,1-insertion of MA is favored, which effectively accounts for the formation of EMA copolymer with a terminal MA unit.

3. Coordination Polymerization of Non-Conjugated Polar Olefin Monomer

Similar to the conjugated olefin polar monomers mentioned above, non-conjugated olefin polar monomers can also directly introduce polar functional groups into the polymer chain, improving the performance of polymers and making them more widely applicable. At present, many transition-metal complexes have been calculated in detail to catalyze the polymerization of non-conjugated polar monomers [59,60,61,62,63,64,65,66]. It is worth noting that Eugene Y.-X. Chen and Hou et al. have successively summarized the experimental and computational work on coordination polymerization of polar olefin monomers by single site metal catalysts [41] and half-sandwich rare-earth catalysts before 2015 [60]. Based on the published work, this minireview mainly provides a comprehensive summary of the theoretical calculations of transition metal and rare-earth metal complexes catalyzed non-conjugated polar monomer polymerization systems that have been published since 2015 (Scheme 4).

In 2016, Lucia Caporaso and Stefan Mecking et al. proposed to use Pd^II-catalyzed copolymerization of ethylene and 2-vinylfuran (VF) generates α,ω-di-furan telechelic polyethylene [67]. Meanwhile, systematic theoretical calculations on the copolymerization process of VF and ethylene have been performed. According to mechanism calculations, following a VF insertion into the chain, a subsequent chain transfer reaction predominantly involves the VF monomer as well. This process leads to the termination of the chain with an unsaturated VF-based unit, observed experimentally, while concurrently initiating a new chain with VF (Figure 17).

The key findings on the heteroatom-assisted olefin polymerization (HOP) strategy of heteroatom (O, S, Se, N, and P)-functionalized polyolefins polymerization, catalyzed by half-sandwich rare-earth catalysts, were made by Hou and Luo [68]. The computational investigations in this study have unveiled that a suitable interaction between the heteroatom in an α-olefin and the catalyst metal center is paramount in attaining olefin polymerization activity and stereoselectivity (Figure 18). The mechanism of heteroatom assisted olefin insertion has been proposed for the first time and seems to be applicable to other polymerization systems, if the suitable interaction between metal and heteroatoms is achievable.

In 2019, density functional theory (DFT) methods were used to investigate the mechanism of organoscandium-catalyzed ethylene and amino olefin (AO, N-(1-butenyl)ⁿPr₂ and N-(1-octenyl)ⁿPr₂) copolymerization by T. Marks et al. [69]. The computational results demonstrate that the short-chain N-(1-butenyl)ⁿPr₂ undergoes enchainment through a self-assisted insertion pathway (energy barrier of 6.0 kcal/mol), whereas the long-chain N-(1-octenyl)ⁿPr₂ enchains through unassisted 1,2-insertion with exogenous s amine coordination (energy barrier of 7.2 kcal/mol) (Figure 19).

The nickel and palladium complexes bearing new diarylamido-based unsymmetrical [NNN^ox] and [SNN^ox] pincer ligands were synthesized and tested for polymerization of norbornene by Luo and Shi [70]. The sidearm effects, which involve quinolino ([NNN^ox] ligands) and phenylthio ([SNN^ox] ligands) moieties, have also demonstrated a noticeable impact on the catalytic performance in the polymerization of norbornene. The study revealed that the increased steric hindrance resulting from the coordinating P-sidearm in the [PNN^ox]-ligated complex could explain its lower catalytic activity compared to the [NNN^ox] ligated complex (Figure 20).

The electron-rich nature of vinyl ethers (CH₂=CHOR) are highly attractive monomers for the synthesis of many polymers and copolymers. Chen et al. developed a system that integrates the palladium-catalyzed dimerization of vinyl ethers to produce β, γ-unsaturated acetals and palladium-catalyzed ethylene copolymerization with the dimerization product to generate polar functionalized branched polyolefins [71]. The mechanism and catalyst optimization for the dimerization reaction were also thoroughly investigated (Figure 21). The calculated results indicate that the energy barrier for the third insertion is 4.7 kcal/mol higher than that of the second insertion. Moreover, the energy barrier for the β-OMe elimination reaction is significantly lower than that of the third vinyl ether insertion (17.8 vs. 25.3 kcal/mol). This finding provides a comprehensive explanation for the experimentally observed high selectivity in the dimerization process.

In 2020, the unconventional highly 1,2-selective polymerization of polar and nonpolar alkylallene compounds has been achieved by Laurent Maron and Dongmei Cui et al. [72]. They conducted detailed mechanism research on this system using DFT theory calculations, and they discovered that the reaction sequence is governed by the readily occurring 1,2-insertion step rather than the coordination of the monomer to the metal center (Figure 22). There is a certain steric hindrance between the growing polymer chain and the allene, which is the main reason for preventing the formation of 1,2 adducts.

Ethylene and vinyl halide (VX, X = F or Cl) copolymerization mechanism in the presence of several neutral palladium phosphine sulfonate catalysts A ((PO^OMe,OMe)PdMe, PO^OMe,OMe = {2(2-MeOC₆H₄)(2-SO₃-5-MeC₆H₃)P}) and A’ ((PO^Bp,OMe)PdMe, PO^Bp,OMe = {(2-MeOC₆H₄)(2-{2,6-(MeO)₂C₆H₃}C₆H₄)(2-SO₃-5-MeC₆H₃)P}) have been comparatively studied via density functional theory (DFT) calculations by Yi Luo et al.; the origin of different chain-end microstructures produced by A and A’ and the discrepancy due to VX insertion into Pd–X (X = F and Cl) species are disclosed [73]. For catalyst A’, more positive charge of metal Pd assisting the β-F elimination and stronger H···OMe and C–H···π interactions are the main reasons for obtaining the chain-end of –CH₂–CHF₂. On the contrary, the strong Pd-Cl bond and the reversible β-Cl elimination are the main reasons why the chain-end unit –CH₂–CHCl₂ cannot be obtained (Figure 23). Moreover, the mechanism of ethylene with vinyl ether (VE, CH₂=CHOEt) copolymerization catalyzed by phosphine-sulfonate palladium complex was also investigated by Yi Luo et al. [74]; the calculation results show that due to the steric hindrance between the coming VE and growing chain, the energy barrier for repeated insertion of VE is too high (29.1 kcal/mol); as a result, the copolymer exclusively incorporates OEt in both the polymer chain and chain end, without the repetitive insertion units of VE (Figure 24).

A scandium dication active species [(IPr)Sc(μ-CH₂SiMe₃)(μ-CH₂CHMe₂)AlⁱBu₂]²⁺[B(C₆F₅)₄]²⁻ (IPr = (2,6-C₆H₃ⁱPr₂NCH)₂C) in situ was synthesized by Li Xiaofang et al. and exhibited unprecedentedly high activity and syndiotactic selectivity in the polymerization of o-methoxystyrene (oMOS) and its silyloxy- or fluorine-substituted derivatives [75]. The polymerization mechanism of oMOS by this Sc dication active species has also been theoretically studied (Figure 25). Computational DFT studies suggest that the increased oxyphilic nature of scandium, which features two vacant coordination sites, enables synergistic coordination of both the heteroatom group and the double bond of oMOS to the scandium center in a σ−π coordination mode. This σ–π-coordination mode can effectively reduce the energy barrier of C=C insertion, and the primary factor contributing to the formation of syndiotactic polymerization products is the coordination of the methoxy groups from the last two inserted oMOS units in the polymer chain with the metal center simultaneously.

Usually, copolymerizing ethylene with industrially significant short-chain alkenoic acids remains a significant challenge. Luo and Tang et al. reported the efficient direct copolymerization of ethylene with vinyl acetic acid by tetranuclear nickel complexes, and the protic monomer can be extended to acrylic acid, allylacetic acid, ω-alkenoic acid, allyl alcohol, and homoallyl alcohol [76]. In addition, they also explored in detail the mechanism of ethylene and polar monomer copolymerization in this catalytic system. According to the calculation results, a proposed rationale involves the enchainment of VA facilitated by unique Ni···Ni synergistic effects (Figure 26). Motivated by the mechanistic insights, a binuclear pre-catalyst with a shorter Ni···Ni distance is synthesized and proves to be significantly more efficient for the copolymerization of ethylene with VA and AA compared to tetranuclear complexes.

In 2022, Luo et al. investigated the detrimental impact of various polar monomers on Brookhart-type catalysts using a combination of DFT calculations and multiple linear regression analyses [77]. They established a connection between the structure and the poisoning effect (ΔΔE(π-σ)) by integrating the heteroatom coordination complex structure with the descriptors of the polar monomer (Figure 27). The results indicate that the descriptors of ^monNMR_C^β and ^B2Bond_Ni/Pd-X are the main factors influencing the poisoning effect. It is important to highlight that the non-covalent interaction between polar monomers and the catalyst could be the primary reason for the deviation of the predicted value by the regression model.

Following this, Luo et al. presented an in-depth computational study on the reaction mechanism and reactivity involved in the polymerization of polar monomers catalyzed by early transition metals, where methylene spacers are present between the vinyl and functional groups [78]. They demonstrated that early transition metal catalysts with strong oxygen affinity, such as titanium(III) and zirconium(III) metallocenes, may not be susceptible to poisoning during the polymerization of certain polar monomers, allowing for successful heteroatom-chelating olefin (Figure 28). This finding offers a novel pathway for catalytic polymerization of polar monomers using early transition metal complexes.

To further address the issues related to the copolymerization system of polar and non-polar monomers, Cui et al. proposed the copolymerization of ethylene with 6-phenoxy-1-hexene (POH) by scandium precatalysts with different steric hindrances [79]. The DFT calculations show that both the polar group and the olefin moiety coordinate to the scandium ion simultaneously, serving as a crucial factor in enhancing the copolymerization process and boosting the incorporation of polar olefins through a self-assisted polymerization mechanism (Figure 29). This strategy can effectively utilize the coordination of heteroatom substituents on polar monomers to improve the polymerization activity and insertion rate of polar monomers, providing an effective approach for the polymerization of other similar polar monomers. Additionally, Luo et al. utilized DFT calculations to investigate the mechanistic aspects of the Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene in a model reaction [80]. Their findings highlight the critical role of the delicate balance between electronic and steric factors in monomer insertions, and they introduced a novel amino dissociated mechanism for AP insertion during chain initiation. Meanwhile, they used the buried volume of the metal center of the preinserted species to support that the alternating monomer-sequences were mainly determined by the “steric matching” principle (Figure 30).

4. Coordination Polymerization of 2-Vinylpyridine

Poly(2-vinylpyridine) (P2VP) is widely utilized in various applications such as thin films, molecular detection, photochemistry, ion exchange, and self-assembly [81,82]. Since 1960 [83], various experimental systems for 2VP polymerization have been continuously proposed, for example, radical polymerization, anionic polymerization, Lewis’s acid–base pair polymerization and coordination polymerization. Among them, coordination polymerization initiated by rare-earth metal catalysts is the most efficient method, and this strategy has also developed rapidly in recent years [84]. With the continuous enrichment of the experimental system, theoretical calculations on the polymerization mechanism of 2VP are constantly being conducted (Scheme 5).

In 2016, Xu et al. proposed novel yttrium bis(phenolate) ether catalysts to mediate 2VP polymerization with highly isotactic (P_m = 0.97) and high molecular weight [85]. Simultaneously, they conducted detailed calculations on the mechanism of 2VP polymerization (Figure 31). The calculation results indicated that coordination insertion of 2-VP on the re-face was more favored than that on si-face in the chain initiation stage, and the chain initiation stage was the rate-determining step in the polymerization process. During the chain propagation stage, 2VP are mainly inserted on the si-face, thus the isotactic polymerization products would be obtained. This work provides a detailed demonstration of the polymerization mechanism of 2VP, providing important ideas for further regulating the polymerization system of 2VP.

In 2017, the chain initiations of β-butyrolactone (BBL) and 2-vinylpyridine (2VP) polymerizations mediated by yttrium complexes were computationally investigated, respectively, by Luo et al. [86]. Through theoretical calculations, they found that the orbital matching degree between the initial alkyl group of a catalytically active species and vinyl group of 2VP is the main factor affecting the chain initiation efficiency (Figure 32). When the orbital matching is good, the initiation efficiency of active species is high, and vice versa. These theoretical findings offer enhanced insights into the distinct activities involved in the polymerizations of BBL and 2VP.

Moreover, the catalytic activity of 2-aminoalkoxybis (phenolate)yttrium and lutetium complexes for polar monomer polymerization was experimentally and theoretically compared by Rieger et al. [87]. Through mechanism calculations and key structural comparisons, they proposed that 2VP is a strong electron donor; when a catalyst with electron-donating ligands was used, an overload of electron density to the metal center is provoked. The presence of an electron-donating nitrogen atom and reduced electron density on the activated CH₂ group of collidine, caused by charge delocalization, did not lead to effective nucleophilic transfer to 2VP. Thus, initiation of catalyst with collidine was lower than that with alkyl (Figure 33). In comparison, catalyst 1b demonstrates versatility as a suitable option for DMAA polymerizations, mainly because DMAA can be added to catalyst 1b to form a stable eight-membered ring intermediate.

In 2018, excellent isoselectivity (mmmm > 99%) and high activity (TOF > 4000 h⁻¹) were achieved for the first time in the polymerization of the polar 2-vinylpyridine using simple lutetium-based catalysts by Xu et al. [88]. Significantly, they identified a novel approach to produce isotactic-atactic stereomultiblock poly(2-vinylpyridine)s through the addition and removal of Lewis bases during polymerization. DFT calculations were utilized to deduce the stereoselective control mechanism during the polymerization process. The calculation results indicate that the introduction of a Lewis base reduces the energy gap between the transition states of re-face and si-face, resulting in a decrease in the tacticity of the polymer (Figure 34).

5. Stereoselective Polymerization of 4-Vinylpyridine

Similar to 2-vinylpyridine, the metal-catalyzed polymerization of 4-vinylpyridine (4VP) presents a substantial challenge due to the restricted geometry of the initiating group and metal center during catalysis [89]. So far, there have been few works on the calculation of the polymerization mechanism of 4-ethylpyridine. In 2021, Xu and colleagues documented the successful polymerization of 4-vinylpyridine (4VP) through the utilization of Lewis pairs consisting of homoleptic rare-earth (RE) aryloxides and ylide-functionalized phosphines (YPhos). Notably, the computational investigation also yielded essential insights into the comparison between YPhos and PEt₃ as the Lewis base component in the polymerization process. DFT calculations conducted by Luo and co-workers revealed that the reduced stability of TS1_PE compared to TS1_P2, attributed to increased steric repulsion and weaker interactions between 4VP and the PEt₃ group, led to lower polymerization activity than that achieved with YPhos (Figure 35) [90].

6. Stereoselective Polymerization of Heteroatom-Functionalized Styrene

Polystyrenes with enhanced surface properties due to functionalization are in high demand for various industrial applications [91]. Compared to post-modification processes, the direct (co)polymerization of heteroatom-functionalized styrene using transition metal complexes is considered one of the most convenient and controllable methods to generate functionalized polystyrenes (Scheme 6) [63,84].

In 2015, the stereoselective coordination/insertion polymerization of the polar ortho-methoxystyrene (oMOS) was successfully accomplished for the first time using the cationic β-diketiminato rare earth-metal species by Cui et al. [92]. Based on the experimental results, they calculated the polymerization mechanism through DFT theory and found that the coordination between ortho-methoxy group and metal center can reduce the coordination energy and insertion energy barrier of the C=C bond of o-MOS, promoting the polymerization reaction (Figure 36). This approach offers a novel pathway for synthesizing homopolymers and copolymers with polar groups by utilizing carefully selected catalysts and polar monomers.

Subsequently, Cui et al. systematically investigated copolymerization of unmasked polar methoxystyrene monomers with nonpolar styrene by using a cationic CGC rare-earth yttrium catalyst [93]. Comparative calculations reveal that the energy barrier for styrene insertion into the active species with a polar styrene unit attached is lower compared to catalysts without such a group. In addition, the electron-donating nitrogen atom from pyridine in the CGC catalyst weakens the interaction between polar styrene and the active species. Meanwhile, due to the different chelation modes between methoxy groups and metal centers, the position of methoxy groups on MOS can affect the sequence distribution of copolymers (Figure 37). In the same year, Cui’s group also illustrated the novel coordination-insertion polymerization of a polar electron-withdrawing fluorine-substituted styrene and its copolymerization with styrene (St) employing rare-earth-metal-based catalysts [94]. The calculation results indicate that the coordination between the electron-donating pyridine side-arm and the metal center could weaken the Lewis acidity of the metal center, thereby reducing the interaction energy between the F atom and the metal center (Figure 38b). Therefore, the coordination energy of the F atom is similar to that of the C=C bond. This can increase the efficiency of the polymerization reaction. By contrast, when the catalyst bearing the unlinked half-sandwich ligand has been used, the metal center tends to coordinate with the F atoms of two monomers at the same time, forming more stable intermediates, resulting in relatively difficult coordination and insertion of C=C bonds (Figure 38a). The above calculation results provide an important theoretical basis for improving the reactivity of the homopolymerization and copolymerization reaction systems of methoxy styrene.

In 2018, Luo and co-authors conducted a comparative analysis of the polymerization mechanism of styrenes substituted with polar functional groups catalyzed by cationic rare-earth metal alkyl complexes using DFT calculations [95]. The underlying reasons for different polymerization mechanisms, namely step growth involving C-H activation and chain growth through vinyl insertion, have been elucidated. They observed that reduced steric hindrance and higher average charge distribution in the four-center transition state are advantageous for C-H activation associated with step-growth polymerization. Additionally, the coordination of the heteroatom from the functional group may promote the insertion of the adjacent vinyl group, leading to a somewhat negative impact on C-H activation (Figure 39). Moreover, due to the steric hindrance and electron donor effects caused by the catalyst side arms, the C-H activation is inhibited. This work mainly conducted a systematic study on the conditions for the occurrence of step-growth polymerization, providing important information for the subsequent development of more efficient step-growth polymerization systems.

In 2019, Cui et al. studied the coordination polymerization of the unmasked polar alkoxystyrenes (AOS) by using the cationic quinolyl anilido yttrium alkyl species that was inert to styrene polymerization, and the mechanism was elucidated by the DFT calculations (Figure 40) [96]. These computational investigations have highlighted the importance of a robust interaction between the polar o-alkoxy group of AOS and the active yttrium center in enhancing the activity and stereoselectivity of AOS polymerization. Importantly, modifying the ortho-oxygen substituent from methyl to benzyl can alter the stereoregularity of the resulting polymer from highly syndiotactic to atactic by weakening the Y-σ-O coordination bond.

By using DFT calculations, Luo et al. demonstrated that the stereoselectivity of Sc-catalyzed polymerization of halogenated styrenes can be co-regulated by C–H···π interactions between a coordinated THF molecule and the phenyl ring of a monomer [97]. The theoretical calculation results suggest that the selectivity of halogenated styrene polymerization products can be adjusted from syndiotactic to random by changing the number and sites of halogen element substitutions on halogenated styrene, which is achieved by changing the interaction between monomers and THF (Figure 41). The strategy of regulating stereoselectivity through a noncovalent interaction can provide a new approach for selective polymerization of olefins.

Afterwards, Cui et al. proposed copolymerizations of ethylene (E) and ortho-/meta-/para-fluorostyrenes by using quinolyl methylene fluorenyl scandium complex (Flu-CH₂-Qu)Sc(CH₂SiMe₃)₂ [98]. According to theoretical calculations, the Lewis acidity of the metal center and the local steric hindrance are influenced by the bulky and electron-withdrawing quinolyl side arm, resulting in a tendency towards continuous insertion of ethylene during copolymerization (Figure 42). Furthermore, it was also found that the insertion of fluorostyrenes is a thermodynamic product.

The chemoselective polymerization of divinyl monomers is a well-recognized approach for producing functional polymers through post-polymerization; however, this method may lead to issues such as polymer cross-linking. Cui and Zhao et al. reported the coordination polymerization of polar divinyl styrenyl monomers 1-(allyloxy)-4-vinylbenzene (AOS), 1-(but-3-en-1-yloxy)-4-vinylbenzene (BOS), 1-(pent-4-en-1-yloxy)-4-vinylbenzene (POS), and 1-(hex-5-en-1-yloxy)-4-vinylbenzene (HOS) by employing rare-earth metal complexes [99]. At the same time, the factors influencing the (co)polymerization behaviors were explored through DFT calculations. The spacer length between the olefinic C=C bond and the oxygen atom of divinyl monomers is the main factor affecting the microstructure of polymerization products. For monomers containing a short spacer, there exists the concurrent Y-μ₁-O: η²-C=C coordination, resulting in polymer chain back-skipping and equal energy barriers for re-insertion and si-insertion, leading to the formation of atactic polymers (Figure 43). In the case of a monomer with a long spacer, the hindrance of polymer chain back-skipping is increased, promoting the preferred alternating re- and si-insertion of the styrenyl C=C bond to yield syndiotactic polymers.

Recently, based on the “self-assisted” theory previously proposed by the Cui’s group, they also presented new racemic ansa-bis(benz[e]indenyl) rare-earth metal complex for isospecific polymerization of polar styrenes, and the isospecific mechanism has been revealed through DFT [100]. According to mechanism calculations, the bulky C₂-symmetric [BIndCMe₂BInd]²–ligand facilitates pMOS si-face coordination, leading to ideal isoselectivity through the enantiomorphic site-control mechanism (Figure 44).

Besides, Luo’s research group systematically investigated the factors contributing to the atactic and syndiotactic selectivity in the polymerization of amino-functionalized styrenes catalyzed by rare-earth-metal catalysts [101]. The computational findings revealed that the stronger the electronic influence of the aromatic rings (-NMe₂ > -OH > -F), the stronger the C-H...π interaction between the monomer and complex. Consequently, the greater the energy barrier difference ΔΔG^≠, the higher the syndioselectivity (Figure 45). Furthermore, when the steric hindrance of functionalized styrene and ligand substituents are both small, energy barriers of re-face and si-face insertion are similar, resulting in random polymerization products. In 2023, Luo et al. also investigated the effect of additives on the stereoselectivity of styrene polymerization catalyzed by rare-earth metal complexes [102]; their findings indicated that the primary factor influencing stereoselectivity was the noncovalent interaction between the additive alloxybenzene and the polymer chain/monomer (Figure 46).

In addition, the ring opening polymerization (ROP) of polar monomers such as cyclic esters, which is out of the scope of this review, has also received widespread attention in the polymer community [103,104,105,106,107,108,109]. Significantly, transition metal complex-catalyzed ROP has always been at the forefront of the development of ROP catalysts for epoxides, cyclic esters, and carbonates, and is even more commonly used than organic catalysis [110,111,112,113]. In experiments, a considerable number of researchers have studied cyclic esters such as ε-Caprolactone (CL), lactide (LA), and other related monomers. There are a certain number of review articles [114,115,116,117,118,119,120] and textbooks [121,122] that have covered the development of ROP of cyclic esters catalyzed by transition metal complexes, documenting the rapid development of this field. Meanwhile, the ROP mechanisms have been comprehensively explored by DFT calculations. In 2019, the DFT calculation results for different ROP mechanisms of various cyclic substrates were systematically integrated by Ilya Nifant’ev and Pavel Ivchenko [117]. In a recent review, Brian K. Long and colleagues discussed the early developments, key recent advancements, and future prospects in the area of redox-switchable ring-opening polymerizations of cyclic esters and cyclic ethers [120]. These two reviews combine theoretical and experimental works to provide a detailed and systematic understanding in this field.

7. Conclusions and Perspectives

In recent years, an increasing number of experimental and theoretical researchers are interested in polar monomer coordination polymerization catalyzed by molecular metal complexes. This is mainly because, unlike radical copolymerization, it can achieve precise control of the microstructure of the polar polymer under mild conditions. At the same time, with the rapid development of computational methods and computer hardware, DFT modeling has gradually become a commonly useful tool in the studies of catalysis, including catalytic polymerization. Theoretical calculations can model various species that may form during the catalytic process, as well as possible non-covalent interactions, which often play an important role in the activity and microstructure regulations.

The technology of polar monomer coordination copolymerization is currently not industrially available yet. Therefore, catalyst development certainly requires fundamental research in this field although it is challenging in increasing activity, molecular weight of resulting copolymer, and the content of polar monomer in the polymer chain. In this sense, there should be considerable opportunities for theoretical calculation studies to design more efficient polymerization catalysts. It is noteworthy that the combination of DFT calculations and data analysis (such as machine learning, multiple linear regression, and principal component analysis) is a more promising approach for rational design of catalysts. It is expected that the combination of experiment and theory speeds up the development of copolymerization systems of polar monomers and nonpolar olefins.