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Groups 3 and 4 single-site catalysts for olefin-polar monomer copolymerization

Chengkai Li ,  Guoqiang Fan ,  Gang Zheng ,  Rong Gao ,  Li Liu

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Citation:  Chengkai Li, Guoqiang Fan, Gang Zheng, Rong Gao, Li Liu. Groups 3 and 4 single-site catalysts for olefin-polar monomer copolymerization[J]. Chinese Chemical Letters, 2025, 36(9): 111297. doi: 10.1016/j.cclet.2025.111297 shu

Groups 3 and 4 single-site catalysts for olefin-polar monomer copolymerization

English

  • 1. Introduction

    Polyolefins, especially polyethylene (PE) and polypropylene (PP), have secured a pivotal position in modern industries and daily life due to their low cost, exceptional physicochemical properties, and superior processability, which are widely used in medical disposables, automotive engineering, and wire & cable fabrication, underscoring their indispensability as core materials [1-7]. Advances in catalyst technology have further propelled the polyolefin industry, enabling the creation of diverse polyolefin with tailored structures and properties, precisely tailored to meet the demands of various industries. Nevertheless, the inherent non-polarity of traditional polyolefins poses limitations to their expansion into high-end applications [8, 9]. Therefore, it is particularly important to introduce functional polar groups into polyolefins, which can significantly improve the adhesion, surface, mechanical and rheological properties of the materials [10-20]. Commercially available functional polyolefins are generally produced by free radical polymerization method, which requires high temperature (150–375 ℃) and high pressure (250–3000 bar) with limited monomer scope [11, 16, 21-23]. Another strategy is post-polymerization modification, which requires harsh reaction conditions and hampers precise control over polymer microstructures [24, 25]. For example, free radical grafting of maleic anhydride frequently needs to be carried out at temperature exceeding 200 ℃, posing risks of polymer degradation or crosslinking [26].

    The limited development of copolymerization of α-olefins with polar olefins stems from formidable "polar monomer problem", wherein the interaction between polar comonomer with Lewis basic heteroatom and Lewis acidic metal active species significantly impairs catalyst efficacy [27]. Hence, numerous studies have focused on attenuating the toxicity of polar groups to metal. Late transition metal catalysts, renowned for their weak oxophilicity and robust tolerance to polar groups, have been widely investigated in recent years [28-42]. A series of novel functional polyolefins have been developed by fine-tuning the catalyst structures and many extremely important achievements have been accomplished, which have been exhaustively described in other reviews and will not be repeated here. However, late transition metal catalysts exhibit moderate copolymerization activity and thermal stability, poor regio- and stereoselectivity, and limited molecular weight due to the chain transfer reaction, leaving scope for improvement in these areas. Early transition metal catalysts have played a pivotal role in the traditional polyolefin industry, enabling the production of high-density polyethylene (HDPE), linear low density polyethylene (LLDPE), isotactic polypropylene (iPP), and other valuable products [43-48]. Nevertheless, their strong oxophilicity constrains their capacity for synthesizing functional polyolefins. Researchers have partially circumvented this limitation through innovative approaches, such as incorporating weakly coordinating polar comonomers, employing bulky substituents to shield heteroatoms, pretreating polar comonomers with Lewis acids to protect the polar groups, and designing novel catalysts, thereby achieving breakthroughs in the copolymerization of α-olefins with polar comonomers using early transition metal catalysts, especially groups 3 and 4 single-site catalysts [49, 50].

    This review focuses on the recent advances of groups 3 and 4 single-site catalysts in catalyzing the copolymerization of α-olefins with polar comonomers. While there are few reports on yttrium catalyst, probably due to the low activity for the α-olefin polymerization [51-54]. Therefore, yttrium catalysts will not be extensively discussed in this context. And in view of the space limitation, we mainly discuss the application of ethylene and propylene as the representatives of α-olefins, and the catalysts are categorized into four groups based on Sc, Ti, Zr, and Hf, and undertake a thorough analysis of effect of different catalysts and polar comonomers on the copolymerization behaviors and the improvement of polymer properties. Finally, an outlook on the potential and future directions of groups 3 and 4 single-site catalysts in fostering efficient copolymerization of non-polar olefins with polar comonomers is provided.

    2. Sc-based catalysts

    Typically, rare-earth metal catalysts are regarded as highly oxophilic, rendering them unsuitable for catalyzing the copolymerization of functionalized polar comonomers. Nevertheless, Sc exhibits unusual bonding character in terms of covalency that could potentially yield different catalytic reactivities [55-61]. In 2017, Luo and Hou et al. reported the first syndiospecific polymerization of a series of heteroatom-containing α-olefins (rrrr > 95%) and the copolymerization with ethylene by using Sc-1−6, which resulted in oxygen-, sulfur-, and phosphorus-containing copolymers with high molecular weights (Mn = 154.2 kg/mol), narrow molecular weight distributions (Ð = 1.39–2.71), and up to 73.5 mol% of comonomer content (Fig. 1 and runs 1–6 in Table S1 in Supporting information) [62]. Interestingly, the copolymerization of polar comonomers with ethylene can be well performed even for some systems where homopolymerization was not extremely efficient. Control experiments and DFT analysis showed that heteroatoms can serve as an efficient promoter for the copolymerization of functionalized polar comonomers, which called heteroatom-assisted olefin polymerization (HOP) mechanism (a mechanism where polar groups stabilize the metal center to promote monomer insertion).

    Figure 1

    Figure 1.  Sc-catalyzed copolymerization of ethylene with various functionalized polar comonomers.
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    Soon after in 2019, they firstly realized the controlled copolymerization of ethylene with anisyl-functionalized propylenes (AP) by sterically demanding half-sandwich Sc-5 catalyst with high catalytic activity (up to 640 kg/(mol h atm)) and yielded copolymers with high comonomer contents (up to 46 mol%) (Fig. S1a in Supporting information and run 7 in Table S1) [63]. As the comonomer AP ratio increased, the molecular weight increased (up to 552 kg/mol) with narrow molecular distribution (Ð = 1.58–1.98), which indicated that the methoxy group plays an important role in promoting the copolymerization (Fig. S1b in Supporting information). The NMR analysis revealed that the copolymers are composed of multiblock sequence containing relatively long amorphous ethylene-alt-anisylpropylene sequences and short crystalline ethylene-ethylene segments, and at same time the phase separation was formed that crystalline ethylene-ethylene segments serve as the physical cross-linking points in the flexible ethylene-alt-anisylpropylene matrix, which is extremely advantageous for the mechanical properties ranging from soft viscoelastic materials to tough elastomers and flexible and rigid plastics (Fig. S1c in Supporting information). In addition, the copolymers with low glass transition temperatures (Tg) exhibit excellent self-healing ability, either in dry environment, water, aqueous acid or alkaline solutions and the copolymers with high Tg can be functioned as shape memory materials (Figs. S1d and e in Supporting information).

    In 2021, they investigated the terpolymerization of ethylene (E) with two different methoxyaryl-functionalized propylenes, such as hexylanisyl propylene (AHexP) and methoxynaphthyl propylene (ANaphP) or methoxypyrenyl propylene (APyrP) by using Sc-5 catalyst, where the hexylanisyl propylene is able to form soft segments and methoxynaphthyl propylene or methoxypyrenyl propylene gives relatively hard segments (Fig. 2, runs 8 and 9 in Table S1) [64]. Analogous to the two-component copolymer, multiblock sequence was obtained containing two alternating sequences and ethylene-ethylene segments and phase separation was formed, which led to improved mechanical properties and self-healing ability.

    Figure 2

    Figure 2.  Terpolymerization of ethylene, hexylanisyl propylene with methoxynaphthyl propylene or methoxypyrenyl propylene.
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    Recently, they reported the terpolymerization of ethylene, AP and 4-[2-(1-pyrenyl)ethenyl]styrene (Pyr) by using Sc-5 catalyst, yielding terpolymers with high comonomer content (42.7 mol%) consisted of relatively long alternating E-alt-AP sequences, isolated Pyr units, and short E-E blocks (Fig. 3a and run 10 in Table S1) [65]. The tercopolymers also exhibit excellent mechanical properties and self-healing ability. However, the difference from the previous example is that the styrenyl C=C bond of the Pyr unit in the terpolymers could undergo [2 + 2] cycloaddition reaction under photoirradiation, which enables the preparation of a self-healable fluorescent two-dimensional image on a terpolymer film through photo-lithography (Fig. 3b).

    Figure 3

    Figure 3.  (a) Terpolymerization of E, AP and Pyr by Sc-5. (b) Photoinduced reversible cycloaddition reaction.
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    The crucial element in achieving these microstructures and properties is the unique interaction between the oxygen atom in the anisylpropylene monomer and the scandium atom in the catalyst. Subsequently, the authors deliberated on the potential of alternative heteroatom functional groups to achieve the same effect, prompting them to employ the same catalyst system to catalyze the terpolymerization of ethylene, styrene (St), and 3-(o-dimethylaminophenyl)propylene (AMP), which also yielded sequence-controlled multiblock copolymers (Table S1, run 11) [66]. The terpolymers, containing AMP-alt-(E-E) sequence and E-E and St-St blocks, exhibit excellent mechanical properties and efficient self-healing capabilities, which are further enhanced when 4‑tert-butylstyrene was used in place of unsubstituted styrene, with elongation at break up to 890%, ultimate tensile strength up to 9.4 MPa, but are slightly weaker than the two materials mentioned above.

    Cyclic polyolefins bearing heteroatom functional groups may show beneficial properties compared to traditional polyolefins, which can be synthesized by cyclopolymerization of α, ω-dienes [67-73]. In 2019, Luo and Hou et al. realized the regio- and stereoregular cyclopolymerization of ether- and thioether-functionalized 1, 6-heptadienes by using Sc-5 catalyst (Fig. 4a) [74]. The polymerization of 4-benzyloxy-1, 6-heptadiene selectively afforded benzyloxy-functionalized cyclic polymer composed of 1, 2, 4-cis-substituted-ethylenecyclopentane (ECP) microstructures in an isospecific fashion by Sc-5. However, 1, 2-trans-1, 4-cis-ECP units with high syndiotacticity were obtained for the polymerization of 4-phenylthio-1, 6-heptadiene. With these results in hand, the copolymerization of ethylene with ether- and thioether-functionalized 1, 6-heptadienes excluding 4-benzyloxy-1, 6-heptadiene was performed efficiently with activity up to 2.4 kg/(mol h atm) and Mn up to 147.9 kg/mol (Table S1, runs 12 and 13). Subsequently, cyclic polymers were transformed to yield Et3SiO-substituted and OH-functionalized polymers with dramatically different physical properties that Et3SiO-substituted polymer exhibited lower Tg (−10 ℃) than the original polymer (11 ℃) and no Tm was detected and it is probably because of the hydrogen bonding interaction that OH-functionalized polymer showed very high Tg (105 ℃) and Tm (261 ℃) (Fig. 4b) [75, 76].

    Figure 4

    Figure 4.  (a) Sc-5-catalyzed cyclopolymerization of heteroatom-functionalized 1, 6-heptadienes. (b) Transformation of cyclic polymer.
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    Another type of polar comonomers is styrene derivatives containing heteroatom functional groups, where the introduction of aromatic groups into polyolefin backbone could improve viscoelastic behavior, thermos-mechanical properties and compatibilities with other polymeric materials [77, 78]. In 2018, Cui et al. reported the direct copolymerization of ethylene with polar styrenes by using Sc-7 catalyst (Fig. 5a) [79]. When styrene or para-methylthiostyrene was used as the comonomer, the catalytic activity remained subdued (TOF = 0.4 × 103 and 0.3 × 103 h-1), and no polymerization took place at all under the same conditions in the case of para-methoxystyrene. Intriguingly, when the comonomer possessed electron-withdrawing substituent, such as para-fluorostyrene, the catalytic activities surged significantly, surpassing the corresponding homopolymerization (TOF = 16.7 × 103 h-1 vs. 4.1 × 103 and 11.1 × 103 h-1), and the comonomer contents could reach up to 50.7 mol%, which is the first example of the positive comonomer effect on the copolymerization of ethylene with polar comonomers. DFT calculations revealed that the electron withdrawing polar group enhances the electrophilicity of scandium center by the Sc3+ and phenyl secondary interaction, which decreases the ethylene insertion energy and stabilizes the transition intermediates. As ethylene is progressively introduced, this interaction weakens until it disappears, resulting an increase in insertion energy, which is consistent with the experimental result that the longest consecutive ethylene sequence is three repeated units and high comonomer contents were obtained (Fig. 5b).

    Figure 5

    Figure 5.  (a) Copolymerization of ethylene with polar styrenes by using Sc-7. (b) The possible copolymerization mechanism.
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    In 2021, they investigated the copolymerization of ethylene with ortho-/meta-/para-fluorostyrenes by using Sc-8 (Fig. 6) [80]. The copolymerization activity of ethylene with para-fluorostyrenes is comparable to the ethylene homopolymerization (1440 vs. 1260 kg/(mol h atm)), but significantly higher than the copolymerization of ethylene with nonpolar styrene under the same conditions (180 kg/(mol h atm)), indicating the polar comonomer positive effect, which probably due to the electron-withdrawing fluorine group increases the Lewis acidity of the Sc3+ active center that favors ethylene insertion and accelerates the copolymerization (Table S2 in Supporting information, run 1). Switching to ortho- and meta-fluorostyrenes, the catalytic activity slightly reduced (1020 and 960 kg/(mol h atm)), whereas the comonomer contents were slightly higher (12.6 mol% and 15.0 mol% vs. 10.0 mol%), indicating that the position of fluoride substituent affects both activity and comonomer insertion (Table S2, runs 2 and 3). The copolymer with a broad melting temperature (Tm) exhibits excellent mechanical properties that the stress achieves up to 39.5 MPa and strain-at-break is above 774% with 75% elastic recovery.

    Figure 6

    Figure 6.  Copolymerization of ethylene with ortho-/meta-/para-fluorostyrenes by using Sc-8.
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    They also synthesized the pseudo- and perfect alternating copolymers of ethylene and polar styrenes with distinguished activities, high molecular weights, and high comonomer conversions (Fig. 7) [52]. To achieve this purpose, five catalysts were investigated. Only a trace amount of homopolymer of poly(para-methoxystyrene) was obtained for catalyst Y-1, probably due to the extremely low activity for ethylene homopolymerization (Table S2, run 4). A strong Lewis acidic scandium analogue Sc-7 exhibited moderate copolymerization activity of 21.5 kg/(mol h atm) and high comonomer content (41.3 mol%) (Table S2, run 5). As for half-sandwich catalyst Sc-9, only trace amount of gel-like product with complicated (topological or cross-linked) structures was isolated (Table S2, run 6). Switching to fluorenyl-carbene scandium catalyst, Sc-10 catalyzed the copolymerization to afford soluble copolymer with low activity (5.75 kg/(mol h atm)) and low comonomer contents (20.7 mol%) (Table S2, run 7). Interestingly, catalyst Sc-11 with smaller substituent on the carbene moiety showed high activity (62 kg/(mol h atm)) and high comonomer incorporation ratio of 49.1 mol% together with high comonomer conversion (85%) (Table S2, run 8). More interestingly, alternating copolymer sequence was formed. In the case of other polar styrene derivatives, pseudo- or perfect alternating copolymers were still obtained.

    Figure 7

    Figure 7.  Copolymerization of ethylene with polar styrenes by using Sc-11.
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    The controlled copolymerization of ethylene with comonomers containing triphenylamine or carbazole groups catalyzed by rare-earth catalysts has rarely been reported, therefore, Leng and Li et al. synthesized functional polyethylene containing triphenylamine or carbazole groups by using half-sandwich Sc-6 catalyst in 2022 (Fig. 8) [81]. This catalytic system exhibits high catalytic activity for the copolymerization, reaching up to 9816 kg/(mol h atm), resulting copolymers with moderate molecular weight, narrow molecular weight distribution, and the comonomer content reached up to 18.9 mol% (Table S2, runs 9 and 10). The introduction of comonomers improves the photophysical and surface properties, which may enlarge the potentials of polyethylene.

    Figure 8

    Figure 8.  Copolymerization of ethylene with comonomers containing triphenylamine or carbazole groups by using Sc-6.
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    Cyclic olefin copolymers are hot topic due to their excellent properties in recent years. In 2023, Cui et al. introduced dicyclopentadiene (DCPD) as comonomer and investigated the terpolymerization of ethylene, DCPD, and styryl monomers by employing Sc-7 catalyst with catalytic activity up to 2200 kg/(mol h atm) (Fig. 9a and run 12 in Table S2) [82]. Substituents on the styryl monomers exert significant influence on the DCPD incorporation that styryl monomer with an electron-donating substituent such as Me (pMeSt), dramatically inhibits the incorporation of DCPD (2.9 mol%), because of the increased electron density of the C=C bond and coordination ability of the corresponding styryl monomer, and with an electron-withdrawing substituent, such as F (p-fluorostyrene), promotes the incorporation of DCPD (26.6 mol%) (Table S2, runs 11 and 12). Subsequently, the effect of the presence of styryl monomer on the properties of terpolymers was investigated. The test results revealed that the presence of styryl monomer has a negative effect on the transparency and moderate amount of introduction of styryl monomer has a positive effect on the refractive index (Figs. 9b-d). In addition, the mechanical properties of terpolymers are superior to those of the copolymer of ethylene and DCPD with tensile strength up to 41.2 MPa and elongation at break up to 9.9% and no cross-linking occurred after hot pressing (Fig. 9e). Therefore, the leaving double bonds can be easily and completely transformed to cyclic units via m-CPBA or Simmons-Smith reaction.

    Figure 9

    Figure 9.  (a) Terpolymerization of ethylene, DCPD, and styryl monomers by using Sc-7. (b) 0.1 mm-thick films. (c) Transmittance curves. (d) Refractive index curves. (e) Stress−strain curves. P1-P3: terpolymers with different monomer contents. P4: copolymer of ethylene with DCPD. P5: copolymer of ethylene with styryl monomer. Reproduced with permission [82]. Copyright 2023, American Chemical Society.
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    Using the similar constrained geometry catalyst (CGC) systems, Cui et al. explored the copolymerization behavior of ethylene with polar 6-phenoxy-1-hexene (POH) by scandium catalysts bearing different steric hindrances and electronic effects in 2023 (Fig. 10) [83]. The half-sandwich scandium catalyst Sc-12 exhibits a high activity (405 kg/(mol h atm)) and the copolymers obtained had high comonomer content (7.0 mol%) and catalyst Sc-13 displays highest activity (1590 kg/(mol h atm)) with 14.6 mol% comonomer incorporation, the reason for which probably due to the electronic effect of the ligand, with minimal influence from the leaving group (Table S3 in Supporting information, runs 3 and 4). To clarify the function of polar group in the copolymerization process, the copolymerization of ethylene and 7-phenylhept-1-ene was conducted under the same conditions, but only polyethylene homopolymer was isolated, which indicated that the polar group plays an important role in facilitating the copolymerization of polar comonomer. The copolymers, synthesized with different comonomer contents, can be efficiently transformed into brominated products, which can further be converted into ethylene-based ionomers that exhibit improved mechanical properties with breaking strain of up to 835% and tensile strength of up to 23.5 MPa.

    Figure 10

    Figure 10.  Copolymerization of ethylene with 6-phenoxy-1-hexene and synthesis of ionomers.
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    Some halogen-containing olefins seem to have relatively weak deactivation effect on transition metal catalysts, which are excellent candidates for the synthesis of functionalized copolymers with improved properties [84-90]. In 2021, Guo and Hou et al. employed Sc-6 to catalyze the copolymerization of 10‑bromo-1-decene (BrDC) with ethylene, propylene, and dienes with high catalytic activity [91]. The homopolymerization of BrDC, copolymerization of BrDC with ethylene or propylene, terpolymerization of BrDC with ethylene and propylene, and tetrapolymerization of BrDC with ethylene, propylene and dienes such as ethylidene norbornene (ENB) can be performed efficiently, yielding poly(BrDC), brominated polyethylene or polypropylene, brominated ethylene-propylene rubbers (EPR), and brominated ethylene-propylene-diene rubbers (EPDM) with controllable compositions and high molecular weights (Table S3, runs 5–8).

    Compared to monometallic catalysts, multinuclear analogues may exhibit some different properties [92-98]. In 2023, Li and Cui et al. synthesized two binuclear half-sandwich Sc-14 and Sc-15 catalysts to catalyze the copolymerization of ethylene with polar comonomers containing oxygen, nitrogen, and sulfur atoms, which shows greater activities than the mononuclear analogue Sc-12 (62.5 and 35 kg/(mol h atm) vs. 5 kg/(mol h atm) (Fig. 11 and runs 9–11 in Table S3) [99]. While, the two binuclear catalysts exhibit different copolymerization behaviors for comonomers with different lengthy spacers and polar groups, for example, for the polar comonomer having a butylene spacer, Sc-14 behaves better than Sc-15, while for those with shorter or longer spacer than the butylene, the catalytic results are reversed, suggesting that the synergistic effect depends not only on the proximity of the two active species, but also on the matching of the polar comonomer and the binuclear catalyst and DFT calculations provided support for the synergetic effects and cooperative mechanism. What is more, the copolymer of ethylene and para-methoxystyrene displays distinct mechanical properties from commercial HDPE, which has a dramatic drop in strength after the yield point, while the copolymer continues to increase before break, probably due to the difference in the molecular weight and microstructure.

    Figure 11

    Figure 11.  Copolymerization of ethylene with polar olefins by binuclear scandium catalysts.
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    3. Ti-based catalysts

    3.1 Comonomers without protecting agent

    In 2008, Fujita et al. successfully accomplished the direct copolymerization of ethylene with 5-hexene1-yl-acetate by using bis(phenoxy-imine) Ti catalysts (Ti-1, Ti-FI catalyst) (Fig. 12) [100]. Experimental results showed that substituents with varying steric hindrance and electronic properties of the FI ligands exert a significant effect on copolymerization behavior that introduction of an electron-donating group (tert‑Bu, OMe) results in higher catalytic activity, reaching as high as 515 kg/(mol h atm), while electron-withdrawing one (CF3) leads to lower catalytic activity, dropping down to as low as 28 kg/(mol h atm), probably because that a less electrophilic Ti center with a more electron-donating ligand exhibits a lower affinity to the polar group. Control experiments demonstrated that Ti-FI catalysts perform better than metallocene catalysts Cp2MCl2 (M = Zr, Ti) and Me2Si(Me4Cp)(NtBu)TiCl2 (CGC), which is consistent with DFT calculations that the energy differences between ethylene-coordinated and carbonyl-coordinated cationic catalysts are much smaller than metallocene catalysts. Therefore, Ti-FI catalysts exhibit much higher functional group tolerance than the metallocene catalysts.

    Figure 12

    Figure 12.  Copolymerization of ethylene with 5-hexene1-yl-acetate by using Ti-1 catalyst.
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    In the same year, Marks et al. employed single-site bimetallic CGCs to catalyze the copolymerization of ethylene and styrene with various para substituents (4-methylstyrene, 4-fluorostyrene, 4-chlorostyrene, and 4-bromostyrene) (Fig. 13) [101]. The organozirconium catalysts are incompetent for the copolymerization, resulting only in mixtures of polyethylene and polystyrene, whereas titanium-based catalysts exhibit remarkable efficiency with catalytic activity up to 2.9 × 104 kg/(mol h atm) (Table S4 in Supporting information, runs 1 and 2). Control studies revealed that bimetallic catalysts facilitate the incorporation of comonomer than the corresponding monometallic catalysts. These findings underscored the ability of multinuclear single-site catalysts to produce macromolecular architectures distinct from those produced by monometallic catalysts.

    Figure 13

    Figure 13.  Copolymerization of ethylene with polar styrenic comonomers by using mono- and binuclear catalysts.
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    Recently, Chung et al. realized the copolymerization of ethylene and 4-bromostyrene by using the [(C5Me4)SiMe2N(t-Bu)]TiCl2 catalyst and then reactive diphenylacetylenyl side groups were grafted onto PE chains and Diels-Alder [2 + 4] cycloaddition with polycyclic aromatic hydrocarbon (PAH) in pitch was performed [102]. The PE-g-PAH/Pitch blend exhibits tunable melt viscosity and achieves a carbon yield exceeding 70% at 1000 ℃. This approach addresses critical limitations of conventional precursors by combining low-cost materials, simplified processing, and high carbon efficiency, offering a promising pathway for scalable carbon fiber manufacturing.

    3.2 Comonomers with protecting agent

    Another strategy for incorporating polar groups into polyolefins involves utilizing protecting agents to protect the functional groups. In 2019, Zhu et al. performed the copolymerization of ethylene with protected vinyl polar comonomer of p‑tert‑butyl‑dimethylsilyloxystyrene by using the [OSSO]-type bis(phenolate) titanium Ti-5 catalyst (Fig. 14) [103]. The catalytic activity gradually increased with increase of the comonomer concentrations in the feed, peaking at 2.2 × 104 kg/(mol h atm) at high comonomer concentration of 1.0 mol/L, which indicated that active species are almost not poisoned by the protected polar group and approximate alternating copolymer could be formed with high molecular weight (43 kg/mol) (Table S4, run 3). Deprotection was conducted efficiently by desilylation based on acidification, resulting in substantial enhancement of the hydrophilicity of the obtained polymer that the water contact angle (WCA) decreased to 85.6° when 13.0 mol% comonomer was incorporated.

    Figure 14

    Figure 14.  Copolymerization of ethylene with p‑tert‑butyl‑dimethylsilyloxystyrene by using Ti-5.
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    Except for non-metallocene catalysts, Ti-based metallocene catalysts are also a good choice for the copolymerization of polar comonomers. In 2020, Nomura et al. realized the efficient copolymerization of ethylene with 9-decen-1-ol, pretreated with AliBu3, by phenoxide-modified half-titanocenes containing SiMe3 (Ti-6) or SiEt3 (Ti-7) group in the phenoxy para-substituent (Fig. 15a) [104]. The catalytic activity is exceptionally high, reaching up to 6.4 × 104 kg/(mol h atm), yielding copolymers with high molecular weight (Mn = 65.5 kg/mol) and narrow molecular weight distribution (Ð = 1.83), probably due to the introduction of SiMe3 or SiEt3 group could stabilize the active species (Table S4, run 4). Subsequently, the hydroxy group was treated with AlEt3 and addition of ε-caprolactone at 80 ℃, yielding amphiphilic graft copolymers via Al-alkoxide initiated ring-opening polymerization and the molecular weight increased with time because of the living manner of this ring-opening polymerization.

    Figure 15

    Figure 15.  (a) Copolymerization of ethylene with 1-decene and 9-decen-1-ol and synthesis of amphiphilic graft copolymer by post-polymerization. (b) Copolymerization of ethylene with 1, 1-disubstituted olefins and deprotection process. (c) Copolymerization of ethylene with AliBu3-protected 9-decen-1-ol by Ti-9.
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    In 2024, Shiono et al. successfully employed Ti-8 to accomplish the copolymerization of ethylene with siloxy-protected 1, 1-disubstituted olefins with relatively low activity (6.45 kg/(mol h atm)), obtaining functional copolymers with low molecular weight (6 kg/mol) and broad molecular weight distribution (Ð = 14.6) (Fig. 15b and run 5 in Table S4) [105]. The silyl groups can be deprotected in a straightforward manner, leading to hydroxyl‑functionalized polymers with higher hydrophilic surface properties.

    In 2025, Cui et al. developed a novel class of half-titanocene catalysts incorporating sterically demanding N-heterocyclic boryloxy ligands, achieving unprecedented efficiency in copolymerizing ethylene with polar comonomer 9-decen-1-ol (7.6 × 104 kg/(mol h atm) at 20 ℃, and 4.0 × 104 kg/(mol h atm) at 100 ℃) (Fig. 15c run 7 and 8 in Table S4) [106]. The strong electro-donating N-heterocyclic boryloxy ligands with tailored steric bulk could stable the active cationic titanium species, enabling exceptionally high polar comonomer incorporation (up to 32.1 mol%) while suppressing chain transfer reactions, yielding high molecular copolymers (Mn = 99 kg/mol). The resulting hydroxyl functionalized copolymers exhibit enhanced mechanical properties (stress at break up to 33.9 MPa, tensile elongation up to 1130%).

    4. Zr-based catalysts

    4.1 Comonomers without protecting agent

    AOs (CH2═CH-(CH2)nNR2) are fascinating comonomers, because the introduction of amino into the polyolefins could confer many interesting properties to materials [107-109]. In 2019, Marks et al. reported the organozirconium catalysts for ethylene and AO (n = 2, 3, 6, R = Et and nPr) copolymerization in the absence of masking reagents with high catalytic activity and comonomer incorporation (Fig. 16a) [110]. For one, the activated Zr-4 catalyzed copolymerization with high activity (3400 kg/(mol h atm)) but with low comonomer incorporation (< 0.5 mol%) (Table S5 in Supporting information, run 1), either using [Ph3C][B(C6F5)4] or B(C6F5)3 as co-catalyst, probably due to the steric encumbrance around the active site being more favorable for ethylene coordination/enchainment instead of bulky comonomer, while use of Zr-5 improves the comonomer incorporation (1.4 mol%) but at the expense of activity (840 kg/(mol h atm)) (Table S5, run 2). Employing the unbridged Zr-6 exhibits high activity (2600 kg/(mol h atm)) but with relatively low comonomer incorporation (1.5 mol%) (Table S5, run 3). Switching to Zr-7-catalyzed copolymerization, both activity (3400 kg/(mol h atm)) and comonomer incorporation (5.0 mol%) are enhanced (Table S5, run 4). Further mechanistic experiments and DFT analyses revealed that the high activity can be attributed to the preferential activation of olefins rather than amine by the active sites.

    Figure 16

    Figure 16.  (a) Copolymerization of ethylene with AO. (b) Copolymerization of propylene with AO.
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    In 2020, they synthesized polar functional isotactic and syndiotactic polypropylenes (PPs) by direct, masking-reagent free propylene and AOs copolymerization by using Zr-7 and Zr-8, respectively, with polymerization activities as high as 4208 and 1055 kg/(mol h atm), which only slightly lower than that propylene homopolymerization, but with relatively low comonomer incorporation (0.4 mol% and 0.6 mol%) (Fig. 16b, runs 5 and 6 in Table S5) [111]. More fascinating, comonomer incorporation significantly improves the stereoselectivity (mmmm: 59.5%→91.0%, rrrr: 66.3%→81.3%). In order to evaluate the effect of polar comonomer introduction on the polymer properties, the advancing aqueous contact angle and melting temperature were tested. The results revealed that upon the introduction of AO, the copolymers exhibit a decreased advancing aqueous contact angle, reflecting the increase in surface energy. While at the same time, the melting temperature do not change significantly. Consequently, the introduction of the AO comonomer not only amplifies the polarity of the copolymer but also spares its melting temperature from detrimental effects.

    Gomez-Elvira et al. introduced the N-alkyl pyrrole into the polypropylene to improve the thermal and thermos-oxidative stability via direct copolymerization of propylene with 1-(undec‑10-ene-1-yl)-1H-pyrrole by using Zr-9 as the catalyst [112]. The catalytic activity decreased from 6489 kg/(mol h) to 77 kg/(mol h) as the comonomer content increased from 0 mol% to 5.3 mol%, while the stereochemistry was scarcely changed, even slightly increased (mmmm = 89.6% vs. 92.4%). Interestingly, crosslinking was formed in heat treatment under air, probably due to N-alkyl pyrroles are able to yield benzyl-like radicals under the action of decomposing peroxides with temperature, which eventually generate dimers by coupling.

    Subsequently in 2023, they investigated the effect of N-alkyl carbazole on the copolymerization of propylene with 9-(undec‑10-en-1-yl)-9H-carbazole using the same catalytic system and the properties of copolymer (photoactivity, thermal and dynamo-mechanical relaxations, dielectric behavior, and the stability) (Fig. 17) [113]. Up to 4 mol% of carbazole units can be incorporated and the presence of comonomer improves the stereoselectivity of copolymers, probably because comonomer incorporation is preferred over racemic misinsertion of propylene. The introduction of comonomer endows the copolymers with a number of attractive properties: fluorescence emission, improved dielectric response and stability due to the chain cross linking through carbazole units.

    Figure 17

    Figure 17.  Copolymerization of propylene with 9-(undec‑10-en-1-yl)-9H-carbazole by using Zr-9.
    DownLoad: Full-Size Img PowerPoint

    In 2023, Jian et al. synthesized the cyclic olefin terpolymers (COT) of ethylene, tetracyclododecene (TCD) and carbazolyl substituted α-olefins (CNAr) by using Zr-8 catalyst with tunable compositions (TCD: 11.5–35.8 mol%, CNAr: 1.2–5.0 mol%) and high molecular weights (up to 144.6 kg/mol) and the catalytic activity is available up to 929 kg/(mol h atm) (Fig. 18a and runs 7–9 in Table S5) [114]. By fine-tuning the ratio of comonomers, COT can be transformed from semi-crystalline to amorphous with high Tg (167 ℃) without affecting the thermal decomposition temperature (Fig. 18b). Furthermore, COT exhibits enhanced mechanical properties (tensile strength: 60.5 vs. 51.2 MPa; strain at break: 7.4% vs. 5.4%) and optical transmittance (93%−95%) compared to copolymers of ethylene and TCD (Figs. 18c and d). What is more, the introduction of CNAr significantly improves the refractive index from 1.533 to 1.569 (Fig. 18e).

    Figure 18

    Figure 18.  (a) Copolymerization of ethylene, TCD and CNAr by using Zr-8. (b) DSC curves. (c) Stress-strain curves. (d) Optical transmittance at 400−800 nm. (e) Refractive index plotted against the incorporation of the carbazolyl group. Reproduced with permission [114]. Copyright 2023, American Chemical Society.
    DownLoad: Full-Size Img PowerPoint

    In addition to the aforementioned comonomers, those containing aluminum as heteroatom have received relatively limited attention. In 2021, Lin et al. introduced aluminum into the polyolefins and realized one-pot synthetic protocol to produce iPP ionomers via direct copolymerization of propylene with 7-octenyldiisobutylaluminum by using a novel C1-symmetric single-site Zr-10 catalyst with high molecular weights (Mn > 68.3 kg/mol), narrow molecular weight distributions and high isotacticities, followed by CO2 quench, yielding iPP ionomers (Fig. 19) [115]. High catalytic activity was achieved (> 2 × 105 kg/(mol h)), but obtained copolymer with low comonomer incorporation (< 0.1 mol%). Incorporation of small amounts of ion content exerted minimal effect on mechanical properties, where the Young's modulus and yield stress are slightly lower than iPP homopolymers, yet the elongation at breaks were all great than 500%. While the presence of comonomer exhibits profound effect on the rheological properties that resultant copolymers manifest high melt strength, strong shear thinning, and extensional strain hardening, which are benefit for applications in blown film, foaming, and thermoforming.

    Figure 19

    Figure 19.  Synthesis of ionomers by using Zr-10.
    DownLoad: Full-Size Img PowerPoint

    4.2 Comonomers with protecting agent

    In 2022, Bouyahyi et al. revealed the effect of four different catalysts on the copolymerization of propylene with AliBu3-passivated 10-undecen-1-ol (Fig. 20a) [116]. Kinetic experiments showed that the catalytic activities of the zirconium-based Zr-9 and Zr-11 are higher than that of the hafnium-based analogues Hf-1 and Hf-2, with Zr-11 reaching a maximum of 1.6 × 105 kg/(mol h atm), while Hf-1 is only 2600 kg/(mol h atm) (Table S6 in Supporting information, runs 1–4). More comonomers can be incorporated into copolymers with higher molecular weights for the zirconocene and hafnocene bearing the substituted indenyl ligand. Among the four catalysts, Zr-11 has the best performance, exhibiting higher catalytic activity, higher molecular weight capability (up to 162.6 kg/mol) and unparalleled stereo-selectivity. Therefore, authors deemed it prudent to evaluate the efficiency of Zr-11 under industrially relevant conditions. While no comonomer was incorporated under slurry and gas-phase conditions and only limited molecular weight and stereo-selectivity capabilities was achieved under high-temperature (130–150 ℃) solution process conditions.

    Figure 20

    Figure 20.  (a) Copolymerization of propylene with AliBu3-passivated 10-undecen-1-ol. (b) Copolymerization of propylene with silane-protected 10-undecen-1-ol.
    DownLoad: Full-Size Img PowerPoint

    Chung et al. utilized silane group to protect the OH group before the polymerization step and employed Zr-11 to catalyze the copolymerization of propylene with 10-undecen-1-oxytrimethylsilane with high catalytic activity (up to 744.5 kg/(mol h atm)) and high comonomer content was incorporated (up to 6.0 mol%) (Fig. 20b and runs 5 and 6 in Table S6) [117]. The deprotections were easily conducted to obtain PP-OH copolymers and subsequently the Steglich esterification reactions were performed using two hindered phenol molecules that contain a carboxylic acid group with PP-OH. The copolymers containing phenol side chains exhibit high thermal-oxidative stability, which is beneficial for the long-term protection of PP productions.

    O'Hare et al. synthesized the phosphonate-functionalized polyethylene via copolymerization of ethylene and 11‑bromo-1-undecene by using commercially available Zr-12 as catalyst and AliBu3 as protecting agent, followed by post-polymerization modification of bromine-functionalized polymers with phosphite esters (P(OiPr)3 and P(OPh)3) (Fig. 21a) [118]. The copolymerization activity of ethylene with 11‑bromo-1-undecene was achieved up to 4322 kg/(mol h atm) and the resultant bromine-functionalized polymers featured high comonomer incorporation (up to 6.1 mol%) but with relatively low molecular weights (Mn = 9.1 and 7.9 kg/mol) (Table S6, runs 7 and 8). The bromide group was converted to phosphonate group in high conversion, produced phosphonate-functionalized PE with high thermal stability and flame resistance. Control experiments showed that the direct copolymerization of ethylene with phosphonate comonomers, CH2═CH(CH2)nP=O(OiPr)2 (n = 2–6), was failed.

    Figure 21

    Figure 21.  Synthesis of (a) functionalized polyethylene by using Zr-12, (b) polypropylene by Zr 11-13 and Ti-10.
    DownLoad: Full-Size Img PowerPoint

    Recently, O'Hare et al. applied the same strategy to synthesize functionalized polypropylene (Fig. 21b). Propylene was copolymerized with 11‑bromo-1-undecene by using Zr-11, Zr-12, Zr-13 and Ti-10 to yield brominated polypropylene with tunable comonomer contents (up to 15.5 mol%), controlled tacticities (isotactic, syndiotactic, atactic), and wide molecular weight range (4–212 kg/mol) [102]. The pendant bromine groups in copolymers serve as latent electrophilic sites, enabling diverse postmodification via nucleophilic substitution. The resulting copolymers exhibit tailored bulk and surface properties, which offers a scalable platform to engineer high-performance polyolefins for different applications.

    Boron functionality shows tolerance to early transition metal catalysts and can be easily converted to other functional groups under mild conditions, whereas the unprotected arylboronic acid can react with methylaluminoxane (MAO) [20, 119-125]. Therefore, Shiono et al. investigated the copolymerization of propylene with diaminonaphthalene-protected alkenylboronic acid by using Zr-14 bearing the NNOO-tetradentate ligand (Fig. 22) [126]. The catalytic activity is relatively low (8 kg/(mol h atm)), but the obtained polymers possess high comonomer incorporation (up to 3.8 mol%) and the stereospecificity is scarcely reduced (mmmm = 90%), thereby yielding polymer with high melting temperature (Tm = 107 ℃) (Table S6, runs 9 and 10). What is more, deprotection of boronic acid amide can be easily proceeded and boronic acid-functionalized copolymers exhibit high Young's modulus and strength, probably because the boronic acids act as cross-linking points via dehy-drotrimerization.

    Figure 22

    Figure 22.  Copolymerization of propylene with diaminonaphthalene-protected alkenylboronic acid by using Zr-14.
    DownLoad: Full-Size Img PowerPoint

    Pan and Li et al. synthesized functional polypropylene via the copolymerization of propylene and functional α-olefins containing trimethylsilyl (TMS)-protected alkynyl groups by using Zr-15 catalyst and click reactions (Fig. 23) [127]. Through ingenious design of the steric and electronic effects of the comonomers, the copolymerization activity reached as high as 5970 kg/(mol h atm), resulting in copolymers with high molecular weight and high isotacticity (mmmm > 94%). The post-functionalization of the copolymers can be readily accomplished through a one-pot process involving complete deprotection of the TMS groups and followed by click reaction (alkynyl/PEG-N3 and thiol/yne). The hydrophilicity of PEG-grafted-iPP is drastically enhanced, as evidenced by a threefold increase in water swelling capacity.

    Figure 23

    Figure 23.  Synthesis approach of the functional iPPs.
    DownLoad: Full-Size Img PowerPoint

    5. Hf-based catalysts

    In 2014, Li et al. compared the copolymerization behavior of propylene with ω-halo-α-alkene by using Zr-12 and dimethyl(pyridyl-amido)hafnium Hf-3, respectively (Fig. S2a in Supporting information) [85, 128, 129]. Zr-12, with larger special opening around the active center, favors the coordination and insertion of monomers, exhibiting high catalytic activity of up to 3050 kg/(mol h atm) but yielding copolymers with low molecular weight (Mn = 12.6 kg/mol) (Table S7 in Supporting information, run 1). Conversely, Hf-3, with narrow special opening around the active center, effectively inhibits the chain-transfer side reaction, ultimately yielding polymer with high molecular weight (up to 70.9 kg/mol) and high isotacticity (mmmm > 99%), but with relatively low catalytic activities (Table S7, run 3). And the comonomer content can be at a maximum of 7.68 mol% for Zr-12 and 11.7 mol% for Hf-3 (Table S7, runs 2 and 4). The copolymers featuring an abundance of pendant iodine groups can be efficiently transformed into various functional PPs through click chemistry. One prime example is the thiol-halogen nucleophilic substitution reaction, which seamlessly introduces the carboxylate group into the side chain of copolymers. This approach significantly facilitates the acquisition of a wide range of novel functional copolymers.

    In 2015, they firstly reported the copolymerization of propylene with 3, 3-dimethyl-3-sila-1, 5-hexadiene (DMSHD) by using Hf-3 to produce Si-containing cyclic olefin copolymers (Fig. S2b in Supporting information) [130]. Hf-3 catalyst exhibits excellent catalytic activity (up to 2940 kg/(mol h atm)), resulting the synthesis of copolymers with high molecular weights reaching up to 462 kg/mol and high comonomer incorporation (up to 25.3 mol%) (Table S7, runs 5–7). By varying the comonomer contents, it is feasible to manipulate the Tm and crystallinity, thereby altering the mechanical properties from thermoplastic to highly flexible to elastomeric.

    In 2017, they found Hf-3 exhibited high catalytic activity for the copolymerization of propylene and 9-hexenylanthracene (up to 3 × 103 kg/(mol h atm)), producing copolymers with high molecular weights (Mn = 127 kg/mol) and tunable comonomer incorporation (up to 5.7 mol%) [131]. The pendant anthryl groups serve as versatile reactive sites for subsequent Diels-Alder functionalization, which could introduce diverse functional groups such as esters, carbonates, and sulfones, thereby tailoring copolymers' polarity and compatibility.

    The utilization of bulky groups to shield the polar group is one of the strategies to introduce the functional groups. In 2019, Pan et al. achieved the direct copolymerization of propylene with AO, which bearing bulky groups to efficiently shield the polar atom, by using Hf-3 (Fig. S2c in Supporting information) [132]. Notably, the catalyst showed remarkable catalytic activity (up to 6580 kg/(mol h atm)), resulting in copolymers with high molecular weight (Mn = 77.1 kg/mol) and comonomer content (up to 7.8 mol%) (Table S7, runs 8 and 9). In addition, the introduction of comonomers can further improve the surface properties that the contact angles decreased from 104.8° to 72.5° as the comonomer contents increased and mechanical properties of copolymers can also be easily tuned by varying the comonomer structures and contents to meet different application requirements.

    In the same year, they realized the synthesis and comprehensive characterization of carbazole-functionalized isotactic polypropylenes via direct copolymerization of propylene with 11-carbazole-1-undecene by using the Hf-3 (Fig. S2d in Supporting information) [133]. The catalyst shows exceptional tolerance to polar comonomer that the catalytic activity achieved up to 4.08 × 103 kg/(mol h atm), yielding copolymers with high molecular weights (up to 317.6 kg/mol) and comonomer incorporation (up to 13.5 mol%) (Table S7, runs 10 and 11). The introduction of comonomers imparts increased thermal stability and tunable mechanical properties, with elongation at break increasing from 14% (pure iPP) to 727% at higher comonomer content. What is more, the resulting copolymers exhibit unique fluorescent properties.

    Soon after in 2020, Zhang and Pan et al. employed two distinct strategies to synthesize polypropylene-graft-poly(lactic acid) (iPP-g-PLA): (1) Propylene was copolymerized with 11-iodo-1-undecene by using Hf-3 to yield functionalized polypropylene backbone containing iodine groups and these pendant iodine underwent quaternization reactions with imidazole-terminated PLA, enabling precise control over graft length, graft density, and ionic content; (2) Propylene was copolymerized with AliBu3-protected 10-undecen-1-ol, where the hydroxyl groups on the resulting copolymer were deprotected and utilized as initiators for the in-situ ring-opening polymerization of lactide [134]. Remarkably, blending PLA with these graft copolymers enhance the toughness and transparency of PLA while minimizing stiffness loss.

    In 2023, Jian et al. employed Hf-3 to catalyze the copolymerization of propylene with O- or S-functionalized long-chain polar olefins with high catalytic activity (up to 2496 kg/(mol h atm)), comparable to propylene homopolymerization (2490 kg/(mol h atm)), yielding highly isotactic copolymers (mmmm = 99%) (Fig. S3a in Supporting information and runs 12 and 13 in Table S7) [135]. Surprisingly, the incorporation of comonomer led to higher molecular weight copolymers than the homopolymers (Figs. S3b and c in Supporting information), which could maximally reach 1195 kg/mol (Table S7, run 14). In order to evaluate the effect of comonomer on the properties of the copolymer, WCA measurement was performed. Notably, the incorporation of 1 mol% of comonomer significantly reduced the WCA by 11°, highlighting the profound impact even a minor incorporation can have on the copolymer properties (Fig. S3d in Supporting information). This phenomenon is more evident when considering the mechanical properties that the tensile strength of copolymer with 1 mol% comonomer is 42 MPa, surpassing the homopolymer of 29 MPa and the elongation at failure of copolymer is 860%, while that of homopolymer is only 13% (Fig. S3e in Supporting information).

    6. Conclusions and perspectives

    Polyolefin materials, as an indispensable and vital component of modern industries, showcase their ubiquitous applications and significance through their immense annual consumption. Functional polyolefins are gradually unveiling their potential, portending even broader application prospects and escalating demand. Over the past decade, significant advancements have been made in the development of groups 3 and 4 single-site catalysts for the copolymerization of olefins with polar monomers. Scandium-based catalysts, for instance, have demonstrated remarkable versatility in enabling the direct copolymerization of ethylene with diverse polar comonomers. These systems leverage unique interactions between the polar groups and the Sc centers, exemplified by the HOP mechanism, to achieve high activity, controlled microstructures, and high-performance materials. Titanium catalysts are rich in ligand structures and can even be synthesized as binuclear catalysts to catalyze the copolymerization of polar monomers. Zirconium and hafnium catalysts, on the other hand, have expanded the scope of functional polyolefins through the incorporation of aluminum, carbazole, boron, or halogenated comonomers, which provide unlimited possibilities for post-polymerization modifications through facile reactions and further expanding the materials' application boundaries. Collectively, these advancements underscore the pivotal role of catalyst design, including ligand steric/electronic tuning, binuclear architectures, and synergistic heteroatom interactions, in mitigating the "polar monomer problem" and enhancing copolymerization efficiency.

    As a domain intimately tied to industry, the future trajectory of functional polyolefins is fraught with both challenges and opportunities. Firstly, the polymerization activities of current catalytic systems are generally low, therefore, enhancing catalyst performance, particularly high catalytic activity, thermal stability, and hydrogen sensitivity, is vital to advancing industrialization and overcoming existing technological hurdles. What is more, enhancing the catalyst's tolerance to polar monomers is equally crucial to facilitate direct copolymerization of α-olefin with polar monomers, thereby eliminating the necessity of alkylaluminum or protective groups. Secondly, the majority of comonomers currently employed are ingeniously designed and synthesized (polar groups are not directly bonded to double bond) and suffer from high costs, difficulties in mass production, and limited comonomer scope. Therefore, invention of low-cost, easily synthesized, green and renewable, multifunctional comonomers is crucial for achieving green production and circular economy transitions. Thirdly, although academic research on low-temperature, homogeneous polymerization conditions is forward-looking, translating these achievements into practical technologies compatible with industrial production conditions remains a subject of intense exploration. Fourthly, optimizing the comprehensive properties of functional polyolefins is directly linked to their market competitiveness and widespread adoption. Achieving breakthroughs in mechanical, processing, and surface properties beyond current commercial offerings is imperative for driving functional polyolefins towards premium and differentiated development. Last but not least, the powerful supporting role of theoretical calculations, machine learning (ML) and artificial intelligence (AI) cannot be overstated. For instance, ML can analyze vast datasets of catalyst structures and polymerization results to identify descriptors (e.g., metal electronegativity, ligand steric parameters) that predict activity/comonomer incorporation. Coupled with DFT, such approaches could accelerate the discovery of next-generation catalysts while minimizing trial-and-error experimentation. In conclusion, functional polyolefins are spearheading the polyolefin field towards greater premiumization and differentiation. Their expansive development prospects and immense market potential undoubtedly infuse fresh vitality and hope into the global industrial landscape.

    Declaration of competing interest

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

    CRediT authorship contribution statement

    Chengkai Li: Writing – review & editing, Writing – original draft. Guoqiang Fan: Validation, Supervision. Gang Zheng: Validation, Supervision. Rong Gao: Supervision, Funding acquisition. Li Liu: Validation, Supervision.

    Acknowledgment

    This work was supported by the National Natural Science Foundation of China (No. U23B6011).

    Supplementary materials

    Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111297.


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  • Figure 1  Sc-catalyzed copolymerization of ethylene with various functionalized polar comonomers.

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    Figure 2  Terpolymerization of ethylene, hexylanisyl propylene with methoxynaphthyl propylene or methoxypyrenyl propylene.

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    Figure 3  (a) Terpolymerization of E, AP and Pyr by Sc-5. (b) Photoinduced reversible cycloaddition reaction.

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    Figure 4  (a) Sc-5-catalyzed cyclopolymerization of heteroatom-functionalized 1, 6-heptadienes. (b) Transformation of cyclic polymer.

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    Figure 5  (a) Copolymerization of ethylene with polar styrenes by using Sc-7. (b) The possible copolymerization mechanism.

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    Figure 6  Copolymerization of ethylene with ortho-/meta-/para-fluorostyrenes by using Sc-8.

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    Figure 7  Copolymerization of ethylene with polar styrenes by using Sc-11.

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    Figure 8  Copolymerization of ethylene with comonomers containing triphenylamine or carbazole groups by using Sc-6.

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    Figure 9  (a) Terpolymerization of ethylene, DCPD, and styryl monomers by using Sc-7. (b) 0.1 mm-thick films. (c) Transmittance curves. (d) Refractive index curves. (e) Stress−strain curves. P1-P3: terpolymers with different monomer contents. P4: copolymer of ethylene with DCPD. P5: copolymer of ethylene with styryl monomer. Reproduced with permission [82]. Copyright 2023, American Chemical Society.

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    Figure 10  Copolymerization of ethylene with 6-phenoxy-1-hexene and synthesis of ionomers.

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    Figure 11  Copolymerization of ethylene with polar olefins by binuclear scandium catalysts.

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    Figure 12  Copolymerization of ethylene with 5-hexene1-yl-acetate by using Ti-1 catalyst.

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    Figure 13  Copolymerization of ethylene with polar styrenic comonomers by using mono- and binuclear catalysts.

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    Figure 14  Copolymerization of ethylene with p‑tert‑butyl‑dimethylsilyloxystyrene by using Ti-5.

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    Figure 15  (a) Copolymerization of ethylene with 1-decene and 9-decen-1-ol and synthesis of amphiphilic graft copolymer by post-polymerization. (b) Copolymerization of ethylene with 1, 1-disubstituted olefins and deprotection process. (c) Copolymerization of ethylene with AliBu3-protected 9-decen-1-ol by Ti-9.

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    Figure 16  (a) Copolymerization of ethylene with AO. (b) Copolymerization of propylene with AO.

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    Figure 17  Copolymerization of propylene with 9-(undec‑10-en-1-yl)-9H-carbazole by using Zr-9.

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    Figure 18  (a) Copolymerization of ethylene, TCD and CNAr by using Zr-8. (b) DSC curves. (c) Stress-strain curves. (d) Optical transmittance at 400−800 nm. (e) Refractive index plotted against the incorporation of the carbazolyl group. Reproduced with permission [114]. Copyright 2023, American Chemical Society.

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    Figure 19  Synthesis of ionomers by using Zr-10.

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    Figure 20  (a) Copolymerization of propylene with AliBu3-passivated 10-undecen-1-ol. (b) Copolymerization of propylene with silane-protected 10-undecen-1-ol.

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    Figure 21  Synthesis of (a) functionalized polyethylene by using Zr-12, (b) polypropylene by Zr 11-13 and Ti-10.

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    Figure 22  Copolymerization of propylene with diaminonaphthalene-protected alkenylboronic acid by using Zr-14.

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    Figure 23  Synthesis approach of the functional iPPs.

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  • 发布日期:  2025-09-15
  • 收稿日期:  2025-01-15
  • 接受日期:  2025-05-09
  • 修回日期:  2025-05-03
  • 网络出版日期:  2025-05-09
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