Potentially Practical Catalytic Systems for Olefin-Polar Monomer Coordination Copolymerization
2023; Chinese Chemical Society; Volume: 6; Issue: 4 Linguagem: Inglês
10.31635/ccschem.023.202303322
ISSN2096-5745
AutoresChen Tan, Min Chen, Chen Zou, Changle Chen,
Tópico(s)biodegradable polymer synthesis and properties
ResumoOpen AccessCCS ChemistryMINI REVIEW2 Apr 2024Potentially Practical Catalytic Systems for Olefin-Polar Monomer Coordination Copolymerization Chen Tan†, Min Chen†, Chen Zou and Changle Chen Chen Tan† Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Key Laboratory of Environment-Friendly Polymeric Materials of Anhui Province, Anhui University, Hefei, Anhui 230601 , Min Chen† Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Key Laboratory of Environment-Friendly Polymeric Materials of Anhui Province, Anhui University, Hefei, Anhui 230601 , Chen Zou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026 and Changle Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026 https://doi.org/10.31635/ccschem.023.202303322 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Studies on transition-metal catalyzed olefin-polar monomer coordination copolymerization and their resulting polar-functionalized polyolefin materials have attracted attention from both academia and industry. After decades of research, recent developments in a variety of high-performance catalytic systems have shown that this field is on the brink of industrialization. This review summarizes representative olefin-polar monomer coordination copolymerization catalyst systems that may be suitable for industrial polyolefin production via homogeneous solution-phase processes or heterogeneous gas-phase/slurry-phase processes. Download figure Download PowerPoint Introduction Polyolefins are one of the most important polymeric materials with huge annual production and wide applications.1 Most polyolefins are nonpolar because they are composed only of C and H atoms, which makes them unsuitable for use in some fields.2 By introducing a small number of polar groups into the polyolefin chain, the compatibility between polyolefins and polar materials can be significantly improved, thereby expanding their utility in fields such as dyeing, bonding, composite materials, and recycling.3–5 Functional groups can also be introduced to produce custom polyolefin microstructures, leading to engineering materials with unique functions and added value.5–8 For example, the introduction of dynamic crosslinking functional groups brings improved sustainability to crosslinked polyolefin materials via the formation of covalent adaptable networks.9–14 Existing industrial routes for preparing polar functionalized polyolefins include postpolymerization modification and free-radical copolymerization.2,15 The former route often involved high temperatures, high shear, and corrosive reaction conditions, and difficult to control the microstructure of the resulting products. The latter route is often limited by the high-temperature and high-pressure polymerization conditions and limited comonomer scope. Also, free-radical polymerization often results in branched copolymers with poor mechanical properties.16 Transition-metal-catalyzed olefin (co)polymerization is used to produce the vast majority of industrially relevant polyolefins, including high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), ethylene propylene diene monomer (EPDM), polyolefin elastomer (POE), and cycloolefin copolymers (COC).16–19 The development of transition-metal catalytic systems has always been a major driving force for the advancement of the polyolefin industry because the microstructures and material properties of polyolefins can be conveniently tuned by altering the catalyst structure.2,8 Unfortunately, these industrial catalysts are easily poisoned by polar comonomers. Academic and industrial efforts to develop transition-metal-catalyzed olefin-polar monomer copolymerization have continued for decades (Figure 1a).16–24 This approach may enable the direct and controlled preparation of polar-functionalized HDPE, LLDPE, iPP, EPDM, POE, and COC-based materials that cannot be obtained by free-radical copolymerization.8 The copolymerization conditions of this route are mild and do not result in undesirable side reactions such as crosslinking or degradation, which are common in high-temperature and high-shear processes.25 Moreover, in terms of industrial polar monomers, the incorporation ratio of polar comonomers in coordination copolymerization routes (mostly ∼0.5–20 mol %)5,8,18 is mostly between industrial free-radical copolymerization26 and free-radical grafting27 routes. Therefore, this approach may lead to new functionalized polyolefin materials that will expand new applications beyond existing commercial materials, rather than just becoming competitors of the latter. The development of these materials will revolutionize the highly developed polyolefin industry.6,8,28 The key challenge in olefin-polar monomer coordination copolymerization is the so-called "polar monomer problem" (Figure 1b),16–28 in which the metal center is poisoned by polar groups or the occurrence of heteroatom-related side reactions.2,18,29 Figure 1 | (a) Transition-metal-catalyzed olefin-polar monomer coordination copolymerization. (b) "Polar monomer problem," caused by the formation of heteroatom-metal coordination bonds or side reactions involving heteroatoms. (c) Representative special polar monomers. (d) Representative fundamental polar monomers (X and Y = polar groups). Download figure Download PowerPoint Thousands of transition-metal catalysts have been recently investigated for olefin-polar monomer coordination copolymerization. As catalysts widely used in polyolefin industries, group IV transition-metal catalysts can catalyze the copolymerization of ethylene or propylene with some polar monomers bearing the C=C double bond not directly connected to the polar functional group (Figure 1c).22 These include polar α-olefins, polar norbornenes, and polar styrenes. These catalysts can achieve high copolymerization activities (up to >108 g·mol−1·h−1), high polar monomer incorporation ratio (up to ca. 30 mol %), and high copolymer molecular weights (up to Mn = 107 g·mol−1).5,22,30,31 Some group III rare-earth-metal catalysts can enable highly-active (up to >106 g·mol−1·h−1) olefin copolymerization with some special polar monomers and produce high-molecular-weight copolymers (Mn > 105 g·mol−1).16 They can also lead to very high polar monomer incorporation ratios of up to >90 mol % and in some cases be used to produce alternating copolymers.5,22,32–35 However, early-transition-metal catalysts cannot catalyze olefin copolymerization using polar monomers in which the C=C bond is directly connected to the polar functional group (Figure 1d).22 In contrast, late-transition-metal-catalyzed olefin copolymerization can be applied to a wide range of polar comonomers, including various special and fundamental polar monomers.2,5,8,16,18 This is because late-transition-metal centers are less oxophilic and not easily poisoned by polar functional groups.20 Late-transition metal catalysts are generally supported by [N,N], [N,O], and [P,O]-type ligands.18 Brookhart-type,36–39 Grubbs-type,40 and Drent-type16,20,41–48 catalysts are the most extensively studied systems. Moderate copolymerization activities and low copolymer molecular weights are the major disadvantages that have limited the development of late-transition-metal catalysts.18 The catalytic performance of late-transition-metal catalysts has been greatly improved via strategies such as the introduction of sterically bulky substituents, smaller chelating rings, rigid ligand frameworks, and solid supports.18,42–59 For olefin copolymerization with polar comonomers, high activities (up to >107 g·mol−1·h−1), high copolymer molecular weights (Mn up to >106 g·mol−1), and high polar monomer incorporation ratios (up to >30 mol %) can be achieved.2,6,18,41–59 Historically, most studies in this field have focused on the development of new catalysts or new strategies to realize a high copolymerization activity, high copolymer molecular weight, and high polar monomer incorporation ratio. As a highly industrially relevant research field, it is surprising that few efforts have been directed toward the commercialization of transition-metal-catalyzed olefin-polar monomer coordination copolymerization. Industrial polyolefin production mainly employs two techniques: a high-temperature (≥120 °C) solution-based process (Figure 2a)60–62 and a heterogeneous gas-phase/slurry-phase process (Figure 2b)28,63–67 to achieve continuous olefin (co)polymerization and avoid reactor fouling.68 The former requires thermally stable catalysts, and the latter requires heterogeneous catalysts. However, most academic studies on olefin-polar monomer coordination copolymerization have reported homogeneous polymerizations at low temperatures (≤80 °C),2,5,8,16,18–24 which often results in significant reactor fouling and is therefore unsuitable for industrial processes. Some recent works have reported some highly stable (≥100 °C) homogeneous and heterogeneous catalytic systems. Some of these systems exhibit high copolymerization activities (>105 g·mol−1·h−1), high copolymer molecular weights (>20 kg·mol−1), and tunable polar monomer incorporation ratios. These high-performance systems represent a potential "drop-in" solution for existing commercial processes, showing that olefin-polar monomer coordination copolymerization is on the brink of industrialization. This mini-review will summarize representative transition-metal catalytic systems that may be applicable for industrial homogeneous or heterogeneous olefin-polar monomer copolymerization. Polar comonomers containing functional groups with weak Lewis basicity (such as borane, silane, N(TMS)2, CF3, and alkyl aluminum) and commercial Ziegler–Natta catalysis are not covered in this review. Figure 2 | (a) High-temperature solution (co)polymerization. (b) Heterogeneous (co)polymerization. Download figure Download PowerPoint Homogenous Systems Industrial homogeneous olefin polymerization requires that the polyolefin product does not foul the reactor to ensure continuous process operation and avoid reactor shutdown. Highly thermally stable catalytic systems (≥120 °C) are required to ensure the polyolefin products remain dissolved in the reactor.53–55 Few catalysts maintain their good olefin-polar monomer copolymerization performance at such high polymerization temperatures. The incorporation of polar monomers can reduce the melting point of products and improve their solubility in organic solvents.52–57 Therefore, copolymerization systems may be able to maintain homogeneous solutions at slightly lower reaction temperatures. Therefore, olefin-polar monomer copolymerization catalysts that maintain their catalytic performance at polymerization temperatures above 100 °C are summarized. Early-transition-metal catalysts As the most commonly used catalysts in industrial high-temperature solution polymerization, half-metallocene constrained-geometry-configuration (CGC) catalysts are unable to catalyze olefin-polar monomer copolymerization.69 In contrast, for some polar comonomers masked by organoaluminum compounds, some metallocene and nonmetallocene group IV catalysts can produce high-molecular-weight copolymers, with activities of over 106 g·mol−1·h−1. Although some reports of olefin-polar monomer copolymerization, catalyzed by metallocene complexes occurred at 80 °C,70,71 most previously reported group IV-metal-catalyzed olefin-polar monomer copolymerization reactions were conducted at low temperatures (≤60 °C).22 Bouyahyi et al.72 recently reported that some Si-bridged ansa-metallocene Zr and Hf catalysts (Figure 3) (Table 1, entry 1) that catalyzed propylene copolymerization with OH-functionalized α-olefins masked by triisobutyl aluminum at 80–100 °C. Activities of 1.3–7.3 × 108 g·mol−1·h−1, moderate molecular weights (Mn = 47 kg·mol−1), and polar monomer incorporation ratios of 0.25–1.05 mol% were achieved at 100 °C. Copolymerization could also be conducted at 130 and 150 °C, leading to high activities (up to 1.8 × 107 g·mol−1·h−1), low molecular weights (Mn > 10 kg·mol−1), and polar monomer incorporation ratios from 0.34–1.40 mol %. Notably, the resulting polar functionalized polypropylene products exhibited high melting points, indicating good stereochemical control of the copolymerization, which is currently difficult to achieve when using late transition metal catalysis. The ligands of metallocene complexes used in this work are classical, making it reasonable to expect the possibility of breakthroughs using some recently discovered thermally-stable metallocene frameworks such as triptycene type ansa-metallocene catalysts62 for olefin-polar monomer copolymerization. Figure 3 | Zr- and Hf-catalyzed high-temperature olefin-polar monomer solution copolymerization. Download figure Download PowerPoint Table 1 | Potentially Practical Catalytic Systems for Olefin-Polar Monomer Coordination Copolymerization Ent. Type Cat.a Polar Monomer Tp (°C) Act.b Mnb Incorp.b Ref. 1 Homogeneous catalysts ∼100–150 ∼40–7330 ∼2.0–47 ∼0.25–1.4 72 2 100 ∼0.82–6.4 ∼8.1–23c ∼0.70–12 43 3 110 ∼23–240 ∼6.5–13c ∼0.30–1.6 54,77 4 CO ∼100–120 ∼0.49–1.9 >30d ∼45–49 78 5 CO ∼110–130 ∼0.15–1.2 ∼403–1340 ∼39–50 79 6 CO 100 ∼0.82–4.8 ∼45–216 ∼0.3–4.4 80 7 100 ∼0.05–3.5 ∼10–107c ∼0.18–6.3,e>∼0.1–2.3f 81 8 Solid-supported catalysts ∼40–80 ∼0.3–47 ∼12–180 ∼0.15–0.94 82,83 9 ∼40–80 ∼0.18–26 ∼17–630 ∼0.4–2.8 84 10 ∼80–150 ∼1–41 ∼30–834 ∼0.1–7.4 85 11 80 ∼0.7–0.9 ∼68–157c ∼0.5–0.8 87 12 ∼50–80 ∼3.5–4.5 ∼140–486 ∼9.4–12 95 13 30 ∼0.9–2.1 ∼21–118 ∼0.17–0.48 96 14 30 ∼1.1–3.1 ∼4–49 ∼0.6–5.1 97 15 30 ∼0.42–5.42 ∼115–844 ∼0.5–3.2 98 16 Precipitation polymerization ∼30–150 ∼1.5–11 ∼23–401c ∼1.0–57 101 aMolecular structure of a transition metal catalyst. For homogeneous polymerization, the copolymerization results with a Tp > 100 °C have not been cited. bCopolymerization activity, number average molecular weight, and incorporation ratio of polar monomer are in units of 105 g·mol−1·h−1, kg·mol−1, and mol %, respectively. cWeight average molecular weight. dIt was not indicated whether it was Mn or Mw. eIncorporation ratio of CO. fIncorporation ratio of vinyl polar monomer. For group III rare-earth-metal catalyzed olefin-polar monomer copolymerization, a low temperature (≤90 °C) has been mostly reported in the literature.22 Despite this, due to the unique heteroatom-assisted olefin polymerization (HOP) mechanism, these catalytic systems often achieve very high polar monomer incorporation ratios (>50 mol %), which may greatly improve the solubility of copolymer products in organic solvents at low temperatures.32–35 Nickel and palladium catalysts Since Brookhart's seminal works on α-diimine systems, nickel and palladium catalysts have been extensively studied for almost three decades for olefin-polar monomer coordination copolymerization.2,5,18 The most attractive advantage of these catalysts is that they can be applied with a wide range of comonomers, including both special and fundamental polar monomers. For [N,N]- and [N,O]-type nickel and palladium catalysts, most olefin-polar monomer copolymerization reactions are conducted below 60 °C due to their poor thermal stability.2,16,18,73 Only some of these catalysts bearing bulky substituents showed low olefin-polar monomer copolymerization activities at 90 °C.74 In contrast, many [P,O]-type nickel and palladium catalysts can catalyze olefin-polar monomer copolymerization at 80–90 °C.41–48 Although these catalysts have long been limited by their low copolymerization activity and low copolymer molecular weight,5,8,16,18,20 these concerns have largely been addressed by introducing very bulky substituents and rigid ligand frameworks or by using bridge ring-type polar monomers such as polar norbornenes.41–48,54–56 However, these catalytic systems have rarely been investigated for olefin-polar monomer copolymerization at temperatures above 100 °C, and they typically show low copolymerization activities at these temperatures.75,76 Agapie's group43 recently reported some SHOP (Shell Higher Olefin Process)-type [P,O]-nickel catalysts, showing high catalytic performance in ethylene/tert-butyl acrylate copolymerization at 100 °C (Figure 4) (Table 1, entry 2). They installed bulky rigid substituents on the O side of the phosphino-phenol ligand, which greatly increased the thermal stability of the catalyst and polar monomer incorporation ratio. These catalysts showed high activities (up to 6.4 × 105 g·mol−1·h−1), moderate molecular weights (Mw up to 23 kg·mol−1), and high polar monomer incorporation ratios (up to 12 mol %) at 100 °C. More recently, they reported a nickel catalyst based on phosphine-enol ligand (Table 1, entry 3) for high temperature (110 °C) homogeneous copolymerization of ethylene and tert-butyl acrylate, leading to high activities (up to 2.4 × 107 g·mol−1·h−1), low molecular weights (Mw up to 13 kg·mol−1), and low polar monomer incorporation ratios (up to 1.6 mol %).54,77 Figure 4 | [P,O]-nickel catalyzed high-temperature olefin-polar monomer solution copolymerization. Download figure Download PowerPoint The alternating coordination copolymerization of olefins and carbon monoxide (CO) can be realized using palladium catalysts. The products of this route, polyketones, were briefly commercialized as engineering plastics, but their production was terminated due to insufficient demand and difficult processing.16 The nonalternating copolymerization of ethylene and CO was realized using a Drent-type palladium catalyst (CO incorporation ratio ∼45–49 mol %), leading to high copolymerization activities (up to 1.9 × 105 g·mol−1·h−1) at ∼100–120 °C and high molecular weights (>30 kg·mol−1) (Table 1, entry 4).78 Recently, Liu's group79 used biphosphazane monoxide (PNPO)-type palladium catalysts to obtain similar nonalternating copolymers (Figure 5) (Table 1, entry 5) (CO incorporation ratio ∼39–50 mol %) with high copolymerization activities (>105 g·mol−1·h−1) at 110 °C and high molecular weights (Mn up to 1340 kg·mol−1). However, the chain structure of these copolymers was still dominated by alternating copolymer units (Tm 147–248 °C, typically >240 °C), which produced copolymers with properties significantly different from those of common polyolefins. Figure 5 | [PNPO]-type palladium-catalyzed nonalternating copolymerization of ethylene and CO at high temperature. Download figure Download PowerPoint Copolymers with a low CO incorporation ratio will be more like polyolefin than polyketone. Recently, Mecking's group80 reported that utilizing the SHOP-type nickel catalysts resulted in low CO incorporation ratios (>5 mol %) during ethylene/CO copolymerization, leading to HDPE-like copolymers with Tm = 133–136 °C (Figure 6) (Table 1, entry 6). High copolymerization activities (up to 2.1 × 105 g·mol−1·h−1) and high copolymer molecular weights (Mn up to 216 kg·mol−1) were achieved. Notably, the HDPE-like material obtained containing carbonyl groups in the main chain was an environmentally friendly polyolefin capable of undergoing photodegradation. Jian's group81 realized the terpolymerization of ethylene, CO, and vinyl polar monomers at 100 °C using a Drent-type palladium catalyst bearing bulky substituents. The terpolymers obtained displayed a low CO incorporation ratio (up to 2.25 mol %) (Figure 7) (Table 1, entry 7), high copolymerization activities (up to 3.5 × 105 g·mol−1·h−1), high copolymer molecular weights (Mw up to 107 kg·mol−1), and vinyl polar monomer incorporation ratios of 0.18–6.3 mol %. The terpolymers obtained exhibited Tm = 107–130 °C, which was close to that of LLDPE. Figure 6 | [P,O]-nickel catalyzed nonalternating copolymerization of ethylene and CO at 100 °C. Download figure Download PowerPoint Heterogenous Systems Heterogeneous olefin (co)polymerization processes dominate the polyolefin industry due to their product morphological control and continuous operation. Compared with high-temperature solution-based (co)polymerization processes, this route is less demanding in terms of the thermal stability of the catalyst and has better versatility.64,68 However, due to the complexity of interactions between polar monomers, supports, catalysts, and cocatalysts, developing heterogeneous catalysts to synthesize polar functionalized polyolefins is challenging.25,28,64 In recent years, some research groups have reported several efficient heterogeneous olefin-polar monomer copolymerization catalytic systems. Figure 7 | Drent-type palladium-catalyzed terpolymerization of ethylene, CO, and vinyl polar monomers at 100 °C (X = CO2R, CO2H, OR, CH2OR, OAc, CN, or CH2OAc). Download figure Download PowerPoint Solid-supported catalysts [N,O]-type catalysts The groups of Cai82 and Liu83 reported heterogeneous anilinonaphthoquinone palladium and nickel catalysts supported on metallocene methylaluminoxane (MMAO)-modified SiO2 or alkyl aluminum-modified SiO2 supports. The catalysts were prepared by linking the carbonyl group of the ligands and the surface hydroxyl groups of silica with aluminum. These catalysts (Figure 8, Ni9@Al-SiO2) (Table 1, entry 8) achieved the copolymerization of ethylene with 5-hexene-1-yl acetate (HAc) and allyl acetate (AA). The heterogeneous nickel catalysts showed high copolymerization activities (up to 4.7 × 106 g mol−1 h−1), high molecular weights (Mn up to 180 kg mol−1), and low incorporation ratio (up to 0.94 mol %) for the HAc comonomer. The Pd catalyst can mediate ethylene copolymerization with methyl acrylate and 5-norbornen-2-yl acetate. For the latter, the copolymerization activity can reach ca. 105 g mol−1 h−1. Recently, Chen's group84 reported heterogeneous anilinonaphthoquinone nickel catalysts (Figure 8, Ni10@SiO2) (Table 1, entry 9) prepared through hydrogen bonding interactions between carbonyl and hydroxyl groups of silica without using aluminum cocatalysts. These heterogeneous nickel catalysts exhibited excellent performance during the copolymerization of ethylene with HAc, AA, and methyl 10-undecylenate (MU), showing a high copolymerization activity (up to 2.65 × 106 g mol−1 h−1) and polar monomer incorporation (up to 2.8 mol %). The prepared copolymer had a molecular weight of up to 630 kg mol−1, which was suitable for commercial polyolefin materials and exhibited outstanding mechanical properties and surface properties. Figure 8 | Solid-supported nickel catalysts for the copolymerization of ethylene and polar monomers. Download figure Download PowerPoint [P,O]-type catalysts [P,O]-type phosphorus phenol nickel catalysts can efficiently copolymerize ethylene and simple polar monomers such as methyl acrylate. Compared with [N,N] and [N,O]-type catalysts, [P,O]-type nickel/palladium catalysts can generate linear copolymers due to their nonsymmetric strong/weak σ-donor ligand frameworks. As a result, they can generate polar-functionalized HDPE, LLDPE, and COC, which are inaccessible in free-radical copolymerization.8,18,19 Chen's group85 introduced ONa groups at the para-position of a phosphorus phenol nickel catalyst and prepared a heterogeneous [P,O]-type nickel catalyst (Figure 8, Ni11@support) (Table 1, entry 10) by using a series of supports (SiO2, MgO, TiO2, Al2O3, ZnO, etc.) using an ion anchoring strategy. These heterogeneous [P,O]-type nickel catalysts efficiently catalyzed the copolymerization of ethylene with acrylates with activities reaching up to 4.1 × 106 g mol−1 h−1. They produced copolymers with molecular weights (Mn) of up to 834 kg mol−1 and a high polar monomer incorporation ratio of up to 7.4 mol %. In addition, these heterogeneous nickel catalysts copolymerized ethylene with trimethoxyvinylsilane, allyl monomers (allyl chloride, acrylonitrile), and polar α-olefins (6-chloro-1-hexene, 10-undecylenol, methyl 10-undecenoate, etc.). These heterogeneous nickel catalysts showed outstanding thermal stability during ethylene/polar monomer copolymerization. Even at a high temperature (120–140 °C), copolymers were obtained with a high molecular weight (Mn = 32–456 kg mol−1) and high polar monomer incorporation (0.4–7.4 mol %). We observed that the activity, polyethylene molecular weight, and comonomer incorporation ratio, followed the order of the alkalinity of the supports, MgO > ZnO > Al2O3 > TiO2 > SiO2, indicating that the catalyst performance could be tuned by changing the support. Chen's group86 developed a coanchoring strategy to combine different catalysts into a single heterogeneous catalyst (Figure 8, Ni11/Ni12@support, Ni11/Ni13@support, and Ni11/Ni12/Ni13@support). The polar bimodal polyethylene in-reactor blend prepared by the copolymerization of ethylene and tert-butyl acrylate/methyl undecylenate catalyzed by coanchored heterogeneous [P,O] nickel catalysts led to tunable molecular weights (Mn 174–2448 kg mol−1), polydispersity index (PDI; 3.7–92.3), and polar monomer incorporation ratios (0.2–1.4 mol %) with high copolymerization activities (up to 3.93 × 106 g mol−1 h−1). Heterogeneous phosphorus benzoquinone nickel catalysts anchored by carbonyl groups87 were prepared by hydrogen bonding interactions between carbonyl groups and supports (SiO2, TiO2, Al2O3) (Figure 8, Ni14@support) (Table 1, entry 11). These catalysts exhibited higher activity (up to 9.1 × 104 g mol−1 h−1) than similar homogeneous catalysts during the copolymerization of ethylene with polar monomers (6-chloro-1-hexene, 10-undecylenol, methyl 10-undecenoate). They achieved copolymers with a molecular weight and polar monomer incorporation ratio of up to 157 kg mol−1 (Mw) and 0.8 mol %, respectively. [N,N]-type catalysts Heterogenization research on [N,N]-type such as α-diimine nickel catalysts has received widespread attention, while heterogeneous type high-performance [N,N]-type olefin-polar monomer copolymerization catalysts have remained largely unexplored.88–94 Compared with [N,O] and [P,O] nickel/palladium catalysts, the use of [N,N]-type nickel/palladium catalysts often leads to significantly increased branching densities of copolymer microstructures due to the fast chain-walking reaction, and the branching density can be tuned in a wide range by the catalyst structures and copolymerization conditions.2,7,36–39,57 Recently, Chen's group95 used an ionic anchoring strategy to install sodium sulfonate ions onto α-diimide nickel and pyridine imine nickel complexes. They then attached them onto solid supports to prepare [N, N]-type heterogeneous nickel catalysts (Figure 8, Ni15@SiO2 and Ni16@SiO2) that were highly tolerant toward polar monomers (10-undecylenic acid). Heterogeneous Ni15@SiO2 exhibited high copolymerization activities (up to 4.5 × 105 g mol−1 h−1), producing copolymers with high molecular weights (Mn up to 486 kg mol−1) and high comonomer incorporation ratios (up to 12.3 mol %) (Table 1, entry 12). Heterogeneous Ni16@SiO2 led to lower activity (up to 1.3 × 105 g mol−1 h−1), molecular weights (Mn up to 8 kg mol−1, Mw up to 18 kg mol−1) and incorporation ratios (up to 5.4 mol %). Chen's group96 introduced potassium trifluoroborate substituents into an α-diimine nickel catalyst (Figure 8, Ni17@SiO2) (Table 1, entry 13), whose ionic nature enabled strong catalyst interactions with solid supports. The trifluoroborate substituents provided strong electron-donating properties, which improved the stability of the catalyst and the molecular weight of polyethylene. During the copolymerization of ethylene with polar monomers (methyl 10-undecenoate, 6-chloro-1-hexene, and 5-hexenylacetate), Ni17@SiO2 showed moderate activities (up to 2.1 × 105 g mol−1 h−1), producing copolymers with high molecular weights (Mn up to 118 kg mol−1) and a slightly lower incorporation ratio of polar monomers (0.17–0.48 mol %) (Table 1, entry 13). In addition to designing special functional groups on catalyst
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