Polar Vinyl Monomer Copolymerizations Using Ni

May 22, 2018 - catalysts: (1) The polar group of the comonomer can coordinate strongly ... trialkoxysilanes are ideal comonomers and circumvent all of...
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Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α‑Diimine Catalysts Zhou Chen and Maurice Brookhart*

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Center for Polymer Chemistry, Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States CONSPECTUS: The most ubiquitous polymer, polyethylene (PE), is produced either through a radical-initiated process or, more commonly, through a coordination/insertion process employing early transition metal catalysts, particularly titanium- and chromium-based systems. These oxophilic early metal catalysts are not functional-group-tolerant and thus cannot be used to synthesize copolymers of ethylene and polar vinyl monomers such as alkyl acrylates and vinyl acetate. Such PE copolymers have enhanced properties relative to PE and are made through radical polymerization processes, requiring exceptionally high pressures and temperatures. Copolymerizations of polar vinyl monomers with ethylene using more functional grouptolerant late metal catalysts potentially offer an attractive alternative for generating such value-added copolymers since ligand variations may provide more control of polymer microstructures and milder reaction conditions would apply. This Account describes our efforts, particularly through detailed mechanistic studies, to probe and develop this potential using Pd(II) and Ni(II) α-diimine catalysts. To inform discussions of the copolymerizations, we briefly review key aspects of ethylene homopolymerizations using these diimine catalysts. These include ligand designs that incorporate axial blocking groups that retard chain transfer and promote production of a high polymer rather than an oligomer. These ligand designs also lead to unique branched polyethylenes via migration of the metal along the chain (“chain-walking”) prior to insertion. Mechanistic investigations of copolymerizations of ethylene with polar vinyl monomers using the diimine complexes have revealed several impediments to developing practical catalysts: (1) The polar group of the comonomer can coordinate strongly to the metal center, blocking coordination of ethylene. (2) Weak binding affinity of the polar monomer relative to ethylene can result in very low levels of comonomer incorporation. (3) A metal alkyl chain bearing a heteroatom, X, on the β-carbon atom can undergo β-X elimination leading to deactivation of the catalyst. (4) Stable chelate formation following insertion of a polar comonomer can greatly retard the rate of chain growth. (5) A metal alkyl chain bearing an electron-withdrawing heteroatom at the □-carbon atom can result in a high insertion barrier. A patent disclosure by the DuPont Versipol group and our extensive mechanistic studies reveal that, remarkably, vinyl trialkoxysilanes are ideal comonomers and circumvent all of the impediments noted above. The Pd-catalyzed copolymerization of vinyl trialkoxysilanes with ethylene produces highly branched, low molecular weight copolymers with activities comparable to those of analgous ethylene homopolymerizations. A 1,2- insertion of the vinyl silane results in the formation of a five-membered Pd−O(R)Si chelate which is readily opened by ethylene and thus does not reduce the rate of chain growth. β-Silyl elimination results in chain transfer and accounts for the lower molecular weight polymer. The nickel α-diimine-catalyzed copolymerizations produce high molecular weight copolymers with structures that vary from nearly linear to moderately branched. Both four- and five-membered chelates are catalyst resting states but are rapidly opened by ethylene, and thus turnover frequencies are only slightly reduced relative to ethylene homopolymerizations. Finally, a convenient and practical nickel-based system has been developed for the efficient synthesis of this copolymer which can be cross-linked to form PEX-b, a commercial PE plastic used for hot water plumbing pipes and power cable coatings.

1. INTRODUCTION Nearly 100 million metric tons of polyethylene (PE) are produced annually via a radical-initiated process or via coordination/insertion polymerizations using early metal catalysts. The use of oxophilic early metal catalysts for copolymerization of ethylene and polar vinyl monomers such as vinyl acrylates or vinyl esters to produce value-added functionalized PEs is precluded due to their functional group incompatibility.1 Incorporation of polar monomers into polyethylene can enhance their properties such as adhesion, the barrier to © XXXX American Chemical Society

oxygen permeability, dyeability, and cross-linking potential. Currently, radical-initiated processes are used industrially to prepare ethylene/polar vinyl monomer copolymers, but since ethylene is far less reactive toward radicals than the functionalized comonomers, these processes require exceptionally high ethylene pressures and high temperatures. Such extreme conditions result in a limited ability to control copolymer Received: May 22, 2018

A

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discovery of these catalysts, much of our work was supported by and carried out in collaboration with the DuPont Versipol group. Focusing on ethylene, eqs 1 and 2 below summarize key features of the polymerization reactions and the basic structures of the Pd(II) and Ni(II) aryl-substituted α-diimine catalysts.6 For palladium, well-defined salts containing noncoordinating counteranions (typically, B(Arf)4− where Arf = 3,5-C6H3(CF3)2) are used. For Ni(II), similar well-defined salts can be prepared and used, but activation of the readily available nickel dibromides with alkyl aluminum reagents is a convenient and preferred method for generating the active cationic alkyl complexes in situ.

properties including microstructures, molecular weights, and dispersities. Given the higher functional group tolerance of late metals and the potential to control polymer structures through ligand variation and to use less demanding polymerization conditions, there has been intense interest in developing late metal catalysts for synthesis of ethylene/polar monomer copolymers.2,3 This Account will focus on common polar vinyl monomers of general structure CH2CHG, which we designate PVMs, not on less practical olefins containing remote functional groups such as CH2CH(CH2)nG4 or substituted norbornenes.5 Two major classes of late metal catalysts have played a key role in probing their potential and limitations for copolymerizations of ethylene with various PVMs. One class is cationic aryl-substituted α-diimine Pd(II) and Ni(II) catalysts first reported by our laboratory in 1995.6 The other class is neutral Pd phosphine-sulfonate “Drenttype” catalysts7,8 together with their various neutral and cationic Ni9,10 and Pd analogues.11 These Drent-type catalysts are capable of copolymerizing ethylene with a wide selection of PVMs including methyl acrylate,8 vinyl acetate,12 vinyl ether,13 and others14,15 to produce functionalized linear polyethylenes. The activities of these systems in copolymerizations are generally substantially lower than those for ethylene homopolymerizations and low molecular weight copolymers are generally produced, suggesting improvements must be realized for practical applications. This chemistry has been reviewed and will not be covered here.7 This Account summarizes our efforts toward the development of Ni and Pd α-diimine catalysts for viable syntheses of E/PVM copolymers. These efforts have entailed in-depth mechanistic studies which have illuminated the details of copolymer chain growth and have provided insight into impediments that need to be addressed to devise practical catalysts. A major focus of this Account will be a full mechanistic description of the copolymerization of ethylene with vinyl trialkoxysilanes catalyzed by Pd/Ni α-diimine complexes. The exceptional rates and high molecular weights exhibited in the nickel-catalyzed copolymerizations provide a potentially practical synthesis of cross-linkable polyethylenes currently industrially prepared by other routes.16 Prior to discussing the copolymerization features of the Ni/Pd α-diimine catalysts, we will briefly review mechanistic aspects of ethylene homopolymerization that will inform subsequent discussions of ethylene/PVM copolymerizations.

In the case of Pd(II) catalysts, turnover frequencies (TOFs) are ca. 500/h at 22 °C.6,19 Polymers with Mn values up to 105 g/mol can be formed,6,20 and in sharp contrast to linear PEs generated from early metal catalysts, these materials are amorphous and exhibit highly branched microstructures. The corresponding Ni(II) catalysts exhibit very high TOFs of (1−3) × 106 /h at 35 °C, comparable to early metal catalysts.6,21 Polymers exhibit Mn values of ca. 105 to 106 g/mol and degrees of branching between 5 and 80 branches/1000 carbons depending on reaction conditions. Increased temperatures increase branching; increased ethylene pressures suppress branching.22 Low temperature NMR investigations coupled with kinetic studies and analysis of polymer microstructures led to the proposed mechanism illustrated in Scheme 1 for ethylene polymerization Scheme 1. Proposed Mechanism for Ethylene Polymerization Catalyzed by Ni/Pd α-Diimine Complexes

2. DEVELOPMENT OF NI/PD α-DIIMINE COMPLEXES FOR ETHYLENE POLYMERIZATION In the 1960s to 1980s, there were scattered reports of Ni(II) catalysts capable of polymerizing ethylene, but these catalysts generally exhibited short lifetimes and/or produced low molecular weight polymers.17 The primary focus of development of Ni(II) catalysts during this period was to produce ethylene oligomers. The prime examples of such Ni(II) catalysts are those developed for use in the Shell Higher Olefin Process (SHOP) for production of α-olefins.18 A breakthrough in late metal olefin polymerization catalysis came in 1995 when Lynda Johnson in our laboratory discovered that cationic α-diimine complexes of Pd(II) in which the imine nitrogens were substituted with bulky aryl groups were effective for polymerization of ethylene and α-olefins to high molecular weight polymers with unique microstructures.6 Our initial report of both Pd(II) and Ni(II) diimine catalysts, now cited ca. 2500 times, stimulated an outburst of activity in late metal-catalyzed olefin polymerization research by both academic and industrial laboratories that continues today. Following our

by Ni(II) and Pd(II) α-diimine complexes.6,23−26 While backbone structures and ortho-aryl substituents play minor roles in determining rates and polymer microstructures, the following key mechanistic features broadly characterize these polymerizations: (1) The catalyst resting states are the alkyl ethylene B

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occurs by an associative process, and exchange rates are greatly retarded by bulky ortho aryl substituents.24

complexes (1, 5, 7). The turnover-limiting step is the migratory insertion of the alkyl ethylene complex, and thus the chain propagation is zero-order in ethylene.27 (2) Low-temperature NMR studies have shown that the barriers28 to ethylene insertion (propagation) for Pd(II) alkyl ethylene complexes are ca. 17−18 kcal/mol,6 while the activation barriers for Ni(II) alkyl ethylene complexes are 13−14 kcal/mol,23 ca. 4−5 kcal/mol lower than the Pd(II) catalysts, accounting for the very significantly higher turnover frequencies in the nickel systems. (3) The branching densities and architectures of the polyethylenes formed are greatly affected by the behavior of the agostic alkyl intermediates (2, 4, 6).29−31 These species undergo “chain-walking” via β-hydride elimination, olefin rotation, and reinsertion, one cycle of which moves the metal one carbon down the chain. NMR studies show the barrier to a 1,2 shift in Ni agostic intermediates is ca. 14 kcal/mol, and once trapped by ethylene, these ethylene alkyl complexes undergo insertion (there is no equilibration between the agostic and the ethylene alkyl species).26 As ethylene concentrations increase, the trapping rate relative to chain-walking rate increases and branching densities decrease. Since chain-walking is a first-order process and trapping is a second-order process (with an assumed negative ΔS‡), increasing polymerization temperature increases the rate of chain-walking relative to trapping, and thus branching increases. As expected for this scenario, methyl branches predominate, and the fraction of longer branches is generally inversely proportional to branch length. The situation with Pd is somewhat more complicated. Relative to Ni, the overall barrier to 1,2-chain-walking in the agostic complexes is lower (9−10 kcal/mol), and the insertion barrier of the alkyl ethylene complexes is higher (17−18 kcal/mol). NMR studies establish that the Pd ethylene alkyl species are in rapid equilibrium with the corresponding agostic complexes.25 These features result in a very large number of 1,2 shifts occurring prior to insertion. NMR studies show that chain-walking through branched carbons is facile; thus these polymers exhibit branch-onbranch or hyper-branched microstructures. While total branches/ 1000 carbons is relatively insensitive to ethylene concentration, increasing ethylene pressure results in a reduced concentration of agostic intermediates, resulting in fewer 1,2 shifts prior to insertion and relatively fewer branch-on-branch structures. These hyper-branched polymers were shown to have a more linear architecture at high ethylene pressures and a less linear more globular architecture at low ethylene pressure.32 Slow rates of chain transfer relative to propagation distinguish these α-diimine catalysts from earlier Ni(II) catalysts which produced oligomers or low Mn polymers. The key to retarding chain transfer and generating high-molecular-weight polymers in α-diimine systems is the introduction of steric bulk in the axial sites of the square coordination plane.6,24 Crystallographic studies demonstrate that the aryl rings lie perpendicular to the square plane, thus positioning the ortho-aryl substituents over axial sites.24,29 Since ligand substitution in d8 square planar complexes occurs associatively, we proposed that chain transfer occurred by associative displacement of the unsaturated polymer chain from an olefin hydride species by ethylene and is slowed by axial bulk (eq 3).24 Alternatively, later calculations33 suggested that chain transfer occurs by concerted β-hydride transfer from the alkyl group of the alkyl olefin complex to bound ethylene. Subsequent associative displacement of the unsaturated polymer chain by ethylene then occurs from an alkyl olefin intermediate rather than from an olefin hydride species (eq 4). Low-temperature NMR kinetic studies demonstrated that exchange of free ethylene and bound ethylene in Pd(II) alkyl ethylene complexes

Recently, we developed “sandwich” Ni(II) and Pd(II) α-diimine complexes of type II incorporating the 8-tolylnaphthylimino group (Figure 1).20,34 These capping aryl groups are

Figure 1. “Sandwich” Ni/Pd α-diimine complexes.

exceptionally effective in shielding axial sites: a comparison of associative ethylene exchange rates in Pd sandwich complex 9 versus the “traditional” complex 8 shows rate differences of more than 2 × 105.20 The further reduction of chain transfer rates in these sandwich systems is illustrated by production of branched ultrahigh molecular weight polyethylene (UHMWPE, Mn’s up to 1.8 × 106 g/mol) from a Ni(II) sandwich complex.34 Incorporation of bulky ligands to retard chain transfer has been the key design feature of all Pd(II) and Ni(II) polymerization catalysts reported since our initial discovery of the aryl α-diiminebased catalysts.2,6

3. ISSUES ASSOCIATED WITH POLAR VINYL MONOMER/ETHYLENE COPOLYMERIZATIONS As noted earlier, this account is focused on mechanistic features of E/PVM copolymerizations using Pd/Ni α-diimine catalysts that illustrate several issues which must be confronted when designing a late metal catalytic system that will exhibit high rates, long catalyst lifetimes, acceptable levels of comonomer incorporation, and high polymer molecular weights. Scheme 2 summarizes various problems that have been encountered in attempting to develop E/PVM copolymerizations using Ni/Pd α-diimine catalysts. Binding through the Polar Group

Strong binding of a polar monomer to the metal center through the functional group will preclude formation of significant concentrations of the olefin-bound complex required for propagation and reduce or even shut down copolymerization activity (Scheme 2i). A prime example is predominant κ-nitrile coordination of the acrylonitrile monomer to a cationic α-diimine Pd complex.2,35 Not only is the nitrile functionality a good ligand for Pd(II), but this electron-withdrawing group deactivates the C

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Scheme 3. Insertion of Vinyl Halides into a α-Diimine Pd(II) Alkyl Complex

Scheme 2. Impediments Associated with Ethylene/PVM Copolymerizations

similar problems of β-OR eliminations in the case of vinyl ethers.38 Strong Chelate Formation

Strong chelate formation following polar monomer insertion can retard the rate of polymerization due to an unfavorable equilibrium between the chelate and the ethylene complex necessary for chain propagation (Scheme 2iv). For example, the rate of methyl acrylate/ethylene copolymerization by (α-diimine)Pd(II) catalysts is substantially retarded relative to the rate of homopolymerization of ethylene.37 Mechanistic studies revealed that 2,1-insertion of methyl acrylate into (α-diimine)Pd-CH3+ generates a four-membered chelate which undergoes further chain-walking to form a stable six-membered chelate, the catalyst resting state (Scheme 4). This chelate is strongly favored Scheme 4. Mechanism of α-Diimine Pd(II)-catalyzed Copolymerization of Ethylene with MA

catalytic Pd(II) system for required π-coordination. In comparison to Pd(II), κ-polar group coordination vs π-coordination is more favored in more electrophilic Ni(II) centers. For example, coordination of vinyl acetate to cationic α-diimine Pd complexes results in formation of π-olefin complexes, while the coordination of vinyl acetate to the Ni analogues prefers the formation of a κ-O bound complex.36 Weak Competitive Binding

over the chelate-opened ethylene complex and thus retards the rate of copolymerization. This mechanism results in no “in-chain” ester groups. The analogous Ni(II) system requires much higher temperatures and ethylene pressures (100−120 °C, >500 psi C2H4) to achieve MA/E copolymerization.40 Rates under these conditions (TOF, ca. 103 h−1, 100 °C) are much lower than those of homopolymerization (TOF, ca. 106 h−1, 35 °C), which is traced to formation of a stable four-membered Ni(II) chelate complex formed by 2,1-insertion of MA. In the more electrophilic Ni(II) complex, isomerization to the six-membered chelate is slow, so propagation occurs predominantly via the four-membered chelate, resulting in “in-chain” acrylate incorporation.

Weak competitive binding of PVMs versus ethylene can prevent viable incorporation of polar monomer into the copolymer (Scheme 2ii). As cationic α-diimine complexes are electrophilic, electron-deficient olefins, particularly ones bearing bulky substituents, are disfavored in competition with ethylene. For example, electron-deficient methyl acrylate (MA) binds much more weakly to the α-diimine Pd center relative to ethylene (Keq, ca. 3 × 103, 35 °C).37 Although this effect is compensated to a degree by a higher rate of insertion, large excesses of methyl acrylate are generally required to produce copolymers with levels of acrylate incorporation of 1.0−12 mol %. In contrast to methyl acrylate, attempts to copolymerize bulkier methyl methacrylate with ethylene using an α-diimine Pd catalyst resulted in formation of only homopolyethylene due to the highly unfavorable binding affinity of methyl methacrylate.

High Subsequent Insertion Barriers

Alkyl ethylene complexes with the polar substituents on the α-carbon atom have a much higher insertion barrier for the subsequent insertion relative to unsubstituted alkyl complexes (Scheme 2v). For example, mechanistic studies of vinyl acetate polymerization revealed that insertion into the (α-diimine)PdCH3+ bond resulted in formation of a stable chelate complex, which is strongly favored over the chelate “opened” ethylene complex (Scheme 5). In addition, the “opened” alkyl ethylene complex, bearing an α-acetoxy group, has a much higher barrier to insertion relative to ethylene homopolymerization, accounting for nearly complete deactivation of the α-diimine Pd catalyst. DFT calculations support high insertion barriers in such complexes.41 Problems itemized above severely limit the use of cationic α-diimine Pd(II) and Ni(II) catalysts for copolymerization of

β-Heteroatom Elimination

Rapid β-X (X = halide, OR) elimination following CH2CHX monomer insertion can lead to decomposition of the catalyst (Scheme 2iii). Sen et al.38 and Jordan et al.39 revealed that vinyl halides undergo 1,2-insertion into (α-diimine)Pd-CH3+ complexes followed by β-halide elimination to generate inactive Pd halide complexes which undergo conversion to dormant halobridged dimers (Scheme 3). Vinyl acetate exhibits primarily 1,2 insertion into (α-diimine)Ni-Me+, resulting in a chelate complex bearing an acetoxy group on the β-carbon. This complex undergoes β-acetate elimination at −34 °C, resulting in catalyst decay.36 Jordan has encountered D

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Accounts of Chemical Research Scheme 5. Inhibition of Chain Propagation in Vinyl Acetate/ Ethylene Copolymerizations using α-Diimine Pd(II) Catalysts

Table 1. Copolymerization Ethylene and VTEoS Using Pd(II) Complex 10a entry [VTEoS] TOF, h−1 1 2 3 4

0.52 M 1.04 M 1.56 M 0

362 227 187 456

incorp. (mol %) 0.26 0.45 0.71 0

Br/ Mn 1000C (×10−3 g/mol) Mw/Mn 106 108 106 109

7.0 5.4 3.1 12.1

2.0 2.1 2.3 2.4

Conditions: V(total) = 10 mL, 10 μmol of Pd catalyst, 22 h, 100 psig ethylene, dichloromethane, 22 °C. a

many common PVMs and ethylene. These same problems, some of which have been clearly demonstrated, likely come into play with other Ni and Pd catalysts. Several groups have shown that less electrophilic neutral Pd phosphine-sulfonate Drenttype catalysts are capable of copolymerizing ethylene with a broad selection of PVMs including methyl acrylate, vinyl acetate, acrylonitrile, vinyl ether, and other PVMs to yield linear copolymers.7 However, these Drent-type catalysts generally produce low molecular weight copolymers with low or moderate activities. More recently, new supporting ligands have been developed.10,11,42−44 Cationic bisphosphine monoxide Pd11 and Ni10 systems and neutral Drent-type nickel analogues9 have led to much enhanced activities for ethylene homopolymerization, but only moderate improvement in PVM/E copolymerizations. Nickel complexes based on modified P,O ligands have exhibited significant improvements in ethylene/MA copolymerization.42,44 The following section outlines our studies aimed at understanding and developing α-diimine Pd(II)- and Ni(II)-catalyzed copolymerizations of ethylene with vinyl trialkoxysilanes, PVMs that serve as ideal comonomers in these catalytic systems.

to afford a highly branched, amorphous copolymer with silane incorporation of 0.25−2.0 mol % depending on silane concentrations and ethylene pressure. The Mn values of oligomers obtained are proportional to ethylene pressure and inversely proportional to silane concentrations. The mole fraction silane observed indicates one silane per chain is incorporated, and NMR analysis reveals the −Si(OEt)3 group is located at a terminal site. In contrast to copolymerization of ethylene and methyl acrylate, the rate of copolymerization of ethylene and VTEoS is not significantly reduced by incorporation of VTEoS relative to ethylene homopolymerization.

Low temperature NMR studies of the copolymerization have led to the proposed chain growth mechanism in Scheme 6.

4. COPOLYMERIZATION OF ETHYLENE AND VINYL TRIETHOXYSILANE Polyethylene (PE) that is lightly functionalized (ca. 0.1−2.0 mol %) with trialkoxysilane groups (−Si(OR)3) is an important commodity PE copolymer.16 It can be cross-linked to form PEX-b, a tough material that is widely used for power cable insulation and hot water piping systems.45 This cross-linkable polymer is produced commercially either by radical-initiated copolymerization of ethylene and vinyl trialkoxysilanes or by radical-induced grafting of vinyl trialkoxysilanes onto preformed polyethylene. The radical-based copolymerizations require exceptionally high pressures of ethylene and high temperatures. Metal-catalyzed copolymerization of ethylene and vinyl trialkoxysilanes potentially offers a method for the synthesis of this copolymer under less extreme conditions and, through catalyst tuning, a method to tailor polymer microstructures and molecular weights. The DuPont Versipol group has disclosed in patent literature that α-diimine-Pd(II) and -Ni(II) complexes are capable of copolymerization of ethylene with vinyl trialkoxysilanes with activities close to those of ethylene homopolymerizations.46 In the case of Pd(II) catalysis, low molecular weight polymers/ oligomers were observed, while Ni(II) systems yielded high molecular weight copolymers. We have built on these observations to develop an in-depth mechanistic understanding of these copolymerizations through analysis of the kinetics of bulk copolymerizations, low temperature NMR spectroscopic characterization of intermediates, and analysis of polymer microstructures and molecular weights.19,47 Moreover, a highly productive Ni-based catalyst system using a convenient initiating procedure has been developed for the expedient synthesis of these copolymers.47

Scheme 6. Proposed Mechanism of α-Diimine Pd(II)catalyzed Copolymerization of Ethylene with VTEoS

Consider the cycle beginning with the five-membered chelate, 11. In the absence of ethylene, this chelate is in equilibrium with silyl complex 12 through β-silyl elimination. The chelate reacts rapidly and essentially completely with ethylene to generate the opened chelate 13. This demonstrates, in contrast to the chelate formed from methyl acrylate, that chelate formation does not inhibit the rate of chain growth in this copolymerization. Opened chelate 13 undergoes a series of ethylene insertions, 13 to 13′. Species 13 and 13′ are the resting states in the cycle, and thus the rate of copolymerization is essentially the same as that of homopolymerization.48 Reaction of 13′ with VTEoS generates

Copolymerizations Catalyzed by α-Diimine Pd(II) Complexes.

As shown in eq 5 and Table 1 below, the α-diimine Pd complex, 10, copolymerizes ethylene with vinyl triethoxysilane (VTEoS) E

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Accounts of Chemical Research Table 2. Copolymerizations Using (α-Diimine)Ni(Me)(MeCN)+ Catalysts 17a,b/B(C6F5)3a entry

C2H4 (psig)

[VTEoS]

TOF (× 10−4 h−1)

branches/1000 C

Mn (kg/mol)

Mw/Mn

incorp. (mol %)

Tm (°C)

1 2b 3 4 5 6 7c 8d

200 200 200 200 400 600 200 600

0.5 M 0.5 M 1.0 M 1.5 M 1.0 M 1.0 M 0.5 M 0.1 M

5.3 4.5 2.7 1.4 4.8 6.8 4.1 45

50 51 51 49 40 32 16 10

104 106 85 61 112 128 14 29

1.6 1.9 1.7 1.7 1.5 1.4 2.2 2.1

0.69 0.68 1.30 2.20 0.72 0.46 3.0 0.23

64 66 57 57 78 89 95 115

V(total) = 50 mL, 1.5 μmol of 17a, 10 equiv of B(C6F5)3, 30 min, toluene solvent. b60 min. c6.0 μmol of 17b. d3.0 μmol of 17b, 20 equiv of B(C6F5)3, V(total) = 200 mL. a

an equilibrium concentration of π-complex 14. (In separate studies, the relative binding affinities of ethylene and the vinylsilane favors ethylene by ca. 30:1 at 22 °C.) Complex 14 undergoes 1,2 migratory insertion to yield the five-membered chelate 15. The insertion barrier of 14, ca. 18 kcal/mol, is similar to the insertion barrier of the analogous ethylene complex. Chelate 15 undergoes β-silyl elimination to form 16. Associative displacement of the silyl-end-capped oligomer from 16 by ethylene results in closure of the cycle and regeneration of 12 ⇌ 11. Branching via chain-walking occurs during chain growth to produce branched oligomers and, for clarity, branching is not shown in Scheme 6. As noted above, rates of associative displacement reactions are dramatically slowed in sandwich complexes such as complex 9 (Figure 1) relative to traditional diimine systems. This property can be used to retard the rate of chain transfer via β-silyl elimination/chain displacement. The sandwich Pd system, 9, catalyzes the copolymerization of VTEoS with ethylene to yield much higher molecular weight copolymers with narrow molecular weight distributions and many silyl groups per chain.19

copolymers are produced in the nickel systems, and each polymer chain contains multiple silane groups. Third, silane groups are not only located at the “chain-end” (on primary carbons) but also at “in-chain” sites (on secondary carbons), and the ratio of in-chain/chain-end silane incorporation increases with the increase of ethylene pressure. The chain growth mechanism shown in Scheme 7 for the Ni-catalyzed copolymerizations has been deduced from analysis Scheme 7. Proposed Mechanism of α-Diimine Ni-catalyzed Copolymerization of Ethylene and VTEoS

Copolymerizations Catalyzed by α-Diimine Ni(II) Complexes

Due to the high cost of Pd and the relatively low activities of α-diimine Pd catalysts, Pd-catalyzed copolymerizations hold little commercial interest. Well-defined (α-diimine)Ni(Me)(MeCN)+ analogues, activated with 10 equiv B(C6F5)3, exhibit very high activities for ethylene homopolymerization with TOFs only slightly less than those of typical (α-diimine)NiBr2/MAO systems. These (α-diimine)Ni systems 17a,b/B(C6F5)3 catalyze copolymerization of VTEoS with ethylene at 60−80 °C to afford, depending on catalyst structure and conditions, nearly linear to moderately branched high molecular weight copolymer with high turnover frequencies (up to 4.5 × 105 h−1) and good lifetimes. Incorporation of the comonomer ranges from 0.23 to 10.0 mol %. Equation 6 summarizes the general copolymerization, and Table 2 shows some specific results.

of TOFs and polymer microstructure analyses and by low temperature NMR experiments. VTEoS inserts into Ni-R bonds in both a 2,1- and 1,2-fashion to generate four- and five-membered chelates, 21a,b. NMR analysis shows these chelates react rapidly with ethylene to yield “opened” alkyl ethylene nickel complexes 22a,b, although the opened chelates are energetically slightly disfavored, which modestly reduces copolymerization TOFs relative to homopolymerization. The open chelate 22b bearing a β-trialkoxysilyl group exhibits a similar insertion barrier to that of a normal alkyl group. Although the insertion barrier for open chelate 22a cannot be determined, complex 22a can isomerize to more reactive complexes 22b,c (vide infra). In addition, VTEoS can compete with ethylene through κ-1 coordination of a silyl ether group to form unreactive ether complex 19, which is a

The behavior of the nickel-catalyzed copolymerizations is remarkably different from the Pd systems in almost every aspect. First, the TOFs of copolymerizations in the nickel systems are first-order in ethylene pressure and inverse-order in silane concentration, and the overall rates are reduced to a modest extent relative to ethylene homopolymerizations. Second, high Mn F

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Accounts of Chemical Research second factor in reducing the rate of chain growth relative to homopolymerization. In contrast to the Pd system in which the alkyl ethylene complex is the resting state, the major resting states for the Ni system are these stable chelates, resulting in lower activities relative to that of ethylene homopolymerization. However, the Ni systems, due to lower ethylene insertion barriers, exhibit much higher activities than the Pd analogues in VTEoS/ethylene copolymerizations. Moreover, the higher thermal stabilities of the chelates allow for compensation of the somewhat reduced turnover frequencies through enhanced catalyst lifetimes relative to homopolymerizations. The copolymers produced by the Ni systems contain both in-chain and chain-end −Si(OEt)3 groups. Chain growth via chelates 21a,b yields “in-chain” silyl group incorporation. As in the Pd system, chelate 21b can undergo β-silyl elimination to generate olefin silyl complex 23, which can collapse via 2,1-insertion to yield chelate 21c in which the silyl group is bound to a primary carbon. Chelate 21c can also be generated via “chain-walking” from 21a. Chain growth from 21c leads to the formation of the copolymer with chain-end −Si(OEt)3 incorporation. This pathway is likely the dominant pathway for chain-end −Si(OEt)3 incorporation. A second pathway for formation of primary −Si(OEt)3 groups involves displacement of the unsaturated chain from silyl complex 23 by ethylene to form ethylene silyl complex 24, which, following silyl migration, yields 25d containing a primary −Si(OEt)3 attachment. This process also results in chain transfer. In contrast to the Pd system in which the chain transfer process is dominant, resulting in a low Mn copolymer, insertion dominates over displacement chemistry in 23, and thus high Mn polymers are generated in the Ni systems. Finally, we have developed an exceptionally convenient Ni-catalyzed copolymerization process using easily synthesized and air-stable α-diimine nickel dibromides in combination with commercial activators, AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4].49 Polymer microstructures (and thus Tm) and comonomer incorporation can be controlled by choice of reaction conditions and catalyst structures. Excellent activities and productivities using this catalyst system can be obtained, and cross-linking during isolation of copolymers does not occur, which is common using other activation procedures. For example, copolymers containing 0.23 mol % triethoxysilyl groups can be produced at 60 °C and 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This copolymer can be cross-linked to form PEX-b via reaction with water, which generates stable siloxane linkages and polymer with a high gel content (gel% = 76%) required for commercial PEX-b (gel% > 65%).

gram of Ni, and the cross-linkable copolymer produced meets the requirements of precursors to commercial PEX-b materials. These studies provide the impetus for expanding late metal catalyst designs to achieve improved copolymerizations of vinyl alkoxysilanes as well as more challenging PVMs such as alkyl acrylates, vinyl esters, and vinyl halides. For example, one can envision monodentate systems in which chelate formation will still leave a vacant coordination site and allow ready ethylene coordination and insertion into metal carbon bonds of chelates. Progress designing catalysts to avoid β-X eliminations using vinyl-X monomers has also been reported.7

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhou Chen: 0000-0003-4345-7070 Notes

The authors declare the following competing financial interest(s): The authors have submitted a provisional patent on portions of this work. Biographies Zhou Chen completed his Ph.D. at SIOC (Yong Tang), CAS, China. He is currently a postdoctoral fellow working with Maurice Brookhart. Maurice Brookhart is currently professor of chemistry at the University of Houston and professor emeritus at the University of North Carolina, Chapel Hill. His research focuses on synthetic and mechanistic organometallic chemistry.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation, DuPont, and the Welch Foundation (Grant E-1893 to M.B.). REFERENCES

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CONCLUSION AND PROSPECTS This Account has illustrated, through mechanistic studies, several impediments to the use of late metal α-diimine Pd and Ni catalysts for efficient copolymerizations of ethylene with many common polar vinyl monomers. Our work, in combination with an earlier DuPont report, shows that vinyl trialkoxysilanes are ideal comonomers. Mechanistic studies reveal full details of polymer chain growth and illustrate why these silanes work so well and circumvent all of the possible pitfalls suffered by other monomers discussed here. A convenient method has been developed for generating active nickel catalysts from easily prepared, air-stable dibromide complexes and commercial activators. These systems display productivities as high as 560 kg of copolymer per G

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Accounts of Chemical Research group on a β-carbon. This species can β-silyl eliminate and, following reaction with ethylene, yield α-olefins and complex 12. Only low carbon number α-olefins are produced, as once ca. 3 or 4 ethylene insertions occur, chain walking back to β-silyl complexes does not compete with further ethylene insertions and chain growth. (49) Compared with the DuPont system, this catalyst system produces similar copolymers, but with a slightly higher (ca. 2-fold) activity.

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DOI: 10.1021/acs.accounts.8b00225 Acc. Chem. Res. XXXX, XXX, XXX−XXX