Nickel-Catalyzed Copolymerization of Ethylene and Vinyltrialkoxysilanes: Catalytic Production of Cross-Linkable Polyethylene and Elucidation of the Chain-Growth Mechanism.

Abstract

Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CH3CN)+ complexes 4a,b/B(C6F5)3 yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of -Si(OR)3 groups whose ratios depend on temperature and ethylene pressure. Detailed low-temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt2)+ reacts rapidly at -60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(C2H4)+ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for formation of chain-end -Si(OR)3 groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the Ni center to the ether moiety, R-O-Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene homopolymerization, but still remarkably high TOFs of up to 4.5 × 105 h-1 and overall productivities can be achieved. Activation of readily available (α-diimine)NiBr2 complexes 2 with a combination of AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4] yields a highly active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example, copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are precursors to the commercial cross-linked polyethylene, PEX-b.