Binuclear Ni catalyzed ethylene copolymerization with short chain alkenol monomers†

Abstract

Ethylene coordination copolymerization with vinyl polar monomers, particularly short chain alkenols, offers an attractive method for controlled synthesis of important hydroxy-functionalized polyethylenes under mild conditions. However, reports on short-chain alkenol copolymerization are limited due to issues like chelating coordination and β-O elimination. Here, we report the synthesis and characterization of binuclear Ni complexes for ethylene copolymerization with various alkenol monomers such as allyl-OH, 3-buten-1-ol, 4-penten-1-ol and 9-decen-1-ol. These complexes, upon activation with Et₂AlCl, achieved notable activity (as high as 592 kg (mol cat h atm)⁻¹) in ethylene/3-buten-1-ol copolymerization, producing copolymers with 1.7 mol% comonomer incorporation and a high molecular weight (M_n = 64.2 kg mol⁻¹). The activity and comonomer content were influenced by Et₂AlCl loading, reaction temperature, and alkenol monomer length, with longer alkenols such as 9-decen-1-ol yielding higher activity, comonomer incorporation and molecular weight. Activities up to 169 kg (mol cat h atm)⁻¹ were also achieved in ethylene/allyl-OH copolymerization with reduced molecular weight (M_n = 17.2 kg mol⁻¹). Microstructural analysis revealed predominant in-chain and chain-end polar monomer incorporation in all cases. Notably, ethylene/allyl-OH copolymers exhibited unique olefinic end groups and microstructures assignable to Friedel–Crafts reactions, which is likely due to an alternative chain termination pathway associated with the short chain length between the O atom and the active Ni center. For comparison, ethylene/allyl-OAc copolymers showed exclusively olefinic groups, indicating a β-OAc elimination mechanism. This process resulted in lower activity and molecular weight, suggesting catalyst poisoning from rapid chain termination.