Norbornene Monomer Effects and Mechanistic Insights in Binuclear Nickel-Catalyzed Olefin Chain Walking Copolymerizations

Chain walking is a unique process in late transition metal-catalyzed ethylene (α-olefin) polymerizations and can be modulated to achieve control over polyolefin branch type, density, topology, branch stereochemistry, and so on, most of which are unattainable without chain walking. Introducing a second monomer could perturb the chain walking process and afford copolymers with unique microstructures. Here, we report the effects of norbornene (NBE) monomer, which is ring strained and cannot undergo chain walking, in binuclear Ni complex-catalyzed ethylene or α-olefin chain walking copolymerization, and have observed significantly reduced molecular weight, promoted chain transfer, suppressed branching, and the appearance of characteristic olefinic end groups versus their respective homopolymerizations. In contrast, the Mw of the NBE copolymers was reduced by 8.6, 9.4, and 16.4 times for ethylene, propylene, and 1-hexene, respectively. The chain transfer was confirmed by significantly increased chains/catalyst numbers in the copolymerizations, by 15.4, 85.1, and 27.1 times versus homopolymerizations, respectively. While various alkyl branches were generated in ethylene and α-olefin homopolymerization, their NBE copolymers showed much reduced branch density and predominant long chain branches. Olefinic end groups also appeared, predominantly norbornyl-linked vinyl (VN) for ethylene copolymer, alkyl-linked vinyl (VE), and vinylene for both propylene and 1-hexene copolymers, suggesting β-H elimination/transfer after ethylene enchainment or propylene/1-hexene 2,1-enchainment. The copolymers' glass transitions could be broadly tuned from 109 to 326 °C with linear relationships against the NBE content and the slopes depending on the monomer type and reaction temperature, indicating a random and uniform distribution of the enchained comonomer. Furthermore, the mechanistic scenarios for the distinctive NBE monomer effects on chain walking, chain growth, and chain transfer were proposed based on DFT studies, confirming the experimental findings.