Nickel-Catalyzed Ethylene Copolymerization with Vinylalkoxysilanes: A Computational Study

Area of Science:

• Organometallic Chemistry
• Polymer Science
• Catalysis

Background:

• α-diimine palladium catalysts excel in ethylene polymerization and copolymerization with polar monomers.
• α-diimine nickel catalysts generally show poor performance in ethylene/polar monomer copolymerization.
• Recent advances show nickel catalysts can effectively copolymerize ethylene with vinylalkoxysilanes, producing functionalized polyethylene.

Purpose of the Study:

• To investigate the mechanism of ethylene copolymerization with vinyltrimethoxysilane (VTMoS) using Brookhart-type nickel catalysts.
• To provide a molecular-level understanding of this unique and cost-effective catalytic system.
• To compare the performance of nickel catalysts with expensive palladium counterparts.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to optimize nickel complexes and active species.
• Mechanistic exploration of chain initiation, propagation, and termination for both homopolymerization and copolymerization.
• Energetic analysis of in-chain and chain-end silane enchainment, including polar substrate distribution.

Main Results:

• Chain initiation is the dominant step in ethylene homopolymerization catalyzed by α-diimine Ni complexes.
• Similar energy barriers for 1,2- and 2,1-insertion of VTMoS explain experimental observations of five- and four-membered chelates.
• Ethylene reinsertion after VTMoS insertion is the rate-determining step, influenced by steric hindrance.

Conclusions:

• Brookhart-type nickel catalysts offer a viable, cost-effective alternative for ethylene/VTMoS copolymerization.
• The dominant pathway for chain-end trialkoxysilane incorporation involves chain-walking, ring-opening, and ethylene insertion.
• Steric repulsion between the silane group and incoming ethylene favors chain-end over in-chain incorporation.