Ethylene–Butadiene Copolymerization by Neodymocene Complexes: A Ligand Structure/Activity/Polymer Microstructure Relationship Based on DFT Calculations

Ethylene/butadiene copolymerization can be performed by neodymocene catalysts in the presence of an alkylating/chain transfer agent. A variety of polymerization activities and copolymer microstructures can be obtained depending on the neodymocene ligands. For a set of four catalysts, namely (C₅Me₅)₂NdR, [Me₂Si­(3-Me₃SiC₅H₃)₂]­NdR, [Me₂Si­(C₅H₄)­(C₁₃H₈)]­NdR and [Me₂Si­(C₁₃H₈)₂]­NdR, we report a DFT mechanistic study of this copolymerization reaction performed in the presence of dialkylmagnesium. Based on the modeling strategy developed for the ethylene homopolymerization catalyzed by (C₅Me₅)₂NdR in the presence of MgR₂, our model is able to account for the following: (i) the formation of Nd/Mg heterobimetallic complexes as intermediates, (ii) the overall differential activity of the catalysts, (iii) the copolymerization reactivity indexes, and (iv) the specific microstructure of the resulting copolymers, including branching and cyclization. The analysis of the reaction mechanisms and the energy profiles thus relates ligand structure, catalyst activity, and polymer microstructure and sets the basis for further catalyst developments

Deciphering the Mechanism of Coordinative Chain Transfer Polymerization of Ethylene Using Neodymocene Catalysts and Dialkylmagnesium

Ethylene polymerizations were performed in toluene using the neodymocene complex (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ or {(Me₂Si­(C₁₃H₈)₂)­Nd­(μ-BH₄)­[(μ-BH₄)­Li­(THF)]}₂ in combination with <i>n</i>-butyl-<i>n</i>-octylmagnesium used as both alkylating and chain transfer agent. The kinetics were followed for various [Mg]/[Nd] ratios, at different polymerization temperatures, with or without ether as a cosolvent. These systems allowed us to (i) efficiently obtain narrowly distributed and targeted molar masses, (ii) characterize three phases during the course of polymerization, (iii) estimate the propagation activation energy (17 kcal mol^–1), (iv) identify the parameters that control chain transfer, and (v) demonstrate enhanced polymerization rates and molar mass distribution control in the presence of ether as cosolvent. This experimental set of data is supported by a computational investigation at the DFT level that rationalizes the chain transfer mechanism and the specific microsolvation effects in the presence of cosolvents at the molecular scale. This joint experimental/computational investigation offers the basis for further catalyst developments in the field of coordinative chain transfer polymerization (CCTP)