Kinetics controlled microstructure and composition in coordinative chain transfer copolymerization of ethylene–propylene

Abstract

The wide applications of ethylene–propylene copolymers, which range from elastomers to oligomeric lubricants and additives can be expanded upon controlling the microstructure specifically its chemical composition. To account for that we use rac-Et(Ind)2ZrCl2 metallocene catalyst in combination with modified methylaluminoxane (MMAO) as the cocatalyst and ZnEt2 as the chain transfer agent (CTA) in a coordinative chain transfer copolymerization. The monotonic decline of molar mass and narrowing of its distribution in the presence of CTA suggest the establishment of the regenerative chain transfer reaction. The end-group analysis by 1H NMR spectroscopy demonstrates the depletion of β-H transfer and replacement of chain transfer to CTA. Microstructure analysis of copolymers by 13C NMR reveals a significant increase in ethylene content, which raises smoothly by CTA content, along with modification of the microstructure at the triad level, suggesting the prevalence of ethylenic sequences. This is in line with the decrease of the glass transition temperature and appearance of a broad melting peak between 30 and 90 °C. We explain these experimental observations by a computational study at the DFT level that compares three reaction pathways of ethylene and propylene insertion into an ethylene-ended propagating chain or chain transfer to the CTA. The outcome shows that propylene insertion is kinetically less feasible compared to ethylene insertion or chain transfer, while thermodynamically the chain transfer reaction is less favored. These results establish a new kinetics control over the chain microstructure in ethylene–propylene copolymers.