Mechanism and Effect of Polar Styrenes on Scandium-Catalyzed Copolymerization with Ethylene

Copolymerization of ethylene (E) and polar vinyl monomers remains a problem because E propagation is hindered. Herein, for the first time, we report the copolymerization of E and polar styrenes (S^R ) by using an oxophilic scandium catalyst that exhibits higher turnover frequencies than both E and S^R homopolymerizations when R is an electron-withdrawing group. This positive comonomer effect was elucidated through computing reaction profiles of E/S^F copolymerization at the DFT (B3PW91) level of theory. It reveals that the secondary interaction between Sc³⁺ and phenyl of the last and penultimate inserted S^F units leads to a decrease of the E insertion barrier, because the electron-withdrawing substituent enhances the electrophilicity of Sc³⁺ by an inductive effect mediated by the secondary interaction. After three consecutive insertions of the E units, the secondary interaction is lost and the S^F insertion is kinetically preferred over the E insertion. This process is in line with the NMR spectrum analyses which show that the resultant copolymers mainly contain S^R (E)_x S^R sequences where x≤3.