Alternating copolymerization of ethylene and propylene: Evidence for selective chain transfer to ethylene

C-1-symmetric ansa-metallocenes Me2E(Ind)(Flu)ZrCl2 (E = Si (1) and C (2)), upon activation with methylaluminoxane (MAO), produce atactic alternating ethylene-propylene copolymers (up to 92% EP dyads) with low molecular weight (less than 20K). Quantitative C-13 NMR analysis of copolymer chain ends produced with 2/MAO indicates that the chain initiation/termination process is highly selective to generate a vinylidene as the unsaturated end group (terminating end) and an ethyl group as the saturated end group (initiating end). The high selectivity for sec-butyl end groups reveals a highly selective chain-initiation process where ethylene insertion is followed by propylene insertion at the initiating end. Chain transfer from a propylene-terminated polymer chain to a coordinated ethylene monomer is proposed to explain this selectivity. The facile cross-monomer chain-transfer process explains the much lower molecular weights obtained for the EP copolymers relative to the ethylene or propylene homopolymers.