The Dual-Site Alternating Cyclocopolymerization of 1,3-Butadiene with Ethylene

The copolymerization of ethylene with 1,3-butadiene was studied with the series of ansa-metallocenes Me₂Si(Cp)(9-Flu)ZrCl₂ (1), Me₂Si(1-Ind)(9-Flu)ZrCl₂ (2), and Me₂Si(9-Flu)₂ZrCl₂ (3) with methylaluminoxane (MAO) as cocatalyst. The catalyst 2/MAO yields a cyclocopolymer composed of two ethylene monomer units for every one butadiene in a novel periodic architecture of 1,2-enchained cyclopentane units separated by three methylenes. The high butadiene content in the copolymer and the high selectivity for alternating cyclocopolymerization to form methylene-1,2-cyclopentane units implicate a dual-site mechanism where butadiene and ethylene are enchained at different coordination sites.