Microstructure of Ethylene−1-Butene Copolymers Produced by Zirconocene/Methylaluminoxane Catalysis

Low molecular weight (ca. 2000) ethylene−1-butene copolymers were synthesized at 90 °C using rac-(dimethylsilyl)bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride (I) catalyst and methylaluminoxane (MAO) cocatalyst. Comonomer composition, triad sequence distribution, and end groups were analyzed by 1H and 13C NMR. The average polymer molecule contains about one double bond and one saturated end group. The unsaturated end groups were formed almost exclusively by transfer from propagating chains containing 1-butene as the terminal unit. At least 98% of the unsaturated end groups are vinylidene and trisubstituted double bonds, which are present in the approximate ratio 3:1. Saturated end groups result from the initiation process. Both ethylene and 1-butene are involved in initiation, but ethylene more than 1-butene. The product is a random copolymer with very short blocks (no longer than 2−3 monomer units) of both ethylene and 1-butene. The comonomer sequence distributions determined by 13C NMR were fitted to first-order Markovian statistics, which allowed calculation of the monomer reactivity ratios. The results are discussed in terms of a reaction mechanism consisting of initiation, propagation, and chain transfer reactions.