Low molecular weight (ca. 2000) ethylene-1-butene copolymers were synthesized at 90 degrees 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 H-1 and C-13 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 C-13 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.