Ethylene polymerization reactions with many Ziegler-Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of alpha olefins: (1) a relatively law ethylene reactivity, (2) markedly higher polymerization rates in the presence of alpha olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions(1) provided the basis for a new kinetic scheme that postulates the equilibrium formation of Ti-C2H5 species with the H atom in the methyl group beta-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any alpha-olefin molecule into the Ti-H bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/alpha-olefin copolymer chains should contain alpha-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers.