The chromium-based Union Carbide catalyst exhibits high activity for the homopolymerization of ethylene. However, it fails to catalyze the homopolymerization of alpha-olefins or even the copolymerization of the latter with ethylene. To understand the origins of this selectivity, density functional theory calculations are used to model the olefin polymerization reactions of a cyclopentadienylchromium(III) complex whose properties recommend it as a structural and functional model for the Union Carbide catalyst. We find that this chromium alkyl binds both ethylene and propene weakly and therefore reversibly, and that the barrier for insertion of propene is significantly higher than that for inserting ethylene. Because olefin binding is reversible, the insertion step discriminates against the incorporation of alpha-olefin into the polymer chain.