Ab initio molecular orbital calculations and transition-state theory have been used to study reactions between ethyl radical and ethylene that model chain transfer in the polymerization of ethylene, and the results rationalized with the aid of the curve-crossing model. It is found that the endothermic transfer of hydrogen from ethylene (abstraction) is kinetically favored over the thermoneutral transfer of hydrogen to ethylene (transfer). A possible explanation lies in the fact that only one bond needs to be broken in the abstraction reaction, in contrast to the two bonds that need to be broken in the transfer reaction.