First, the activation energies for the addition of 3 1 and 3 2 to the corresponding S0 dienes are lower than those of 3 -6. For 4 -6 the reactive alkyl radical site is a secondary radical, which explains the slightly higher activation barriers for these dienes. On the other hand, the higher activation energy and lower dimerization yield of 3, as compared to 2, could result from larger steric congestion at the transition state of the dimerization of 3. This can be seen in several properties. First, the reactant molecule 3 3 at the transition state for dimerization of 3 is more distorted than the reactant molecule 3 2 at the transition state for dimerization of 2