The reaction energies become gradually less exergonic when going from 1 to 5 and 6, which in part can be described by an increased hyperconjugative stabilization of the triplet diene reactants. In 3 1, there is no hyperconjugative stabilization while in 5 and 6 the triplet dienes experience hyperconjugative methyl group stabilization of both the alkyl and allyl radical moieties. Accordingly, the gain in energy upon dimerization is smaller in the latter species. A second contributing factor may be a greater loss in entropy in the dimerization of 5 and 6 than of 1, as the products of the two first dienes will be more structurally confined due to steric clashing between methyl substituents.