for the first dimerization step, which should stem from the fact that the reactive alkyl radical part is a secondary radical in 3 4. This explains the lower dimerization yield of 4 than of 2. For the three dienes 4 -6, which have most stable conformers with secondary alkyl radical moieties, there are higher-energy conformers with primary alkyl radical moieties. For 4, such a conformer is at a relative energy 0.7 kcal mol -1 above that most stable one, while for 6 such a conformer is less accessible at 2.8 kcal mol -1 above the most stable. In this less stable conformer of 6 with primary radical sites, the allyl radical moieties either experience steric congestion between methyl substituents on two adjacent C atoms or adopt a less favorable Z-configuration of the methyl substituted allyl radical. Thus, conformers with reactive primary alkyl radical moieties are less populated in the case of 6 (Fig. , ESI). In diene 5, the methyl substituents are placed at the ends of the diene segment, and as a result, the alkyl radical moiety of this diene in its T1 state will always be a secondary radical moiety.