Ab initio molecular orbital calculations have been used to study the effect of gamma substituents (X = H, F, or CN) on the addition of 1-Y,3-X-disubstituted propyl radicals (Y = F or CN) to ethylene. It is found that, although the gamma substituent (i.e. penultimate unit) can exert a significant effect on the stability of the propagating radical, only a small fraction of this effect appears to carry over to the reaction barrier. Furthermore, it appears that when penultimate unit effects on radical stability are large enough to result in significant penultimate unit effects on the reaction barrier, polar interactions are also likely to be significant. The implications of these results for the implicit penultimate model in free-radical copolymerization are discussed.