High-level ab initio calculations at the QCISD/6-311G** + ZPVE level have been carried out to study the addition reactions of tert-butyl radical to a set of substituted alkenes, CH2=CHX (X=H, NH2, F, Cl, CHO, and CN), and the results analyzed with the aid of the curve-crossing model. The reactivity of the tert-butyl radical is found to be governed by a combination of enthalpy and polar factors. The polar factor leads to tert-butyl radical displaying strong nucleophilic character which stabilizes the transition states by 20-25 kJ mol(-1) compared with those for the relatively nonpolar reactions of methyl radical. Consequently, barrier heights are significantly lower for radical addition reactions of tert-butyl (1.9-21.6 kJ mol(-1)) than for methyl (24.3-39.8 kJ mol(-1)). A transition state structure-enthalpy correlation is found for the addition reactions of tert-butyl radical but is shifted slightly from the correlation line previously found for the CH3., CH2OH., and Ch(2)CN(.) radicals, reflecting the increasing importance of polar contributions.