The theoretical enthalpies of propagation reactions at 0 K without zero-point vibrational energy corrections according to terminal and penultimate models of the radical copolymerization of styrene with acrylonitrile are reported from molecular orbital calculations at the following levels of theory and basis sets: HF/6-31G(d); B3-LYP/6-31G(d); B3-LYP/6-311G(d,p) and B3-LYP/6-311+G(3df)//6-311G(d,p). Both the enthalpic terminal and penultimate unit effects, determined according to the theoretical thermochemistry, depend on the level of theory and basis set used for the molecular orbital calculations. The best performing B3LYP/6-311+G(3df)//B3LYP/6-311G(d,p) procedure gives theoretical enthalpies for the addition of styrene and acrylonitrile to CH3. that differ from experimental values by 0.6 and 1.6 kcal mol(-1), respectively. An analysis of the results obtained here leads to the conclusion that at least for the styrene-acrylonitrile monomer system, that is, a monomer system known as one of the few systems that do not conform to terminal model composition and microstructure equations, the enthalpic terminal unit effects seem to depend on the penultimate units of the growing radical. This finding, together with the outcome from our previous work on the dependence of the penultimate effects on the terminal units in a growing macroradical, indicates the inseparability of the enthalpic terminal (implicit) and explicit penultimate unit effects on the radical copolymerization. (C) 2003 Wiley Periodicals, Inc.