Enthalpic and electronic terminal and penultimate unit effects in the free-radical copolymerization of styrene (S) with methyl methacrylate (M) were investigated by quantum mechanical calculations at 0 and 298 K. Total energies, zero-point energies scaled by a 0.96 factor, and thermal enthalpy corrections for all optimized structures were computed at the B3-LYP/6-31G(d) level of theory. Differences in enthalpies for elementary propagation reactions at 0 and 298 K did not exceed 0.6 kcal/mol. Enthalpic effects of the replacement of S by M in the penultimate position of the growing radicals in elementary copolymerization propagation reactions (enthalpic penultimate unit effects) were always positive, ranging from 1.2 to 3.3 kcal/mol at 298 K. The values suggested that the elementary propagation reactions involving more S units in the growing polymer chain ends should be slightly thermodynamically preferred. A comparison of these results with those for the S-acrylonitrile monomer system showed that the most crucial feature differentiating enthalpic effects for the two monomer systems is the replacement of M by acrylonitrile in the reaction pair CH3-S-M . + M --> CH3-S-M-M . and CH3-M-M . + M --> CH3-M-M-M .. (C) 2004 Wiley Periodicals, Inc.