The geometries and electronic properties of substrates, transition structures (TS), and product radicals in modeled elementary propagation reactions were studied for the styrene–acrylonitrile monomer system by use of quantum‐mechanical calculations: (DFT/B3‐LYP/6–31G(d), ROMP2/6–311+G(3df,2p)//DFT/B3‐LYP/6–31G(d), and DFT/B3‐LYP/6–311+G(3df,2p)//DFT/B3‐LYP/6–31G(d)) and for some parameters, the high‐level composite method G3 (Gaussian‐3, G3/MP2). Activation enthalpies (ΔHact) and reaction enthalpies (ΔHr) for modeled propagation reactions at 298.15 K were evaluated. The enthalpy of activation energy (ΔHact, kJ/mol) for the investigated elementary reactions rises for the B3‐LYP calculation in the following order: (CH3A•+S) < (CH3A•+A) < (CH3S•+A) < (CH3S•+S). For three propagation reactions, (CH3A•+A), (CH3A•+S), and (CH3S•+A), correlation between reaction enthalpy and enthalpy of activation suggests weak or negligible polar effects reflecting the Evans–Polanyi relation. However, from the electron affinities and ionization energies values data, it is not excluded that at least for [CH3A•+S[b]] and [CH3S•+A[b]] reactions, nucleophilic and electrophilic polar effects, respectively, can also be expected. The dependencies between TS geometries, electronic parameters, and enthalpic effects suggest the presence of a steric factor in all TS, including its exceptionally high contribution to the activation enthalpy for the CH3S•+S addition. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1827–1844, 2005