Ab initio molecular orbital calculations on the Q-e scheme for predicting reactivity in free-radical copolymerization

Ab initio molecular orbital calculations have been performed on the reactions of unsaturated monomers with both hydrogen and methyl radicals. These calculations have utilized several basis sets, namely STO-3G, SV3-21G, SV Dunning and triple zeta valence. The correlation between the experimental Q and the energy of formation of the product radical was examined, whilst the electronegativity of the monomer was correlated with the e parameter. In addition, a Q-e-e*-type scheme was analysed to correct for the simplification in the standard Q-e scheme that the monomer and the radical are identical with respect to their charge distributions. In this instance experimental e parameters were correlated with the average electronegativity of monomer and radical. This work shows that correlations exist between these parameters as other simpler studies (based on Huckel calculations) have reported. However, while the correlations with both e and the e-e* average parameter were predictive, the relationship between Q and a general reactivity of the radical proved only qualitative. A statistical analysis of the data revealed a significant polar influence on this relationship; by taking this into account we were able to improve the correlation. We also rescaled the Q-e scheme as suggested by other authors and correlated the results of our calculations with the new numbers. This work emphasizes the difficulties in correlation analysis, which requires the separation and parameterization of polar, steric and general reactivity effects. It also stresses the requirement for accurate experimental data.