Dependence Of Copolymer Composition In Radical Polymerization On Solution Properties: A Quantitative Thermodynamic Interpretation

The thermodynamic formulation of transition state theory is applied to compute the effect of the medium (solvent type and concentration) on the copolymer composition obtained in radical copolymerization. The magnitude of the medium effect on the deviation of the reactivity ratios (of the terminal model) with respect to the value observed in a reference state is shown to be linked to the degree of the thermodynamic nonideality of the system and, more specifically, to the activity coefficients of the two monomers and transition states involved in each reactivity ratio. To illustrate and validate this thermodynamic approach, the quantum chemistry-based solvation model COSMO-RS (Conductor-like Screening MOdel for Real Solvents) was used to estimate the relevant activity coefficients for a variety of comonomer-solvent combinations for which copolymer composition curves are reported in the literature. The evaluation was based on a limited (by necessity) but broad selection of systems for which different qualitative explanations for the medium effect are proposed in the literature. Overall, the comparison between measurements and predictions shows that the qualitative trends are well captured, and in many cases, the quantitative agreement is also quite remarkable, demonstrating the potential of our unified approach. There is one exception: for reasons not yet understood, our method considerably fails for copolymer systems involving amide monomers.