Determining the Kinetic and Thermodynamic Parameters of Anionic Polymerization of Styrene Using Linear Free-Energy Relationship

The effect of solvents on kinetic parameters of anionic polymerization is complex and a comprehensive theoretical study has been rare. In this work, four solvent polarity descriptors (i.e., polarizability, dipole moment, nucleophilic index, electrophilic index) are correlated with solvent parameters (E_a, A, ΔH‡, and ΔS‡) by multiple linear regression using the Catalan linear free-energy relationship (LFER) equation for the anionic polymerization of styrene. The results show that E_a, ΔH‡, and ΔS‡ have a low correlation with dipole moment, whereas A is strongly correlated with dipole moment. Given the fact that the larger A the larger effective collision frequency Z, it is hypothesized that in polar solvents, the polymer chains are more extended, enabling more effective collisions between monomers and active anionic species during polymerization. In contrast, in nonpolar solvents, the polymer chains collapse, making it more difficult for the monomer to be inserted into the ion pairs. Subsequently, n-pentane and tetrahydrofuran are chosen as representatives to confirm this conjecture by molecular simulations. Lastly, E_a, A and k_p are predicted for 173 solvents using well-established descriptive relationships.