A Mechanistic Perspective on Solvent Effects in Free-Radical Copolymerization

1. INTRODUCTION

The ability of solvents to affect the homopropagation rate of many common monomers has been widely documented. For example, Bamford and Brumby [1] showed that the propagation rate k_p of methyl methacrylate (MMA) at 25°C was sensitive to a range of aromatic solvents. Burnett, Cameron, and Joiner [2] found that the k_p of styrene (Sty) was depressed by increasing concentrations of benzonitrile, bromobenzene, diethyl phthalate, dinonyl phthalate, and diethyl malonate, while in other studies [3, 4], they found that the k_p for MMA was enhanced by halobenzenes and naphthalene. More recent work by Zammit et al. [5] has shown that solvents capable of hydrogen bonding, such as benzyl alcohol and N-methyl pyrrolidone, have a small influence on both the activation energy E_a and preexponential factor A in Sty and MMA homopropagation reactions. These are but a few of the many instances of solvent effects in the homopolymerization reactions of two typical monomers, Sty and MMA. For these monomers, solvent effects are relatively small; this is indicative of the majority of homopropagation reactions. However, in some instances, much larger effects are observed, especially when specific interactions such as hydrogen bonding or ionization occur. Examples of this type include the polymerization of N-vinyl-2-pyrrolidone (for which water has been found to dramatically increase k_p [6]) and the polymerization of acrylamide (for which pH plays a strong role [7]).