279896 Effect of Bulky or Extended Side Chains On the Autoacceleration Behavior of Methacrylate-, Acrylate- and Styrene-Based Monomers During Polymerization
Autoacceleration, or more commonly referred to as the gel effect, is a classical aspect of free radical polymerization wherein conversion rises rapidly with time which can lead to a dangerous buildup of heat and reactor explosions if not properly anticipated. In contrast to methyl methacrylate (MMA) which shows classical autoacceleration behavior, we analyzed the homopolymerization kinetics of several longer and bulkier methacrylate-based monomers, such as ethyl to n-butyl methacrylate and others, which can exhibit significantly different severity and onset of autoacceleration than expected, and in some cases a form of autoacceleration in solution-based polymerizations is observed.
Autoacceleration behavior in conventional free radical polymerization is marked by a rapid decrease in termination with increasing conversion which results in a rapid rise in polymerization rate and a potentially dangerous increase in temperature due to the exothermic nature of the reaction. The cause of this decrease in termination is generally attributed to a decrease in the radical chain diffusion coefficient, resulting in a decrease in termination and yielding a higher concentration of radicals in the system which leads to a rapid increase in reaction rate. Traditional systems such as the polymerization of MMA and styrene have been well represented in the research literature, and the mechanisms relating to the cause and severity of autoacceleration in these systems are relatively well understood.
There are a number of systems, however, that yield much different autoacceleration behavior in comparison to MMA. Here we analyze the effects of changes to repeat unit structure on polymerization kinetics, which can result in significant changes to the onset and severity of the autoacceleration behavior observed during polymerization. We show how methodical changes to the subunit structure as in methyl to n-butyl methacrylate, as well as the addition of bulky sidegroups such as tert-butylstyrene in comparison to styrene, increase the likelihood for chain transfer or branching which in turn affect the concentration dependence of the radical chain diffusion coefficient and thus the termination rate parameter and severity of autoacceleration. We also discuss circumstances in which the observed autoacceleration of free-radical polymerization may be largely dominated by branching effects.