Free radical polymerization kinetics - revisited

Recent developments in the kinetic modellingof free radical polymerization are reviewed withanemphasisonworkinthe author's laboratory. At low conversions the chain length dependence of the termination rate constant has been experimentally demonstrated and theoretically modelled. The impact of this chain length dependence on other aspects of free radical polymerization kinetics has also been explored both theoretically and experimentally. At higher conversions the gel effect has beenmodelledwith some success by using the concept of chain entanglement to describe the influence of high polymer concentrationontheterminationreactionrate. The state of knowledge concerning copolymerization kinetics is briefly discussed. INTRODUCTION The free radical polymerization of vinyl monomers is probably the most.common synthetic reaction carried out by man. As such, it has been studied long and well. In broad outline the nature of the reactions involved has been well understood for more than three decades, and this has led many to believe that the subject is one in which there is not much new to be found. However, interest does continue in this area because its complexity has so far prevented anyone from giving a complete description of the course of a polymerization in quantitative terms. Interest in modelling free radical polymerization kinetics has also received added stimulus in recent years becuase of the need for kinetic models for computer control of processes (1). A complete kinetic model must be able to describe the individual reaction rates for initiation, propagation, chain transfer and termination of the propagating chain. Only from such information can one deduce the molecular weight distribution or other important properties of the polymer chain. Conversely, only by using the molecular weight distribution (MWD) or at least averages of it can one hope to sort out the highly coupled influences of the various elementary reactions on the overall rate and extent of polymerization. The common use of gel permeation chromatography (GPC) to determine MWD in the last decade has provided the polymerization kineticist with what is now an indispensable tool for experimental work. As a result of its use the "classical" kinetic model, based almost exclusively on rates and number average degrees of polymerization, must be modified. In this paper, the work done toward such a modification during the last six years in the author's laboratory is reviewed and summarized. When a vinyl monomer undergoes free radical polymerization the simplest possible system consists of an initiator which is soluble in the monomer and a resultant polymer which is also soluble in its monomer. A wide variety of techniques have been employed to measure the kinetics of polymerization for such systems using characteristics such as shrinkage,