A model of frontal polymerization using complex initiation

Frontal polymerization is a process in which a spatially localized reaction zone propagates into a monomer, converting it into a polymer. In the simpl est case of free-radical polymerization, a mixture of monomer and initiator is placed in a test tube. A reaction is then initiated at one end of the t ube. Over time, a self-sustained thermal wave, in which chemical conversion occurs, is produced. This phenomenon is possible because of the highly exo thermic nature of the polymerization reactions. Though there are certain advantages to this polymerization process over the more traditional methods, one of the drawbacks is that conversion tends to be incomplete. One way to increase conversion is by using greater amounts of initiator. The disadvantage to using this method is that more initiator results in the production of more free radicals, leading to large numbers o f undesirably short polymer chains. A second method is to use a mixture of unstable and stable initiators. In this paper we develop and study a mathem atical model of the propagation of free-radical polymerization fronts using such a complex initiation. We compare the propagation velocity, maximum te mperature and degree of conversion of fronts with a stable initiator, an un stable initiator and a mixture of the two. In addition, we examine how alte ring the stability of the stable initiator affects these quantities. We sho w that it is indeed the case that a mixture of unstable and stable initiato rs does have many advantages over using either type of initiator individual ly, in agreement with the existing experimental data.