Radical polymerization of pure styrene causes the viscosity to increase. This retards the deactivation of the free radicals by radical-radical reactions, but it does not have such a great effect on the reaction of the radicals with styrene. Hence the concentration of free radicals increases and the rate of reaction increases, which can lead to thermal runaway. While the autoacceleration seen with pure styrene is the classic example taught to chemistry undergraduates, this effect can occur during the polymerization of other monomers such as methyl methacrylate.[1]
Avoiding thermal runaway is one of the reasons why suspension polymerization is such a popular way of making polystyrene. The droplets dispersed in the water are small reaction vessels, but the heat capacity of the water lowers the temperature rise, thus moderating the reaction.