a reaction that takes place at such a rapid rate that large amounts of energy are released

Polymerization is a process of forming a polymer by combining large numbers of chemical units or monomers into long chains ( polyethylene from ethylene or polystyrene from styrene). Uncontrolled polymerization can be extremely hazardous. Some polymerization processes can release considerable heat or can be explosive.

The free radical polymerization of pure styrene will cause 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, this can lead to a thermal runaway.