Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the reaction proceeds via a free-radical intermediate. The free radical is formed from radical initiators, which spontaneously generate free radicals by homolytic fission. Organic peroxides (such as dibenzoyl peroxide, as shown in Figure 1) or azo compounds are popular radical initiators. A low concentration ratio of radical initiator to monomer is used to minimize radical coupling.

Figure 1: The homolytic fission reaction of radical initiator dibenzoyl peroxide to generate benzoyloxy radicals and the subsequent decomposition to phenyl radicals

This free radical initiates polymerization by reacting with a monomer molecule. Unsaturated monomers are suitable for radical chain-growth polymerization; a substituent group across the unsaturated bond that stabilizes the free radical increases the reaction feasibility. Ethylene, propylene, vinyl chloride, and styrene are examples of monomers that can be polymerized via free-radical chain-growth polymerization.

The reaction between a monomer and a free radical generates a new free radical. This new radical reacts with another monomer and creates yet another free radical. The polymer chain grows as the reaction between free-radical intermediates and monomers repeats sequentially. Typically, the propagation step repeats one thousand to ten thousand times before the termination of polymerization.

Often, chain transfer reagents, such as thiol, are used to control the molecular weight (Figure 2). A chain transfer agent must be sufficiently reactive to transfer a hydrogen atom to the growing chain to terminate the polymer growth. The resultant radical must add to a double bond in the monomer to initiate polymerization.

Figure 2: (Top): The termination reaction of chain transfer agent thiol and a growing polymer chain; (Bottom): Subsequent reaction of thiol radical and monomer molecule to initiate polymer growth

Inhibitors are the reagents used to reduce the reactivity of radicals in the growing polymer chain. Figure 3 depicts the reaction of benzoquinone as an inhibitor.

Figure 3: The reaction between chain inhibitor benzoquinone and a growing polymer chain to generate a less reactive free radical