Radical Chain-Growth Polymerization: Mechanism

During initiation, a free radical generated from a radical initiator undergoes homolytic fission and reacts with the monomer's double bond. This creates a sigma bond between the initiator and monomer, shifting the radical site to the newly bonded monomer. This activated monomer is now ready to begin the propagation phase of polymerization.

During propagation, the radical species adds to another monomer's unsaturated bond, creating a new radical site on the freshly added monomer. This step repeats cyclically, with each addition extending the polymer chain. The number of propagation cycles directly determines the final molecular weight of the polymer chain produced.

Monomers preferentially bond such that the radical site remains on the more substituted carbon. In monosubstituted monomers like vinyl chloride and styrene, this positioning places the unpaired electron on the substituted carbon (tail), stabilizing the growing chain and favoring head-to-tail addition throughout polymerization.

Termination occurs through radical coupling, where two growing chains' radical sites combine directly, or through radical disproportionation, where one chain's radical abstracts hydrogen from the other chain's alpha carbon. Coupling produces a single chain with a saturated bond, while disproportionation yields one alkyl-terminated and one alkenyl-terminated polymer.

In radical coupling, two radical sites bond directly to form a single polymer with a saturated linkage. In radical disproportionation, one radical abstracts hydrogen from the alpha carbon of another chain, producing two distinct polymers: one ending in an alkyl group and one ending in an alkenyl group.

Chain transfer reagents terminate polymer chain growth by removing the radical site from the growing chain or reducing its reactivity. Unlike coupling and disproportionation, chain transfer does not eliminate the radical entirely but instead transfers it elsewhere, allowing polymerization to continue on a new chain or with reduced efficiency.

Radical chain-growth polymerization uses free radicals to initiate and propagate chain growth through three distinct stages. Other mechanisms like anionic chain-growth polymerization and cationic chain-growth polymerization employ different reactive intermediates and initiation strategies, offering alternative pathways for synthesizing polymers with distinct properties.