Radical Chain-Growth Polymerization: Overview

Radical chain-growth polymerization, also called addition polymerization, is the successive addition of monomers to a growing polymer chain via free-radical intermediates. A radical initiator generates a free radical with an unpaired electron, which reacts with the first monomer. As each new monomer adds, the unpaired electron shifts to the opposite end, allowing the chain to grow sequentially. This process typically repeats one thousand to ten thousand times before termination.

Radical initiators such as organic peroxides like dibenzoyl peroxide or azo compounds spontaneously generate free radicals through homolytic fission, breaking a bond to create two radicals with unpaired electrons. These free radicals then react with monomer molecules to initiate polymerization. A low concentration ratio of radical initiator to monomer is used to minimize unwanted radical coupling and ensure controlled chain growth.

Unsaturated monomers with double bonds are suitable for radical chain-growth polymerization. Monomers like ethylene, propylene, vinyl chloride, and styrene work well because substituent groups across the unsaturated bond can stabilize the free radical intermediate, increasing reaction feasibility. The stability of the resulting radical determines how readily the monomer will participate in the polymerization process.

During propagation, the free radical reacts with a monomer molecule, generating a new free radical at the opposite end of the added monomer. This new radical then reacts with another monomer, creating yet another free radical. The polymer chain grows as this cycle repeats sequentially, with the reactive propagation site shifting to each newly appended monomer until termination occurs.

Chain transfer reagents like thiols control polymer chain length by terminating the growth of the polymer chain while simultaneously initiating new polymerization. The chain transfer agent transfers a hydrogen atom to the growing chain, stopping its growth. The resulting radical must be sufficiently reactive to add to a monomer double bond, initiating polymerization of a new chain and controlling the final molecular weight.

Inhibitors reduce the reactivity of growing polymer chain radicals through stabilization, slowing or stopping polymerization. Reagents like benzoquinone react with the free radical at the chain's propagation site, converting it into a less reactive species. This stabilization prevents further monomer addition, allowing chemists to control reaction rates and prevent unwanted side reactions during radical chain-growth polymerization.

A low concentration ratio of radical initiator to monomer minimizes radical coupling, where two free radicals combine prematurely and terminate chain growth without producing useful polymer. By keeping initiator concentration low relative to monomer, more radicals react with monomers rather than with each other, ensuring efficient chain propagation and maximizing polymer yield and chain length control.