Chain-Growth Polymerization

Chain-growth polymerization builds macromolecules by the rapid, repeated addition of monomer to a small population of active centers, so high-molar-mass chains form even at low overall conversion.

Definition

Chain-growth polymerization is a polymerization in which monomers add one at a time to the reactive end of a growing chain, so that the chain extends only at its active center rather than by reaction between two arbitrary species.

Scope

This topic covers the elementary steps of chain polymerization—initiation, propagation, chain transfer, and termination—chiefly for free-radical systems but with the same framework applying to ionic and coordination variants. It includes initiator decomposition and efficiency, propagation and termination rate constants, the steady-state rate law, kinetic chain length, the gel (Trommsdorff) effect, and the way transfer reactions limit molar mass.

Core questions

• How do the rates of initiation, propagation, and termination determine the polymerization rate and average molar mass?
• Why does radical polymerization rate depend on the square root of initiator concentration?
• How do chain-transfer reactions cap molar mass without stopping the polymerization?
• What causes autoacceleration at high conversion?

Key theories

Steady-state kinetics of radical chain polymerization

Assuming radical generation and consumption are balanced gives a rate of polymerization proportional to monomer concentration and the square root of initiation rate, and a kinetic chain length set by the ratio of propagation to termination.

Chain transfer and the Mayo equation

Transfer of the radical to monomer, solvent, initiator, or a deliberately added agent terminates one chain and starts another; the Mayo relation links the reciprocal degree of polymerization to transfer constants, allowing molar mass to be tuned without changing rate.

Mechanisms

An initiator decomposes to give primary radicals that add to monomer, creating chain-carrying radicals. Propagation adds monomer units rapidly to this radical center. Termination occurs when two radicals combine or disproportionate, removing both active centers. Because active-center concentration is tiny and turnover is fast, each chain forms in a fraction of a second, and the bulk of unreacted monomer remains until late in the reaction. At high conversion, increasing viscosity slows termination more than propagation, producing the autoacceleration known as the gel or Trommsdorff effect.

Clinical relevance

Chain-growth radical polymerization is the dominant industrial route to commodity plastics such as polyethylene, polystyrene, poly(vinyl chloride), and poly(methyl methacrylate), and underlies emulsion processes for paints, adhesives, and synthetic rubber. Mastery of its kinetics lets manufacturers target a specific molar mass, conversion, and heat-release profile.

History

Free-radical chain polymerization was placed on a quantitative footing in the 1930s and 1940s as the radical chain mechanism, steady-state kinetics, and chain-transfer constants were established, work that supported the wartime synthetic-rubber programs and the subsequent expansion of commodity thermoplastics.

Key figures

• Hermann Staudinger
• Frank Mayo
• Ernst Trommsdorff

Seminal works

• odian2004
• young2011

Frequently asked questions

Why does a long chain form even when most monomer is still unreacted?

Only a very small number of active centers exist at any instant, and each adds thousands of monomers in a fraction of a second before terminating. So full-length chains are produced continuously while the large pool of monomer is consumed gradually.

What is the gel or Trommsdorff effect?

At high conversion the medium becomes viscous, which slows the diffusion-controlled termination of radicals far more than it slows propagation. Radical concentration rises, the rate accelerates, and the reaction can self-heat, an important process-safety consideration.