Copolymerization

Copolymerization links two or more different monomers into a single chain, and the relative reactivities of the monomers toward each growing chain end determine whether the product is random, alternating, gradient, or blocky in sequence.

Definition

Copolymerization is the simultaneous or sequential polymerization of two or more chemically distinct monomers to form a copolymer, a single macromolecule containing more than one type of repeat unit.

Scope

This topic covers the synthesis and sequence control of copolymers: the copolymer composition (Mayo-Lewis) equation, monomer reactivity ratios and their measurement, the classification of copolymers as statistical, alternating, gradient, block, or graft, composition drift with conversion, and how controlled and living methods are used to build well-defined block and graft architectures.

Core questions

• How do reactivity ratios determine the instantaneous composition of a copolymer?
• What conditions produce alternating, random, gradient, or blocky sequences?
• Why does copolymer composition drift as conversion proceeds, and how is it controlled?
• How are well-defined block and graft copolymers synthesized?

Key theories

Mayo-Lewis copolymer composition equation

Two reactivity ratios, each comparing a chain end's rate of adding its own monomer versus the comonomer, fix the instantaneous copolymer composition as a function of feed; their values predict azeotropic compositions, alternating tendency, and the drift of composition with conversion.

Sequence control via reactivity ratios

Reactivity-ratio products near zero give alternating copolymers, near one give random copolymers, and much greater than one give a tendency to homopolymerize or block, so the same monomer pair can yield very different microstructures and properties.

Mechanisms

At each propagation step a growing chain end chooses between the available monomers according to its reactivity ratio, a kinetic preference set by the stability of the resulting radical or ion and by steric and polar effects. Over the course of the reaction the more reactive monomer is consumed faster, so the feed composition and hence the chain composition drift unless monomer is replenished. Block copolymers instead are made by living or controlled methods, adding the second monomer only after the first is exhausted; graft copolymers are made by polymerizing branches from reactive sites on a backbone.

Clinical relevance

Copolymerization is the principal way to tune polymer properties continuously: styrene-acrylonitrile and ABS combine toughness with rigidity, ethylene-vinyl acetate adjusts flexibility and adhesion, and block copolymers such as styrene-butadiene-styrene act as thermoplastic elastomers and as self-assembling templates for nanostructured materials, membranes, and drug carriers.

History

The quantitative theory of copolymerization was developed in the 1940s when Mayo and Lewis derived the copolymer composition equation and Alfrey and Price introduced the Q-e scheme to rationalize reactivity ratios, giving chemists predictive control over copolymer composition that underpinned the postwar growth of synthetic copolymers.

Key figures

• Frank Mayo
• Frederick Lewis
• Turner Alfrey
• Cheves Walling

Seminal works

• odian2004
• young2011

Frequently asked questions

What are reactivity ratios?

They are the ratios of how fast each growing chain end adds its own monomer versus the other monomer. Together they predict the copolymer's composition and sequence: values near zero give alternating copolymers, near one give random ones, and large values favor blocky structures.

Why does copolymer composition change during a batch reaction?

The more reactive monomer is incorporated faster and depletes sooner, so chains formed early are richer in it and chains formed later are richer in the slower monomer. This composition drift is controlled by feeding monomer continuously or by using living methods.