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Chemistry · Glossary

What is Steps of a chain polymerisation?

Also known as: initiation · radical initiator · propagation · termination

Definition 29.9 University Chemistry — Year 2 · Chapter 29 — Polymers: Synthesis, Structure and Properties

A radical chain polymerisation runs in three kinds of steps. In initiation, a radical initiator (a molecule with a weak bond, such as a peroxide or an azo compound) splits on heating into radicals, one of which adds to a monomer. In propagation, the radical at the chain end adds a monomer molecule and remains a radical. In termination, two radicals destroy each other, by combination (they bond) or disproportionation (one takes a hydrogen atom from the other).

initiation:I−I⟶2 I∙,I∙+CH2=CHPh⟶I−CH2−C˙HPhpropagation:∼CH2−C˙HPh+CH2=CHPh⟶  ∼CH2−CHPh−CH2−C˙HPhtermination:∼CH2−C˙HPh+PhC˙H−CH2∼⟶∼CH2−CHPh−CHPh−CH2∼\begin{align*} &\text{initiation:} && \mathrm{I{-}I} \longrightarrow 2\,\mathrm{I^{\bullet}}, \qquad \mathrm{I^{\bullet} + CH_2{=}CHPh \longrightarrow I{-}CH_2{-}\dot{C}HPh} \\ &\text{propagation:} && {\sim}\mathrm{CH_2{-}\dot{C}HPh + CH_2{=}CHPh \longrightarrow}\; {\sim}\mathrm{CH_2{-}CHPh{-}CH_2{-}\dot{C}HPh} \\ &\text{termination:} && {\sim}\mathrm{CH_2{-}\dot{C}HPh + Ph\dot{C}H{-}CH_2}{\sim} \longrightarrow {\sim}\mathrm{CH_2{-}CHPh{-}CHPh{-}CH_2}{\sim} \end{align*}
Radical polymerisation of styrene. The initiator I−I\ce{I-I} (dibenzoyl peroxide, for instance) splits into two radicals; each adds to the CHX2\ce{CH2} end of styrene, giving the more stable benzylic radical; propagation repeats the addition thousands of times; two chain radicals combine to end both chains. The dot marks the unpaired electron.
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