Radical polymerization — termination
Radical polymerization: termination by disproportionation
How the Radical polymerization works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Two growing polymer radicals (e.g. poly(methyl methacrylate)). Conditions: Radical–radical encounter, diffusion-controlled. Bonds formed: C–H (saturated end); C=C (unsaturated end); bonds broken: β C–H of one chain. Stereochemistry: Two chains: one saturated, one terminal alkene. Rate and selectivity: Favoured by hindered radicals and higher temperature (PMMA).
At a glance
Step 1: Radical B sits near an H on chain A
Two growing polymer radicals meet, as in combination termination — but this time, instead of pairing directly, one radical's chain-end hydrogen is positioned near the other radical's unpaired electron.
Step 2: H atom transfer
A single fishhook arrow transfers that hydrogen atom from one chain to the other, quenching both radicals at once — one chain gains an H, the other loses one to form a new terminal double bond.
Step 3: Two dead chains
The result is two separate polymer chains — one saturated, one with a new terminal alkene — rather than combination's single longer chain. Which pathway dominates depends on the specific monomer and reaction conditions.
Abbreviations and terms
- DEAD
- Diethyl azodicarboxylate. Mitsunobu oxidant: picks up PPh₃ to form a betaine.
How it compares with related reactions
| Radical polymerization | Radical polymerization | Radical polymerization | |
|---|---|---|---|
| Reagents | Two growing polymer radicals (e.g. poly(methyl methacrylate)) | Two growing polymer radicals (e.g. polystyrene) | AIBN (or benzoyl peroxide), vinyl monomer (styrene) |
| Conditions | Radical–radical encounter, diffusion-controlled | Radical–radical encounter, diffusion-controlled | 60–80 °C (or hν), inert atmosphere |
| Stereochemistry | Two chains: one saturated, one terminal alkene | Head-to-head linkage | Attack at the less substituted CH₂ gives the benzylic radical |
| Rate / selectivity | Favoured by hindered radicals and higher temperature (PMMA) | kt ≈ 10⁷–10⁸ M⁻¹ s⁻¹; dominant for polystyrene | Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Radical polymerization?
Two growing polymer radicals (e.g. poly(methyl methacrylate)). Conditions: Radical–radical encounter, diffusion-controlled.
Which bonds are formed and which are broken?
Formed: C–H (saturated end); C=C (unsaturated end). Broken: β C–H of one chain.
What is the stereochemical outcome?
Two chains: one saturated, one terminal alkene
What controls the rate or the selectivity?
Favoured by hindered radicals and higher temperature (PMMA)
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make