Radical polymerization — termination
Radical polymerization: termination by combination
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. polystyrene). Conditions: Radical–radical encounter, diffusion-controlled. Bonds formed: new C–C σ bond (chain-to-chain); bonds broken: none (two radicals consumed). Stereochemistry: Head-to-head linkage. Rate and selectivity: kt ≈ 10⁷–10⁸ M⁻¹ s⁻¹; dominant for polystyrene.
At a glance
Step 1: Two chain radicals meet
Two growing polymer chain radicals, each carrying an unpaired electron, encounter each other in solution.
Step 2: Electrons pair up
Their two unpaired electrons pair up directly, forming a single new C–C σ bond between the two chain ends — radicals simply combining, with no arrow-pushing needed since there's no charge separation to track.
Step 3: One chain of doubled length
The result is one long polymer chain, roughly double the length of either radical chain — this termination pathway increases the average molecular weight of the polymer product.
Abbreviations and terms
- M
- Metal. A generic metal centre.
How it compares with related reactions
| Radical polymerization | Radical polymerization | Radical polymerization | |
|---|---|---|---|
| Reagents | Two growing polymer radicals (e.g. polystyrene) | Two growing polymer radicals (e.g. poly(methyl methacrylate)) | 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 | Head-to-head linkage | Two chains: one saturated, one terminal alkene | Attack at the less substituted CH₂ gives the benzylic radical |
| Rate / selectivity | kt ≈ 10⁷–10⁸ M⁻¹ s⁻¹; dominant for polystyrene | Favoured by hindered radicals and higher temperature (PMMA) | 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. polystyrene). Conditions: Radical–radical encounter, diffusion-controlled.
Which bonds are formed and which are broken?
Formed: new C–C σ bond (chain-to-chain). Broken: none (two radicals consumed).
What is the stereochemical outcome?
Head-to-head linkage
What controls the rate or the selectivity?
kt ≈ 10⁷–10⁸ M⁻¹ s⁻¹; dominant for polystyrene
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make