Radical polymerization — propagation
Radical polymerization: propagation
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: Growing chain radical, vinyl monomer (styrene). Conditions: 60–80 °C, bulk or solution. Bonds formed: C–C σ (one per monomer added); bonds broken: C=C π of monomer. Stereochemistry: Head-to-tail addition; tacticity set by chain end. Rate and selectivity: kp ≈ 10²–10⁴ M⁻¹ s⁻¹; far faster than initiation.
At a glance
Step 1: Chain radical adds to a monomer
The growing chain's radical end adds across the π bond of another monomer molecule, forming a new C–C bond via the same fishhook mechanism used in every radical addition.
Step 2: Radical regenerated one unit longer
This regenerates a radical, now one monomer unit longer, at the new chain end.
Step 3: Repeat thousands of times
This cycle repeats thousands of times per initiation event, rapidly building a long polymer chain — propagation is by far the fastest of the three stages, which is why radical polymerizations produce high-molecular-weight polymer almost immediately.
Abbreviations and terms
- M
- Metal. A generic metal centre.
How it compares with related reactions
| Radical polymerization | Radical polymerization | Radical polymerization | |
|---|---|---|---|
| Reagents | Growing chain radical, vinyl monomer (styrene) | AIBN (or benzoyl peroxide), vinyl monomer (styrene) | Two growing polymer radicals (e.g. polystyrene) |
| Conditions | 60–80 °C, bulk or solution | 60–80 °C (or hν), inert atmosphere | Radical–radical encounter, diffusion-controlled |
| Stereochemistry | Head-to-tail addition; tacticity set by chain end | Attack at the less substituted CH₂ gives the benzylic radical | Head-to-head linkage |
| Rate / selectivity | kp ≈ 10²–10⁴ M⁻¹ s⁻¹; far faster than initiation | Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting | kt ≈ 10⁷–10⁸ M⁻¹ s⁻¹; dominant for polystyrene |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Radical polymerization?
Growing chain radical, vinyl monomer (styrene). Conditions: 60–80 °C, bulk or solution.
Which bonds are formed and which are broken?
Formed: C–C σ (one per monomer added). Broken: C=C π of monomer.
What is the stereochemical outcome?
Head-to-tail addition; tacticity set by chain end
What controls the rate or the selectivity?
kp ≈ 10²–10⁴ M⁻¹ s⁻¹; far faster than initiation
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make