Radical polymerization — initiation
Radical polymerization: initiation
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: AIBN (or benzoyl peroxide), vinyl monomer (styrene). Conditions: 60–80 °C (or hν), inert atmosphere. Bonds formed: 2 R• (and N₂); first C–C bond; bonds broken: weak C–N (AIBN) or O–O (BPO). Stereochemistry: Attack at the less substituted CH₂ gives the benzylic radical. Rate and selectivity: Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting.
At a glance
Step 1: Homolysis of AIBN
An initiator like AIBN or benzoyl peroxide decomposes under heat or light, homolytically cleaving a weak bond to generate two radicals — the same fishhook-arrow homolysis seen in radical halogenation.
Step 2: Radical adds to the monomer
One of these initiator radicals adds to the terminal carbon of a vinyl monomer, forming a new C–C bond and generating a new, larger carbon radical.
Step 3: New active chain end
This new radical is now the active chain end, ready to add to further monomer units — the initiation step that kicks off the entire polymer chain.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- AIBN
- Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.
- BPO
- Benzoyl peroxide. Radical initiator that cleaves at the weak O–O bond.
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Radical polymerization | Radical polymerization | Radical polymerization | |
|---|---|---|---|
| Reagents | AIBN (or benzoyl peroxide), vinyl monomer (styrene) | Growing chain radical, vinyl monomer (styrene) | Two growing polymer radicals (e.g. polystyrene) |
| Conditions | 60–80 °C (or hν), inert atmosphere | 60–80 °C, bulk or solution | Radical–radical encounter, diffusion-controlled |
| Stereochemistry | Attack at the less substituted CH₂ gives the benzylic radical | Head-to-tail addition; tacticity set by chain end | Head-to-head linkage |
| Rate / selectivity | Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting | kp ≈ 10²–10⁴ M⁻¹ s⁻¹; far faster than initiation | 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?
AIBN (or benzoyl peroxide), vinyl monomer (styrene). Conditions: 60–80 °C (or hν), inert atmosphere.
Which bonds are formed and which are broken?
Formed: 2 R• (and N₂); first C–C bond. Broken: weak C–N (AIBN) or O–O (BPO).
What is the stereochemical outcome?
Attack at the less substituted CH₂ gives the benzylic radical
What controls the rate or the selectivity?
Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make