Polyacrylonitrile (PAN) is formed by free‑radical addition polymerisation of acrylonitrile. The reaction starts with a radical initiator, proceeds by rapid monomer addition, and ends by termination, usually in a precipitation polymerisation process.
Monomer structure and reactivity
Acrylonitrile (CH2=CH–CN) has a vinyl group next to a strongly electron‑withdrawing cyano group. The nitrile polarises the double bond, making the β‑carbon electron‑deficient and highly susceptible to radical attack.
Free‑radical polymerisation mechanism
The reaction follows the classic chain‑growth scheme: initiation, propagation, termination.
Initiation
- Thermal initiators such as AIBN or benzoyl peroxide decompose to give radicals.
- Redox systems (persulfate/bisulfite) are common in aqueous or suspension media.
- A radical adds to the less substituted carbon of the vinyl bond, forming a new chain‑initiating radical.
Propagation
- The chain‑end radical attacks the double bond of another acrylonitrile, opening it and extending the chain.
- Each addition regenerates a new radical at the chain end, allowing rapid growth.
- The nitrile group stabilises the radical intermediate by resonance, keeping the rate high.
Termination
- Combination: two radical chains couple, forming a single longer polymer.
- Disproportionation: one chain abstracts a hydrogen from another, giving one saturated and one unsaturated chain.
Precipitation polymerisation
Because PAN is poorly soluble in its monomer and many solvents, the polymerisation is usually carried out in a homogeneous phase until the growing chains reach a critical size. At that point the chains precipitate as micro‑particles, forming a solid phase that continues to swell with monomer. Termination is slowed by the physical restriction of radicals inside the precipitate, leading to a slight auto‑acceleration effect.
Industrial relevance
Free‑radical polymerisation is favoured industrially for its tolerance to impurities, scalability, and ability to control molecular weight via chain‑transfer agents. PAN produced in this way is the main precursor for high‑strength carbon fibres used in aerospace, automotive, and wind‑energy applications.