Q7(d). Radical chain polymerisation of vinyl chloride with benzoyl peroxide

Introduction. PVC is made by radical chain-growth polymerisation of vinyl chloride, CH₂=CHCl. Industrially it is a suspension process: droplets of liquid monomer (under pressure) dispersed in water, about 50–70°C, with an oil-soluble peroxide initiator. The chain grows in three stages: initiation, propagation and termination, with chain transfer as a fourth step that sets the chain length.

Scheme: Initiator and the head-to-tail polymer.

1. Initiation

1. The weak O–O bond of benzoyl peroxide breaks on heating (homolysis): (PhCOO)₂ → 2 PhCOO•. Some benzoyloxy radicals lose CO₂: PhCOO• → Ph• + CO₂.
2. The initiator radical (R• = PhCOO• or Ph•) adds to the CH₂ end of the monomer: R• + CH₂=CHCl → R-CH₂-CHCl•. Addition at CH₂ gives the radical on the Cl-bearing carbon, which is more substituted and is stabilised slightly by the Cl lone pair.

2. Propagation

The chain radical adds to further monomer, each time at the CH₂ end: head-to-tail enchainment.

R(CH₂CHCl)_(n)CH₂CHCl• + CH₂=CHCl → R(CH₂CHCl)_(n+1)CH₂CHCl•

Thousands of monomers add in a fraction of a second; the step is strongly exothermic, and the heat is removed through the water phase.

3. Termination

1. Combination: two chain radicals combine, Pₘ• + Pₙ• → Pₘ₊ₙ.
2. Disproportionation: one radical takes an H atom from the other, giving one saturated and one unsaturated chain end.

4. Chain transfer (important in PVC)

The growing radical abstracts Cl or H from a monomer molecule more easily than in other vinyl polymerisations: the chain stops and a new monomer radical starts another chain. Chain transfer to monomer therefore controls the molar mass of PVC, which is set mainly by the temperature. Transfer to polymer gives occasional branches.

Structural features that follow

• Mainly head-to-tail -CH₂-CHCl-CH₂-CHCl⁻; a few head-to-head links and allylic chloride or tertiary chloride defects are the weak points from which HCl loss (thermal degradation) starts.
• The polymer is largely atactic, with low crystallinity; it is insoluble in its own monomer and precipitates as grains inside the droplets.

Conclusion. Benzoyl peroxide homolyses to PhCOO•/Ph•, which add to CH₂=CHCl; the chain radical adds monomer head-to-tail (propagation); chains stop by combination or disproportionation, and chain transfer to monomer limits the molar mass: -[CH₂-CHCl]_(n)⁻.

Application. Rigid PVC is used for water pipes, window frames and cards; plasticised PVC (with phthalate or other plasticisers) for cable insulation, flooring and medical tubing; heat stabilisers are added because of the allylic chloride defects.