Question

Give the initiation and propagations steps for the radical polymerization of styrene using benzoyl peroxide as the initiator.

Answer 1

Radical Polymerization of Styrene with Benzoyl Peroxide

1. Initiation Step

Benzoyl peroxide (BPO) decomposes thermally to form phenyl radicals, which initiate the reaction:

$${\text{(PhCOO)}}_2\stackrel{\mathrm{\Delta }}{\to }2{\text{PhCOO}}^{\bullet }\text{(Peroxide decomposition)}$$$${\text{PhCOO}}^{\bullet }\to {\text{Ph}}^{\bullet }+{\text{CO}}_2\text{(Decarboxylation)}$$$${\text{Ph}}^{\bullet }+{\text{CH}}_2=\text{CHPh (Styrene)}\to {\text{Ph-CH}}_2-{\text{CH}}^{\bullet }\text{Ph}\text{(Radical addition to monomer)}$$

2. Propagation Step

The styrene radical reacts with additional monomers to grow the polymer chain:

$${\text{Ph-CH}}_2-{\text{CH}}^{\bullet }\text{Ph}+n{\text{CH}}_2=\text{CHPh}\to {\text{Ph-(CH}}_2-{\text{CHPh)}}_n-{\text{CH}}_2-{\text{CH}}^{\bullet }\text{Ph}$$

• Mechanism:

• Radical attacks the vinyl group ($\text{C=C}$) of styrene.

• Chain extends via repeated addition.

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Key Notes:

• BPO Role: Provides radicals to start polymerization (half-life depends on temperature).

• 
  

• Termination: Occurs via radical coupling/disproportionation (not shown here).