Chapter 26: Q3P (page 1326)

Give a mechanism, using Figure 26-1 as a guide, showing chain branching during the free-radical polymerization of styrene. There are two types of aliphatic hydrogens in the polystyrene chain. Which type is more likely to be abstracted?

Short Answer

Expert verified

That hydrogen in the polystyrene chain will be abstracted after which radical becomes stable. Benzylic hydrogen will get abstracted and not secondary hydrogen as benzylic radical is tertiary as well as resonance stabilized.

Step by step solution

Step-1. Abstraction of benzylic hydrogen from polystyrene chain:

Benzylic hydrogen will get abstracted rather than secondary hydrogen as radical formed after benzylic hydrogen abstraction will be more stable than radical formed after secondary hydrogen abstraction, reason being resonance stabilised and it is tertiary. The radical from growing polystyrene chain abstracts benzylic hydrogen which leads to formation of new benzylic radical and growing polystyrene chain gets terminated.

Abstraction of benzylic hydrogen and formation of new benzylic radical

Step-2. Free-radical polymerisation of styrene and branching:

Styrene monomer gets attached to newly formed benzylic radical from polystyrene chain via coupling reaction of free radicals and this also leads to branching in the polystyrene chain as benzylic radical lies in middle of the chain.