Question

Explain why radical polymerization of styrene forms branched chains with 4° carbons as in A, but none with 3° carbons as in B.

Short answer

Answer

The reason behind the above phenomenon is that the formation of polystyrene chain requires a radical from the second polymer chain. The abstraction of Ha will result in the formation of resonance-stabilized radical A'.

Secondary radical B' is formed by abstraction of H_b without added resonance stabilization. Therefore, the abstraction of H_a is favorable. Thus, this radical goes on to form products with $4°$C’s (A).

Step-by-step solution

Radical polymerization

The process of formation of polymers by adding monomeric free radical units is called radical polymerization.

The formation of polystyrene using a radical inhibitor azoisobutyronitrile (AIBN) is an example of radical polymerization process.

Radical polymerization in A

The formation of a long polystyrene chain takes place when the phenyl group gets bonded to every other carbon atom. Branch formation takes place when there is abstraction of the H atom.

There will be the formation of resonance stabilized radical when H_a is abstracted. The abstraction of H_b will not result in resonance stabilization. Therefore, abstraction of Ha is favored, and this radical goes on to form products with $4°$C’s (A).

Diagrammatic representation

The diagrammatic representation of the phenomenon is shown below.

Abstraction of hydrogen atoms

Representation of favored product

Resonance forms