Question

The mechanism given for cationic polymerization of isobutylene (Mechanism 26-2) shows that all the monomer molecules add with the same orientation, giving a polymer with methyl groups on alternate carbon atoms of the chain. Explain why no isobutylene molecules add with the opposite orientation.

Short answer

Tertiary carbocation

In case of isobutylene, the growing chain will react at the less substituted carbon generating more highly substituted carbocation and polymer thus formed will have methyl groups on alternate carbon atoms of the chain.

Step-by-step solution

Step-1. Cationic polymerisation:

Cationic polymerisation is given by vinyl monomers which have electron rich double bond. It is used for the preparation of polymers which offers technological importance such as polybutene, polyisobutylene etc. It is a chain-growth type of polymerization in which monomer is made reactive by the help of cationic initiator.

Step-2. Monomer orientation in cationic polymerisation of isobutylene:

Addition of the monomer will occur in such a way that more stable intermediate gets generated. In case of isobutylene, the growing chain will react at the less substituted carbon generating more highly substituted tertiary carbocation and polymer thus formed will have methyl groups on alternate carbon atoms of the chain. All the monomer molecules add in the same orientation. If isobutylene monomer adds in opposite orientation, then primary carbocation will get generated which is not stable.