21.13: Cationic Chain-Growth Polymerization: Mechanism

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation, thereby producing a dimer that acts as a new carbocation. The propagation step repeats itself and builds the polymer chain. In the termination step, the propagating chain gets terminated either by adding a base that deprotonates the carbocation and forms a new π bond or by the attack of a nucleophile on the carbocation, where the nucleophile adds to the cationic end of the chain.

The cationic polymerization mechanism involves initiation, propagation, and termination steps.

In the initiation step, the Lewis acid catalyst, formed from boron trifluoride and water, protonates the π bond of a monomer, generating a carbocation stabilized by the electron‐donating group.

In the propagation step, the generated carbocation is attacked by the π bond of a second monomer, forming a dimer that acts as a new carbocation.

The propagation step repeats itself and enables the polymer chain to grow.

In the termination step, the growing polymer chain is terminated by adding a base that deprotonates the carbocation, forming a new π bond.

Alternatively, a nucleophile that attacks the carbocation could also be used.