Cationic Chain-Growth Polymerization: Mechanism

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.