Cationic Chain-Growth Polymerization: Mechanism

The Lewis acid catalyst, formed from boron trifluoride and water, protonates the π bond of a monomer to generate a carbocation. This carbocation is stabilized by an electron-donating group on the monomer, initiating the polymerization process and enabling subsequent chain growth through repeated monomer additions.

During propagation, the π bond of a second monomer attacks the carbocation generated in initiation, forming a dimer that acts as a new carbocation. This step repeats continuously, with each new monomer addition extending the polymer chain through successive carbocation formation and nucleophilic attack.

Termination occurs through two mechanisms: adding a base that deprotonates the carbocation to form a new π bond, or using a nucleophile that attacks the carbocation directly. Both methods stop chain growth by neutralizing the reactive carbocation at the polymer chain end.

Carbocation stability is critical because the electron-donating group on the monomer stabilizes the carbocation formed during initiation and propagation. This stabilization ensures the carbocation remains reactive enough to attack successive monomers, allowing efficient chain growth and polymer formation throughout the polymerization process.

Cationic polymerization uses a Lewis acid catalyst to generate carbocations that drive chain growth, whereas anionic chain growth polymerization uses different initiators and mechanisms to generate carbanions. Understanding both mechanisms helps distinguish between positively charged and negatively charged intermediates in polymer synthesis and their respective roles in chain propagation.

In initiation, the π bond of the first monomer is protonated by the Lewis acid catalyst to form a carbocation. In propagation, π bonds of subsequent monomers act as nucleophiles attacking the growing carbocation. This dual role of π bonds drives both chain initiation and elongation throughout the polymerization mechanism.

In nucleophile-mediated termination, a nucleophile directly attacks the carbocation at the end of the growing polymer chain, adding to the cationic end and stopping further monomer incorporation. This provides an alternative to base-mediated deprotonation for controlling chain termination and determining the final structure of the polymer product.