Anionic Chain-Growth Polymerization: Overview

Anionic chain-growth polymerization is an addition polymerization process that proceeds through a carbanion intermediate. A strong nucleophile such as an organolithium or Grignard reagent initiates the reaction. The monomers must be ethylene derivatives with electron-withdrawing groups that stabilize the carbanion intermediate formed during polymerization.

Monomers for anionic polymerization must be vinyl compounds bonded to one or two electron-withdrawing groups. Examples include acrylonitrile, vinyl chloride, and styrene with single electron-withdrawing groups, or methyl-α-cyanoacrylate with two groups. The electron-withdrawing substituents stabilize the carbanion intermediate generated during the polymerization process.

Polyacrylonitrile, produced from acrylonitrile, is used in knitted textiles under trade names like Orlon and Acrilan. Poly(vinyl chloride) from vinyl chloride is used in shower curtains, garbage bags, and water pipes. Polystyrene from styrene is widely used for foam cups and disposable food containers.

Methyl-α-cyanoacrylate contains two electron-withdrawing groups, making it highly reactive. This allows even weak nucleophiles, such as trace amounts of water or base in the surroundings, to initiate rapid polymerization into superglue. The dual electron-withdrawing groups stabilize the carbanion intermediate exceptionally well.

Electron-withdrawing groups on the monomer stabilize the carbanion intermediate formed during anionic polymerization. Monomers with one electron-withdrawing group require strong nucleophile initiators, while those with two groups are highly reactive. Even weak nucleophiles can trigger polymerization when two electron-withdrawing groups are present on the monomer.

Strong nucleophiles such as organolithium compounds or Grignard reagents initiate anionic polymerization. Butyl lithium is the most commonly used initiator for this process. These nucleophiles attack the electron-deficient double bond of the monomer to generate the carbanion intermediate that propagates the polymer chain.

Cyanoacrylate esters like Dermabond, which contain a 2-octyl group, undergo rapid anionic polymerization when exposed to moisture or bases on skin. This fast polymerization creates a strong adhesive bond suitable for closing wounds as a replacement for traditional stitches in medical applications.