21.13: Anionic Chain-Growth Polymerization: Overview
The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance, acrylonitrile, vinyl chloride, styrene, with one electron-withdrawing group, and methyl-α-cyanoacrylate, with two electron-withdrawing groups, are suitable monomers for anionic polymerization. The electron-withdrawing groups of the monomer stabilize the carbanion generated during the polymerization process. The polymerization of acrylonitrile, vinyl chloride, and styrene yields anionic polymers such as polyacrylonitrile, poly(vinyl chloride), and polystyrene. Polyacrylonitrile is used in knitted clothes as fibers, and globally it is popular under the trade names Orlon, Creslan, and Acrilan. Poly(vinyl chloride) finds application in shower curtains, garbage bags, and water pipes. Polystyrene is widely used for making foamed cups and disposable food containers.
The presence of two electron-withdrawing groups makes the monomer highly reactive. Consequently, the polymerization process gets initiated by even weak nucleophiles. For instance, polymerization of methyl-α-cyanoacrylate into superglue gets triggered by a trace amount of water or base present in the surroundings. Compounds similar to superglue also find medical applications as an adhesive used for closing wounds as a replacement for regular stitches. For example, Dermabond, a cyanoacrylate ester with a 2-octyl group, is often used to close wounds instead of stitches.
