Step growth reaction
The step growth reaction involves condensation reaction of two different functions A and B, present on two different molecules. The linkage between the molecules is formed by elimination of smaller molecules such as water, alcohol, HCl, CO2 and other molecules. For example the polyester oligomer is formed by condensation reaction of an acid function with an alcohol eliminating H2O molecule and can react further with either a monomer or an oligomer. Condensation reactions are typically catalyzed by acid, base and/or metal ions.
Polymers such as polyester , polyurethane or polyamides are formed by step growth condensations. Polyester is formed from the reaction of a diacid with a dialocohol catalyzed by toluene sulfonic acid or metal salts. Polyurethane is produced from condensation of di-isocyanate and di-alcohol catalyzed by tertiary amines (1,4 –diazabibicyclo(2,2,2) octane and metal salts while polyamides (nylons) are synthesized by condensation of diamine and diacid.
Chain growth reaction
Chain growth polymerization involves reaction of unsaturated monomer compounds such as vinylic, acrylic or dienic or strained heterocyclic monomers. Reaction is initiated by formation of an active site on a monomer and is a slow process. It is followed by rapid propagation by addition of monomers to active sites by opening of double bond or ring. Then there is transfer of active site to macromolecular polymer to end its growth and begin the further growth of another. Finally, in the termination step destruction of active sites occurs. The active sites for chain growth polymerization include unpaired electrons as in free radical polymerization, anions having carbon –metal or alkoxide , cations such as carbenium or oxonium ions and co-ordination bonds with transition metals in Ziegler –Natta or metallocene catalysts.
Polymers produced via chain polymerization include polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyvinyl esters, acrylonitrile etc.