The important factors which control the stereochemistry of polymerization, and thereby the crystalline properties of the polymers so formed, are considered for f o u r different types of reactions including:( 1 ) the cationic polymerization of substituted a-methylstyrenes to form syndiotactic polymers; ( 2 ) the anionic ring-opening polymerization of substituted B-propiolactones to form crystalline polyesters; (3) the metathesis ring-opening polymerization of lr5-cyclooctadiene to form poly(l,3-butadiends of controlled cis-trans contents; and (4) the condensation polymerization of a-methylbenzyl chloride to form isotactic poly(a-methylben-ZYl).