Reactions of ring systems, such as aziridines, oxetanes and oxiranes with carboxylic acids, anhydrides, imides, lactones and carbonate esters are catalyzed by C3-C60, substituted or unsubstituted, straight or branched-chained, alkyl, aryl, or aralkyl carboxylate Cr+3 salts, preferably chromium +3 octoate. The catalysts also accelerate the reaction of hydroxy compounds with anhydrides, and of thiiranes with anhydrides. The catalysts selectively enhance the conversion of ring systems to form monomers, prepolymers, copolymers, functional end-group monomers, functional end-group prepolymers, and functional end-group polymers rather than forming homopolymers. By varying the catalyst concentration, molar ratios, and reaction temperatures, the reaction time required to form the desired product can be controlled.