Process for the modification of a polymer or copolymer having the following general structure for one or more of the repeating units: (1) wherein n is an integer, m is an integer in the range 0 to 6, and R is selected from hydrogen, substituted or unsubstituted C.sub.1-C.sub.20 alkyl, C.sub.3-C.sub.20 cycloalkyl, C.sub.6-C.sub.20 aryl, C.sub.7-C.sub.20 aralkyl, and C.sub.7-C.sub.20 alkaryl, which groups may include linear or branched alkyl moieties; the optional one or more substituents being selected from the group consisting of hydroxy, alkoxy, linear or branched alk(en)yl, aryloxy, halogen, carboxylic acid, ester, carboxy, nitrile, and amido groups, which process involves contacting the polymer or copolymer with a cyclic organic peroxide under conditions whereby at least some of said peroxide is decomposed. This process results in a (co)polymer with a high degree of branching but free of gel formation. ##STR00001##