A process for modifying a polypropylene (co)polymer wherein said process comprises melt mixing the polypropylene (co)polymer in the presence of an initiator wherein said initiator is selected from the group defined by formula (1), wherein R is selected from the group consisting of optionally substituted C.sub.1 to C.sub.18 acyl, optionally substituted C.sub.1 to C.sub.18 alkyl, aroyl defined by formula (2), and compounds of formula (3), wherein U, V, X, Y, Z, U', V', X', Y' and Z' are independently selected from the group consisting hydrogen; halogen; C1-C18 alkyl; C1-C18 alkoxy, aryloxy, acyl, acyloxy, aryl, carboxy, alkoxycarbonyl, aryloxycarbonyl, trialkyl silyl, hydroxy, or a moiety of formula (4), and wherein T is alkylene.
