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 C₁ to C₁₈ acyl, optionally substituted C₁ to C₁₈ 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.