Metallocene-catalyzed propylene-ethylene copolymer (PER) having a terminal vinylidene group, reacted with maleic anhydride without free radical conditions to prepare terminally MAH-functionalized PER. Through the analysis of the obtained polymer by C-13 NMR spectroscopy, it was shown that a large part of a terminal vinylidene group isomerizes to a more stable internal double bond in this reaction and that the PER-MAH is formed via the reaction of MAH with isomerized PER and original PER.