Fragmentation was observed in the polymerization process for the preparation of a polypropylene (PP)/poly(ethylene-co-propylene) (EPR) in situ alloy. Composition distributions of different polymer particles were analyzed by preparative temperature-rising elution fractionation. The fractions eluted at room temperature and 96, 110, and 117 degrees C were selected for C-13-NMR characterization. There was more propylene homopolymer and ethylene-propylene block copolymer in the large particles, whereas the small particles contained more ethylene-propylene random copolymer and copolymer with a transition microstructure. On the basis of the formation mechanism of various components in the PP/EPR alloy, we inferred that the fragmentation of the polymer particles mainly took place in the copolymerization step. (c) 2005 Wiley Periodicals, Inc.