The main place of solid-state graft polymerization in polypropylene (PP) granules has been believed to be the amorphous region of PP. In this work, the solid-phase morphology of nascent spherical PP (N-PP) granules was found to be markedly changed by an annealing treatment. The crystallinity of PP granules was almost doubled after annealing at 150 degrees C for 12 h, whereas the porosity of the granules was unchanged. Solid-state grafting polymerizations of styrene initiated by tert-butyl perbenzoate in both N-PP and annealed polypropylene (A-PP) granules were compared under different reaction conditions. The formation of gel in the product could be completely depressed at a low concentration of the initiator when A-PP granules were used as the matrix and graft-polymerized at 120 degrees C. Both the introduction of styrene and the annealing treatment of the PP granules led to a depression of polymer degradation in the process of the grafting reaction. However, using A-PP as the matrix caused an increase in the grafting degree at a relatively high concentration of the initiator. A reduction in the amorphous phase in the PP granules was thought to be the main reason for the effects of the annealing treatment on the structure of the graft polymerization products. (c) 2008 Wiley Periodicals, Inc.