Polypropylene (PP)/Ti-MCM-41 nanocomposites were prepared by isospecific propylene polymerization with Ti-MCM-41/Al(i-C4H9)(3) catalyst. The cross polarization/magic angle spinning (CP/MAS) C-13 NMR spectrum of the composite was similar to that of the conventional isotactic PP, and the decrease in the pore volume of Ti-MCM-41 in the nanocomposites, as measured by N-2 adsorption, was consistent with the value calculated from the weight loss in the thermogravimetric analysis (TGA) curve; both these facts attest to propylene polymerization within the mesopores of Ti-MCM-41. Alkali treatment followed by extraction with o-dichlorobenzene allows us to extract the confined PP out of the Ti-MCM-41 mesopores. Although the PP/Ti-MCM-41 nanocomposites do not exhibit a crystalline melting point, the same PP when extracted from the mesopores showed a clear melting point at 154.7degreesC. this indicates that the crystallization of PP confined in mesopores is strongly hindered. For the PP polymerized within the confinement, the molecular weight (M-w) and molecular weight distribution (M-w/M-n) were 84,000 and 4.3, respectively; these values were considerably smaller than those of the PP polymerized concurrently outside the Ti-MCM-41 mesopores (M-w=200,000-450,000, M-w/M-n=40-75). Therefore, the confinement also has a marked effect on the molecular weight of the PP. (C) 2003 Wiley Periodicals, Inc.