Supramolecular inclusion complexes (ICs) involving polyhedral oligomeric silsesquioxane (POSS) capped poly(epsilon-caprolactone) (PCL) and alpha-cyclodextrin (alpha-CD) were investigated. POSS-terminated PCLs with various molecular weights were prepared via the ring-opening polymerization of epsilon-caprolactone (CL) with 3-hydroxypropylheptaphenyl POSS as an initiator. Because of the presence of the bulky silsesquioxane terminal group, the inclusion complexation between alpha-CD and the POSS-capped PCL was carried out only with a single end of a PCL chain threading inside the cavity of alpha-CD, which allowed the evaluation of the effect of the POSS terminal groups on the efficiency of the inclusion complexation. The X-ray diffraction results indicated that the organic-inorganic ICs had a channel-type crystalline structure. The stoichiometry of the organic-inorganic ICs was quite dependent on the molecular weights of the POSS-capped PCLs. With moderate molecular weights of the POSS-capped PCLs (e.g., M, = 3860 or 9880), the stoichiometry was 1:1 mol/mol (CL unit/alpha-CD), which was close to the literature value based on the inclusion complexation of alpha-CD with normal linear PCL chains with comparable molecular weights. When the PCL chains were shorter (e.g., for the POSS-capped PCL of M-n = 1720 or 2490), the efficiency of the inclusion complexation decreased. The decreased efficiency of the inclusion complexation could be attributed to the lower mobility of the bulky POSS group, which restricted the motion of the PCL chain attached to the silsesquioxane cage. This effect was pronounced with the decreasing length of the PCL chains. (c) 2007 Wiley Periodicals, Inc.