A series of polylactides tethered with polyhedral oligomeric silsesquioxane (POSS-PLAs) were synthesized via the ring-opening polymerization of L-lactide with 3-hydroxypropylheptaisobutyl polyhedral oligomeric silsesquioxane (3-hydroxypropylheptaisobutyl POSS) at a concentration of 0.02-2.00 mol % in the presence of a stannous(II) octoate catalyst. H-1-NMR spectra and a composition analysis of the POSS-PLA hybrids confirmed that 3-hydroxypropylheptaisobutyl POSS served as an initiator for L-lactide in the ring-opening polymerization. X-ray diffraction patterns evidenced that polyhedral oligomeric silsesquioxane (POSS) molecules of POSS-PLA hybrids were well dispersed without the formation of their crystalline aggregates. The POSS-PLA hybrid with 0.50 mol % POSS content was solution-blended with a neat polylactide (PLA) homopolymer to obtain PLA/POSS-PLA nanocomposites with various POSS-PLA contents of 1-30 wt %. The X-ray diffraction results of the PLA/POSS-PLA nanocomposites demonstrated that the POSS-PLA was well dispersed in the neat PLA matrix. The thermal and thermooxidative degradation properties of the nanocomposites were found to be improved at POSS-PLA contents of 1-20 wt %, compared to the neat PLA. The crystallization rates and crystallinities of the PLA/POSS-PLA nanocomposites were faster and higher, respectively, with increasing POSS-PLA content because of the nucleation effect of the POSS molecules in the neat PLA matrix. (C) 2009 whey Periodicals, Inc. J Appl Polym Sci 115: 1039-1046, 2010