A polyhedral oligomeric silsesquioxane containing seven isobutyl groups and one mercaptopropyl group (POSS-SH) was linked via a free radical addition reaction to the side-chain double bonds of bacterial poly [3-hydroxyalkanoate-co-3-hydroxyalkenoate], (PHAE). PHAE with 11.5, 55 78. and 97 mol% of double bonds were used to produce a series of inorganic-organic hybrid materials, PHAE-POSS, with increasing amounts of POSS. The reactions proceeded with high yields and limited side reactions, confirmed with NMR and GPC experiments. X-ray diffraction and C-13 solid-state NMR analysis of these nanocomposite materials suggested the favoured crystallization of POSS. with PHAE functioning as a non-crystalline combining matrix. This is in contrast to blends of POSS-SH and PHAE, where incompatibility occurred and the two components crystallized independently. The covalent links between PHAE and POSS imposed spatial constraint which resulted in disordered crystalline POSS domains and limited crystal sizes. With increasing POSS content, the appearance of these POSS-based biopolyesters changed from non-sticky and elastic to brittle and glass-like. The covalent linking of POSS-SH to PHAE increased the heat stability such as glass transitions, and melting points could be tailored between 48 and 120 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.