The biosynthesis of medium chain length poly(3-hydroxyalkanoates)mcl PHAs by Pseudomonas guezennei using glucose, sodium octanoate, and 10-undecenoic acid as sole or mixed carbon sources was investigated. Chemical composition of polyesters was analyzed by GCMS and NMR. The copolyester produced by P. guezennei from glucose mainly consisted of 3-hydroxyoctanoate and 3-hydroxydecanoate, and the presence of 3-hydroxydodec-5-enoate was demonstrated. Using sodium octanoate as the sole nutrient, the microorganism produced a poly(3-hydroxyoctanoate) (PHO) polymer containing up to 94 mol% 3-hydroxyoctanoate. Biosynthesis of poly[(3-hydroxyoctanoate)-co-(3-hydroxyundecenoate)] (PHOU) copolymers bearing terminal reactive double bonds on its side chains with unsaturation degree ranging from 8.8% to 78.2% was obtained by tuning the ratio of sodium octanoate/10-undecenoic acid in the medium. Thermal analysis indicated semi-crystalline polymers with melting temperatures (T-m,) ranging from 46 to 55 degrees C, fusion enthalpy (Delta H) comprised between 3 and 35 J/g and glass transition temperature (T-g) from -36 to -44 degrees C, except for the highly amorphous 78.2% unsaturated PHOU with a low T-g (-50 degrees C). Molecular weights determined by GPC ranged from 119 000 and 530000 g/mol. The biosynthesis of natural polyesters with controlled ratio of vinyl-terminated side chains is of great interest for further chemical modifications. (C) 2012 Elsevier B.V. All rights reserved.