Chemical modifications of bacterial polyesters: From stability to controlled degradation of resulting polymers

Abstract

Polyhydroxyalkanoates (PHAs) form an enlarged family of biopolyesters, which are biocompatible, biodegradable and non-toxic. Polyhydroxyalkanoates biodegradation corresponds to a hydrolysis involving endo- or exo-enzymatic systems in the breaking cleavage of esters bonds. This type of degradation is needed for environmental applications. In the case of therapeutic and biomedical uses, a simple hydrolysis is required. Hydrolytic degradation of PHAs is not evident as shown on poly(3-hydroxyoctanoate) (PHO) and is depending on the structure of the polyester and more particularly on the nature of the side chains. In some cases, blending with others polymers has decreased PHAs crystallinity and has made easier hydrolysis. Another route consists in the preparation of unsaturated PHAs which can be chemically modified. Pendant double bonds have been turned into carboxylic, hydroxyl or epoxy groups. Moreover these reactive functions were used for grafting oligomers of hydrolysable polylactic acid (PLA) or hydrophilic polyethylene glycol (PEG). Otherwise block copolymers with polycaprolactone (PCL) have been prepared, aiming at nanoparticles formation in the view of drug release. Therefore, the hydrophilic/hydrophobic balance of these materials was controlled by chemical modification and their stability/hydrolysis has been studied. Results have shown that the most suitable products in regard to hydrolysis concern PHAs containing carboxylic groups in side chains, noted poly(3hydroxyoctanoate-co-9-carboxy-3-hydroxydecanoate) and its derivatives. Carboxylic groups promote water penetration into the polymer and participate to ester groups hydrolysis through better water penetration and catalysis.