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.

Bacterial polyhydroxyalkanoates-eco-friendly next generation plastic: Production, biocompatibility, biodegradation, physical properties and applications

“…All PHAs are susceptible to degrade by hydrolysis to some extent and under normal conditions they are water stable (231). But the high degree of polymerization, high crystallinity, isotacticity (only the enantiomer of absolute configuration R is present in these polymers) and hydrophobicity are among the factors limiting the bioavailability of PHAs which can affect the rate of biodegradation in natural environment (231,232).…”

“…But the high degree of polymerization, high crystallinity, isotacticity (only the enantiomer of absolute configuration R is present in these polymers) and hydrophobicity are among the factors limiting the bioavailability of PHAs which can affect the rate of biodegradation in natural environment (231,232). PHA biodegradation corresponds to hydrolysis involving endo-or exo-enzymatic systems in the breaking cleavage of esters bonds.…”

“…In the case of therapeutic and biomedical uses, a simple hydrolysis is required. Hydrolytic degradation of PHAs is not evident and is depending on the structure particularly on the nature of the side chains and materials crystallinity (231). Therefore, PHAs need to have tunable hydrophilicity, chemical functionalities and appropriate hydrolytic stability to expand their therapeutic applications towards more advanced areas.…”

“…The first way to obtain more hydrophilic PHAs is synthetic strategies which consist in the preparation of unsaturated PHAs with polar functional groups such as carboxylic, hydroxyl or epoxy groups. The functionalization turns the reactive pendant double bonds into hydrophilic functions (231,232). Functional PHAs with hydrophilic groups such as carboxylic, amine or hydroxyl on the side chains were prepared by biotechnological syntheses but the fermentation conditions are difficult and generally polymers are obtained with very low yields (231,87).…”

“…The functionalization turns the reactive pendant double bonds into hydrophilic functions (231,232). Functional PHAs with hydrophilic groups such as carboxylic, amine or hydroxyl on the side chains were prepared by biotechnological syntheses but the fermentation conditions are difficult and generally polymers are obtained with very low yields (231,87). The most suitable products in regard to hydrolysis concern are PHAs containing carboxylic groups in side chains.…”