Aliphatic hydrolytic degradation

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydro-... 

Aliphatic polyesters degrade chemically by hydrolytic cleavage of the backbone ester bonds [38,92,93,143-145] and by enzymatic promotion [35,146]. Hydrolysis can be catalyzed by either acids or bases [38]. Polyester hydrolysis is schematically illustrated and exemplified for PLA in Fig. 5. Carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is denoted autocatalysis [147] and makes polyester matrices truly bulk eroding [38,43]. Degradation products are resorbed by the body with a minimal reaction of the tissues [8,15,95,148]. 

Polyanhydrides Polyanhydrides have a hydrophobic backbone with a hydrolytically labile anhydride linkage. These polymers widely vary in chemical composition and include aliphatic, aromatic, and fatty acid-based polyanhydrides. The rate of degradation depends on the chemical composition of the polymer. In general, aliphatic polyanhydrides degrade more rapidly than the aromatic polymer. Hence, copolymer blends with varying ratios of aliphatic-to-aromatic polyanhydrides can be synthesized to suit specific applications. 

Polymers do not normally biodegrade until they are broken down into fairly low molecular weight chemical species that can be assimilated by microorganisms. Aliphatic polyesters degrade in the presence of water by sequential hydrolytic cleavage of the backbone ester bonds leading to production of monomeric hydroxyl carboxylic acids which in turn can be metabolized by bacteria (5). 

This would support the fact that PEG-HZ-PE conjugates containing hydrazone bond derived from the aliphatic aldehyde are more prone to hydrolytic degradation. Aromatic aldehyde-derived hydrazone bond is too stable for the purpose of pH-triggered drug release. Careful selection of an aldehyde and an acyl hydrazide would be necessary for the application of the hydrazone-based chemistry for the development of pH-sensitive pharmaceutical nanocarriers. 

The aliphatic-aromatic diacid monomers were prepared from the reaction of bromoalkanoic acid methyl ester and p-hydroxy benzoic acid methyl ester. The polymers of carboxyphenoxy alkanoic add of n = 3, 5, and 7 methylenes were soluble in chlorinated hydrocarbons and melted at temperatures below 100 °C. Copolymers of these monomers melted at lower temperatures than the respective homopolymers. These polymers displayed zero-order hydrolytic degradation profile ranging from 2 to 10 weeks. Increasing the length of the alkanoic chain, decreased the degradation rate of the polymer (Fig. 3). 

Poly(lactic acid)s and their copolymers belong to the family of aliphatic polyesters therefore, their ester groups are hydrolytically degraded in the presence of water according to the following reaction ... 

Li, S. (1999) Hydrolytic degradation characteristics of aliphatic polyesters derived from lactic and glycolic acids. Journal of Biomedical Materials Research, 48,342-353. 

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