Polymers with Hydrolyzable Backbones: A Review

Polymers are large molecules composed of repeating subunits called monomers, and their unique properties make them widely used in various industrial and biomedical applications. However, the accumulation of non-biodegradable synthetic polymers in the environment has become a major concern. The development of biodegradable polymers, therefore, has become an active area of research in recent years. In this article, we will discuss the properties and biodegradability of several polymers with hydrolyzable backbones.

Aliphatic polyesters are among the few synthetic chemical compounds of high molecular weight that have been shown to be biodegradable. The strongly hydrolyzable backbones of these compounds make them susceptible to biodegradation by fungi, such as Aspergillus niger and Aspergillus flavus. It has been observed that polyesters derived from diacids of medium-sized monomers (C6-C12) are more easily degraded than those derived from longer or shorter monomers. Flexible aliphatic polymers can be degraded under enzyme catalysis, whereas their rigid counterparts may not be degraded.

Polyglycolic acid (PGA) is the simplest linear aliphatic polyester and is widely used as a degradable and absorbable suture. PGA and its copolymer poly(glycolic acid-co-lactic acid) (PGA/PL) can be degraded through simple hydrolysis of the ester backbone in aqueous surroundings, such as body fluids. The breakdown products are eventually metabolized to carbon dioxide and water or excreted from the body through the kidney.

Polycaprolactone (PCL) is another aliphatic polyester that has been thoroughly examined as a biodegradable medium and as a matrix in controlled drug-release systems. PCL can be broken down enzymatically by fungi. The ε-caprolactone polymerization process is used to produce PCL.

Polyamides, which contain the same amide linkage as polypeptides, have slow biodegradation rates and are regarded as non-biodegradable. However, their degradation to low-molecular-weight oligomers under the influence of enzymes and microorganisms has been reported. Introduction of benzyl, hydroxy, and methyl substituents greatly improves polyamide biodegradation. The higher crystallinity of polyamides caused by strong interchain relations is responsible for the low observed biodegradation levels. Copolymers containing both amide and ester groups are easily degraded, and the degradation rates increase with increasing ester content.

Unlike natural protein structures that are composed of non-repeated units, synthetic polyamides have short and regular repeating units. Their higher symmetries and strong hydrogen interchain bonds give rise to highly ordered crystalline morphologies that decrease their accessibility to enzyme attack. However, polyamide esters and polyamide urethanes with long repeating chains undergo degradation at rates intermediate between those of proteins and synthetic polyamides.

Polyurethanes combine the structural characteristics of polyesters and polyamides, and their susceptibilities to biodegradation depend on their structures. Generally, it has been found that polyurethane biodegradation is conditioned by whether a basic polymer is a polyester or a polyether. Polyurethanes with structures based on polyethers are resistant to biodegradation, whereas polyester polyurethanes are susceptible to it. Many microorganisms and enzymes are highly effective in polyurethane degradation.

In conclusion, the biodegradability of polymers with hydrolyzable backbones is determined by their structures, especially the lengths of their repeating units and the presence of specific functional groups. The development of biodegradable polymers is essential for reducing the environmental impact of synthetic polymers

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