Aliphatic polyesters biodegradation mechanisms

At the end of the 1990s, BASF commercialized Ecoflex F, a completely biodegradable statistical copolyester based on the fossil monomers 1,4-butanediol (BDO), adipic acid and terephthalic acid (see Fig. 3). Ecoflex F combines the good biodegradability known from aliphatic polyesters with the good mechanical properties of aromatic polyesters. 

Aliphatic polyesters occupy a key position in the field of polymer science because they exhibit the remarkable properties of biodegradability and biocompatibihty, which opens up a wide range of applications as environmentally friendly thermoplastics and biomaterials. Three different mechanisms of polymerization can be implemented to synthesize aliphatic polyesters (1) the ring-opening polymerization (ROP) of cyclic ketene acetals, (2) the step-growth polymerization of lactones, and (3) the ROP of lactones (Fig. 1). 

The polymerization of substituted lactones is an attractive strategy for extending the range of aliphatic polyesters and for tailoring important properties such as biodegradation rate, bioadherence, crystallinity, hydrophilicity, and mechanical properties [100]. Moreover, the substituent can bear a functional group, which can be very useful for the covalent attachment of drugs, probes, or control units. 

Aliphatic polyesters are the most economically competitive of the biodegradable polymers moreover, synthetic polyesters are expected to be degraded nonspecifi-cally by lipases. Although these polyesters are biodegradable, they often lack good thermal and mechanical properties. On the other hand, aromatic polyesters - such as... 

A controllable biodegradability, desirable mechanical properties, suitable gas permeability and selectivity would extend the potential application areas of aliphatic polyesters not only in agriculture or in the greenhouse or packaging industry but also as a substitute for human skin. There is a need for such focused studies in the future. 

Polyglycolide was one of the first synthetic polymers used as a degradable surgical suture [122]. Fig. 8 shows the glycolide monomer and polymer structures. This aliphatic polyester is biodegradable and exhibits negligible toxicity when implanted in tissue. It is also possible to fabricate a strong fiber of this polyester with satisfactory mechanical properties. 

Poly(butylene succinate) (PBS) is an important member of biodegradable aliphatic polyester family. PBS and related copolymers have shown considerable promise for uses as environmentally biodegradable thermoplastics, as well as bioabsorbable/biocompatible medical materials (/). In both cases, practical applications require that the polymer possess a high molecular weight (M >20,000) so that it can have useful mechanical properties. 

Aliphatic polyesters (such as PLA, polyglycolic acid and their copolymers) are the most important class of biocompatible polymers used in biomedical applications. This class of polymers has shown superior properties over conventional polymers, such as excellent biocompatibility, biodegradation, and thermal, physical and mechanical properties, which make them suitable for applications in drug delivery and tissue engineering [19-21]. 

A variety of copolymers having high molar ratios of PDS compared to other monomers within the same linear aliphatic polyester family have been reported for the purpose of improving the mechanical and biodegradation properties [Shalaby, 1994]. For example, copolymer of PDS (80%) and PGA (up to 20%) has an absorption profile similar to Dexon and Vicryl sutures but it has compliance similar to PDS. Copolymer of PDS (85%) and PLLA (up to 15%) results in a more compliant (low modulus) suture than homopolymer PDS but with absorption profiles similar to PDS [Bezwada et al., 1990). 

PBS is a linear aliphatic polyester with excellent mechanical properties and biodegradability. Its molecular formula is as follows ... 

Taniguchi I, Nakano S, Nakamura T, El-Salmawy A, Miyamoto M, Kimura Y (2002) Mechanism of enzymatic hydrolysis of poly(butylene succinate) and poly(butylene succinate-co-L-lactate) with a lipase from Pseudomonas cepacia. Macromol Biosci 2 447-455 Tomita K, Kuroki Y, Hayashi N, Komukai Y (2000) Isolation of a thermophile degrading poly(butylene succinate-co-butylene adipate). J Biosci Bioeng 90 350-352 Tserki V, Matzinos P, PavUdou E, Panayiotou C (2(X)6a) Biodegradable aliphatic polyesters. Part II. Synthesis and characterization of chain extended poly(butylene succinate-co-butylene adipate). Polym Degrad Stab 91 377-384... 

In an attempt to gain an element of control over the mechanical, physical and degradation properties of PLA, large-scale efforts are currently underway to chemically and physically modify PLA throng copolymerization and physical blending with prudently chosen secondary constituents. Without immediate consideration for the issue of financial accessibility of PLA-based products, a logical, con lementary conqx)nent to PLA is poly(8-caprolactone)[PCL]. PCL is a flexible, semi-crystalline(Tg -60°C, Tm 60°C) aliphatic polyester with unique biodegradation characteristics and superior thermal and hydrolytic stabihty. ... 

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