Amorphous biodegradable polyesters polymer

Polylactide is a kind of biodegradable polyester that possesses many desirable properties such as non-toxicity, hydrolyzability and biocompatibility for use for varied biomedical purposes such as sutures, fracture fixation, oral implant and drug delivery microspheres.It is popularly synthesized by the ringopening polymerization of lactide monomers which are the cyclic dimers of lactic acid. Polymerization of racemic D,L-lactide typically results in atactic, amorphous polymers named poly(D,L-lactide) (rg 60 °C), whereas polymerization of L-lactide or D-lactide results in isotactic, semicrystalline polymers called poly(L-lactide) or poly(D-lactide) (7" 180 PLA fractures... 

The control of biodegradation rate is of critical importance for many applications of degradable polymers. Amorphous polyesters absorb water and hydrolyse much more rapidly than crystalline materials. Consequently, in partially crystalline polymers, hydrolysis occurs initially in the amorphous phase and continues more slowly in the crystalline phase. This selective degradation leads to an increase in crystallinity by chemicrystallisation. A very similar selective abiotic oxidation process occurs in the semi-crystalline polyolefins which fragment rapidly due to failure at the crystallite boundaries. 

There are many kinds of bio-based and biodegradable polymers, among which one of the most promising is poly(lactic acid) (PLA), a biocompatible thermoplastic aliphatic polyester. PLA is a linear thermoplastic polyester produced by the ring-opening polymerization of lactide. Ln general, commercial PLA grades are copolymers of poly(L-lactic acid) and poly(D,L-lactic acid), which are produced from L-lactides and D,L-lactides, respectively. The ratio of L-enantiomers to o,L-enantiomers is known to affect the properties of PLA, i.e. if the materials are semicrystalline or amorphous. Until now, most of the efforts reported in order to improve the properties of PLA have been focused on the semicrystalline material. ... 

The biodegradation of polyesters is also affected by the chemical structure, hydrophilic/hydrophobic balance within the main chain, MW and crystallinity, which acts as a hindrance to biodegradability. In fact, biodegradation first takes place in the amorphous region of the polymer, where the erosion rate is much higher than in... 

The accessibility of the polymer to water-borne enzyme systems is vitally important because the first step in the biodegradation of plastics usually involves the action of extracellular enzymes which break down the polymer into products small enough to be assimilated. Therefore, the physical state of the plastic and the surface offered for attack, are important factors. Biodegradability is usually also affected by the hydrophilic nature (wettability) and the crystallinity of the polymer. A semicrystalline nature tends to limit the accessibility, effectively confining the degradation to the amorphous regions of the polymer. However, contradictory results have been reported. For example, highly crystalline starch materials and bacterial polyesters are rapidly hydrolysed. 

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