Aliphatic polyesters are a group of synthesized homopolymers or copolymers. They are nontoxic and can easily be fabricated into a variety of novel devices, such as rods, screws, nails, and cylinders. The polymers are commercially available in varying molecular weights as both homopolymers and copolymers. Molecular weights of polyesters range from 2000 Da to greater than 100 000 Da.
Co-monomer ratios of lactic acid and glycolic acid (or lactide and glycolide) for poly (DL-lactide-co-glycolide) range from 85: 15 to 50: 50. Table I shows the chemical and trade names of different commercially available aliphatic polyesters.
Empirical Formula and Molecular Weight
Aliphatic polyesters are synthetic homopolymers or copolymers of lactic acid, glycolic acid, lactide, glycolide and e-hydroxycaproic acid. Typically, the molecular weights of homopolymers and copolymers range from 2000 to >100 000 Da.
Functional Category: Bioabsorbable material; biocompatible material; biodegradable material.
Applications in Pharmaceutical Formulation or Technology
Owing to their reputation as safe materials and their biodegradability, aliphatic polyesters are primarily used as biocompatible and biodegradable carriers in many types of implantable or injectable drug-delivery systems for both human and veterinary use. Examples of implantable drug delivery systems include rods, cylinders, tubing, films, fibers, pellets, and beads. Examples of injectable drug-delivery systems include microcapsules, microspheres, nanoparticles, and liquid injectable controlled-release systems such as gel formulations.
For typical physical and mechanical properties of the aliphatic Polyesters. Polymer composition and crystallinity play important roles in the solubility of these aliphatic polyesters. The crystalline homopolymers of glycolide or glycolic acid are soluble only in strong solvents, such as hexafluoroisopropanol. The crystalline homopolymers of lactide or lactic acid also do not have good solubility in most organic solvents.
However, amorphous polymers of DL-lactide or DL-lactic acid and copolymers of lactide or lactic acid with a low glycolide or glycolic acid content are soluble in many organic solvents. Aliphatic polyesters are slightly soluble or insoluble in water, methanol, ethylene glycol, heptane, and hexane.
Method of Manufacture
Generally, aliphatic polyesters can be synthesized via polycondensation of hydroxycarboxylic acids and catalytic ring-opening polymerization of lactones. Ring-opening polymerization is preferred because polyesters with high molecular weights can be produced. Moreover, the dehydration of hydroxycarboxylic acids to form lactones does not have to be carried to a high degree of completion.
Lactones can easily be purified owing to the differences of their physical and chemical properties from those of the corresponding hydroxycarboxylic acid. The esterification of the carboxylic acid end group makes polymers more hydrophobic, which decreases the hydrolytic degradation rate of the polymers in the presence of water or moisture.