The instability of polylactic acid hydrolysis is specifically due to the presence of ester bonds in its main chain. The degradation process is erosive and appears to have two steps in the conversion of the polymer into its monomer, lactic acid: first, water diffuses into the amorphous region of the polymer matrix, causing the ester bonds to break; secondly, after the erosion of the amorphous region, the crystalline region of the polymer becomes easily hydrolyzed and splits. The polymer chains in the crystalline region break down and collapse. When exposed to physical conditions, the polymer degrades via free water, which is also obviously broken down by some specific enzymes, particularly those with ester activity. The degradation product, lactic acid, is non-toxic and can enter the tricarboxylic acid cycle, after which it is converted into water and carbon dioxide and excreted. Some lactic acid is also excreted through urine.
Research indicates that sutures made from polyglycolic acid lose half of their material strength within two weeks, and almost all by four weeks. The polymer is completely absorbed by the body's tissues within four to six months. The degradation rate is faster in the body than in vitro, a phenomenon attributed to the activity of cellular enzymes.
The polymerization of glycolic acid is a simple process for producing PGA, but it is not effective as it yields low molecular weight products. The process involves the following simple steps: heating glycolic acid under atmospheric pressure to approximately 175-185°C until no more steam is released, followed by reducing the pressure to 150 mmHg while maintaining the temperature for about two hours, resulting in the production of low molecular weight PGA.
The common process for producing high molecular weight polymers involves lactide ring-opening polymerization. Lactide can be obtained by减压加热low molecular weight PGA, and collected through distillation. The ring-opening polymerization of lactide can be catalyzed by various catalysts, including antimony compounds like antimony trioxide or trichloride antimony, zinc compounds (zinc lactate), and tin compounds such as stannous octoate or stannous alcohol.
