Development of methods to study features of intermediates and final products in synthesis of lactides and glycolides

Copolymers of lactide and glycolide (PGLA) are widely used in medicine: surgery, orthopedics, oncology, etc. These polymers can be used to produce surgical sutures, orthopedic implants, thermoplastics, plates, screws, pins, etc. The given paper represents the results of the development of measurement techniques to monitor the quality of glycolides and lactides, starting materials for the synthesis of biodegradable polymers. The main controlled parameters of the monomer, lactide and glycolide affecting the process of PGLA copolymer polymerization, are the presence of carboxyl groups in monomers, the ratio between L and D isomers in lactide, and oligomer presence of glycolide. It was stated that the presence of carboxyl group in monomers influences negatively on the process of lactide and glycolide polymerization. To produce highly molecular polymer the residual concentration of carboxyl group should not exceed 10 mol/g. In case the monomer does not comply with this requirement, the recrystallization from hot solution is used to purify the monomer. Ethyl acetate is usually used. If the conditions are correct, the purity of monomer after single recrystallization is improved by one order of magnitude. We developed a measurement technique based on potentiometric method of determination of pH of glycolide or lactide solutions after dissolving in methanol and recalculation of the obtained value into mass fraction of impurities containing carboxyl group. To polymerize the PGLA copolymer it is necessary to have the L/D isomer ratio in lactide of 50/50. If L isomer is predominant the polymer receives the crystalline structure. If D isomer prevails the polymer receives amorphous properties. We have developed a measurement technique to control the ratio between mass fractions of isomers in lactide based on optical rotation. The direction and angle of rotation of initial linearly-polarized emission depend on the nature and concentration of active component in the medium. It was shown that L and D lactides have opposite directions of optical rotation. During the work-out of the glycolide synthesis it was stated that the obtained glycolide contains the residual amounts of glycolic acid and oligomers adulterating the properties of the received polymers and copolymers. The determination of the presence/absence of oligomers in monomers was carried out on the basis of changing of dissipation in the solution. Visual observation of glycolide/lactide solution opalescence was carried out in ethyl acetate. Automatic comparison of transmission spectra was also carried out prior to and after the centrifuge processing of the solution. The molecular-mass features of the PGLA copolymer, produced from glycolide and lactide, influencing the mechanical properties and time of decomposition of the polymer. If the molecular mass of the copolymer is less than 60,000 a.m.u. it will not be suitable to produce monofilament thread, will be crispy and unductile. The time of copolymer resorption will also shorten. The molecular mass distribution of copolymer was determined using liquid chromatography according to the developed measurement technique based on the difference between the time of eluation of polymers with different molecular mass from chromatographic column with subsequent calculation of molecular mass distribution according to the method of absolute standardization. A mixture of acetonitrile and water was used as an eluent.

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