Polymer CommunicationMelt/solid polycondensation of glycolic acid to obtain high-molecular-weight poly(glycolic acid)
Introduction
Poly(α-hydroxy acid)s are a group of aliphatic polyesters that are now attracting great attention as biodegradable polymers having wide applications [1]. These polymers can be assimilated into water and carbon dioxide in the natural environment and microorganisms. In particular, poly(glycolic acid) (PGA) and its copolymers have been most widely used as biomedical materials such as absorbable suture [2].
PGA is usually synthesized by ring-opening polymerization of glycolide that is a cyclic diester monomer prepared from glycolic acid (GA) via an oligomer Eq. (1)[3].
In this process, exhaustive purification of glycolide is needed to obtain high polymers to make the PGA products more expensive. For easier synthesis of PGA, simple polycondensation of GA should be established, although it has been believed to be an inadequate method to obtain high polymers [4]. In the polycondensation system of PGA, two principal equilibrium reactions exist. One is dehydration equilibrium for esterification Eq. (2), and the other is ring-chain equilibrium involving depolymerization to glycolide Eq. (3).
Ordinary melt polycondensation of GA gives a low-molecular-weight oligomer, which likely decomposes into glycolide to prevent the chain-growth of PGA. This is because the reaction conditions of high temperature and high vacuum can induce not only dehydration, but also the formation of glycolide in equilibrium with PGA. One possible way to obtain high polymer of PGA is to use catalysts that can enhance the dehydration rate of oligomer without stimulating its depolymerization to glycolide. In this study, we disclose a new method of polycondensation of GA which can be promoted by the catalysis of Zn(II) and by the polymer crystallization in solid state Eq. (4).
Section snippets
Materials
GA was purchased from Nacalai Tesque (Kyoto, Japan). 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was supplied by Central Glass Co. Ltd (Tokyo, Japan). Nafion® NR-50 was a product of Du Pont Co. SnO was prepared by alkaline hydrolysis of SnSO4[5]. Other reagents were commercially obtained. A commercial PGA sample, which was prepared by the ring-opening polymerization of glycolide, was supplied by Mitsui Chemical Corp. (Tokyo). Its number-average (Mn) and weight-average molecular weights (Mw) were
Results and discussion
When the polycondensation of GA was conducted above 200°C, the condensation product quickly became dark in color probably because of its thermal decomposition. When the same reaction was done below 180°C, the molecular weight (Mw) of the condensation product did not increase above 20,000 Da in spite of being free from discoloration. These data limited us to conduct the polycondensation around 190°C that was around the crystallization temperature (Tc) of PGA. In the early stage of the
Conclusion
PGA was prepared by the melt/solid polycondensation of GA. This dehydrative polycondensation was discovered to be effectively catalyzed by zinc acetate dihydrate. The weight-average molecular weight of PGA reached 91,000 that was similar to that of PGA prepared by the ring-opening polymerization of glycolide. This synthetic method will give a simple manufacturing process for making PGA and its copolymers.
Acknowledgements
This work was partly supported by a Grant-in-aid for Scientific Research on Priority Area, “Sustainable Biodegradable Plastics”, No. 11217210 (1999) from the Ministry of Education, Science, Sports and Culture (Japan).
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