Melt/solid polycondensation of glycolic acid to obtain high-molecular-weight poly(glycolic acid)

doi:10.1016/S0032-3861(00)00282-2

Polymer

Volume 41, Issue 24, November 2000, Pages 8725-8728

https://doi.org/10.1016/S0032-3861(00)00282-2 Get rights and content

Abstract

Bulk polycondensation of glycolic acid (GA) was studied to obtain high-molecular-weight poly(glycolic acid) (PGA). At first, a solid oligocondensate was prepared by melt-polycondensation of GA at 190°C, and then it was subjected to solid-state polycondensation at the same temperature to increase the molecular weight. After the catalyst screening, zinc acetate dihydrate was discovered to be the best catalyst. The weight-average molecular weight of PGA reached 91,000, which was at the same level with that of PGA prepared by the conventional ring-opening polymerization of glycolide. This process can afford a facile route to large-scale synthesis of PGA.

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 (M_n) and weight-average molecular weights (M_w) 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 (M_w) 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 (T_c) 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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Polymer CommunicationMelt/solid polycondensation of glycolic acid to obtain high-molecular-weight poly(glycolic acid)

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusion

Acknowledgements

J Polym Sci Part A: Polym Chem

J Chem Soc Perkin Trans I

J Biomed Mater Res

Macromol Chem Phys

Polymer Communication
Melt/solid polycondensation of glycolic acid to obtain high-molecular-weight poly(glycolic acid)