Synthesis of Biodegradable Polymers Using Supercritical Carbon Dioxide As Reaction Medium

Biodegradable polymers are of high interest for applications in the medical sector due to their decomposition into safe compounds under physiological conditions.^[1-3] Applications in the biomedical sector require medical grade polymers. Therefore, polymerization processes that avoid the use of toxic substances are mandatory. The use of supercritical carbon dioxide (scCO₂) as reaction medium is particulary interesting. Firstly, CO₂ is non-toxic and gaseous at ambient conditions. Consequently, it may easily be removed from the product without chromatographic techniques and/or any energy intensive drying processes. Secondly, the physico-chemical properties may be tuned in a wide range by changes in temperature and pressure. Of particular interest is the density and the ability to dissolve other chemical compounds, which is directly related to the phase behavior of the systems. Another aspect is the processing of the material following the synthesis. Various processes are already established for the generation of particulate material.^[4] Recently, it was also shown that the type of crystalline phases may be influenced.^[5] In addition to the choice of reaction medium the selection of catalyst is crucial.

The focus of this contribution is on the ring-opening polymerization of diglycolide. It will be shown that scCO₂ has various advantages for the synthesis of polyglycolide (PGA). Besides using metal-organic compounds as catalyst enzymatic polymerizations were carried out. The polymerizations was performed in an optical high pressure cell, allowing for reaction pressures up to 1500 bar. Since the decomposition rate of the polymer strongly depends upon its molecular weight, molecular weight control during polymerization is of high importance. Thus, the reaction parameters such as temperature, pressure, monomer concentration, and reaction time were varied to obtain polymer material with well-defined molecular weights. In addition, the degradation rate of the polymer is strongly affected by the chemical structure of the polymer.^[3] Therefore additional monomers such as ε-caprolactone and ω-pentadecalactone were employed. Due to the low ring strain in ω-pentadecalatone enzymatic polymerizations of this monomer are particularly attractive. The enzymes Candida antarctica and Burkholderia cepacia were used.

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[2]        W. D. Hovis, R. W. Bucholz, Foot Ankle Int. 1997, 18, 128.

[3]        D. Hofmann, M. Entrialgo-Castaño, K. Kratz, A. Lendlein, Adv. Mater. 2009, 21, 3237.

[4]        S.-D. Yeo, E. Kiran, J. Supercrit. Fluids 2005, 34, 287.

[5]        D. Bolten, M. Türk, J. Supercrit. Fluids 2012, 66, 32.

[4]        T. Meyer, M. F. Kemmere, „Supercritical Carbon Dioxide- in Polymer Reaction Engineering“, 2005, Wiley-VCH, Weinheim.