(429b) 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.

[1]        K. A. Athanasiou, G. G. Niederauer, Biomaterials 1996,
17, 93.

[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.