Polymerization procedure supercritical carbon dioxide

Supercritical carbon dioxide has been used as a dispersing medium for the manufacture and processing of polymeric materials. The process allows for the synthesis of high molar mass acrylic polymers in the form of micrometer-sized particles with a narrow size distribution. This procedure represents an environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations (Fox, 1994). 

We investigated the chemoenzymatic synthesis of block copolymers combining eROP and ATRP using a bifunctional initiator. A detailed analysis of the reaction conditions revealed that a high block copolymer yield can be realized under optimized reaction conditions. Side reactions, such as the formation of PCL homopolymer, in the enzymatic polymerization of CL could be minimized to < 5 % by an optimized enzyme (hying procedure. Moreover, the structure of the bifunctional initiator was foimd to play a major role in the initiation behavior and hence, the yield of PCL macroinitiator. Block copolymers were obtained in a consecutive ATRP. Detailed analysis of the obtained polymer confirmed the presence of predominantly block copolymer structures. Optimization of the one-pot procedure proved more difficult. While the eROP was compatible with the ATRP catalyst, incompatibility with MMA as an ATRP monomer led to side-reactions. A successfiil one-pot synthesis could only be achieved by sequential addition of the ATRP components or partly with inert monomers such as /-butyl methacrylate. One-pot block copolymer synthesis was successful, however, in supercritical carbon dioxide. Side reactions such as those observed in organic solvents were not apparent. 

One of new trends today in sol-gel chemistry is the use of supercritical carbon dioxide as a solvent during sol-gel polymerization or for extraction of the resulting product. The initial aUcoxysilane is mixed with anhydrous 99% HCO2H and sol-gel polymerization of the mixture is then carried out in supercritical CO2 in an autoclave (40°C, 41.4-55.2 MPa). Gel formation proceeds over 12h and then CO2 is slowly removed (8 h). This procedure favors for the formation of highly porous monolithic composites with the mesopore and macropore architectures. Components that sharply enhance the ability of Si02 to adsorb water may also be introduced at the stage of formation. For example, selective water adsorbents, such as CaCl2 and LiBr, bind up to 53 wt% of H20. " " ... 

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