Chemical Structure Branched structures
Substituents on simple organic molecules the reaction mixtures then need dilution with water, extraction, and evaporation and cause difficulties in solvent recovery and generate contaminated effluent water for treatment. However, using a 10% solution of DMF or DMSO in toluene might be worthwhile because the increased reaction rates can be combined with easier solvent recovery.
DeSimone and colleagues demonstrated the use of supercritical CO2 as a medium for dispersion polymerizations. They used CO2 and a specially engineered, free-radical initiator and polymeric stabilizer to effect the polymerization of methyl methyacrylare.
The stabilizer contains a carbon-dioxide-phobic background, which attaches to the growing polymer particle, and a carbon-dioxide-philic side-chain, which is soluble in the supercritical medium and stabilizes the polymer colloid as the reaction proceeds. Use of the stabilizer allows high degrees of polymerization, leading to micrometer-size particles with a narrow size distribution.
Carbon dioxide can be easily separated from the reaction mixture since the pressure can be released and the gas vented to the atmosphere. This scheme offers the possibility of avoiding the generation of hazardous waste streams, including aqueous streams contaminated with leftover monomer and initiator, which is one of the plastic industry's greatest cleanup problems.
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