Homogeneous free-radical polymerization of styrene in supercritical CO2

The free-radical polymerization of styrene has been studied in the homogene ous phase of supercritical (sc) CO2 at 80 degrees C and pressures between 2 00 and 1500 bar. 2,2'-Azobisisobutyronitrile is used as initiator and CBr4 as chain-transfer agent. The polymerization is monitored by means of online FT-IR/NIR spectroscopy. In the presence of CO2 a solution polymerization m ay be carried out up to a considerable degree of monomer conversion. At 500 bar, for example, maximum styrene conversions of 34.4 and 11.9% may be rea ched in homogeneous phase at CO2 contents of 16.8 and 44.5 wt.-%, respectiv ely. Analysis of the measured conversion-time profiles yields termination r ate coefficients, k(t), which are by one order of magnitude larger than k(t ) for styrene bulk polymerizations at identical temperature and pressure. T he enhanced termination rate in fluid CO2 is assigned to the poor solvent q uality of scCO(2) for polystyrene.