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Propagation rate coefficients of styrene and methyl methacrylate in supercritical CO2

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Propagation Rate Coefficients of Styrene and Methyl Methacrylate in Supercritical CO2 Alex M. van Herk* and Bart G. Manders Eindhoven University of Technology, Laboratory of Polymer Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Dorian A. Canelas, Murat A. Quadir, and Joseph M. DeSimone* University of North Carolina at Chapel Hill, Department of Chemistry, CB# 3290, Venable and Kenan Laboratories, Chapel Hill, North Carolina 27599-3290 Received March 12, 1997 Revised Manuscript Received June 11, 1997 Introduction. Near-critical or supercritical carbon dioxide is rapidly becoming an attractive alternative as a continuous phase in homogeneous1 and heterogeneous radical polymerizations.2 A review on chain polymer- izations in supercritical fluids appeared recently.3 At present, CO2 offers an environmentally sound solvent choice. Advantages offered include that the polymer can be easily separated from the reaction medium, the CO2 can easily be recycled, and there is effectively no chain transfer to CO2 for free-radical reactions.3 In order to model the kinetics of homogeneous and heterogeneous free-radical polymerizations, it is necessary to obtain important kinetic coefficients like the initiation, propa- gation, and termination rate coefficients in CO2. It must be realized that CO2 is in general a poor solvent for most polymers except for amorphous or low-melting fluo- ropolymers and siloxanes. Therefore, the first refer- ences to free-radical polymerizations in CO2 typically deal with precipitation polymerizations.4 Homogeneous polymerizations in supercritical CO2 have been performed for fluorinated monomers.1 The decomposition kinetics and initiator efficiencies in CO2 differ from that in other media. The rate constant for AIBN decomposition was 2.5 times slower in CO2 than in benzene. This was attributed to the low dielectric constant of CO2 relative to benzene. Initiator efficien- cies were very high (>80%) as a consequence of negli- gible solvent cage effects in supercrit

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