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, J. M. (1997). Propagation rate coefficients of styrene and methyl methacrylate in supercritical CO2. Macromolecules, 30(13), 4780-4782. DOI: 10.1021/ma9703416 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Introduction. Near-critical or supercritical carbon dioxide is rapidly becoming an attractive alternative as a continuous phase in homogeneous 1 and heterogeneous radical polymerizations. 2 A review on chain polymerizations in supercritical fluids appeared recently. 3 At present, CO 2 offers an environmentally sound solvent choice. Advantages offered include that the polymer can be easily separated from the reaction medium, the CO 2 can easily be recycled, and there is effectively no chain transfer to CO 2 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, propagation, and termination rate coefficients in CO 2 . It must be realized that CO 2 is in general a poor solvent for most polymers except for amorphous or low-melting fluoropolymers and siloxanes. Therefore, the first references to free-radical polymerizations in CO 2 typically deal with precipitation polymerizations. 4 Homogeneous polymerizations in supercritical CO 2 have been performed for fluorinated monomers. 1 The decomposition kinetics and initiator efficiencies in CO 2 differ from that in other media. The rate constant for AIBN decomposition was 2.5 times slower in CO 2 than in benzene. This was attributed to the low dielectric constant of CO 2 relative to benzene. Initiator efficiencies were very high (>80%) as a consequence of negligible solvent cage effects in supercritical CO 2 , as this medium has a very low viscosity. 1,6 Some early kinetic studies on the γ-radiation-induced precipitation polymerization of ethylene in liquid carbon dioxide 5 showed that there were no large effects on the propagation rate as compared to bulk polymerizations. In heterogeneous polymerization in the presence of CO 2 , the large plasticization effects have a dramatic effect on diffusivities 7,8 and hence termination rate coefficients.In order to understand dispersion polymerizations in CO 2 , 1 as well as the effect of CO 2 in emulsion polymerizations in hybrid CO 2 /aqueous...

The free-radical propagation rate coefficients for methyl methacrylate (MMA) in liquid and supercritical carbon dioxide have been successfully determined using pulsed-laser polymerization (PLP) method. The effect of carbon dioxide solvency on the propagation rate coefficient, kp, is examined at 180 bar for a temperature range of 20-80°C. The temperature dependency of the obtained kp data is evaluated by use of the Arrhenius equation. The resulting activation energy is (25.4 ( 1.2 kJ‚mol -1 , and the preexponential factor is (5.2 ( 3.0 × 10 6 L‚mol -1 ‚s -1 . These results are compared to the bulk activation parameters for the propagation rate coefficient of MMA as set forth by the IUPAC working party on "Modeling of Kinetics and Processes of Polymerization". The discrepancy in the propagation rate coefficients at the lower temperature regime is attributed to a solvent effect of CO2. This is consistent with the results from a recent PLP study of this monomer.

The development of a new reaction medium is reported, based on a biphasic mixture of carbon dioxide and water for emulsion polymerizations:  a surfactant-free aqueous emulsion polymerization of methyl methacrylate using potassium persulfate under a varying head pressure of CO2 (0−350 bar) at 75 °C. The resulting polymer is a stable latex with particles of submicron size. The effect of CO2 on polymerization is relatively small, until there is a significant change at high CO2 pressure. This is seen by examining the molecular weight distribution in the form of the log(number distribution), P(M) (readily obtained by GPC). At pressures of 140 bar and below, the P(M) show the form expected for chain-stopping events dominated by transfer and by diffusion-controlled termination. At 280 bar, ln P(M) is significantly steeper than its lower-pressure counterpart at relatively low conversion (45%). This is attributed to swelling by supercritical CO2 reducing the viscosity of the particles, allowing more rapid termination.