Propagation Rate Coefficients of Styrene and Methyl Methacrylate in Supercritical CO₂

Abstract

, 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...

Homogeneous free-radical polymerization of styrene in supercritical CO2

“…Van Herk et al 8) showed that the propagation rate coefficient for styrene polymerization in CO 2 solution at 65 8C and pressures between 164 to 181 bar is very close to bulk k p of styrene at the same T and p. Although propagation rate coefficients in CO 2 solution and in bulk are not always identical, as has been demonstrated 10) for lower temperature where the CO 2 solvent quality is rather poor, they seem to be close to each other at higher T and p and at polymerization conditions that yield relatively low 8) . Along these lines it appears justified to use the bulk styrene k p data 14) for the kinetic analysis of our solution experiments at 80 8C carried out in the presence of significant amounts of chain-transfer agent.…”

“…Pulsed laser polymerization (PLP) experiments directed toward measuring the propagation rate coefficient of styrene in fluid CO 2 , however, showed that PS is soluble in concentrations of at least 5 wt.-% in the presence of styrene monomer which obviously acts as a cosolvent 8) . The solubility may be further enhanced by choosing polymerization conditions such that polymer molecular weight is not too large.…”

Solvent Influence on Propagation Kinetics in Radical Polymerizations Studied by Pulsed Laser Initiated Polymerizations

“…To improve the situation, over the last couple of years more systematic studies into the solvent influence on k p for various monomers were conducted. In addition to several organic solvents, water [19,[49][50][51][52][53][54][55][56][57][58] and rather unconventional solvents such as supercritical carbon dioxide [59][60][61][62][63][64][65][66] and ionic liquids [67][68][69] were used. Because of the widely differing physico-chemical properties of the solvents, which are significantly different from monomer properties, solvent induced changes were expected to be large, and thus, were expected to contribute to a better understanding of the solvent influence on k p .…”

“…[61] Van Herk et al reported that MMA k p is not affected for reaction conditions, at which L 1 is around 10 000 g Á mol À1 . [59] Such low molecular weight poly (methyl methacrylate) (PMMA) shows better solubility in the monomer-scCO 2 mixture than high molecular weight material. Moreover, the tendency of lower molecular weight polymer to form coils is less pronounced, which may disfavor intramolecular segment-segment interactions.…”

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

“…83,84 The understanding that has emerged based mainly on the work of Buback and Beuermann is that k p is not significantly affected by the presence of scCO 2 compared to in bulk. However, the propagation rate in scCO 2 for MMA, 85 butyl acrylate, 85 as well as other acrylates/methacrylates 86 (but not S 87,88 ) is reduced by $40% (i.e., compared to what would be expected based on dilution only), which has been proposed to be caused by a reduction in the local monomer concentration in the vicinity of the radical chain end (PLP yields the product of k p and local monomer concentration). 83,84,86 At low conversion (in the conversion regime where segmental diffusion is rate determining for termination 89 ), the value of k t is much higher in scCO 2 than in bulk for S 90 and butyl acrylate.…”