SUMMARYPulsed-laser polymerization (PLP) in conjugation with molecular weight distribution (MWD) measurement has emerged as the method of choice for determining the propagation rate coefficient k, in free-radical polymerizations. Detailed guidelines for using this technique (including essential internal consistency checks) and reporting the results therefrom are given by the authors, members of the IUPAC Working Party on Modeling of kinetics and processes of polymerization. The results for PLP-MWD k, measurements from many laboratories for bulk free-radical polymerization of styrene at low conversions and ambient pressure are collated, and are in excellent agreement. They are therefore recommended as constituting a benchmark data set, one that is best fitted by (the confidence ellipsoid for the Arrhenius parameters is also given). These benchmark data are also used to evaluate the merits of several other methods for determining k,; it is found that appropriately calibrated electron paramagnetic resonance spectroscopy appears to yield reliable values of k, for styrene.

The results are reported for a series of measurements of the propagation rate coefficient (k p) of butyl acrylate obtained from pulsed-laser polymerization (PLP). Previous attempts reported in the literature to use PLP for this monomer have failed because the data did not satisfy the internal consistency tests afforded by PLP. The problem was obviated by carrying out measurements at very low temperatures and with very short times between laser pulses. Data for k p were obtained over the range −65 to −7 °C which satisfy PLP consistency tests (invariance of the apparent k p value to laser pulse frequency, etc.). The results fit k p (dm3 mol-1 s-1) = 107.2 exp(−17.3 kJ mol-1)/RT); the confidence ellipse for these parameters is provided. These data extrapolate to a value of k p = 2.7 × 104 dm3 mol-1 s-1 at 50 °C. The higher value of the frequency factor of butyl acrylate compared to that of butyl methacrylate can be rationalized in terms of hindered rotations in the transition states.

, 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...

Propagation rate coefficient values, k p , reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed-laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in k p is observed as laser pulse repetition rate is increased from low (25-100 Hz) to high (300-500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5-70 °C. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 1.35 × 10 7 L mol −1 s −1 and an activation energy of 20.4 kJ mol −1 , with 95% confidence ellipsoids for the parameters also presented.

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