Critical evaluation of propagation rate coefficients for free-radical polymerization of alkyl acrylic acid esters with this kp data being derived from pulsed-laser initiated polymerizations in conjunction with polymer molecular weight analysis by size-exclusion chromatography.
First-principles modeling of free-radical polymerizations remains an important objective for polymer science and industry, in order to efficiently introduce process modifications and new products. The lack of reliable rate coefficients remains a major impediment in reaching this goal. For measurement of individual propagation rate coefficients, kp, the IUPAC Working Party on “Modeling of Polymerisation Kinetics and Processes” recommended pulsed-laser initiated polymerization (PLP) in conjunction with size-exclusion chromatography (SEC) as the method of choice. This technique has been successfully used to determine reliable propagation rate coefficients for several monomers. After having established benchmark value data of kp for styrene and a series of methacrylates in three previous publications of the IUPAC Working Party (fourth manuscript in preparation), the intention of the present project is to collate reliable kp data for alkyl acrylates. The knowledge of kinetic data for these monomers is of great importance, as they are frequently used in technical polymerizations, e.g. in the coatings and adhesives industries.
Acrylate chain-growth kinetics have been measured using PLP techniques, but only at low temperatures (<30 +ALA-C); it was found kp is 50 times greater than that for methacrylates. Recent studies, while not questioning the accuracy of these data, conclude that PLP-measured values could not be used to effectively model polymerizing acrylate systems. This discrepancy can be explained by invoking a more complex set of mechanisms that include intramolecular hydrogen abstraction followed by slower propagation of the resulting mid-chain radical or by chain b-scission. Thus a goal of this present effort is to discuss and clarify the conditions under which the PLP-measured values may be effectively used in modeling studies.
The project has resulted in the publication of a 5th paper in theseries of benchmark data sets of critically evaluated propagationrate coefficients: "Criticallyevaluated rate coefficients in free-radical polymerization – 5.Propagation rate coefficients for butyl acrylates", J.M. Asua,S..Beuermann, M. Buback, P. Castignolles, B. Charleux, R.G. Gilbert,R.A. Hutchinson, J.R. Leiza, A.N. Nikitin, J.-P. Vairon, and A.M.van Herk, Macromol. Chem. Phys. 205, 2151-2160 (2004).> abstract(MCP website)