Kinetics and Modeling of the Radical Polymerization of Acrylic Acid and of Methacrylic Acid in Aqueous Solution

Abstract

The radical polymerization of methacrylic acid, acrylic acid and acrylamide in aqueous solution has been investigated. Detailed kinetic models for both acrylic acid, AA, and methacrylic acid, MAA, have been developed applying the program PREDICI. Good representation of experimental conversion vs. time profiles and molar mass distributions as well as, in case of AA, the branching level could be achieved. The polymerization of MAA has been studied at 35 and 50 °C with focus on the influence of 2-mercaptoethanol, ME, as chain transfer agent, CTA, on reaction kinetics. The rate coefficient of transfer to CTA, ktr,CTA was measured for different monomer levels by the Mayo and the chain length distribution procedure. The ratio of ktr,CTA to the propagation rate coefficient, kp , is independent of monomer to water ratio while both rate coefficients increase by approximately one order of magnitude in passing from bulk to dilute aqueous solution. It was found that addition of CTA reduces the rate of MAA polymerization by two effects on kt. At negligible monomer conversion, kt increases towards higher content of CTA, because average chain length is reduced by the CTA. Chain-length dependent termination may be represented by adopting the composite model, which is a well-established theory to describe chain-length dependency of termination of macroradicals of identical size. The composite model could be applied to average chain length. The reduction of kt towards higher degrees of monomer conversion (Norrish–Trommsdorff or gel effect) becomes weaker towards higher levels of CTA, which could be described by correlating the intensity of the gel effect to molar mass of polymer in solution. The polymerization of non-ionized AA in aqueous solution has been studied between 35 and 80 °C with and without ME as CTA. Chain-length dependent termination was taken into account for modeling as for MAA. During AA polymerization a 1,5-hydrogen shift (backbiting) takes place transforming the secondary propagating radical, SPR, into a tertiary midchain radical, MCR, the kinetics of which were included into the model. The backbiting reaction was quantified via 13C-NMR the other MCR reactions were estimated from conversion vs. time profiles. By measuring the MCR fraction during butyl acrylate, BA, polymerization via electron paramagnetic resonance, EPR, it could be shown that the transfer of MCRs to CTA is not an important reaction path. BA can be used as AA model compound so that the same finding should also apply for AA polymerizations in aqueous phase. Chain transfer of SPRs of AA was measured by the Mayo method. The model was extended towards high-temperature polymerization of AA between 90 and 170 °C, where beta-scission and propagation of macromonomers need to be considered. Moreover, a model for the polymerization of ionized AA was developed, which takes numerous dependencies of rate coefficients on ionization and ionic strength into account, e.g., propagation is reduced by ionization of monomer, but to a higher extent for lower monomer concentration. Moreover, propagation of ionized monomer augments towards higher ionic strength. MCRs were found during acrylamide polymerization via EPR revealing the backbiting reaction to apply for this monomer as well. Thus, the kinetic scheme is the same as for AA polymerization.