Radical Polymerization Kinetics of Non-Ionized and Fully-Ionized Monomers Studied by Pulsed-Laser EPR

Abstract

The radical polymerization kinetics of non-ionized and fully-ionized monomers in organic and aqueous solution was investigated by SP–PLP–EPR, i.e., highly time-resolved single-pulse–pulsed-laser–polymerization (SP–PLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy. Dicumyl peroxide as the photoinitiator allowed for investigations into the chain-length dependence of the termination rate coefficient, kt, of styrene bulk polymerization. The Composite Model perfectly represents the chain-length-dependence of kt for styrene and for all other monomers studied within the present investigation. The temperature dependence of the termination rate coefficient of two monomeric radicals, kt(1,1), scales with inverse viscosity, η−1, of the reaction mixture prior to polymerization. The product kt(1,1)∙η thus turns out to be a temperature-independent quantity for non-ionized radicals, which allows for estimates of kt(1,1) on the basis of easily accessible viscosity data. The impact of monomer concentration and temperature on the termination kinetics of charged radicals, as studied for fully-ionized methacrylic acid (NaMAA) at 5 wt.% and 10 wt.% monomer, is distinctly different. The measured activation energy, EA(kt(1,1)), is far below EA(η−1) and kt(1,1) for the more viscous solution at 10 wt.% NaMAA is higher than at 5 wt.%. This effect is assigned to the action of counter ions which is also seen with quaternary ammonium trimethylaminoethyl acrylate chloride (TMAEA) radicals. A novel SP–PLP–EPR method has been developed for investigation into the propagation kinetics of slowly terminating radicals. The integral over radical concentration measured after applying a single laser pulse is related to the separately measured monomer-to-polymer conversion per laser pulse thus providing the propagating rate coefficient, kp. The technique is illustrated for the fully-ionized methacrylate trimethylaminoethyl methacrylate chloride (TMAEMA) and the reliability is checked by investigations into di(n-butyl) itaconate (DBI) bulk polymerization. For the acrylate-type TMAEA radicals, the remarkably low value of the pre-exponential factor, A(kp), demonstrates the large entropy penalty associated with the formation of the transition state for propagation due to the restricted internal mobility induced by the charged side group. The bulky side groups in DBI cause a similar mobility restriction. First evidence for mid-chain radicals (MCRs) formed from end-chain radicals (SPRs) by the backbiting process was provided for TMAEA and acrylamide (AAm) polymerizations via the EPR spectra recorded during stationary polymerization. AAm exhibits a molar fraction of MCRs, xMCR, which is significantly lower than with butyl acrylate polymerization. The analysis of MCR concentration vs time profiles reveals the relatively high activation energy for the rate coefficient of backbiting, kbb, as the main reason behind the low value of xMCR. The MCR propagation and the cross-termination kinetics of SPRs and MCRs of AAm are similar to the associated values for acrylates. Although kbb is similarly low, significantly higher numbers for xMCR are found in TMAEA polymerization, which is due to the small rate coefficients of MCR propagation, kpt, and of SPR-MCR cross-termination, ktst(1,1), in the case of fully-ionized species. The comprehensive kinetic picture obtained for TMAEA and AAm homo-polymerizations underlines the enormous potential of the SP–PLP–EPR technique.