The propagation rate coefficient, k p , for free-radical polymerization of nonionized methacrylic acid (MAA) in aqueous solution has been studied via pulsed laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion chromatography (SEC). The PLP-SEC experiments were carried out between 20 and 80 °C within the entire concentration range from dilute solution containing 1 wt % MAA up to bulk MAA polymerization. The k p values which are determined under the assumption that the relevant monomer concentration at the radical site is identical to the known overall MAA concentration decrease by about 1 order of magnitude between 1 and 100 wt % MAA. This significant lowering is almost entirely due to a reduction in the Arrhenius preexponential factor, A(k p ), whereas the activation energy, E A (k p ), stays essentially constant. The decrease in A(k p ) is assigned to intermolecular interactions between the transition state (TS) structure for MAA propagation and an MAA environment being significantly stronger than the ones between this TS structure and an H 2 O environment. In an MAA-rich environment, the barrier to rotational motion of the relevant degrees of motion of the TS thus experiences enhanced friction, which is associated with a lowering of the preexponential factor and thus of k p .

Butyl acrylate (BA) solution polymerization (1.5 M in toluene) was investigated via singlepulse pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR) with emphasis on the termination and transfer kinetics of the system in which two distinctly different types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), are present. MCRs are produced by intramolecular hydrogen transfer (backbiting). They may react back to SPRs by monomer addition. The evolution of SPR and MCR concentrations after photoinitiation with an intense laser pulse was measured via highly time-resolved EPR at temperatures between -40 and þ60°C. At very low temperatures the MCR concentration is negligible, enabling the chain-length-dependent rate coefficient of SPR termination, k t ss (i,i), to be directly determined. At higher temperatures it was necessary to use PREDICI simulation of the radical concentration vs time traces, a process which yields the chain-length-dependent rate coefficient of SPR termination for monomeric radicals, k t ss (1,1), as well as the rate coefficients for backbiting, k bb , for monomer addition to an MCR, k p t , and for SPR-MCR cross-termination, k t st . The composite model adequately represents k t ss (i,i), with the power-law exponents R s = 0.85 ( 0.09 and R l = 0.16 ( 0.07 for shortchain and long-chain radicals, respectively, and a crossover chain length between short-chain and long-chain behavior at around i c = 30. The activation energy for both k t ss (1,1) and k t st (1,1) is found to be as one would expect for translational diffusion of small molecules.

A novel method to extract individual free-radical polymerization rate coefficients for butyl acrylate intramolecular chain transfer (backbiting), k bb , and for monomer addition to the resulting midchain radical, k p t , is presented. The approach for measuring k bb does not require knowledge of any other rate coefficient. Only the dispersion parameter of SEC broadening has to be determined by fitting measured MWDs or should be available from separate experiments. The method is based upon analysis of the shift in the position of the inflection point of polymer molecular weight distributions produced by a series of pulsed-laser polymerization (PLP) experiments with varying laser pulse repetition rate. The coefficient k bb is determined from the onset of the sharp decrease of the apparent propagation rate coefficient (k p app ) with decreasing repetition rate, an approach verified by simulation. With experiments performed between -10 and +30°C, the estimated values are fitted well by an Arrhenius relation with pre-exponential factor A(k bb ) ) (4.84 ( 0.29) × 10 7 s -1 and activation energy E a (k bb ) ) (31.7 ( 2.5) kJ‚mol -1 . At low pulse repetition rates, the experimental k p app values are related to an averaged propagation rate coefficient, k p av , that is dependent on the relative population of chain-end and midchain radicals. Evaluated by comparing simulated and experimental molecular weight distributions, k p av provides an estimate for k p t . The Arrhenius parameters are: A(k p t ) ) (1.52 ( 0.14) × 10 6 L‚mol -1 ‚s -1 and E a (k p t ) ) (28.9 ( 3.2) kJ‚mol -1 .