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 .

Propagation rate coefficients, k p , of N-vinyl pyrrolidone (NVP) radical polymerization in aqueous solution have been measured via the pulsed-laser polymerization -size exclusion chromatography method (PLP-SEC) over an extended concentration range from 1.8 to 100 wt % NVP at temperatures between 2 and 60°C. The SEC analyses have been carried out by a modified procedure using dimethyl acetamide as the eluent. An about 20-fold increase of k p is observed in passing from NVP bulk polymerization to reaction in dilute aqueous solution. As with nonionized methacrylic acid (MAA), for which a similarly strong change in k p has recently been reported, the large solvent effect in NVP polymerization is assigned to intermolecular interactions resulting in a significant hindrance of internal rotational motion in the transition state structure for propagation. Some contribution from a minor change in activation energy may however not be ruled out. PLP-SEC studies carried out on aqueous NVP solutions to which either polyNVP or N-ethyl-2-pyrrolidone (NEP), the saturated analogue of NVP, have been added, demonstrate that k p depends on the molecular environment at the reactive site, which is affected by NVP (or NEP) content, but not by the polyNVP content. The lowering of monomer concentration during NVP polymerization to higher degrees of monomer conversion results in an increase of k p . Variation of pH in the range 3 to 10 does not affect k p .

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Pulsed-laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion-chromatography (SEC) was employed to determine the propagation rate coefficient, k p, of methacrylic acid (MAA) free-radical polymerization in aqueous solution. This was done over a wide range of degree of MAA ionization, at MAA concentrations between 5 and 40 wt %, and at temperatures from 6 to 80 °C. Depending on monomer concentration, the degree of ionization, α, may largely affect k p. At 5 wt % MAA, k p is lowered by about 1 order of magnitude in passing from nonionized to fully ionized MAA, whereas the associated decrease in k p at 40 wt % MAA is only by 20%. The changes of MAA k p with α are assigned to intermolecular interactions affecting the friction that is experienced by the relevant internal rotations of the transition state structure for propagation. Increasing hindrance of rotational motion is associated with a lowering of the pre-exponential factor, A(k p). The observed effects are primarily of entropic origin, but slight changes in activation energy, E A(k p), also seem to play a role. An expression is given which allows for estimates of MAA k p as a function of degree of ionization, monomer concentration, and temperature.

The chain-length dependence of the termination rate coefficient, k t, of bulk homopolymerizations of n-butyl methacrylate (n-BMA) and tert-butyl methacrylate (t-BMA) at ambient pressure and temperatures between −30 and 60 °C has been studied via the single pulse−pulsed laser polymerization−electron spin resonance (SP-PLP-ESR) technique. The decay of radical concentration, c R, after laser SP initiation is monitored with a high time resolution of microseconds by ESR spectroscopy. Radical chain length, i, increases linearly with time t after applying the laser pulse. The experimental k t i,i values refer to rate coefficients for termination of two radicals of identical chain length i. The variation of k t i,i with chain length is adequately represented via the composite model proposed by Smith et al., in which two power-law expressions, k t i,i ∝ i −α, are contained with the exponents αs and αl referring to short-chain and long-chain radicals, respectively. The transition between the two regimes occurs at the crossover chain length, i c. The rate coefficients extrapolated for termination of two radicals of chain length unity, k t 1,1, are almost identical for n-BMA and t-BMA with an activation energy of E A(k t 1,1) ≈ 10 kJ mol−1. The αs values are close to each other: 0.65 ± 0.10 (n-BMA) and 0.56 ± 0.10 (t-BMA). Both αl values are found to be 0.20 ± 0.05, which is close to the theoretical value of αl = 0.16. The crossover chain lengths are i c ≈ 50 for n-BMA and i c ≈ 70 for t-BMA. The minor differences in composite-model parameter values of n-BMA and t-BMA are assigned to differences in chain mobility.

Via electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n-butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at -50 °C and by the 7-line spectrum of MCRs at +70 °C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs allows for estimating the rate coefficient k pt for monomer addition to a mid-chain radical. For 70 °C, k pt is obtained to be 65.5 L · mol(-1)  · s(-1) .