Determination of the Mode of Radical Termination from Pulsed Laser Polymerization Experiments in the Presence of Retardation and Chain Transfer to Agent

Abstract

The pulsed laser polymerization–high termination rate limit–size‐exclusion chromatography (PLP–HTRL–SEC) technique is used to estimate the mode of termination (δ) for n‐butyl methacrylate (n‐BMA) polymerization at 30 °C. It is found that molecular mass distributions measured in these experiments are influenced by an unknown side reaction such as retardation or chain transfer that results in a marked decrease of intensity of the PLP peak. A new approach is developed to evaluate δ, with numerical experiments used to demonstrate the robustness of the methodology in the presence of retardation or chain transfer to agent. Application to the experimental n‐BMA polymerization at 30 °C leads to an estimate for δ of 0.60 ± 0.03.

Detection of PLP Structure for Accurate Determination of Propagation Rate Coefficients over an Enhanced Range of PLP-SEC Conditions

“…Thus, the more difficult situation arises at the HTRL if termination takes place exclusively by disproportionation (δ = 1), as there is no higher molar mass shoulder (caused by termination by combination) that appears, − and thus no consistency check is possible to confirm that the L 1 peak is delivering k p accurately. Figure illustrates this situation for distributions simulated for PLP of MMA at 25 °C at the HTRL calculated for different values of x with δ = 1; the value of x is increased from the previously used HTRL boundary value of x max = 5 to and finally x = 10 x max .…”

“…As presented in the Introduction, however, it is difficult to choose parameters that precisely represent the limits of applicability. In addition, the boundary values of applicability will be different for experimental variations that apply PLP for estimation of additional kinetic parameters (e.g., for estimation of the mode of termination − ). In this work, we use the newly defined parameter x to explore these limits.…”

“…Consider now another example, presented in Figure 14, of a distribution generated by PLP of butyl methacrylate at 30 °C with high photoinitiator concentration, with further details of this PLP-SEC experiment given elsewhere. 27 As the PLP structure occurs on the high-MM side of the distribution peak, the negative value of γ = −2 is necessary to reveal the maximum in the derivative curve related to L 2 . The k p value estimated from the inflection point of the transformed distribution is 306 L mol −1 s −1 , in excellent agreement with the IUPAC value of 317 L mol −1 s −1 ), 7 while the ratio L 1 /L 2 is found to be a reasonable value (0.537).…”

“…As shown in Figure , if termination occurs partially by combination (δ < 1) under HTRL conditions, two maxima are expected in the first-derivative plot of the distribution, allowing the estimation of k p according to IUPAC recommendations irrespective of how high is the value of x (or β). − This general finding can be explained as follows. At the HTRL the concentration of radicals created by the previous pulse is approximately equal to 1/( k t t d ) when the next pulse arrives, independent of the value of [R 0 ] (or x ); the instantaneous termination of these radicals inevitably creates a sharp peak in distribution.…”

“…The position of this peak located at L 1 is also fixed relative to the background w (log M ) distribution and independent of [R 0 ]. In addition, the appearance of the second peak in the first-derivative curve of the distribution is guaranteed by the shoulder that arises from the stretching of the background distribution of height (1 – δ) k p [M]/ k t from L 1 up to L 2 . − This feature at 2 L 1 is actually a cliff rather than a peak, arising from the fact that the termination of radicals having chain length L by combination results in dead macromolecules with chain length 2 L . When new radicals are introduced by the next pulse, they quickly combine with those of chain length L 1 , resulting in a sharp decrease in both their concentration and the background distribution at chain length 2 L 1 .…”