Free‐Radical Termination Kinetics Studied Using a Novel SP‐PLP‐ESR Technique

Abstract

Summary: A novel method for measuring termination rate coefficients, kt, in free‐radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator‐derived radicals. During subsequent polymerization, radical concentration is monitored by time‐resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain‐length dependence of kt to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

“…[37] First, the data were plotted as log(c R 0 /c R -1) versus logt, where c R 0 is the value of c R immediately after the laser pulse. The rationale for this approach [38] is the equation…”

“…[39] It suggests looking for linear regions in a plot of log(c R 0 /c R -1) versus logt, and taking the slope of these as 1-α, from which it is trivial to determine α. [38] INSERT Figure 2 An example of this procedure is given in Figure 2. Two straight-line regions are evident, suggesting composite-model behavior.…”

“…Two straight-line regions are evident, suggesting composite-model behavior. [38] From the long-time slope the value of α l is obtained, while the crossover chain length i c = t c /(k p c M ) is determined from t c , the time at which the two straight lines intersect. However only an estimate of α s may be obtained from the slope of the linear fit at short times.…”

The Cutthroat Competition Between Termination and Transfer to Shape the Kinetics of Radical Polymerization

“…The discussed trends hold for RP in general, the presented equations for steady state only. Using the latter it has been shown that simple steady-state experiments can yield good information on CLDT, although there is no disputing that single-pulse PLP remains the method of choice for such studies [10,11] (see Table 1). In particular the transfer limit is recommended as an important but little realized phenomenon: it can have the guise of 'classical' kinetics (e.g., 〈k t 〉 invariant with R init ) where actually CLDT is occurring.…”

“…Although recent theoretical [9] and experimental [10,11] work has shown that this two-parameter model is an oversimplification of reality, it is a nice model to use for calculations, as it clearly exposes the general effects of CLDT on RP kinetics, [12][13][14] and these trends are essentially the same for more complex homo-termination models. [9] The same also holds for cross-termination models, [12][13][14] and so the simplest one will be employed here unless otherwise stated:…”