Recent Advances in the Understanding of Termination in Radical Polymerization from Using the SP-PLP-EPR Technique
Since at least the 1940s, workers have been seeking to understand the kinetics of the fundamental radical-polymerization reaction of termination. This quest has not proven to be easy. Just under a decade ago, two game-changing advances were made that have unleashed rapid progress in the field: (1) The so-called ‘composite model’ was proposed: short radicals and long radicals are characterized by different scaling-law behavior in their rate of termination. (2) Single-pulse pulsed-laser polymerization (SP PLP) was coupled with electron paramagnetic resonance (EPR) spectroscopy to produce a remarkably potent new method for measuring chain-length-dependent termination (CLDT) rate coefficients, in particular those of short radicals. Essentially without exception, the SP-PLP-EPR method has revealed termination behavior in accord with the composite model. Here we summarize three recent SP-PLP-EPR studies, all involving new directions for the application of this technique.