Electron spin resonance (ESR) spectroscopy, also referred to as EPR (electron paramagnetic resonance), allows for the direct observation of radicals, thus being of eminent importance for detailed investigations into radical polymerization. ESR spectra provide information on the structures, properties, and concentrations of the radicals. ESR detection of radicals has been difficult under stationary polymerization conditions at low radical concentration. The quality of ESR spectrometers has, however, been largely improved. Moreover, with the advent of pulsed lasers, large radical concentrations may be instantaneously generated and subsequently monitored at high time resolution.
Steady-state ESR (SS ESR) measurements have been a major technique for the direct detection of radicals during radical polymerizations. In this project two more techniques of ESR in combination with pulsed laser are employed. One is time-resolved (CIDEP) ESR (TR ESR) and the other is single pulse-pulsed laser polymerization-EPR (SP-PLP-EPR). These techniques have different time resolution. The time regions are from nsec (10-8-10-7 sec) to μsec (10-6-10-5 sec) for TR ESR and from μsec to msec (10-5-10-4 sec) for SP-PLP-EPR. The time resolution of SS ESR is longer than msec. Initial radical addition reactions can be observed by TR ESR and the initial stage to first several steps of radical addition reactions can be detected by the SP-PLP-EPR technique. SS ESR can provide information on propagating radicals with long propagated chain. Various aspects of radical polymerization reactions can be observed by these three techniques. The resulting ESR spectra can provide rate coefficients of each elementary process and the activation energy (Ea) of the reactions based on temperature dependent measurements. Figure 1 shows a typical 3D image of the TR ESR spectrum of radical polymerizations of tert-butyl methacrylate (a) with a scheme of observed radical addition reaction (b) as an example. Slice at constant time provides a spectrum with hyperfine splitting (Fig. 1(c)) and detailed structure can be determined from the spectrum. Time profiles of the each spectroscopic line (Fig. 1(d)) are strongly correlated with reaction kinetics. When the same polymerization was observed by SS ESR, the ESR spectrum of propagating radicals can be detected, as shown in Figure 2. Initiation and propagating processes of the same polymerization system were observed by TR and SS ESR spectroscopies.
A typical 3D image of the TR ESR spectrum of radical polymerizations of tert-butyl methacrylate.
SS ESR spectrum of propagating radicals
Moreover, single pulse experiments provide access to chain-length-dependent termination, to transfer, and even to propagation rate coefficients. The method is also applicable to reversible addition fragmentation transfer (RAFT) and atom-transfer radical polymerization (ATRP) kinetics. The multitude of powerful ESR techniques enables the quantitative detection of specific radicals over the entire course of a polymerization reaction. Application of these methods requires the accurate knowledge of the ESR spectra and associated hyperfine coupling constants (hfcs). Since values of hyperfine splitting constants were determined very precisely from the spectrum, it was reasonably well simulated as the corresponding propagating radical. Absolute values of the hyperfine splitting constants are proportional to the spin densities of each element and the spin density is strongly correlated with electron density. The precisely determined hfcs data will provide significant information on theoretical calculations of properties of radicals. No hfcs on polymerization-related radicals have been tabulated since Rånby and Rabek´s book on ESR Spectroscopy in Polymer Research (Springer, 1977).
A critically evaluated update of the available data and of recommended experimental methods is urgently required, in particular as novel monomers and polymerization techniques have been introduced. Experimentalists, methodologists, and theoreticians will cooperate within the project to establish a database of reliable ESR spectra for use in academia and industry.
For more information, contact the Task Group Chair Atsushi Kajiwara <kajiwara@nara-edu.ac.jp>
©2016 by Walter de Gruyter Berlin/Boston
