PHOTODEGRADATION OF ALTERNATING P-TERT-BUTYLSTYRENYL-CO AND EXO-NORBORNYL-CO COPOLYMERS STUDIED BY MULTIPLE-FREQUENCY TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

The photodegradation of two alternating R-CO copolymers (R = P-tert-butylstyrenyl and exo-norbornyl) was studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy at two different frequencies, X-band (9.5 GHz) and Q-band (35.1 GHz), and by product analysis. For the tert-butylstyrenyl-CO copolymer the EPR spectra displayed strong chemically induced dynamic spin polarization (CIDEP) expected for triplet-born radical pairs from the Norrish I alpha-cleavage reaction. Simulation of the spectra allowed unambiguous assignment of the radicals present as early as 200 ns after the laser flash. The C-13-substituted ((CO)-C-13) tert-butylstyrenyl copolymer showed unusual magnetic field effects on its spin polarization patterns and an additional long-range C-13 hyperfine interaction in the benzylic fragment. The exo-norbornyl-CO copolymer unexpectedly showed weak EPR signals of endo-substituted 2-norbornyl radicals from Norrish I alpha-cleavage, indicating that the polymer is not completely stereoregular. Product analysis suggests that this polymer also undergoes Norrish type II degradation processes, producing 1,4-biradicals, which are unobservable by TREPR. A discussion of the mechanisms of photodegradation for both polymers and a spectroscopic analysis of the samples before and after photolysis are presented.