PHOTODEGRADATION OF ALTERNATING P-TERT-BUTYLSTYRENYL-CO AND EXO-NORBORNYL-CO COPOLYMERS STUDIED BY MULTIPLE-FREQUENCY TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY
Mde. Forbes et al., PHOTODEGRADATION OF ALTERNATING P-TERT-BUTYLSTYRENYL-CO AND EXO-NORBORNYL-CO COPOLYMERS STUDIED BY MULTIPLE-FREQUENCY TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY, Macromolecules, 27(4), 1994, pp. 1020-1026
The photodegradation of two alternating R-CO copolymers (R = P-tert-bu
tylstyrenyl and exo-norbornyl) was studied using time-resolved electro
n paramagnetic resonance (TREPR) spectroscopy at two different frequen
cies, 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 str
ong 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 radic
als present as early as 200 ns after the laser flash. The C-13-substit
uted ((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-no
rbornyl-CO copolymer unexpectedly showed weak EPR signals of endo-subs
tituted 2-norbornyl radicals from Norrish I alpha-cleavage, indicating
that the polymer is not completely stereoregular. Product analysis su
ggests that this polymer also undergoes Norrish type II degradation pr
ocesses, 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 a
re presented.