Isomers of polystyrene cyclohexadienyl radical: Structure and photochemical behavior

Abstract

Low-temperature (77 K) radiolysis of polystyrene (PS) was found to yield a mixture of the isomers of alkyl-substituted cyclohexadienyl (CHD) radical ( R CHD ) formed upon addition of in situ generated hydrogen atoms to the double bonds of aromatic ring in positions 2–6. Two kinds of R CHD radicals with different thermal stability and photosensitivity have been identified by EPR and optical absorption spectroscopy. The radicals with unpaired electron on C 2, C 4, and C 6 carbon atoms of the phenyl ring, whose relative amount exceeds a statistically expected value (>3/5), exhibit a lower stability and higher photosensitivity. Their EPR spectra exhibit a triplet structure with the hyperfine splitting constant α 3 = 1.0 mT while the absorption spectra, a band peaked at λ max = 330 nm ( ɛ ≈ 9.0 × 10 3 M −1 cm −1). The relative amount of radicals on C 3 and C 5 atoms of the ring is below 2/5. Their EPR spectra are quadruplets with α 4 = 1.0 mT, while the absorption spectra represent a band peaked at λ max = 320 nm ( ɛ ≈ 7.2 × 10 3 M −1 cm −1). The individual EPR spectrum of C 3 and C 5 isomers has been obtained for the first time.