Isomers of polystyrene cyclohexadienyl radical: Structure and photochemical behavior

Low-temperature (77 K) radiolysis of polystyrene (PS) was found to yield a mixture of the isomers of alkyl-substituted cyclohexadienyl (CHD) radical (RCHD)(RCHD) formed upon addition of in situ   generated hydrogen atoms to the double bonds of aromatic ring in positions 2–6. Two kinds of RCHDRCHD radicals with different thermal stability and photosensitivity have been identified by EPR and optical absorption spectroscopy. The radicals with unpaired electron on C2, C4, and C6 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 α3 = 1.0 mT while the absorption spectra, a band peaked at λmax = 330 nm (ɛ ≈ 9.0 × 103 M−1 cm−1). The relative amount of radicals on C3 and C5 atoms of the ring is below 2/5. Their EPR spectra are quadruplets with α4 = 1.0 mT, while the absorption spectra represent a band peaked at λmax = 320 nm (ɛ ≈ 7.2 × 103 M−1 cm−1). The individual EPR spectrum of C3 and C5 isomers has been obtained for the first time.