Photodegradation mechanisms in poly(2,6-butylenenaphthalate- co-tetramethyleneglycol) (PBN–PTMG), Part III: Photodegradation induced by the carbonyl group in n, π * excited states

Abstract

In order to identify the initiation step in the photodegradation of poly(2,6-butylenenaphthalate)- block-poly(tetramethyleneglycol) (PBN–PTMG), we undertook its photolysis with monochromatic irradiation. We discuss the initiation reaction on the basis of the analytical results for the PBN–PTMG in the presence or absence of 2,2′-dihydroxy-4-methoxybenzophenone as a UV absorber, 2-hydroxy-4-methoxybenzophenone as a UV absorber, β-carotene as a quencher of singlet oxygen or methylene blue as a photosensitiser, respectively. The PBN–PTMG containing 2,2′-dihydroxy-4-methoxybenzophenone exhibits better resistance to the incident light of ca. 370 nm corresponding to the absorption of the n, π * transition of the carbonyl group in the PBN block than the PBN–PTMG containing 2-hydroxy-4-methoxybenzophenone. The PBN–PTMG containing β-carotene shows similar photodegradation tendencies as that of the PBN–PTMG without β-carotene. In contrast, the PBN–PTMG with methylene blue is not degraded by monochromatic radiation in the range λ = 600–690 nm. These facts indicate that singlet oxygen does not participate in the initiation reaction of the photodegradation of the PBN–PTMG. Therefore, we concluded that the photodegradation of the PBN–PTMG is induced through the hydrogen abstraction by carbonyl groups in n, π * excited states.