关键词: 电子自旋共振, 自由基中间体, 自旋捕捉, 低密度聚乙烯, 紫外辐照

Abstract: Radical intermediates formed by UV-irradiation of low density polyethylene (LDPE) in the presence of benzophenone (BP) as a photoinitiator have been trapped by using 2,4,6-tri-t-butylnitrosobenzene (BNB) as a spin-trapping reagent. The resulting spin-adduct radicals have been characterized in situ by ESR spectroscopy. Two kinds of spin-adducts are detected and identified:a tertiary carbon radical and a secondary carbon radical which are formed by addition of the spin-trap (BNB) to two polymer radicals. These intermediates are produced by hydrogen abstraction of the BP excited triplet state from branch points and methylene groups of the LDPE chain, respectively. This is the evidence that the photocrosslinking of LDPE mainly takes place at both tertiary carbons (branch points) and secondary carbons along the chains and that H-shaped links dominate. Secondary bonded hydrogens are less reactive but occur in much larger number than tertiary bonded hydrogens in LDPE.

Key words: Electron spin resonance, Radical intermediates, Spin trapping, Low density polyethylene, UV irradiation