Photoinitiated crosslinking of low-density polyethylene. 6. Spin-trapping ESR studies on radical intermediates

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 2,3,5,6-tetramethylnitrosobenzene (ND) and 2,4,6-tri-tert-butylnitrosobenzene (BNB) as spin-trapping reagents. 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 traps (ND or 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 photo-cross-linking 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 a much larger number than tertiary bonded hydrogens in LDPE.

Development of new dyeing photoinitiators based on 6H-indolo[2,3-b]quinoxaline skeleton

“…From literature it is known that nitro compounds in the presence of electron donors undergo photoreduction yielding corresponding nitroso derivative [31], which are known to be efficient free radical scavengers [32]. It is also noteworthy fact that nitro compounds themselves are considered as efficient inhibitors of free radical polymerization [33].…”

Resorbable elastomeric networks prepared by photocrosslinking of high-molecular-weight poly(trimethylene carbonate) with photoinitiators and poly(trimethylene carbonate) macromers as crosslinking aids

“…Polyethylene and poly(ethylene oxide) have also been shown to crosslink upon UV irradiation in the presence of a PI that undergoes Norrish type II photolysis, such as benzophenone, and acts as a hydrogen abstracting agent [35,36]. It was proposed for both polymers that crosslinking occurs by the combination of two macroradicals to form H-type crosslinks.…”

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

“…[1][2][3] The alkyl radical intermediates and microstructures of crosslinks produced in the photocrosslinking of LDPE and its model compounds have been studied by spin-trap electron spin resonance (ESR) 4,5 and 13 C nuclear magnetic resonance (NMR). 6 However, the photolytic products and photoreduction mechanism of BP as a photoinitiator in the photoinitiated crosslinking of polyethylene have not yet been investigated.…”