Photochemical Transformation of Benzil Pendant Groups of Polystyrene Copolymers into Benzoyl Peroxide Moieties and Their Subsequent Thermal Decomposition. Cross-Linking or Chain Scission?

Abstract

Films of the copolymer, 11 wt % 1-{4-(2-methacroyloxyethoxyphenyl}-2-phenyl-1,2-ethanedione (BZMA) and styrene (S) (BZMA/S), as well as polystyrene (PS) doped with either the BZMA monomer (BZMA-PS) or 1-{4-(2-acetyloxyethoxy-phenyl}-2-phenyl-1,2-ethanedione (BZAc-PS) in concentrations that match the composition of the copolymer, have been irradiated (λ > 400 nm) in the presence of molecular oxygen at ambient temperatures. The rates of consumption of BZMA and BZAc and the concurrent formation of the corresponding benzoyl peroxide-containing units (BPMA and BPAc) were followed by infrared spectroscopy. The rates of benzil-group consumption and peroxide formation matched each other and were virtually the same in the three film types. Larger concentrations of oxygen increased the rate of consumption of BZMA. From a kinetic treatment of data at two concentrations of oxygen in PS, it is concluded that BZMA photooxidation is more than 10 times faster than that of benzil. At 91 °C, the first-order rate constants for thermal decomposition of BPMA and BPAc in PS are equal to and are larger than in the BPMA/S copolymer. The lower rate constant of BPMA/S, as well as the worse fit of the rate data from BPMA and BPAc in PS to a unimolecular decomposition model, is ascribed to some bimolecular decomposition, probably from aggregated peroxides. There is no indication of a bimolecular decomposition component in the copolymer. A very large portion (91 wt %) of BPMA/S, from irradiation of BZMA/S, is THF insoluble (i.e., cross-linked). The insoluble part increases to about 99 wt % after the BPMA/S film is treated at 91 °C for 6 h. Both of the corresponding doped polymers remain completely soluble in THF after irradiation and thermolysis. Cross-linking during the irradiation and heating is ascribed to formation of ester linkages (through abstraction of H atoms from -O-CH 2-CH2-O-groups by acyloxy radicals) and combination of pendant acyloxy radicals with radical sites on neighboring chains; abstraction from benzylic carbon atoms along PS chains leads to scission. By contrast, irradiation and subsequent heating of BZMA-PS or BZAc-PS films results in more chain scissions than cross-linking since the average molecular weights are decreased.

1,4‐Bisbenzil: Visible‐Light‐ and Heat‐Assisted Crosslinking of Polystyrene Films

“…However, formation of two additional bands in the region near 1700 and 1740 cm −1 (Figure ) was observed too. The two absorptions not observed during irradiation of BZ were attributed to benzoic acid and benzoic acid esters structures bound to phenyl ring of PS, both known as decomposition products of BPO in PS . Despite a rapid decrease of UV–vis absorption in the λ > 400 nm region of the n →π* absorption band (caused by irradiation and different starting optical densities at equal mass concentrations), the rates of dicarbonyl consumption in the case of simultaneously irradiated PS films containing either BZ or BZBZ differ only slightly (0.042 and 0.035 h −1 , respectively).…”

“…The photochemical formation of PS bearing pendant BPO from BZ structures was studied earlier for copolymers of styrene with monomers containing BZ structures and was exploited for crosslinking of PS …”

“…Benzil (BZ) in aerated glassy polymer films can be converted almost quantitatively to benzoyl peroxide (BPO) by insertion of molecular oxygen upon irradiation at λ > 400 nm, that is to say, at the long‐wavelength edge of the n →π* absorption band where BPO does not absorb . By irradiation of copolymer films containing BZ structures, covalently attached BPO pendant groups have also been formed . Whereas a decomposition of low molecular peroxides doped into polystyrene (PS) film results in a decrease in polymer molecular weight, the decomposition of pendant BPO groups in copolymers is an efficient method of crosslinking .…”

“…Crosslinked PS is usually obtained by irradiation with γ rays with or without added crosslinking agent, post‐modification of PS to form reactive pendant groups, copolymerization of styrene with a monomer bearing reactive group to form a self‐crosslinkable PS, or less commonly by using low molecular crosslinkers such as difunctional Friedel–Crafts reagents or difunctional azides, especially bis(perfluorophenyl azides) …”

Photooxidation of benzil groups in the presence of hindered amine stabilizers in the polystyrene film

“…[7][8][9][10][11][12] Transformation is clearly demonstrated by the decrease in absorbance of 1,2-dicarbonyl band at 1650 -1700 cm Ϫ1 and by the formation of new absorption band corresponding to BP structures in the region 1750 -1800 cm Ϫ1 . The effect of parent TMP and its N-oxy radical NO on the BZ photooxidation products was compared ( Figs.…”