γ-Irradiation of maleic anhydride in benzene

Abstract

γ-Irradiation of a benzene solution of maleic anhydride produced a polyanhydride of low molecular weight, a small quantity of a maleic anhydride–benzene 2:1 adduct (II)(G∼ 0·06), carbon dioxide (G= 0·29), carbon monoxide (G= 0·26), acetylene (G= 0·08), and traces of ethylene and methane. Addition of a catalytic amount of benzoyl peroxide increased the yield of the polyanhydride by about 50% but its composition and the yield of (II) were virtually unchanged. The presence of cyclohexane increased the yield of the polyanhydride and decreased that of (II). Analysis of the polymer obtained in the absence of additives suggested incorporation of 3–4 benzenes per 6 maleic units; 2–3 benzenes had lost their aromaticity, and one, the terminal phenyl, retained it. The following were calculated: G(maleic anhydride → polymer)∼ 2·6–2·2; G(benzene → polymer)∼ 1·2–1·6. The latter consisted of two terms: G(benzene → terminal phenyl)∼ 0·4; G(benzene → aliphatic C₆H₆)∼ 0·8–1·2. These two terms were compared with the literature data on the yields of radicals and of benzene triplets produced in liquid benzene by the low LET radiation. It was concluded that (II) was formed by energy transfer from solvent to maleic anhydride, either free or complexed, and the subsequent reactions of the excited maleic anhydride with benzene. Formation of the polymer occurred by way of a short radical chain. The initiation step involved scavenging of radicals, mainly phenyl, by maleic anhydride, and the propagation consisted of copolymerisation of maleic anhydride with either triplet benzene or tautomers of excited benzene such as benzvalene.