Peroxide-initiated grafting of maleimides onto hydrocarbon substrates

The grafting of N-phenethyl-maleimide (1) onto squalane and eicosane was investigated. As reference substances for spectroscopical investigations homopolymer, N-phenethyl-succinimide (2), t-butyl-(4) and cyclohexyl-N-phenethyl-succinimide (6) were synthesized. The grafting reactions were carried out at 150 °C in 1,2-dichlorobenzene with Luperox 130 as initiator (molar ratio hydrocarbon substrate:1:initiator = 10:1:0.2). Two fractions of graft products were isolated and analysed by 1H NMR, 13C NMR, FTIR and UV spectroscopy, SEC and MALDI-TOF MS to determine the average number of grafted residues per substrate molecule and to elucidate the structure of the grafts and the grafting sites. Overall grafting yields were found to be >90%. Only a small percentage of the total amount of substrate was grafted (2–3%). First fraction of both oligomers (approx. 25 wt%) showed to be a mixture of homopolymers (average degree of polymerization 6) and graft products (approx. 1:3), the latter containing mainly long-chain grafts with an average chain length of 7. The major fraction of graft products contained predominantly single units. As an average number of units per substrate molecule very similar results were obtained for eicosane and squalane (n = ∼ 3). In the case of squalane single units were found to be linked mainly to tertiary carbon atoms, long chain grafts mainly to secondary C-atoms. Apart from the homopolymers resulting from radical transfer, homopolymers terminated with methyl groups resulting from secondary radicals formed by the decomposition of Luperox were also observed. Homopolymers as well as graft products were found to contain small amounts of maleimide groups. The results suggest that as in the mechanism proposed for maleic anhydride, both inter- and intramolecular hydrogen abstraction occurs as part of the chain process. Termination proceeds mainly by hydrogen transfer and also by disproportionation, to a lesser extent. The formation of long chain grafts on tertiary carbons seems to be sterically hindered.