Chemical modification of molten polyethylene by thermal decomposition of peroxyesters

Abstract

ABSTRACT:The functionalization of polyethylene was realized by the thermal decomposition of peroxyesters in molten polymer in the absence of any other additive. Ester and acid functions were introduced onto the polymer operating with various peroxyesters. This grafting resulted from the combination of a polymer radical, generated in the abstraction of a hydrogen from polyethylene by an alkoxy radical, with an acyloxy or carbon-centered radical arising from the perester. A complete methodology was set up to identify and to titrate the functions present onto the polyethylene and to determine the extent of crosslinked polymer.

Crosslinking of high‐density polyethylene in the presence of organic peroxides

“…Taking this remark into account and the efficient ester function grafting on PE with a peroxide/acrylate system, [27][28][29][30] we can conclude that the functionalization by peroxysuccinic derivatives may originate from a mechanism other than the coupling previously advanced. 18 This analysis shows that it is actually very difficult to simply explain the obtained results. A fundamental study of the behavior of alkyl and acyloxy free radicals in a viscous medium is an important goal if we would like to properly use free-radical reactions for the chemical modification of polyolefins.…”

“…Indeed, the thermolyses of various peralkanoates in PE produced very little crosslinking, but efficient functionalization was observed. 18 Even if higher crosslinking extents were observed for t-butyl perhexanoate and 3-dodecyloxycarbonyl perpropanoate, it does not appear reasonable to believe that increasing the number of carbon atoms in alkyl group R would be very much responsible for the changing of the reactivity. An explanation must be found in the difference in the behaviors of the various acyloxy radicals and alkyl radicals generated in the thermolyses of the different peresters.…”

“…Unlike the previous carboxy radicals mentioned in this article, aroyloxy and alkoxycarboxy, acyloxy radicals are prone to fast decarboxylation, 26 generating alkyl radicals (way f). Nevertheless, their coupling reactions with PE macroradicals in the cage (way e), advanced to explain the grafting of ester functions during the decomposition of peralkanoates in PE, 18 show that decarboxylation is not a single reaction produced by these radicals in a viscous medium, isomerization and coupling with a macroradical being mentioned. The 1 H-NMR study of the soluble (noncrosslinked) polymer did not allow the identification of larger amounts of protons borne by carbons linked to an oxygen atom (ester or ether) than in the starting PE.…”

“…Then, whatever the explanation is, we now have to consider the behavior of alkyl radicals R. The one generated from 4 is primary, like the others produced from the linear peresters. 18 The main difference from the pure alkyl ones arising from t-butyl and cumyl perpropanate or tbutyl perhexanoate remains the presence of a methyl substituent on the ␣-carbon in the radical center. If we consider that such a radical would disproportionate more readily than a linear primary alkyl one, a possible explanation can be advanced.…”

“…A preliminary study 16 showed that, depending on the type and structure of the peroxy derivative, function grafting or crosslinking of the polymer mainly took place. The decomposition of peroxyketals 17 and t-butyl peroxyalcanoates 18 allowed the grafting of ester functions; meanwhile, the use of hydroperoxides 19 induced the formation of carbonyl and hydroxyl on the macromolecule. The crosslinking and functional grafting involved the combination of a radical produced by hydrogen abstraction onto PE, with another macroradical for the former or a small radical arising from the peroxide for the latter.…”