ABSTRACT

Heavy hydroaromatic hydrocarbons (178 is used as a model) were recently reported as effective melt and long-term heat stabilizers for polyolefins in strictly oxygen-deficient environments (73). The H-donor mechanism is based on an easy aromatization by reaction with reactive alkyl radicals [Eq. (32)].cxo +nR ~

©rD +nRH (32)

With the exception of the hemiester 30 and its analogues, none of the commercially available phenolic and aminic antioxidants, in their original chemical form, trap alkyls (2,56). It was recognized, however, that the complementary alkyl-trapping activity of some sacrificial transformation products arising from phenols (see Sec. IV.A.l: quinone methide, yielding 118); aromatic amines (see Sec. IV.A.2: aminyls, nitroxides, nitrones, and quinone imines, yielding N-alkylates 140, 167, 168; O-alkylate 166; and ring C-alkylates 141, 142, 165; O-alkylhydroxylamines 157,158; or secondary nitroxide 159); hydroxylamines (see Sec. IV.A.3, Scheme 9); and HAS (see later in Sec. IV.E.3) plays an important role in polymer protection and results from a complex chemistry of stabilizers capable of scavenging alkylperoxyls and alkyls in a cascade process (2,15,54,56).