ANTIOXIDANTS AND LIGHT STABILIZERS .2. ON THE THERMAL-STABILITY OF INDOLINONIC NITROXIDES

1,2-Dihydro-2-methyl-2-phenyl-3-phenylimino-3H-indole-N-oxyl acts as a process stabilizer in the extrusion of polypropylene, its antioxidant activity being greater than that of the phenolic antioxidant pentaerythritol tetra[3-(4-hydroxy-3,5-di-tert-butylphenyl)propionate] (Anox 20 or Irgonox 1010). In view of the high temperatures employed in the extrusion processes, the thermal stability of seven 3-aryliminoindoline N-oxyls and seven 3-oxoindoline N-oxyls bearing different substituents in position 2 has been investigated. Only the two 2-methyl-2-phenyl-, the two 2-ethyl-2-phenyl-, and the two 2,2-diphenyl-substituted derivatives do not undergo significant decomposition at 200-degrees-C, while all other nitroxides are converted to the corresponding indolenine N-oxide and indolenines depending upon the temperature. The decomposition process appears to take place through initial loss of an R. radical from position 2, the rate constants varying with the nature of the R substituent. The ease of fragmentation of the nitroxides could be related to the BDE of the R-H bond of the hydrocarbon corresponding to the R substituent with the exception of the 2-butyl-2-phenyl substituted derivatives which undergo degradation much faster than expected.