Polymer reactions. VI. Inhibited autoxidation of polypropylene

The autoxidation of polypropylene inhibited by 2,6-di-tert-butyl-p-cresol (AH) and dilauryl thiodipropionate (S) was studied by the combined methods of electron spin resonance, oxygen absorption, and chemical analysis. With AH alone, there is a critical concentration of about 6 X 10^-3 mole/l. below which there is no inhibition. This critical concentration agrees with that determined for inhibited squalane autoxidation and that calculated from known rate constants. Above the critical concentration there is a well-defined induction period during which the ROO· concentration is estimated to be 10^-8 mole/l. ROOH] decreased rapidly as did A·] and AH]; the latter are kinetically related. The rate constant for the reaction between A· and ROO· is estimated to be 7 X 10⁷ l./mole-sec. at 130°C. At the end of the induction period, ROOH], ROO·], and –dO₂]/dt increased rapidly until steady-state values were attained for all of them. With S alone, there are only retarded oxidation but no well-defined induction periods. ROOH] is greatly reduced by S. In all systems where the oxidation rates were appreciably suppressed there was formed a very stable paramagnetic species, S·, which was inert toward AH and I₂ but reactive toward triethyl phosphite. Because of its similarity with spin centers in carbon black, S· is postulated to be a delocalized polysulfide spin center. With both S and AH present, the combined effect of stabilization is synergistic. The observed time-dependent variations of ROOH], ROO·], and A·] follow familiar mechanisms. Mathematical relationships describing each of these three systems are included.