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Using recently reported data on the kinetics of decomposition of species R203 and R204 in solution (R = f-Bu, D) and in the gas phase (R = SF5), it is possible to show that the difference, A/f°f(0-(0)2) -A7í°f(R02) = 14.5 -AH°f(RO) where the first term represents the contribution of the group 0-(0)2 to the heats of formation of polyoxides. Since, for many alkoxy radicals (RO), heats of formation are now known with good accuracy, either one value of Aff°f(R02) or one absolute value of AH°f(0-(0)2) from °{ of any polyoxide will serve to fix the other. The assumption DH°(R02-H) = DH°(H02-H) = 89.6 ± 2 kcal/mol is shown to be in good agreement with independent solution measurements and fixes the group value ° (0-(0)2) = 16.8 ± 2 kcal/mol, an appreciable improvement over earlier deductions. New values derived from this are listed for AH°C of R20" and RÓn ( > 2) where R = H, Me, and t-Bu. The radical group values M/°f[0-(Ó)(H)] = 5.0 and M7°f[0-(0)(C)] = 16.4 kcal/mol are derived from the data. Group additivity permits the calculation of AH°t for other polyoxy radicals.

A chain mechanism is proposed to account for the very rapid termination reactions observed between alkyl peroxy radicals containing a-C-H bonds which are from lo4 to lo6 faster than the termination of tertiary alkyl peroxy radicals. The new mechanism is 2RCH20i kRCHzOOH + R m RCH20 + RCH20i +RCHO + RCH202H:3"-KCHOO is the zwitterion originally postulated by Criegee to account for the chemistry of 0 3-olefin addition. Heats of formation are estimated for CH200, RCHOO, and (CH:hCOO and it is shown that all steps in the mechanism are exothermic. The second step can account lor ('A)Oz which has been observed. k l is estimated to he 109-2'H literlM sec where 0 = 2.303RT in kcal/mole. The second and third steps constitute a chain termination process where chain length is estimated a t from 2 to 10. This mechanism for the first time accounts for minor products such as acid and ROOH found in termination reactions. Trioxide (step 3 ) is shown to he important below 30°C or in very short time observations (< 10 s at 30OC). Solvent effects are also shown to he compatible with the new mechanism.-___