Self-Reactions of 1,3-Diphenylpropyl and 1,3,5-Triphenylpentyl Radicals: Models for Termination in Styrene Polymerization

Quantitative determination of the products from thermal decomposition of 1,1?,3,3'- tetraphenylazopropane (2) and 1,1',3,3',5,5?-hexaphenylazopentane (5) shows that the respective derived title radicals (1) and (4) undergo self-reactions with a similar preference for combination over disproportionation. The proportion of combination increases from 86 to 93% ( E+c-E+d = 9.6 ± 1.2 kJ mol-1 and ?Sc+:- ? Sd+ : =43 ±7 JK-1 mol-l) for (1), and from 86 to 93% ( E+c-E+d = 10.6 ± 1.4 kJ mol-1 and ?Sc- ? Sd : = 45 ± 6 J K-1 mol-1) for (4) over the temperature range 80-161°. The relevance of these results to the termination mechanism in the radical polymerization of styrene is discussed. Some minor by-products (28)-(30) which arise by addition of 1,3-diphenylpropyl (1) to 1,3-diphenylpropene (14)/(15) have also been detected. The syntheses of the azo compounds (2) and (5), the former as separate, pure meso - and rac-diastereomers (2m) and (2r), and of their various expected decomposition products are described.