Radiolysis of polyisobutene IV: A study of the role of free radicals in the fracture reaction using radioactive additives

Abstract

The influence of additives on the formation of polymer radicals and macromolecular fractures in polyisobutene by exposure to 4 MeV electrons has been studied using phenyl- 3 H- sec-butyl- 35 S 1-disulphide, thiophenol- 3 H, thiophenol- 35 S, and nitrobenzene- 14 C. G (polymer radicals) was found to decrease from a maximum value of 7·3 at a concentration of thiophenol of 3 to 5 × 10 −4 mole/g to 4·5 to 5·4 at about 15 × 10 −4 mole/g and then to remain constant. As this decrease may be due partly to the scavenging of H · and CH 3 · precursors, it is supposed that the primary polymer radicals are formed by processes which are relatively insensitive to the presence of large concentrations of a ‘protective agent’. Comparison of the effects of thiophenol on yields of radicals and fractures leads to the conclusion that two types of primary polymer radical are formed. One is due to the fracture of CH and CCH 3 bonds, 4·2 ⩾ G(—) ⩾2.7, and the other to direct fracture of the main chain of carbon atoms, G(—·) ⩾ 3·1. Nitrobenzene- 14 C, which is only slightly soluble, combines with the polymer, G (combined nitrobenzene) = 1·2, and increases the G (fractures) value by 0·6 after a dose of 5 Mrad. An explanation is suggested for the contrasting effects of thiophenol and nitrobenzene at low concentrations. Thiophenol reduces the yield of fractures by stabilizing the radical — and preventing β-bond scission. Nitrobenzene fails to do this but instead further increases the yield of fractures by preventing the ‘fracture-offsetting’ combination of the radicals, —·, formed by β-bond scission of —.