Electron spin resonance study of copolymerization of hexafluoroisobutylene with vinylidene fluoride

The Copolymerization reaction of hexafluoroisobutylene (HFIB) and vinylidene fluoride (VF₂) was studied using X-band electron spin resonance (ESR) by applying a photo-in situ method. Owing to the toxicity of HFIB monomer, the flow ESR method could not be directly applied without extensive modification. The observed ESR signal with 21-line hyperfine structure was assigned to the copolymer radical with head-to-head configuration. Although the HFIB monomer radical possesses the same hyperfine pattern, the ambiguity has been removed by using CF₃ as an initiator radical. Owing to the high steady-state concentration of the observed copolymer radical with head-to-head configuration as well as the nature of the static photo-in situ ESR method, we believe the actual molecular configuration for HFIB/VF₂ copolymer must be head-to-tail. The observed hyperfine constant for A_βF = 1.74 mT suggests that the geometry for HFIB/VF₂ copolymer radicals with both head-to-head or head-to-tail configurations is possibly similar to that of (CF₃)₃C· radical. The small value for A_βH indicates steric hindrance to rotation about the C_α? C_β bond, and this is also supported by the experimental results of nonalteration in linewidth during the temperature dependence study from ?40 to about 90°C. Attempts to measure directly the monomer reactivities have been unsuccessful owing to the fact that not all the possible radicals were detected, but nevertheless the relative reactivities of VF₂ and HFIB could be estimated. The relative reactivities of VF₂ and HFIB monomers and the steric hindrance effect indicate that the conformation of the copolymer is head-to-tail; this has been further confirmed by infrared analysis.