Determination of H-crosslinks in γ-irradiated polybutadiene by ¹³C-NMR

Summary

The solid-state ¹³C NMR spectra of a 99% cis-1,4-polybutadiene after γ-irradiation at 25°C showed a reduction in the intensity of the -CH=resonance at 130.1 ppm and the development of a broad resonance at 45.8 ppm due to H-crosslinks. The -CH₂-signal comprises the original narrow resonance at 28.2 ppm due to -CH₂-groups remote from crosslinks, and a broad resonance centred at 30.5 ppm attributable to -CH₂-groups adjacent to crosslinks. There was an increase in the intensity of the -CH₂-resonance at 33.6 ppm due to trans -CH₂-groups resulting from radiation-induced isomerization. Quantitative determinations of (i) the decrease in -CH= and (ii) the increase in crosslinked carbon atoms both gave G values of about 14 after 3 MGy. The crosslinks are evidently located in clusters, since literature G(X) values derived from gel contents and swelling ratios range from 3 to 6. The high G(X) value from NMR is consistent with a chain reaction for the crosslinking process.