NMR Characterization of Cross-Linked Polystyrene Gels

Direct polarization magic angle spinning (DP/MAS) NMR is ideally suited for the characterization of solvent-swollen cross-linked polystyrene gels. These gels exemplify a larger group of polymeric materials that undergo rapid but anisotropic reorientation on the molecular scale. The line broadening due to the residual dipolar coupling (¹H NMR) or chemical shift anisotropy (¹³C NMR) in the swollen gel can be removed by moderate-rate magic angle spinning to give highly resolved ¹H and ¹³C NMR spectra. Application of these methods permits full characterization of chemically modified polystyrene resins including well-resolved signals attributed to the cross-link junctions. The complementary nature of DP/MAS and CP/MAS NMR is discussed, as is the possible application of DP/MAS NMR to directly monitor cross-linking polymerization kinetics around the gel point.