Differential Line Broadening in Coupled Carbon-13 Magic-Angle Sample-Spinning Nuclear Magnetic Resonance Spectra of Solid Polymers

We have observed large differential line broadening (DLB) of the individual components of ¹³C-¹H coupled multiplets in ¹³C “magic-angle” sample spinning (MAS) nuclear magnetic resonance (NMR) spectra of solid poly (cis-butadiene), poly(cis-isoprene), poly(cts, trans-butadiene), butadiene-styrene and acrylonitrile-butadiene-styrene block polymers, and poly(trans-isoprene). Differential line broadening of olefinic methine carbons is manifest as broadening of the high-field doublet component in poly(cis-butadiene) and poly(cis-isoprene), while in trans segments in poly(cts,trans-butadiene) and in poly(trans-isoprene) the low-field component broadens first, on cooling. DLB effects for isolated vinyl segments (due to 1,2-addition) in polybutadienes are more pronounced than those seen in the polybutadiene backbone and suggest restricted motion of the vinyl substituent. Results with the block copolymers are very similar to those found with poly(cts,trans-butadiene) and are thought to be due to dipolar and dipolar-chemical shift anisotropy interference effects. Such DLB effects are largest at high magnetic field strengths and decrease with MAS spinning rate, consistent with such interferences. The observation of differential line broadening using proton-coupled ¹³C MAS NMR offers a novel approach to investigating molecular motion in highly mobile solids and may be a useful additional structural characterization technique to the more conventional MAS NMR methods.