Double labeled isocyanate resins for the solid-state NMR detection of urethane linkages to wood

Polymeric methylenebis(phenylisocyanate) resins prepared with 15N and 13C labeled functional groups allow the detection of urethane linkages to wood using solid-state NMR. Resins prepared with only 15N do not reveal urethane formation because of urea and urethane nitrogen signal overlap. Likewise, resins having only 13C suffer from overlapping carbonyl signals. However, in the case of the doubly labeled isocyanate group, 15N causes narrowing that seems to provide partial resolution of the urethane and urea carbonyl carbons. Furthermore, the double label gives complimentary nitrogen and carbon spectra which are required for a complete interpretation. This method will be useful for the characterization of the adhesive mechanism in wood-based composites bonded with isocyanate resins.