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

Abstract Polymeric methylenebis(phenylisocyanate) resins prepared with 15 N and 13 C labeled functional groups allow the detection of urethane linkages to wood using solid-state NMR. Resins prepared with only 15 N do not reveal urethane formation because of urea and urethane nitrogen signal overlap. Likewise, resins having only 13 C suffer from overlapping carbonyl signals. However, in the case of the doubly labeled isocyanate group, 15 N 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.