The reaction of polymethylene diphenyl diisocyanate (pMDI) with pyrolysis oils (PO) was studied by differential scanning calorimetry (DSC) and rheology. Chemical reactions between pMDI and PO occur under 100degreesC, as shown in DSC scans. DSC analysis showed that the peak temperature of the reaction decreased as the PO content of the PO-pMDI blends increased. The heat of reaction is at its maximum around 30-40% PO content. A rheological study of various PO-pMDI blends was done to evaluate the evolution of viscosity with time for different PO-pMDI hybrid mixtures. The initial viscosity of the blends is directly proportional to the PO content. An exponential increase of viscosity was demonstrated for all PO-pMDI mixtures. Rheological and chemical analysis results confirmed that chemical reactions occur between pMDI and PO at room temperature. (C) 2003 Wiley Periodicals, Inc.