Rigid polyurethane foams (PUFs) were prepared from polymeric 4,4-diphenylmethane diisocyanate (PMDI; having functionality of 2.9), polyether polyols, silicone surfactant, amine catalysts, and distilled water. The effects of reactivity on the properties such as density, compressive and flexural strength, and glass-transition temperature (T-g) of the PUF samples were studied. The kinetic rate of forming the PUF samples was increased with the catalyst and water content. With increasing OH value and functionality of the polyols, the density and compressive strength of the PUF samples also increased. For the PUF samples synthesized with polyols having high functionality (>5), the flexural strength of the PUF samples decreased with the functionality of the polyols. With increasing OH value and functionality of the polyols, the T-g of the PUF increased because of an increase in the degree of crosslinking of the PUF samples. The T-g value and compressive strength of the PUF samples were observed to increase with the NCO index. From this result, it was suggested that the increase in the T-g value and compressive strength of the PUF samples may be attributable to the additional crosslinks that arose from allophonate and biuret formation by the supplementary reactions of excess PMDI. (C) 2004 Wiley Periodicals, Inc.