A high-molecular weight hydroxylated acylglycerol-based molecule was initially synthesized by thermally polymerizing soybean oil in a Diels-Alder formation, a mechanism whereby crosslinking of acylglycerols generally occurs at the 2-position of the fatty acid moieties. Hydroxylation was achieved by direct heat treatment of the polymerized or bodied soybean oil and pure glycerol. This thermal route involves transesterification in a closed reaction of the macromolecule with glycerol at temperatures greater than 200oC. Tunability in the functionality of the molecule through hydroxylation was achieved. In addition, the method effects a significant increase in molecular weight of the macromolecule with respect to processing time. Control of the parameters mentioned translate to a synthesized product with the desired functionality and molecular weight for vast practical chemical applications that include polyols in polyurethane production. Rigid foaming results of the processed soy-based polyol showed excellent physicochemical properties suggesting comparability with a petroleum-based polyether polyol.