A method of making a biobased-petrochemical hybrid polyol is provided. This method includes reacting a cyclic ether with a vegetable oil-based polyol in the presence of a cationic catalyst or a coordinative catalyst that includes a vegetable oil-based polyol ligand to form the biobased-petrochemical hybrid polyol. The biobased-petrochemical hybrid polyol that is created has a number average molecular weight of about 3,000 to about 6,000 and has a structure that is about 22% to about 36% biobased. In one aspect of the present invention, the cyclic ether is propylene oxide, and the propoxylated polyol formed from the propylene oxide and vegetable oil-based polyol is then reacted with ethylene oxide in the presence of a superacid catalyst to create a block copolymer with a terminal polyethylene oxide block having a high percentage of terminal primary hydroxyl groups. If the cyclic ether is a homogeneous mixture of ethylene oxide and propylene oxide, then propylene oxide-ethylene oxide random copolymers are formed by polyaddition to vegetable oil polyols. The biobased-petrochemical hybrid polyol may be reacted with an isocyanate so as to create a flexible polyurethane that has good hydrolytic resistance.