Copolymers of propylene oxide and ethylene oxide have an inner block that contains from 65-90 weight percent oxyethylene units and from 10 to 35 weight percent oxypropylene units. This block has a molecular weight of from 150 to 350. The copolymer has an outer block which contains at least 95 weight % oxypropylene units and from 0 to 5% oxyethylene units. The equivalent weight of the copolymer is from 800 to 2000. The copolymers are useful in making polyurethane foams that have unexpectedly high tensile and/or tear strengths.