A method of making a polyether-polydialkylsiloxane block copolymer is provided. The method comprises providing a mixture comprising at least one alkylene oxide and at least one silanol-terminated polydialkylsiloxane; and polymerizing the mixture in the presence of a double metal cyanide complex catalyst, and optionally in the presence of an aprotic solvent and/or an antioxidant. A polyether-polydialkylsiloxane block copolymer having a molecular weight of 150-50,000 g/mole and having a polydispersity index of 1.0 to 1.5 made by the above method is also provided.