A thermoplastic composition that includes a poly(arylene ether)-polysiloxane block copolymer is prepared by a method that includes oxidatively copolymerizing a monohydric phenol and a hydroxyaryl-terminated polysiloxane. The oxidative polymerization includes a monohydric phenol addition period characterized by a first temperature, a build period following the addition period and characterized by a second temperature greater than the first temperature, and a temperature ramp period between the addition period and the build period. During the temperature ramp period, the temperature is increased at an average rate of about 0.01 to about 0.35° C. per minute, which improves the efficiency with which the hydroxyaryl-terminated polysiloxane is incorporated into the poly(arylene ether)-polysiloxane block copolymer.