Continuous method of treating polymer reaction mixture

An electrophotographic toner comprising styrene polymer or copolymer with other vinyl monomer, having a number average molecular weight (Mn) of 2,000-15,000, a Z average molecular weight (Mz) of between 400,000 and 9,000,000, and Mz/Mn of 50-600, and comprising 70-30 parts of a low molecular weight polymer having a number average molecular weight of 1,000-5,000 and a glass transition temperature of 40.degree.-80.degree. C., and 30-70 parts of a high molecular weight polymer which gives said styrene polymer or copolymer having said Mn, Mz and Mz/Mn when mixed with said low molecular weight polymer, said styrene polymer or copolymer being produced by flowing down and preheating a reaction mixture containing said styrene polymer or copolymer by causing said mixture to flow through a circular-ring-shaped space of a heat-exchanger, said heat-exchanger consisting of an outer double pipe as a jacket of a circulating heat medium, a concentrically loaded inner pipe of the heat-exchanger for circulating the heat medium and the circular-ring-shaped space formed in between, and flashing the reaction mixture containing said styrene polymer into a reaction mixture containing with said heat-exchanger to continuously separate the styrene polymer with 0.3% or less of volatile ingredient.