For emulsion polymerisation the reactor type has a strong influence on the final product properties, for example the particle size (distribution) and the polymer composition. A batch copolymerisation of styrene and methyl acrylate shows strong composition drift. The course of the batch reaction has been explained with model calculations of the polymerisation rates of the monomers. In a single CSTR it has been shown that a homogeneous copolymer is formed. It has been demonstrated that in a series of three CSTR's copolymers with a three modal chemical composition distribution are produced.