The better understanding of the degree of mixing in highly viscous media is of crucial interest with respect to the product quality during a polymn. reaction. A special reactor composed of a loop tubular reactor followed by a tubular part, all being filled with motionless mixers, was developed in our labs in collaboration with a manufacturer. To investigate and det. the degree of mixing, a special test reaction using an inline sensor was developed. Application of these results to very fast copolymn. was realized using a technol. avoiding solvents and obtaining good product quality. [on SciFinder (R)]