AbstractThe calculation of the “theoretical” minimum modifier requirement, TMMR, for butadiene‐styrene (75:25) copolymers of given intrinsic viscosities at various conversions is described. The TMMR of 75% conversion polymer of intrinsic viscosity 2.0 is R0= 0.28, which is only about 60% of the value of R0for commercial primary dodecyl mercaptan (C.M.) in large‐size reactors. The efficiency, E, of a modifier for 75:25 butadiene‐styrene copolymerization using soap as emulsifier and persulfate as catalyst has been defined by and calculated from the equation:\documentclass{article}\pagestyle{empty}\begin{document}$$E = \frac{{P\left( {0.2 + P} \right)}}{{R_0 \left( {M_v \times 10^{ - 5} } \right)}}$$\end{document}where Mvis the intrinsic viscosity molecular weight of the polymer at conversion, P, with an amount of mercaptan, R0, charged with the monomers. For all modifiers the efficiency is low at low conversions and gradually increases to a maximum with increasing conversion. For different modifiers this maximum may be as at low as 10% conversion or at higher than 80% conversion. After the maximum the efficiency decreases with further increase of conversion. The most efficient modifier for the production of polymers of intrinsic viscosity, [η], at conversion, P, will meet the following conditions, R = 1 at conversion, P, and dP/dR is constant. Inefficient modifiers may be made more efficient by changing the conditions or procedure of polymerization in such a way that the modifier more nearly complies with the above