An exptl. method to det. reactivity ratios by DSC expts. with stopped polymn. is presented. In different DSC expts., the polymn. is interrupted at different times, by plunging the crucible into liq. nitrogen. The crucibles are then opened, and the content (polymer and monomers) is dissolved and the compn. of the monomers is analyzed by gas chromatog. By using the integrated equation of Meyer and Lowry the reactivity ratios can be estd. by nonlinear parameter fitting. The method was illustrated for Bu acrylate-2-hydroxyethyl methacrylate system with Bz2O2 catalyst. [on SciFinder (R)]