A model system of styrene (St) and methyl methacrylate (MMA) was copolymerized in an NMR tube at 60 degrees C using 2,2'-azobis(isobutyronitrile) as the initiator and pyridazine as an internal standard to optimize an in situ( 1)H NMR spectroscopic method for determining reactivity ratios by generating data at hundreds of instantaneous comonomer compositions (244 data points from 8 to 91 mol % St) starting with only nine initial comonomer compositions. The radical reactivity ratios of styrene (r(st) = 0.697 +/- 0.010) and methyl methacrylate (r(MMA) = 0.491 +/- 0.007) were determined by nonlinear least-squares fitting of a Mayo-Lewis plot of the instantaneous copolymer composition as a function of the comonomer feed composition using the terminal model and MINITAB statistical software, in which the copolymer composition was calculated by assuming that all comonomer consumed was converted to copolymer without side reactions; the results were similar to accepted literature values for the terminal and implicit penultimate models. After correcting for changes in the "lock" value at the initial stages of the copolymerization (because of solids formed in the sealed NMR tube), the same technique was used to determine the reactivity ratios of 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA; r(FOSA) = 1.624 +/- 0.048) and 2-(N-ethylperfluoro-octanesulfonamido)ethyl methacrylate (FOSM; r(FOSM) = 2.876 +/- 0.083) in their radical copolymerizations with N,N-dimethylacrylamide (DMA; r(DMA) = 1.126 +/- 0.031 with FOSA; r(DMA) = 0.859 +/- 0.026 with FOSM).