Copolymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS, monomer 1) with 2-hydropropyl methacrylate (monomer 2) was conducted in pure water at 80 degrees C. The reactivity ratios estimated from the compositional data of the copolymers at low conversion are r(1)=0.04+/-0.04 and r(2)=6.30+/-0.48, and values of Q(1) and e(1) are 0.16 and 1.37, respectively. Copolymer microstructure predicted by statistical calculation shows mean sequence length of M(1) shorter than 2. These results can be attributed to the strong repulsion between the ionized chain radical and charged monomer of AMPS.