Copolymers with anti-thrombogenic properties based on N,N-dimethylacrylamide (A) and para-acryloyloxyacetanilide (B) were prepared by radical polymerization in DMF solutions at 50 degrees C using 2,2＇-azobisisobutyronitrile (AIBN) as initiator. The reactivity ratios of both monomers were determined by the application of the non-linear least-squares method suggested by Tidwell and Mortimer. The obtained results (r(A) = 0.47 and r(B) = 0.63) confirm that this comonomer pair polymerizes statistically but with a moderate tendency to form alternate sequences, depending on the composition of the monomer feed.