Novel comb copolymers were prepared by the free-radical copolymerization of acrylamide (AM) and poly(ethylene glycol) (PEG) macromonomers in aqueous solution using potassium persulfate as an initiator. The reactivity ratios of acrylamide (M(1)) toward macromonomers (M(2)), r(1) = K-p11/K-p12, were estimated at low conversion by using H nuclear magnetic resonance (n.m.r.) spectroscopy and high performance liquid chromatography (h.p.l.c.) techniques. The values of r(1) ranged between 1.35 and 2.2 and were dependent on the PEG pendant chain length in the macromonomers. The reactivity of the macromonomers in copolymerization, characterized by (1/r(1)), decreased with the PEG pendant chain length. The reactivity of the macromonomer in homopolymerization, expressed as K-p/K-t(1/2), increased with the PEG pendant chain length. The changes of K-p and K-t are probably caused by the shielding effect on the growing radicals by the macromonomer pendant chains. The molecular weights of the copolymers were found to be sensitive to the polymerization temperature. Higher-molecular-weight (> 3 x 10(6)) copolymers were obtained at lower temperatures.