The surface activity of cationic DMBAC (dimethyl methacryloyloxyethyl benzyl ammonium chloride) was revealed by measuring surface tension and estimating of critical micelle concentration (CMC).The copolymerization of acrylamide (AM) and DMBAC was studied in aqueous solution at above and below CMC of DMBAC

respectively.The reactivity ratios r1 and r2 were determined by the Fineman\|Ross method.IR and 1H\|NMR spectra afford information about polymer structure.The compositions of copolymers were calculated by UV spectrophotometer.The intrinsic viscosity values of the copolymers were measured by the Ubbelohde\|type Viscometer.Molecular weight (Mw) and polydispersity index (PDI) of the copolymers were measured by gel permeation chromatography (GPC).The effect of DMBAC concentration on the microstructure of the copolymers was analyzed by 13C\|NMR and DSC spectra.The property of hydrophobic association of the copolymers was characterized by using pyrene fluorescence probe technique.The experimental results of copolymerization showed that the reactivity ratios of monomers were different at above and below CMC of DMBAC

r1(AM)=0.65

r2(DMBAC)=1.41 and r1(AM)=0.74

r2(DMBAC)=0.91

respectively.These results suggested that DMBAC monomers may distribute in a microblock sequence in the copolymer chain at above CMC of DMBAC

and in a random sequence at below CMC of DMBAC.The structures of copolymers were proved again by 13C-NMR and DSC spectra.The copolymer CP-1 with microblock sequences showed stronger hydrophobically associating than the copolymer CP-2 with random sequences.