The aqueous two—phase copolymerization(ATPC)of(2-methacryloyloxyethyl)trimethylammonium chloride(DMMC)and acrylamide(AM)in the aqueous sdution of poly(ethylene glycol)(PEG)is studied and compared with homogeneous aqueous solution copolymerization(HASC)．The reactivity ratios of DMMC／AM homogeneous aqueous solution copolymerization are rDMMC=1．77 and

rAM=0．32．The 95％ and 99％ joint confidence regions are also calculated．The conversion versus time curves and the composition of DMMC—AM copolymers are compared in two copolymerization systems

which shows that the reaction rate of ATPC is more quickly than that of HASC

but the final conversions are almost the same；the composition of DMMC—AM copalymers in ATPC is obviously different from that of in HASC．The partition coefficients of DMMC

AM and the copolymer of DMMC—AM at 65℃

under the reaction condition are determined

K1=0．035

K20．125 and Kp=0．011

respectively．Base on the polymerization mechanism of ATPC and the partition coefficients of monomers and copolymer

a new method of calculating the copolymer composition is presented for ATPC．The copolymer composition calculated by this method is in agreement with the experiment data．