The　reactivity　ratio　of　monomer　and　the　microstructure　of　copolymer　as　polycarboxylate　ether　(PCE)　superplasticizer　were　investigated.　Polycarboxylate　ethers　(PCEs)　were　synthesized　from　methyl　allyl　polyethylene　glycol　(MAPEG,　M-w=2　400　g/mol)　and　methacrylic　acid　(MAA)　via　aqueous　free　radical　copolymerization　in　low　conversion　(P＜20%).　Gel　permeation　chromatography　(GPC)　and　high　performance　liquid　chromatography　(HPLC)　were　used　to　track　the　residual　concentration　of　the　reactants　and　the　amount　of　copolymer　formed　during　the　copolymerization.　The　reactivity　ratios　of　monomers,　MAPEG　and　MAA,　were　determined　as　r(1(MAPEG))　=0.0489　and　r(2(MAA))　=1.6173,　respectively,　by　employing　the　K-T　and　YBR　methods.　According　to　the　obtained　monomer　reactivity　results,　the　sequence　distribution　of　the　copolymer,　the　number　average　length　of　MAA　in　the　polymerisate　were　found　to　decline　with　the　decrease　of　the　mole　fraction　of　MAA　in　the　polymerization　system.　As　a　result,　the　distribution　of　chain　segments　becomes　narrower　and　the　copolymer　structure　becomes　more　uniform.　Therefore,　uniform　polymers　could　be　obtained　by　slowly　adding　MAA　monomer　during　copolymerization　process.