A　series　of　novel　comb-like　poly(butyl　acrylate)-g-poly(dimethylaminoethyl　methacrylate)　(PBA-g-PDMAEMA)　with　different　side　chain　lengths　were　designed　and　successfully　synthesized　by　the　＂first　main　chain　then　side　chain＂　method.　Infrared　Spectroscopy　(IR),　H-1　Nuclear　Magnetic　Resonance　(H-1　NMR),　and　gel　permeation　chromatography　(GPC)　were　used　for　structural　confirmation　and　molecular　weight　characterization.　This　polymer　exhibited　responsive　behavior　from　hydrophilicity　to　hydrophobicity　under　the　alkaline　environment　of　cement-based　materials,　with　the　contact　angle　of　105.6　degrees,　a　decreased　evaporation　rate,　and　a　hydrophile-lipophile　balance　(HLB)　value.　A　significant　internal　hydrophobic　effect　on　cement　mortar　was　shown　in　the　water　absorption　rate,　which　decreased　by　75.2%,　and　a　dry　shrinkage-reducing　rate　of　more　than　30%.　Furthermore,　this　polymer　can　effectively　slow　the　exothermic　rate,　reduce　the　heat　release,　and　delay　the　exothermic　peak　of　cement　hydration.　It　was　interesting　that　these　properties　showed　a　direct　correlation　with　the　side　chain　length　of　the　comb　polymer.　The　aims　of　this　study　are　to　provide　a　new　avenue　to　synthesize　polymers　with　the　spontaneous　hydrophilicity-hydrophobicity　transition　in　the　cement　system,　achieving　excellent　internal　hydrophobicity　of　cement-based　materials,　and　to　offer　a　promising　alternative　to　resist　external　erosion　for　improving　the　durability　and　service　life　of　cement-based　materials.