The aim of this study is the synthesis and characterization of the novel copolymers of poly(isopentenyl PEG-co-allyl PEG) (TPEG-co-APEG) and the investigation of their influence on the properties of the cement mortar. The performance of the copolymers was compared with the physical blends of TPEG and APEG having the similar TPEG/APEG ratio. In addition, a commercial MPEG type PC was used as a control in the experiments. The FTIR and 1H-NMR proved that the copolymerization was successfully achieved. GPC enlightened the change of molecular weight of the copolymers. The dispersion, flow retention and the strength of the mortars prepared with copolymers or physical blends were compared. It was found that the strength and the adsorption of the copolymers were better than that of blends, whereas flow retention of the blends was better in comparison to that of the copolymers.