Copolymers of methoxy polyethyleneglycol (meth) acrylate (mPEG(M)A) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) were synthesised, characterised with Fourier transform infrared spectroscopy and gel permeation chromatography, and the effects of molecular architecture on the properties of cement-based systems were investigated. Superplasticiser (SP) adsorption was determined with a total organic carbon analyser, while spread and mechanical properties were assessed according to European standards. The rheological behaviour of cement pastes at different copolymer dosages was also studied. Sufficient fluidity and fluidity retention over time are requirements for the mixing, transportation, pumping and moulding of concrete at construction sites. The synthesised SPs with long side chains provided mortars with high fluidity, high dispersing power per adsorption ratio and low mechanical strength compared with the SPs with short side chains. They were also effective at decreasing the shear yield stresses and plastic viscosities (at high dosages) of the cement pastes. Additionally, mortars dosed with mPEG(M) A-co-AMPS had higher fluidity and fluidity-retaining properties compared with mortars prepared with lignosulfonate-type SP.