Two urethane poly(ethylene oxide) dimethacrylates based on polyether diol of variable length (M-n: 300 and 1000 g/mol), isophorone diisocyanate and 2-hydroxyethyl methacrylate, were synthesized and characterized to be used as comonomers for dental composites. The polymerization process initiated with benzoyl peroxide/amine or camphorquinone/tertiary amine couple was followed by a combination of FTIR spectroscopy and fluorescence technique using a pyrene probe for the in situ monitoring of microscopic changes in the polymeric systems. Difference in the polymerization kinetics of the oligomers and low-molecular-weight monomer is presented in connection with the chemical structure of the urethane dimethacrylate, taken together with another partners (bisGMA, TEGDMA, filler) with effect on the mobility and reactivity of the formed polymeric network. The results suggest a higher reactivity of the oligomeric forms and are in agreement with the fluorescence measurements induced of the pyrene molecule, whose monomer fluorescence emission significantly increased with irradiation time as the network formation occurred. Measurements of the contact angle showed the maintenance of the hydrophilic nature of these resins, while the values of polymerization shrinkage and diametral tensile and compressive strengths of the three specimens indicated that the incorporation of dimethacrylate urethane oligomers in the organic matrix (18 wt%) led to materials with good properties. POLYM. ENG. SCI., 49:1127-1135, 2009. (C) 2009 Society of Plastics Engineers