Dimethacrylate based networks usually have a poor impact resistance. In this work, we have tried to understand the origin of this brittleness. Two systems based on a same dimethacrylate monomer polymerised with styrene and divinylbenzene comonomers, respectively, have been chosen to correlate structural parameters with mechanical properties. The increase of structural heterogeneity, characterized by the width of relaxation time distribution, was measured as a function of double bonds conversion using dynamic mechanical tests. Atomic force microscopy observations of network structure after laser ablation show that the heterogeneity of networks is spacially organised due to the formation of microgels and their agglomeration. The presence of microgels strongly affects the polymerisation kinetics and controls the mechanical behaviour. Results show that the more densely crosslinked the network, the more heterogeneous is its structure. The impact resistance is shown to be related to the level of networks heterogeneities.