Simultaneous interpenetrating networks from poly(methyl methacrylate-co-ethyleneglycol dimethacrylate) (PA) and a hydroxyl-terminated polybutadiene-based polyurethane (PU) were prepared with various hard-segment contents (X) in the PU and different ratios (PU/PA) between the components. The level of the reinforcement, the mechanism of molecular failure, and the phase inversion depended strongly on X. Dynamic mechanical results indicated that the interpenetration occurred in the rigid blocks of the PU. The improved thermal and mechanical properties observed with higher values of X were interpreted in terms of the molecular weight and polydispersity of the hard blocks in the PU.