FT-LR spectroscopy, synchrotron SAXS, and dynamic rheometry have been employed to monitor, in-situ, structure development during the reactive processing of model flexible polyurethane foam systems. The following combinations of components were investigated : (I) diisocyanate, polyether polyol, and water and (II) diisocyanate, polyether monol, and water. The formation of urethane, soluble urea, and hydrogen-bonded urea species during the fast bulk copolymerization has been studied using the adiabatic reactor method and forced-adiabatic, time-resolved FT-IR spectroscopy. The decay of isocyanate is correlated with the polymerization kinetics, and the evolution of hydrogen-bended urea is analyzed emphasizing the onset of microphase separation of urea hard segment sequences. FT-IR spectroscopy indicated that the microphase separation transition (MST) occurred at a critical conversion of isocyanate functional groups and followed the kinetics associated with nucleation and growth. The dynamics of microphase separation during the fast bulk copolymerization have also been investigated employing forced-adiabatic, time-resolved synchrotron SAXS experiments. Microphase separation was observed to occur at a critical conversion of isocyanate functional groups and is shown to follow the kinetics associated with spinodal decomposition. Forced-adiabatic rheological measurements have been conducted during the fast bulk copolymerization. Four main regions of rheological development during the formation of polyurethane foam were identified. These were : (I) bubble nucleation, (II) liquid foam and microphase separation, (III) physical gelation, and (TV) foamed elastomer. The use of model systems demonstrated that the presence of covalent cross-links delay the onset of microphase separation of the urea hard segment sequence lengths. Although foam stability is not dependent upon the formation of urethane covalent cross-links in the early part of the foaming reaction, molecular connectivity between the microphases via urethane covalent cross-links is an essential requirement with regard to long-term dimensional stability and the mechanical/physical properties of the foam.