In-situ sequential semi-interconnected interpenetrating polymer networks (IPNs) based on well-defined unsaturated poly(ester-urethane) (PU) and polystyrene (PS) were synthesized at PU/PS ratios of 100/0, 90/10, 80/20, 70/30, and 50/50, in order to study the effect of length of PS intranetwork bridges on the structure. The structure of these IPNs was studied using various characterization techniques such as swelling behavior in binary solvent mixtures, wide angle X-ray diffraction (WAXD), and solid state C-13 nuclear magnetic resonance (NMR) spectroscopy. The swelling studies showed that PS intranetwork bridges influence the size of the network cages formed. The swelling studies in a binary solvent mixture indicated that the PS intranetwork bridges of up to 6 styrene units (SU) in length are present in the stretched configuration. WAXD studies showed that the effective distance between the chains (d(eff)) increased up to the intranetwork bridge length of 6 SU and decreased on a further increase in the bridge length. Thus, both swelling and WAXD studies indicated that in the IPNs containing longer PS intranetwork bridges the PS chains are in a coiled configuration. This fact was confirmed by C-13 NMR spectroscopy which showed the increased mobility of the PS bridges in the case of the IPNs containing a higher PS content. On the basis of these results, it is proposed that the PU cages are amorphous and present in a coiled configuration, the IPNs containing up to 30% PS had cages containing stretched PS bridges and the IPN containing 50% PS had cages containing coiled PS bridges.