Polymer electrolytes are alternatives to liquid electrolytes traditionally used in electrochemical devices such as lithium-ion batteries and fuel cells. In particular, block polymer electrolytes are promising candidates because they self-assemble into well-defined microstructures, in which orthogonal properties can be integrated into a single material (e.g., high modulus in domain A, fast ion transport in domain B). However, the performance of block polymer electrolytes often falls short, due to the lack of long-range continuity of both domains and relatively low strength. We recently reported a simple, one-pot synthetic strategy to prepare polymer electrolytes with the highest reported combination of modulus and ionic conductivity, attributes enabled by a co-continuous, cross-linked network morphology. In this work we aim to understand the mechanism by which this nanoscale morphology is formed by performing a series of in situ, time-resolved experiments-small-angle X-ray scattering, conductivity, rheology, and reaction kinetics-to monitor the electrolyte as it transitions from a macroscopically homogeneous liquid to a microphase-separated solid. The results suggest that the chain connectivity of the diblock gives rise to isotropic concentration fluctuations that increase in amplitude and coherence such that the network morphology is ultimately produced. The kinetic trapping of this network morphology by chemical cross-linking prior to the ordering transition is shown to be critically important to the resulting advantageous bulk electrolyte properties.