Hydrogels are a desired material for biomedical and pharmaceutical applications. To better control the synthesized hydrogels for various applications, it is necessary to have a thorough understanding of hydrogel structure and reaction mechanism. In this study, pH-sensitive hydrogel networks consisting of methacrylic acid (MAA) crosslinked with tri(ethylene glycol) dimethacrylate (TEGDMA) were synthesized by free-radical photopolymerization in the water/ethanol mixture. Reaction rate was measured using Photo-Differential Scanning Calorimetry (PhotoDSC) with a modified sample pan designed for handling volatile reagents. A photo-rheometer and a dynamic light scattering (DLS) goniometer were used to follow the changes in viscosity and molecule size of the resin system during photopolymerization. It was found that the rate of polymerization increased and more compact and less swelling gels would form with a higher water fraction in 50wt% solvent/reactant mixture. This is because the weaker interactions between the MAA and the solvent give a higher opportunity for propagation and a higher reaction rate. And the hydrophobic TEGDMA and initiator tend to form aggregates in the higher water solution, contributing to the inhomogeneous microgel formation. The copolymerization of the MAA/TEGDMA system was enhanced as the light intensity increased, especially at the low light intensity range and low conversion. The use of the high light intensity significantly shortened the reaction time to reach macrogelation and increased the swelling ratio of formed hydrogels, which can be explained by the mechanism for the relative rates of intra- and intermolecular reactions during microstructure formation.