The recent introduction of semicrystalline acrylated oligomers exhibiting fast photoinitiated free radical polymerization in the solid state calls for a deeper understanding of the mechanisms behind the reaction kinetics. The photoinduced polymerization of an acrylated urethane-based poly(ethylene glycol) precursor was studied in detail at temperatures below the melting point using differential photocalorimetry. In isothermal conditions, the exothermal heat flow profile is characterized by an acceleration step followed by a gradual deceleration. In contrast to liquid-state photopolymerization, the well-known gel effect cannot be invoked to account for the reaction acceleration in the crystallized resin. By revisiting the kinetics of free-radical polymerization, it appears that the acceleration results from the buildup of the radical concentration toward steady state in a reaction diffusion driven process. The kinetic behavior is examined in terms of conversion for which any structure-dependent kinetic effect is described by a power-law approximation based on scaling arguments from experimental evidence and polymer physics. This results in a closed-form analytical expression that compares well to experimental data for the photopolymerization kinetics of a semicrystalline acrylated urethane precursor upon adjustment of three parameters. The model is extended to include the additional kinetic complexity for liquid (meth)acrylates and provides a unified approach to free-radical polymerization built on fundamental insights.