Published approaches to analyze the kinetics of radical photo-polymerization are mainly based on in situ IR or Raman measurements. But these techniques only monitor the monomer decay. Additional information particularly about solidification cannot be obtained with these methods. Electron spin resonance, ESR, spectroscopy allows monitoring of radicals. The presented investigation gives an introduction to additional applications of ESR to monitor the solidification of multifunctional acrylic esters by successive in situ irradiation. This general approach to monitor the termination rate of the midchain radical which is not involved in propagation but whose lifetime is correlated via mobility with the solidification which is demonstrated with a set of commercially available multifunctional resins without an initiator. A detailed analysis of ESR signals is required to prove that the presented method is applicable for acrylic esters and to clarify published contradictory statements. The properties and the curing of the resins are characterized by additional methods, such as UV-Vis and Raman spectroscopy. The first results based on this innovative technique indicate that the functionality of the ester group has a significant and, most of all, measurable effect on the mobility of the small reactive molecules within the cured(-ing) polymer.