Polymerization of neat vinyl acrylate (VA) photoinitiated by phosphine oxide initiator in a nitrogen atmosphere was studied by photo differential scanning calorimetry (photoDSC). The study revealed a new phenomenon in fast free-radical polymerization: spontaneous temporary cessation of heat release (i.e., temporary termination of the polymerization) or temporary reduction of the polymerization rate. Such a phenomenon, named "stumbling polymerization", includes a fast initial photopolymerization of VA up to a monomer conversion of xi approximate to 10%, with the first maximum rate of polymerization achieved not later than at 3 s after the beginning of irradiation under our experimental conditions; a temporary cessation of polymerization; and a resumed photopolymerization up to xi approximate to 100%. Experiments on photopolymerization of neat VA without a photoinitiator allowed us to obtain the ratio of the termination rate coefficient (2k(t)) to the propagation rate coefficient (k(p)); 2k(t)/k(p), decreased from similar to 300 at the beginning of polymerization to a value of similar to2 at the final stage of polymerization.