Fluorescence probes for polymerization reactions: Bulk polymerization of styrene,n-butyl methacrylate,ethyl methacrylate,and ethyl acrylate

Julolidine malononitrile 3 was used as a fluorescent probe for high-conversion (free-radical) bulk polymerization of methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, ethyl acrylate, styrene, and the copolymerization of styrene/n-butyl methacrylate. The fluorescence of the probe increased gradually as polymer conversion increased. This was followed by an abrupt rise in fluorescence intensities by a factor of 3 to 40 depending on the polymer formed. Finally the fluorescence intensities leveled off as the polymer limiting conversion was reached. The polymerization region in which fluorescence intensity increases sharply seems to correspond to the increase of the rigidity of the medium at the glass transition. A correlation between the limiting quantum yield of fluorescence of the dye and the polymer glass transition T_g and expansion coefficient α was found. These results were interpreted in terms of rotation-dependent nonradiative decay which links the excited-state conformation to the rigidity of the medium.