A new approach for monitoring in situ the progress of an addition polymerization has been developed based on the fluorescence quenching of fluorene. Fluorene is quenched by the enone functionality on acrylates and methacrylates, but is not quenched after the carbon-carbon double bond in this group is broken by incorporation into the polymer backbone. Ethyl (2-fluorenyl)methacrylate was used as a self-quenching comonomer during the 2,2’-azobis(2-methylpropionitrile)-initiated free-radical copolymerization of methyl methacrylate at 60 degrees C. The fluorescence intensity increases by 60% up to the onset of the gel effect (defined as the sudden increase in the temperature profile). The system shows sensitivity well into the glassy state, with fluorescence increasing more than two orders of magnitude from the beginning of the reaction. This sensitivity is compared with that of two free-volume-dependent probes, 1,3-bis(1-pyrene)propane and dimethylaminobenzylidenemalononitrile, and to fluorene. The temperature profile of the test-tube-scale reaction was used as an internal reference for characterizing the sensitivity of the probes with respect to the gel effect region.