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Radiation-Induced Polymerization Monitored with Fluorogenic Molecular Probes

Each year over one billion pounds of acrylic-based polymeric products are produced world wide. Such products include windows in aircraft, lenses in eyeglasses and CD players, coatings on parquet flooring and various architectural structures such as skylights and domes. Often these products are made by injecting monomer or monomer solutions into molds in which it is polymerized, i.e. cured, by the application of heat. Alternatively, acrylic-based products are produced as coatings by exposing the polymerizable medium to UV light or high-energy radiation. Obviously, it is of importance to such processes that the curing stage is optimized. Insufficient curing will lead to the production of materials with inferior properties. Alternatively, excessive heating or exposure can lead to both slow rates and high costs of production. In this thesis a novel method is introduced by which the polymerization process may be monitored. This non-invasive manner of monitoring the polymerization process involves the use of trace concentrations of molecular probes in the polymerizing media. Unique to the molecular probes discussed in this thesis is their fluorogenic character. These probe molecules which are initially non-fluorescent chemically react with growing polymer chains and thereby become fluorescent. The fluorescence from the polymerizing system is therefore a direct measure of the overall extent of polymerization. Because of the associated chemical change to the probe molecule, this is a highly sensitive technique for monitoring the polymerization process. This is especially true for the early stage of polymerization, a stage in which the physical properties (viscosity, dielectric constant) of the polymerizing media do not change significantly (see chapter seven). Furthermore, because of the high sensitivity of fluorescence techniques, only a very small (millimolar or less) probe concentration is required. The polymerization of methyl methacrylate, MMA, was chosen as the system to study for two reasons; the ready availability of scientific data on this system and the associated Trommsdorff effect observed as a rapid autoacceleration of polymerization when the monomer conversion reaches a certain extent. Obviously, the bulk of scientific data present in the literature enabled rapid comparisons between the experimental results obtained with those from research institutes throughout the world.