Kinetics of Cap Separation in Nitroxide-Regulated “Living” Free Radical Polymerization: Application of a Novel Methodology Involving a Prefluorescent Nitroxide Switch Olga Garcı ´a Ballesteros, † Luca Maretti, ‡ Roberto Sastre, † and J. C. Scaiano* ,‡ Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), Juan de la Cierva 3. Madrid 28006, Spain, and Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Received March 1, 2001; Revised Manuscript Received June 7, 2001 ABSTRACT: A novel approach employing a prefluorescent paramagnetic sensor has been employed to determine the absolute kinetics for end-cap cleavage in TEMPO-capped polystyrene obtained by “living” free radical polymerization. This new methodology takes advantage of the suppression of coumarin fluorescence when this chromophore is tethered to a paramagnetic nitroxide. This coumarin-nitroxide is an excellent free radical trap for carbon-centered radicals; upon radical trapping, the resulting diamagnetic alkoxyamine is strongly fluorescent. Thus, fluorescence buildup is a direct measure of free radical formation and can be employed to quantify their formation or to study their kinetics. Studies of the temperature dependence of the process can be employed to determine activation parameters and bond dissociation energies. This simple technique can be employed to study the dynamics in the actual polymer systems, overcoming the frequent need to resort to model compounds. Introduction The methodology of “living” free radical polymeriza- tion (LFRP) offers the possibility of controlling polydis- persity in free radical polymerization and the convenient design of block copolymers. 1-3 The “living” properties of LRFP derive from the reversible capping of the growing polymer chain with nitroxides. This cap readily dissociates at temperatures around 125 °C to allow further polymer growth. 2,4 The overall dynamics of LFRP is largely determined by the cap cleavage reaction and the small concentrations of nitroxide that ac- cumulate in the system. Yet, determinations of the absolute kinetics of this key step have been difficult, and researchers have frequently resorted to small mimetic compounds that more easily overcome analyti- cal problems and the inherent complexity of the poly- meric system. Nitroxides are persistent free radicals frequently used as radical scavengers or as quenchers of excited states. In particular, 2,2,6,6-tetramethylpiperidine-N-oxyl, known as TEMPO, has been used as a quencher of singlet and triplet excited states. Covalently tethering a nitroxide to a chromophore with a short link results in the intramolecular quenching of the chromophore’s excited state. 5 In the case of fluorescent moieties, the tethered nitroxide results in essentially quantitative suppression of the emission. Radical trapping leads to a diamagnetic alkoxyamine which acts as a switch, thus restoring the chromophore’s fluorescence. Blough and co-workers have shown that these covanlently linked nitroxide-fluorophore adducts can be employed as very sensitive optical sensors of radical/redox reactions. 5-8 In this work, TEMPO has been covalently tethered via a short link, to a fluorescent chromophore (cou- marin), leading to a nonflourescent paramagnetic com- pound (CU-T • ). When a TEMPO-capped polystyrene produced by LFRP is thermally decomposed in the presence of the sensor, the resulting macroradicals (PS • ) can be trapped by either TEMPO (T • ) or the prefluores- cent probe (CU-T • ) with essentially the same kinetics. Thus, by adequately choosing the concentrations, it is possible to favor trapping by the probe, which results in a dramatic enhancement of the fluorescence (CU- T-PS) (Scheme 1). This radical-switch methodology has been used for the first time in the determination of the dynamics of macroradicals derived from TEMPO-capped polystyrene using fluorescence spectroscopy. The rates of radical formation were determined; from these data we also obtained the kinetic Arrhenius parameters and the corresponding bond dissociation energy for the terminal TEMPO group in the polymeric chain. Experimental Section Synthesis of the Thermal Initiator: 2,2,6,6-Tetra- methyl-1-(1-phenylethyoxy)-piperidine (EB-T). Typical conditions for the photochemical synthesis of the initiator were similar to those described in the literature. 10 One equivalent of nitroxide, TEMPO (Aldrich), was dissolved in 5 mass equivalents of di-tert-butyl peroxide (Aldrich), previously puri- fied by passing it through a plug of neutral alumina (Aldrich). The red-orange solution was then diluted with 30-35 mL of ethylbenzene (99% purity, Aldrich) as solvent and reagent, loaded into a Pyrex test tube, and purged with nitrogen for 30 min. The solution was then irradiated with 300 nm broad- band lamps until the color was discharged, typically 4-5 days. The lightly colored yellow solution so obtained was concen- trated by bubbling a stream of nitrogen through the sample and then further concentrated under vacuum. Recrystalliza- tion from methanol afforded white crystals in greater than 90% yield. † Instituto de Ciencia y Tecnologı ´a de Polı ´meros. ‡ University of Ottawa. 6184 Macromolecules 2001, 34, 6184-6187 10.1021/ma0103831 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/31/2001