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Poly(9,9-dialkylfluorene-2,7-diyl) derivatives end-capped with 2-fluorenyl, 9,9-di-n-hexyl-2-fluorenyl, and 9-fluorenone-2-yl substituents have been synthesized through Ni(0)-mediated polymerization. Absorption and emission spectra of these polymers were measured after annealing at elevated temperatures in air as a function of annealing time. The fluorescence stability of the polymers end-capped using 2-bromo-9,9-di-n-hexylfluorene was much better than for the polymers end-capped with 2-bromofluorene. The exceptional sensitivity of the luminescence of the polymers end-capped with 2-fluorenyl substituents to thermal annealing in air is rationalized by a combination of π-stacking and chemical oxidation of the end groups to fluorenones. The latter was demonstrated by the IR spectra of annealed films and comparison of the results with the absorption, emission, and IR spectra of the polymers deliberately end …