Four kinds of sterically hindered groups-based hyperbranched polyfluorene copolymers containing different branching units such as benzene or triphenylamine were designed and synthesized. All the monomers such as 2,7-dibromo-9,9-dioetylfluorene,2,7-dibromospiro(fluorene-9,9'-xanthene), 9,9-dioetylfluorene-2,7-bis(4,4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane) and 9, 9-dioetylfluorene-2-boronic acid were prepared according to the literature procedures. Suzuki polycondensation (SPC) with the branching units and linear units was conducted in biphasic system (toluene/aqueous Na2CO3) with Pd(PPh3)(4) as a catalyst precursor. The reaction mixture containing monomers was kept stirring at 72 degrees C under nitrogen for 60 h at the polymerization stage. Then, the end-capped group was added to the reaction mixture together with additional Pd(PPh3)(4) in order to reduce the heavy atom effect which quenched fluorescence. The reaction was continued for further 12 h. After cooling to room temperature, some amount of thiourea was added, and then the mixture was vigorously stirred for another 3 h. The mixture was filtrated to remove the catalyst residues and extracted with toluene. After removal most of the solvent, the crude polymer in certain amount of toluene was added dropwise to methanol and the formed precipitate was recovered by filtration. The solid was washed with methanol, water and again with methanol. then followed by Soxhlet extraction with acetone for 48 h to remove oligomer. The yellow powder was collected and dried under vacuum. All the resulting hyperbranched copolymers are soluble in common organic solvents, such as tetrahydrofuran, chloroform and toluene. The structures of the hyperbranched polymers were fully characterized by NMR, GPC, elementary analysis and thermogravimetric analysis. The degrees of branching of the polymers were calculated through the integration of the characteristic signals of the branching units and linear units from the high-resolution NMR spectroscopy. The GPC experimental results showed that the number-average molecular weights of the polymers were from 11200 to 17200.The thermal degradation temperatures (weight loss 5 %) of the polymers were nearly at 400 degrees C. Cyclic voltammetry (CV) was used to assess the electrochemical behavior of the polymers. The polymers show a reversible oxidation/reduction wave which can be assigned to the oxidation/reduction potential of copolymer main chain from the onset. The HOMO level of the polymer containing triphenylamine is lower than that containing benzene, which indicated less difficulty in hole injection. The photophysical characteristics of the polymers were investigated in solution and in solid films . Solid films on quartz plates used for UV-Vis fluorescence were prepared by spin-coating with 10(-3) mol/L tetrahydrofuran solution .The absorption maxima of the polymers are blue-shifted within 10 nm compared to that in solution. In addition, the PL maxima remain unchanged or are only slightly red-shifted. Their thermal stability was investigated from the change of fluorescent spectra in solid films after annealing at elevated temperature in air. There was no obvious change in the PL spectra, and no additional green-blue band emission was observed even after the film was annealed at 200 degrees C in air. Apparently, with the introduction of spirofluorene/large aromatic units to the hyperbranched structure,the red-shifted phenomena and the undesirable low-energy long wavelength emission band are reduced rearkably. All results indicated that the tendency to form aggregate/excimers in the solid state was efficiently suppressed,and the spectral stability of the polymers was greatly improved.
