4.1 Synthesis and characterization of 4,4´-bis[poly(4-
4.1.4 Thermal properties: DSC and TGA
The thermal properties of the series 16 c – f of 4,4´-bis[poly(4-bromostyryl)methyl]-2,2´-bipyridines have been characterized by Differential Scanning Calorimetry (DSC) and Thermo-Gravimetric Analysis (TGA). The data obtained by these measurements are summarized Table 4-3.
Table 4-3: Molecular weights and thermal properties of different 4,4´-bis[poly(4-bromostyryl)methyl]-2,2´-bipyridines (16 c -f) prepared via ATRP in bulk at 60 °C using 2 as initiator and CuCl / PMDETA as catalytic system.
polymer Mn [g/mol]a) Mw [g/mol]a) PDIa) Tg [°C]b) Tonset [°C]c)
16c 2080 2890 1.39 104.7 230
16d 3107 4816 1.55 112.2 226
16e 10340 15322 1.48 126.8 236
16f 26610 31481 1.18 135.6 233
a) Determined with SEC with THF + 0.25 wt% TBAB as eluent, calibration versus polystyrene standards, UV-detection, PDI = Mw/Mn; b) determined with DSC with heating / cooling rate of 10 Kmin-1; c) defined as temperature at which weight loss is starting; determined with TGA via heating from 30 to 650 °C with 10 Kmin-1.
The absence of any melting or recrystallization peaks in DSC points towards the amorphous nature of polymers 16 c – f. In all cases, glass transitions were evident in the DSC-curves: The glass transition temperatures Tg of polymers 16 c – f range between 104.7 °C for 16c which exhibits the lowest molecular weight and 135.6 °C for 16f which represents the polymer with the highest molecular weight in this series. A constant increase in Tg with molecular weight is recognizable in this series of 4,4´-bis[poly(4-bromostyryl)methyl]-2,2´-bipyridines.
4 Bifunctional polymers carrying Ruthenium (II) core and poly(vTPA) chains
All 4,4´-bis[poly(4-bromostyryl)methyl]-2,2´-bipyridines (16c – f) show appreciably good thermal stability having an onset of weight loss at temperatures around 230 °C.
4.2 Synthesis and characterization of
4,4´-bis[poly(4-vinyltriphenylamino) methyl]-2,2´-bipyridine (17 c – f)
In order to obtain a hole transport functionality the bromophenyl groups in polymers 16 were converted into triphenylamine moieties. For this purpose a method of Pd-catalyzed amination reaction with diphenylamine as coupling reagent was adopted. Similar reactions were reported to proceed smoothly and fast with a high degree of substitution using coupling reagents like carbazoles or phenazines104.
The general method of Pd-catalyzed amination reactions was intensively studied in the groups of Buchwald105 and Hartwig106 and it is known from literature that side reactions involving incorporation of phosphor and dehalogenation are possible if aryl halides containing electron-poor substituents are used. In order to avoid any undesired product and to obtain complete conversion in appreciably short periods, different combinations of Pd-catalysts and ligands were tested in model reactions of bromo benzenes with diphenylamine to find out the most efficient system. In this way it turned out that a combination of Pd(OAc)2 and P(tBu)3 in the presence of NaOtBu gave the best results with the reaction proceeding in extraordinarily short time of one to two hours. The Pd-catalyzed polymeranalogous amination reaction of the poly(4-bromostyrene) chains of macroligands 16 c – f was carried out in a general procedure as shown in Scheme 4-2 and described in detail in the experimental part of this thesis. In this way poly(4-vinyltriphenylamine) carrying a bipyridine unit in the center could be obtained (17 c – f ).
104 T. Kanbara, Y. Yokokawa, K. Hasegawa J. Polym. Sci.: Part A: Polymer Chemistry 2000, 38, 28.
105 B. H. Yang, S. L. Buchwald J. Organomet. Chem. 1999, 576, 125.
106
4 Bifunctional polymers carrying Ruthenium (II) core and poly(vTPA) chains
With the described catalytic system it was possible to achieve very high conversion of the aryl bromide to triphenylamine in less than two hours using the following procedure:
In a three-neck flask equipped with a condenser, an argon inlet and a septum the starting polymer and diphenylamine were dissolved in dry toluene. One after another the base NaOtBu, the ligand P(tBu)3 and Pd(OAc)2 were added. It must be noted that prior to use NaOtBu has to be dried properly under vacuum at 110 °C. The ligand P(tBu)3 in toluene solution (1 g P(tBu)3 in 30 ml dry toluene) was stored under inert gas atmosphere and used as such making sure that water and oxygen is absolutely excluded. The reaction mixture was refluxed for 2 h before terminating the reaction. The working up procedure for further purification is also not very complicated.
N
Scheme 4-2: Schematic representation of the Pd-catalyzed polymeranalogous amination of 4,4´-bis[poly(4-bromostyryl)]-2,2´-bipyridine (16) with diphenylamine as coupling reagent and NaOtBu, Pd(OAc)2 and P(tBu)3 as catalytic system to yield 4,4´-bis[poly(4-vinyltriphenylamino)methyl]-2,2´-bipyridine 17.
4 Bifunctional polymers carrying Ruthenium (II) core and poly(vTPA) chains
After cooling the reaction mixture to room temperature and filtration over alumina to remove the residual Pd, half of the solvent was evaporated and the poly(4-vinyltriphenylamine) (17) was precipitated in methanol.
Further reprecipitation from THF solution of the polymer into MeOH yielded about 90 % of the desired product. Thus 16 c – f were converted into the corresponding 4,4´-bis[poly(4-vinyltriphenylamino)methyl]-2,2´-bipyridines 17 c – f the characterization of which will be discussed here.
4.2.1 NMR-spectroscopy
The polymeranalogous amination reaction was followed via NMR-spectroscopy and the spectra of bromostyryl)methyl]-2,2´-bipyridine 16f and 4,4´-bis[poly(4-vinyltriphenylamino)methyl]-2,2´-bipyridine 17f are both discussed here for comparison.
The NMR-spectra of educt 16f and product 4,4´-bis[poly(4-vinyltriphenyl-amino)methyl]-2,2´-bipyridine 17f are given in Figure 4-5.
After about one hour of reaction, the signals of the aryl protons of polymer 16f at 6.29 ppm and 7.17 ppm have completely disappeared. The new multiplets corresponding to triphenylamine protons appeared at about 6.5 – 7.0 ppm. As it is already described for polymers 16 the resonance signals of the bipyridyl protons are getting non-detectable with increasing molecular weight due to their low weight fraction within the polymer chain: For polymer 17d broad singulets at 7.96 ppm and 8.29 ppm are indicating the bipyridine centre of the polymer chain whereas in 17f the bipyridine protons could not be detected.
4 Bifunctional polymers carrying Ruthenium (II) core and poly(vTPA) chains
Br Br
N N
m n
N N
m n
N N
17g 16g
8 7 6 5 4 3 2 1
δ [ppm]
Figure 4-5: 1H-NMR spectra of polymers 16f and 17f recorded in CDCl3 (250 MHz, 298 K) showing the disappearance of the two broadened dublets from polymer 16f (6.3 ppm, 7.2 ppm) and the appearance of the multiplets (6.5 – 7.0 ppm) corresponding to the aryl protons of polymer 17f after about one hour of reaction.
Taking the results from the observation of the polymeranalogous reaction via NMR-spectroscopy into account it can be assumed that about 95 % of the amination reaction is complete after a reaction time of ca. one hour.
4 Bifunctional polymers carrying Ruthenium (II) core and poly(vTPA) chains