The synthesis of isotactic poly(N-butenyl-carbazole) (
The recent growing interest in carbazole containing polymers is due to the discovery of polymeric light emitting diodes [
The first and the most widely studied organic polymeric photoconductor is the poly(N-vinylcarbazole) (PVK) [
Polymers with pendant or in chain isolated carbazolyl groups were synthetized by using different polymerization techniques [
It is well known that a precise stereocontrol of microstructures for polymers obtained by vinyl monomers can be achieved by coordination Ziegler-Natta polymerization [
Recently, we reported the polymerization of a vinyl monomer with a pendant carbazole group (9-(pent-4-en-1-yl)-9H-carbazole). Isotactic poly(N-pentenyl-carbazole) (
In this paper we present the synthesis of isotactic poly(N-butenyl-carbazole) (
The achieved polymer was structurally analyzed by 13C NMR, FTIR, and X-ray analysis; moreover, thermal analysis, UV-Vis, and photoluminescence analysis were also carried out. In addition, the infrared and fluorescence behaviors of
All manipulations involving air and moisture sensitive compounds were carried out under an atmosphere of dried and purified nitrogen with standard vacuum-line, Schlenk, or glovebox techniques.
All reagents and solvents were purchased from Sigma-Aldrich s.r.l. (Milan, Italy). Toluene was refluxed over sodium for 48 h and distilled before use, methylalumoxane was used as a solid after distillation of solvent (toluene), and other reagents were used without further purification. The catalytic precursor
In a 100 mL round bottom flask, to a suspension of carbazole (0,0598 mol) and 4-bromo-1-butene (0,598 mol), benzyltriethylammonium bromide (TBEA) (0,00179 mol) and 35 mL of 16 M NaOH solution were added. The mixture was left stirring for 24 h at 60°C. Afterwards the mixture was poured into water and extracted twice with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered and the solvent removed by evaporation to give the crude product. The resulting residue was purified on a silica gel column chromatography in hexane/ethyl acetate (8 : 2) to give a white solid (87% yield). 1H NMR (300 MHz, CDCl3)
Isotactic PBK was prepared by introducing 7 mL of dry toluene, 4.52 × 10−3 mol (1 g) of monomer, and 8.27 × 10−3 mol (480 mg) of MAO into a 50 mL round bottom flask equipped with magnetic stirrer. Polymerization was started by introducing 1.43 × 10−5 mol (6.4 mg) of
NMR spectra of monomer were recorded on an AX 300 Bruker spectrometer operating at 298 K. The sample was prepared by dissolving 5 mg of monomer in 0.5 mL of deuterated chloroform (CDCl3). Tetramethylsilane was used as internal chemical shift reference.
NMR spectra of polymer were recorded on an AX 300 Bruker spectrometer operating at 75 MHz and 373 K. The samples were prepared by dissolving 40 mg of polymer in 0.5 mL of 1,1,2,2-Tetrachloroethane-d2 (TCDE). Hexamethyldisiloxane was used as internal chemical shift reference.
The molecular weight distribution and polydispersity index were estimated by GPC at 140°C using 1,2,4-trichlorobenzene as solvent and narrow molecular weight distribution polystyrene (PS) standard samples as reference. The measurements were performed on PLGPC210 with four PL-Gel Mixed A Columns, a refractive detector, and a DM400 data manager (Viscotek, Malvern Instruments Ltd., Worcestershire, UK).
Calorimetric measurements were carried out on a DSC Q20 apparatus manufactured by TA Instruments, in flowing N2. Polymer samples of 5–10 mg were placed in aluminum pans and heated/cooled at a rate of 10°C/min. Measurements were done in the range 30–280°C.
X-ray diffraction patterns of polymer powder sample were obtained by an automatic Bruker D8 Advance diffractometer, in reflection, by using the nickel filtered Cu-K
Infrared spectra were obtained at a resolution of 2.0 cm−1 with a Tensor 27 Bruker spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter. The frequency scale was internally calibrated to 0.01 cm−1 using a He-Ne laser. Thirty-two scans were signal averaged to reduce the noise. Before infrared analysis, samples were prepared on KBr pellets, at room temperature. The pellets were then dried in vacuum at room temperature.
Thin polymer films have been prepared by spin coating on quartz slides substrate. The film thickness and roughness were measured by KLA Tencor P-10 surface profiler. UV-Vis measurement was performed by a Varian Cary 50 spectrophotometer and photoluminescence recorded by a Varian Cary Eclipse spectrophotometer.
Monomer, 9-(but-3-en-1-yl)-9H-carbazole, was synthesized following a synthetic procedure accurately described in the experimental part.
Polymerization of monomer was performed at 20°C in toluene solution by using the
The obtained polymer (PBK) was fractionated by exhaustive extraction with boiling acetone and two different fractions, an acetone insoluble (65% of whole starting material) and a soluble (35% of whole starting material), were obtained.
Both PBK fractions, analyzed by GPC, showed monomodal curves and narrow molecular weight distributions. The molecular weight of the acetone insoluble fraction (
The microstructure of PBK fractions was fully determined by 13C NMR analysis, utilizing the carbon attributions in similar environments, reported in literature [
13C NMR spectra of PBK fractions, (a) acetone insoluble, (b) acetone soluble (solvent: tetrachlorodideteuroethane; scale: hexamethyldisiloxane).
The acetone soluble PBK fraction 13C NMR spectrum (Figure
In Figure
X-ray diffraction patterns of (a) as polymerized sample, (b) acetone-soluble fraction, (c) acetone-insoluble fraction, and (d) crystalline fraction after annealing at
The X-ray pattern shows a small but well-defined reflection at 2
X-ray spectra of acetone soluble and insoluble fractions of
The pattern of the acetone-soluble fraction, Figure
The results of WAXD analysis of the amorphous and crystalline fractions of the
It is worth noting that the two humps in the X-ray pattern of the amorphous fraction correspond to those observed in the spectrum of the as polymerized sample, and the few indistinct crystal reflections observed in the X-ray pattern of the as polymerized sample correspond to peaks of the crystalline fraction pattern.
In Figure
Second heating of DSC runs of (a) as polymerized (unfractionated) sample, (b) acetone-soluble fraction, and (c) first heating of DSC run of acetone-insoluble fraction.
The first DSC heating run of the polymer crystalline fraction, is also shown in Figure
Between the two endothermic peaks a weak exothermic peak at
In Figure
These differences can be rationalized by assuming that the two patterns correspond to two different crystalline phases, which are probably very similar.
Both of these two crystalline forms cannot be obtained by crystallization from the melt, so that in the second DSC heating run (not showed in Figure
In detail, as for the PVK, the carbazole units directly connected to the main chain make the polymer chain extremely rigid [
The different
As for the infrared characterization of the
FTIR spectra of PVK (line (A)); crystalline (line (B)); and amorphous (line (C)) fractions of
The same phenomenon has already been reported for
In addition to the C-H stretching region, the region in which there are more differences between the PVK infrared spectrum and those of the amorphous and crystalline fractions of
In Figure
On the contrary, the PVK absorption bands at 1224 and 1236 cm−1, assigned, respectively, to
These differences are probably due to the influence that the alkylene linker has on
Finally, it should be noted that additional bands at 1195 and 1224 cm−1 are present only in the spectrum of
The good solubility of
Figure
Absorption spectrum of
Fluorescence spectra of
The
As in
The formation of these two excimers was previously detected in PVK films and was also described in literature [
Partially isotactic PVK commercial sample, used for comparison in this work, shows an emission spectrum where the fluorescence generated by the “sandwich-like” excimer state is predominant. Instead,
Isotactic poly(N-butenyl-carbazole) (
The microstructures of PBK fractions were fully determined by 13C NMR analysis and, for the first time, the isotactic microstructure of PBK (
WAXD analysis showed that
DSC analysis of
As for the infrared analysis,
To optical analysis,
It is worth noting that OLEDs showing white light emission, although of low efficiency, were obtained by using
The development of OLED devices based on
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank Dr. Patrizia Oliva and Dr. Ivano Immediata for technical assistance. Financial support of the Relight project within PON 2007–2013 program by the Ministero dell’Università e della Ricerca (MIUR) is gratefully acknowledged.