Synthesis and Characterization of Triarylamine-Based Block Copolymers by Combination of C-N Coupling and ATRP for Photorefractive Applications ()
1. Introduction
Because of the excellent charge transporting properties, triarylamine (TAA) derivatives have been intensively studied and been widely used in thin layer electrooptical devices such as electroluminescent, solar cell and photorefractive holograph [1-4]. Especially, the polymeric TAA derivatives exhibit extensive application prospects due to the potential advantages such as possibility of extended p-conjugation and excellent film-forming property. The common reactions for the synthesis of poly(triarylamine) (PTAA) where TAA units are linked together in the main chain are Ullman reaction [5], palladium or nickel catalyzed coupling reactions [6], and simple oxidative polymerization using ferric chloride as an oxidizing agent [7,8]. Recently, our group have. reported a new approach to synthesize PTAA from A-B type monomer by C-N coupling polymerization using palladium based system as a catalyst [9-11]. A serious of PTAA-based block copolymers have been successfully prepared by adding terminal modifying macroinitiators during the polymerizations. The physical and electrical properties were significantly influenced not only by the chemical structure of polymers, but by their morphologies, which suggests the possibility of designing new TAA-based materials to meet the requirements of applications.
Generally, photorefractive (PR) materials possess both the photoconductivity and electro-optical (EO) activity. It is known that the high mobility of EO active component plays an important role in the high performance PR materials [12]. Therefore it is important that the material has low glass transition temperature (Tg). Nevertheless better photoconductive nature of PTAA, high Tgs of PTAA derivatives prevent the utilization as PR materials. Moreover, the alkyl substituted PTAA show poor miscibility with EO chromophore, which could lead to the poor stability of the host-guest PR systems. In order to solve these problems, it is proposed to utilize block copolymers. It is possible that introduction of soft block with high polarity to PTAA improves the miscibility of EO chromophore. Furthermore, the Tgs for both domains could be controlled independently in a microphase sepatated sample [13].
The process of ATRP can provide well-defined polymers from numerous monomers [14-16]. Therefore polymers prepared by other methods such as cationic, ROMP polymerization and GRIM polymerization have been reported to be employed as macroinitiators for ATRP [17- 19]. However, the application of maroinitiator synthesized by C-N coupling to ATRP has not yet been reported. In this paper, the block copolymers containing PTAA segment and poly (n-butyl acrylate) (PBA) or poly (ethyl acrylate) segment were prepared by combination of C-N coupling and ATRP for the first time. The PR properties of composites consisting of synthesized block copolymers, EO chromophore, and photosensitizer were preliminarily investigated. The unique structures of these block copolymers are expected to improve the performance of PR materials as described above.
2. Experimental
2.1. Materials
4-(4'-Bromophenyl)-4"-butyldiphenylamine, 3 and 2-(4-bromophenoxyl) ethanol, 1 were synthesized according to reported procedures [9,20]. n-Butyl acrylate (n-BA) and ethyl acrylate (EA) were distilled from calcium hydride under reduced pressure and stored in freezer. Tetrahydrofunan (THF) was purified by distillation under nitrogen from sodium benzophenone ketyl. Toluene, anisole and N-methylpyrrolidone (NMP) were distilled from calcium hydride and stored under nitrogen at room temperature. The other reagents and solvents were used as purchased without any further purification.
2.2. Characterization
1H NMR spectra were recorded with a JEOL ECX300 spectrometer operating at 300 MHz. Molecular weight and its distribution were determined by gel permeation chromatography (GPC) with JASCO UV-970 and RI- 2031 detectors. Chloroform was used as an elute at a flow rate of 0.5 mL/min and the GPC system was calibrated with polystyrene standards. Differential scanning calorimetric (DSC) analyses were performed on a Rigaku DSC-8230 under nitrogen atmosphere at heating and cooling rates of 10˚C /min. Thermogravimetric analyses (TGA) were performed on Shimadzu DTA-60 under nitrogen atmosphere at heating rates of 10˚C/min. Atomic force microscopic (AFM) measurements were performed on JEOL JSPM 4200 system in trapping mode (phase and topographic modes) with an MPP-11100-10 silicon probe (resonant frequency: 300 kHz, force constant: 40 N/m). All thin film of polymers were spin-cast onto glass slide with a MIKASA 1H-D7 spin coater from THF solutions at 1500 rpm for 30 s.
2.3. Synthesis of 2-(4-Bromophenoxy)EthoxyTert-Butyldimethylsilane 2
To the mixture of 2-(4-bromophenoxyl)ethanol (5.38 g, 24.8 mmol), tert-butyldimethylchlorosilane (3.92 g, 26 mmol) and dichloromethane (30 mL), pyridine (2.23 mL, 28 mmol) and 4-dimethylaminopyridine (0.01 g, 0.08 mmol) were added slowly under nitrogen atmosphere at 0˚C. The solution was stirred at 0˚C for 30 min, and then the temperature was increased to 25˚C. After 24 h, solvents were evaporated, and the products were extracted with diethyl ester. The organic layer was washed by water and dried over MgSO4. The concentrated residue was purified by column chromatography (silica/toluene). Transparent liquid (5.62 g) was obtained. Yield: 62%. 1H NMR [300 MHz, CDCl3, δ(ppm)] δ7.39 (d, 2H), δ6.81 (d, 2H), 4.02 (d, 2H), 3.98 (d, 2H), 0.90 (s, 9H), 0.11 (s, 6H).
2.4. Synthesis of Tert-ButyldimethylsilylTerminated Poly (4-Butyltriarylamine) (PBTPA-TBS) 4
The example of synthetic details was as follows: a 20-mL flask equipped with a magnetic stirrer and a nitrogen inlet was charged with the mixture of 4-(4'-bromo-phenyl)-4"-n-butyldiphenylamine 3 (1.0 g, 2.7 mmol), sodium tert-butoxide (0.28 g, 3.0 mmol) and dry THF (5 mL). To this mixture, the solution of 2 (0.57 mg, 1.8 mmol), palladium (II) acetate (0.012 g, 0.054 mmol) and tri-tert-butyphosphine (0.044 g, 0.22 mmol) in THF (5 mL) was added under nitrogen atmosphere. After the mixture was stirred under reflux for 24 h, the solution of diphenylamine (0.34 mg, 2 mmol) in THF (0.5 mL) was added. The solution was continued stirring for 1 h under reflux, and then poured into methanol. The precipitate was dissolved in chloroform and then reprecipitated in acetone. After filtration and dried, 0.91 g of gray powder was obtained. Polymerization conditions and characteris tics of the resulting polymers were listed in Table 1. Yield: 60%. 1H NMR [300 MHz, CDCl3, d (ppm )] d7.43 (d, 50H), 7.15 - 7.03 (m, 104H), 6.84 (d, 2H), 4.02 (d, 2H), 3.97 (d, 2H), 2.58 (t, 26H), 1.61 (m, 26H), 1.38 (m, 26H), 0.94 - 0.90 (m, 48H), 0.11 (s, 6H).
2.5. Synthesis of Hydroxyl-Terminated Poly (4-Butyltriarylamine) (PBTPA-OH) 5
The mixture of PBTPA-TBS (0.7 g, 0.16 mmol), THF (40 mL) and hydrogen fluoride pyridine (pyridine/HF)
Table 1. Characteristics of PTPA-TBS prepared by C-N coupling under various reaction conditionsa.
(1.8 mL) [21] was placed into a 100-mL flask and stirred for 48 h at room temperature. After reaction, the mixture was poured into methanol. The precipitate was dissolved in chloroform, and then reprecipitated in acetone. Finally, 0.62 g of gray powder was obtained. Yield: 90%. 1H NMR [300 MHz, CDCl3, δ (ppm )] δ7.43 (d, 50H), 7.15 - 7.03 (m, 104H), 6.84 (d, 2H), 4.02 (d, 2H), 3.97 (d, 2H), 2.58 (t, 26H), 1.61 (m, 26H), 1.38 (m, 26H), 0.94 (t, 39H).
2.6. Synthesis of Bromo Ester Terminated Poly (4-Butyltriarylamine) (PBTPA-MI) 6
The reaction was carried out under nitrogen atmosphere. PBTPA-OH (0.5 g, 0.12 mmol) was dissolved in THF (62 mL), followed by addition of triethylamine (3.53 mL, 25.6 mmol) to the solution. Then 2-bromopropionyl bromide (2.33 mL, 23.3 mmol) was added slowly by a syringe at 0˚C. The solution was stirred at 0˚C for 30 min, and then the temperature was increased to 25˚C. After 48 h, the mixture was poured into methanol. The precipitate was dissolved in chloroform and then reprecipitated in acetone. Finally, 0.43 g of gray powder was obtained. Yield: 86%. 1H NMR [300 MHz, CDCl3, δ (ppm )] δ 7.43 (d, 50H), 7.15 - 7.03 (m, 104H), 6.80 (d, 2H), 4.45 (d, 2H), 4.35 (m, H), 4.10 (d, 2H), 2.58 (t, 26H), 1.78 (d, 3H), 1.61 (m, 26H), 1.38 (m, 26H), 0.94 (t, 39H).
2.7. Synthesis of PBTPA-Based Diblock Copolymers
The example of synthetic details was as follows: a 5-mL flask equipped with a magnetic stirrer and a nitrogen inlet was charged with PBTPA-MI (0.1 g, 0.023 mmol) and CuBr (72.9 mg, 0.51 mmol). After the evacuation followed with backfilling of nitrogen, the mixture of anisole/NMP (0.36 mL/0.36 mL), distilled n-BA (1.64 mL, 11.5 mmol) and N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA) (127 μL, 62 mmol) were added. Then the reaction mixture was stirred for 5 min. After five freeze-pump-thaw cycles, the flask was placed in a 95˚C oil bath for polymerization. After dilution with THF, the reaction mixture was filtered with alumina column in order to remove the copper catalyst. After concentration, the polymer was obtained by precipitation from methanol. Polymerization conditions and characteristics of block copolymers are listed in Table 2.
2.8. Photorefractive Measurement
PR devices were prepared as follows: THF solutions of PBTPA-b-PBA block copolymer, 2-[[4-(diethylamino) phenyl]methylene]propanedinitrile (DDCST) and 2,4,7- trinitro-9-fluorenone (TNF) were filtered through a 0.45 μm filter and cast onto indium tin oxide (ITO, 30 W)
Table 2. Characteristics of PTPA-b-PBA prepared by ATRP under various reaction conditionsa.
covered glass substrate at room temperature. The viscous solutions were left to evaporate the residual solvent under vacuum for 6 h. Then the composite was sandwiched with another ITO-covered glass. The sample thickness was controlled to be approximately 80 μm by a Teflon spacer.
The photorefractive properties of the composites were preliminarily studied by the two beam coupling (2BC) experiment using a conventional experiment setup [22]. The coherent beam from an NEC GLS-5410 He-Ne laser operating at 633 nm was split into two writing beams with the equal intensity. The two p-polarized writing beams with an angle of 20˚ were overlapped on the sample, which was tilted at an angle of 50˚ with respect to the bisector of the two writing beams. Asymmetric energy transfer between the two beams can be obtained by the following experiment: Beam 1 was switched on/off, while the transmitted intensity of beam 2 was monitored with photodiodes (Hamamatsu Photomics, S2281), and the intensity of beam 1 was monitored as beam 2 was switched on/off. The 2BC gain is calculated according the Formula [23]:
(1)
Where d is the thickness of the sample, is the tilting angle inside the sample. The experiment was carried out under room temperature, 
is the beam coupling ratio where I0 is the signal intensity without the pump beam and I is the signal intensity with the pump beam and is the light intensity ratio of the two incident beams.
3. Results and Discussion
A new synthetic route was developed for the introduction of polyacrylate at PBTPA chain end using TBS terminated PBTPA as a precursor (shown in Scheme 1). The TBS end group was easily converted to hydroxyl, and then reacted with 2-bromopropionyl bromide to obtain PBTPA with bromo ester end group that was used as a macroinitiator for ATRP.
3.1. Synthesis of PBTPA-TBS Precursors
Our group has reported a self-condensing type monomer 3 with bromophenyl and diphenylamine moieties for building up well-defined polymer via C-N coupling reaction, which could be functionalized at both terminals separately by adding terminal modifying derivatives like an aryl bromide or arylamine derivatives as a terminator [9]. As a result, PBTPA-b-poly(ethylene oxide) [9] and PBTPA-b-polystyrene [11] were synthesized using polymer modified aryl bromides as terminal modifiers. However, preliminary studies revealed that the preparation of PBTPA-b-PBA via C-N coupling polymerization in the presence of a terminal modifier with PBA chain or ATRP initiating 2-bromopropionate moiety afforded PBTPA homopolymer without designed chain end in a relatively low yield. This is probably due to the prohibition of oxidative insertion of Pd in the terminator [9]. As a result, PBTPA-b-poly(ethylene modifier by ester groups. Therefore, different synthetic strategy is necessary, and new synthetic routes are explored. Here, aryl bromide with hydroxyl group and that protected with TBS group (1 and 2) were prepared (as shown in Scheme 1). The polymerization was carried out using palladium acetate and tri(tert-butyl)phosphine ligand as the catalyst, and tertbutoxide as base in THF for 24 h.
The 1H NMR spectrum of the product initiated by 1 was shown in Figure 1(A). The signals assignable to hydroxyl proton and methylene protons of the initiating end were almost negligible which indicated the expected product 5' was not obtained. While the signal derived from the diphenylamino group at 5.7 ppm was clearly observed. This suggested that aryl bromide with hydroxyl group hardly modified the diphenylamino terminal of 3, and the products were mainly the byproducts obtained via the self-polymerization of 3.
The structure of 2 was confirmed by 1H NMR in which the characteristic signals at 0.9 ppm and 0.11 ppm corresponding to the TBS group were observed, respecttively. The signals derived from methylene protons at 4.02 ppm and 3.98 ppm were also observed.
The structure of the product initiated from compound 2 (PBTPA-TBS) was confirmed by 1H NMR spectrum (Figure 1(B)). All the signals can be assigned, including the PBTPA chain and TBS end group. Moreover, no signal derived from the diphenylamino terminal at 5.7 ppm can be found, which indicated the complete modification of diphenylamino group by 2. Furthermore, unimodal and symmetric peaks were observed in the GPC traces of all the PBTPA-TBS products (Figure 2). In a previous study, it is reported that the GPC traces of the polymers modified by other aryl bromide in some cases exhibited a shoulder at a higher retention time region, which corresponds to the cyclic oligomers [9]. It is suggested that the aryl bromide derivate with TBS end group possesses afforded no cyclic oligomer because of high initiation efficiency of 3 by C-N coupling polymerization.
Scheme 1. Reaction scheme for synthesis of TPA-based macroinitiator and block copolymers.
Figure 1. 1H NMR spectra of (A) PTPA initiated by 1, (B) PTPA initiated by 2.
Figure 2. GPC traces of PTPA-TBS obtained under various conditions.
Table 1 shows the characteristics of PBTPA-TBS prepared with various reaction conditions. All the polymers showed high yields and narrow polydipersity in dexes around 2. The degrees of polymerization (DPn) were estimated from 1H NMR spectra by the comparing the integration of methylene protons from BTAA moiety (2.58 ppm) with that from end group (3.97 ppm). As ex pected, with the decrease of the ratio of monomer to initiator, lower polymerization degree was obtained. Poly mers with different polymerization degree ranging from 13 to 42 were obtained by adjusting the feed ratios. As suming that the reactivities in the oxidative insertion of modifier 2 and monomer 3 are comparable, the average degree of polymerization should be equal to the ratio of 3 to 2. It is noteworthy that the feed monomer ratios were obviously lower than the corresponding polymerization degree, which indicated the reactivity probability of monomer 3 is higher than modifier 2. It is possible that Pd migration after the reductive elimination is limited in the vicinity of reaction center, i.e., the subsequent oxidative insertion occurs preferentially at C-Br linkage in the same growing chain end. According to this mechanism, it is reasonable that higher reactivity of monomer com pared with the modifier. The detail about the reaction mechanism is now under investigation.