Synthesis of Diblock Copolymers Containing Poly(N-vinylcarbazole) by Reversible Addition-Fragmentation Chain Transfer Polymerization

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1 Synthesis of Diblock Copolymers Containing Poly(N-vinylcarbazole) by Reversible Addition-Fragmentation Chain Transfer Polymerization NAN HU, WEN-XI JI, YIN-YIN TONG, ZI-CHEN LI, ER-QIANG CHEN Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing , China Received 30 March 2010; accepted 19 June 2010 DOI: /pola Published online in Wiley Online Library (wileyonlinelibrary.com). KEYWORDS: block copolymers; poly(ethylene glycol); poly(n-vinylcarbazole); poly(vinyl acetate); poly(vinyl alcohol); reversible addition-fragmentation chain transfer INTRODUCTION Poly(N-vinylcarbazole) (PVK) has attracted great academic and industrial interests since 1950s. 1 As known to possess photoconductivity and the hole transport property, PVK with a rather simple chemical structure also plays a significant role in the current researches on electrical and optical properties of advanced polymeric materials. 2 4 To further expand its application and to investigate the effect of phase morphology on its functionalities, one can consider synthesizing PVK block copolymers, which in fact is still a challenge remaining in PVK study. Block copolymers are composed of different polymer chains connected covalently, which may self-assemble into a variety of ordered structures with domain size ranged from a few to hundreds nanometer. 5,6 Properly tuning the microphase-separated morphology and the domain size can be used as a promising strategy to optimize the properties of block copolymers with electro-optical functions Here, we intend to provide a facile and feasible synthetic method of block copolymers containing PVK. In recent years, the rapid development of controlled/living radical polymerization (CLRP) offers the possibility to prepare PVK with predetermined molecular weight and narrow molecular weight distribution Reversible addition-fragmentation chain transfer (RAFT) polymerization mediated by xanthates is among the most effective methods. 18,19 Our purpose is to prepare diblock copolymer containing PVK using RAFT polymerization mediated by xanthates. Three diblock copolymers are targeted, poly(ethylene glycol)-b-pvk (PEGb-PVK), PVK-b-poly(vinyl acetate) (PVK-b-PVAc), and PVK-bpoly(vinyl alcohol) (PVK-b-PVA) (Scheme 1). Our results indicated that a PEG-based xanthate (PEG-X), like the structurally similar small molecule xanthate (X1), was very effective to mediate the RAFT polymerization of NVK, affording a series of well-defined PEG-b-PVK with different chain lengths of PVK. While for the synthesis of PVK-b-PVAc, successive RAFT polymerization of NVK and VAc mediated by a small molecule xanthate (X1) was confirmed to be the effective selection. Hydrolysis of PVK-b-PVAc under basic conditions afforded PVK-b-PVA. EXPERIMENTAL Materials N-Vinylcarbazole (NVK, Aldrich, 98%) was purified by recrystallization twice from methanol. Vinyl acetate (AR, Beijing Chemicals Co.) was dried over calcium hydride and distilled under nitrogen. 2,2 0 -Azobis(isobutyronitrile) (AIBN, AR, Beijing Chemicals Co.) was recrystallized from methanol. All the other reagents, including potassium O-ethyl xanthate (CP, Beijing Chemicals Co.), ethyl 2-bromopropionate (Aldrich, 99%), poly (ethylene glycol) monomethyl ether (Aldrich, PEG, M n,nmr ¼ 5000, PDI ¼ 1.04), 2-bromopropionyl bromide (Aldrich, 97%), and tributylstannane (Aldrich, 97%), were used as received. 2-Ethoxythiocarbonylsulfanyl-propionic acid ethyl ester 20,21 (X1) and PEG-X 22 were synthesized according to literature procedures. All solvents were purified using common methods. Characterization Gel permeation chromatography (GPC) was carried out in tetrahydrofuran (THF) (flow rate: 1 ml/min) at 35 C with a Waters 515 pump equipped with a Waters 2410 refractive index detector and three Waters Styragel HR columns (1 10 4, , and 500 Å pore sizes). Monodisperse polystyrene standards were used for calibration. NMR spectra were recorded on a Bruker ARX-400 spectrometer or a Varian Gemini 300 spectrometer. Elemental analysis was carried out on an Elementar Vario EL instrument. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was performed on a Daltonics Additional Supporting Information may be found in the online version of this article. Correspondence to: Z.-C. Li ( zcli@pku.edu.cn) or E.-Q. Chen ( eqchen@pku.edu.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, (2010) VC 2010 Wiley Periodicals, Inc. SYNTHESIS OF DIBLOCK COPOLYMERS CONTAINING PVK, HU ET AL. 4621

2 JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI /POLA SCHEME 1 Synthetic Approaches for PEG-b-PVK and PVK-b-PVAc by RAFT Polymerization. Autoflex III spectrometer equipped with a 337 nm nitrogen laser. 2,5-Dihydroxybenzoic acid (DHB) was used as the matrix, and the mass spectra were acquired in linear mode at an acceleration voltage of þ19 kv. Typical Procedure for the Synthesis of PVK, PEG-b-PVK, and PVAc Homopolymer PVK was synthesized by RAFT polymerization with X1 as the chain transfer agent. Thus, a mixture of X1 (9.28 mg, mol), AIBN (1.37 mg, mol), NVK (0.605 g, mol), and chlorobenzene (1.8 ml) was degassed by three freeze-pump-thaw cycles, sealed under vacuum, and heated at 60 C for 24 h. Then the reaction mixture was precipitated in a large excess of hexane, and the polymer was recovered by filtration. This step was repeated two more times to remove all residual monomers. NVK conversion was determined by weighing the vacuum dried polymer. A similar procedure was used for the synthesis of PEG-b-PVK with PEG-X instead of X1 as the chain transfer agent. Bulk RAFT polymerization of VAc mediated by X1 was conducted under similar conditions as in the synthesis of PVK. The polymerization was stopped after 2 h, the purified PVAc-X was used as a macromolecular chain transfer agent for the synthesis of PVAc-b-PVK. Typical Procedure for the Synthesis of PVK-b-PVAc Using PVAc-X or PVK-X A mixture of PVAc-X (0.201 g, M n,theory ¼ 5600, M n,gpc ¼ 7000, PDI ¼ 1.23), AIBN (1.170 mg, mol), NVK (0.252 g, mol), and 1, 4-dioxane (0.75 ml) was degassed by three freeze-pump-thaw cycles, sealed under vacuum and heated at 60 C for 12 h. Then the reaction mixture was precipitated in a large excess of hexane, and the resultant polymer was isolated by filtration. Typical Procedure for Xanthate End Group Removal with Tributylstannane A mixture of PEG-b-PVK (0.204 g, M n,nmr ¼ 23,300, PDI ¼ 1.14), tributylstannane ( g, mol), AIBN ( g, mol), and chlorobenzene (0.6 ml) was degassed by three freeze-pump-thaw cycles, sealed under vacuum and heated at 80 C for 3 h. Then the reaction mixture was precipitated in a large excess of hexane, and the polymer was isolated by filtration. This step was repeated three more times to remove all the residual tin-products. Typical Procedure for the Synthesis of PVK-b-PVA and Acetylation of PVK-b-PVA A methanol solution (10 ml) of sodium hydroxide (20 mg, 0.50 mmol) was added to the THF solution (10 ml) of xanthate-free PVK-b-PVAc (150 mg, M n,nmr ¼ 6400, M n,gpc ¼ 7100, PDI ¼ 1.21, mmol of VAc units), and the mixture was stirred at room temperature for 12 h. The precipitate was obtained by filtration and washed three times with methanol to obtain PVK-b-PVA as a white powder. A mixture of PVK-b-PVA (20 mg, mmol of VA units), acetic anhydride (40 mg, 0.39 mmol), acetic acid (0.040 ml), and 4622 WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

3 NOTE TABLE 1 Synthesis of PVK by Solution Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization of N-vinylcarbazole (NVK) Mediated with X1 a Entry [NVK] 0 /[X1] 0 Conv (%) b M n,theory c M n,gpc d PDI d ,700 2, ,000 7, ,000 15, ,800 22, a In chlorobenzene, [X1] 0 :[AIBN] 0 ¼ 5:1, [NVK] 0 ¼ 1.74 mol/l. 60 C, 24 h. b Measured by gravimetry. c M n,theory ¼ [NVK] 0 /[X1] 0 M NVK conversion þ M X1. d Gel permeation chromatography (GPC) data were based on polystyrene standard calibration, PDI: polydispersity index, M w,gpc /M n,gpc. pyridine (0.020 ml) was stirred at 90 C for 6 h under nitrogen. PVK-b-PVAc was obtained by precipitating the solution in water and dried under vacuum. 23 RESULTS AND DISCUSSION RAFT Polymerization of NVK RAFT polymerization of NVK has been reported to be well controlled with xanthates as the chain transfer agents. 19 Before we carried out the block copolymer synthesis, we used xanthate X1 to mediate the RAFT polymerization of NVK to get a series of PVK with different molecular weights. The solution polymerization of NVK was conducted in chlorobenzene (1.74 mol/l) at 60 C, using a constant molar ratio (5:1) of X1 to AIBN. During the polymerization process, the reaction solution remained transparent with a slight increase in the viscosity with time. After 24 h, the polymers were recovered by precipitation, dried under vacuum, and characterization of the polymers is summarized in Table 1. In all cases, the conversions of NVK are very high, and polydispersities of all the polymer samples are lower than 1.30 as evidenced by the GPC traces (Supporting Information Fig. S1). PVKs with different molecular weights are thus obtained by varying the molar ratio of NVK to X1. We also conducted the polymerization in 1,4-dioxane and the results were similar (data not shown). 1 H NMR spectrum of one PVK sample was measured and shown in Supporting Information Figure S2. It can be seen that the typical signals of PVK are quite diffusive due to the restricted rotation of the bulky carbazole groups combined with ring current effects of neighboring rings. 24,25 The signals of the end groups are overlapped with those of the main chain and the pendant carbazole groups. Therefore, it is hard to calculate the absolute molecular weight (MW) of PVK by NMR. The theoretical number-average MW (M n,theory ) is then calculated. As can be seen from Table 1, the M n,gpc of PVK is much smaller than the M n,theory, especially when the MW is large. The reason is that excluded volume of poly(nvinylcarbazole)s is smaller than that of polystyrene standard samples with the same molecular weights. 26 To know the exact MW of PVK, a polymer sample (entry 1 in Table 1) was analyzed via MALDI-TOF-MS (Fig. 1). A family of peaks is clearly detected, and the difference between two adjacent FIGURE 1 MALDI-TOF mass spectrum of PVK (entry 1, Table 1). Matrix: 2,5-dihydroxybenzoic acid (DHB), MALDI-TOF: matrix assisted laser desorption/ionization time-of-flight. peaks is 193, corresponding to the molecular weight of NVK monomer. This means that the polymers are composed of PVK with different degree of polymerization (DP). However, when we examined the molar mass of each peak more closely, we found that the molar masses are not in accord with those calculated from the expected structure of PVK obtained from an ideal RAFT polymerization. For instance, the peak of PVK 17 is expected at m/z ¼ (C 5 H 9 O 2 - (NVK) 17 -C 3 H 5 OS 2 þ Na þ ), whereas no such peak is observed in Figure 1, instead, a peak at m/z ¼ (C 5 H 9 O 2 -(Nvinlycarbazole) 17 -Hþ Na þ ) was observed. This peak can be considered from a PVK with a hydrogen instead of a xanthate group as the end group. Since the xanthate end groups of PVK have been confirmed to be existed by elemental analysis before the measurement of MALDI-TOF-MS (data not shown), we believe that the elimination of the xanthate end groups of PVK occurred during MS measurement due to the weak CAS bond. A similar phenomenon has also been reported in literatures From this figure, we can calculate the average MW of this PVK to be 3600, very close to FIGURE 2 GPC traces of PEG-b-PVK prepared by RAFT polymerization of NVK with PEG-X as a macro-cta. Conditions are the same as in Table 2. SYNTHESIS OF DIBLOCK COPOLYMERS CONTAINING PVK, HU ET AL. 4623

4 JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI /POLA TABLE 2 Synthesis of PEG-b-PVK by RAFT Polymerization of NVK with PEG-X as the Macro-CTA a Entry [NVK] 0 /[PEG-X] 0 Conv. (%) b M n,theory c M n,gpc d M n,nmr e PDI d Composition EG:NVK f ,700 10,300 9, : ,500 15,200 23, : ,600 24,400 32, : ,700 24,500 39, : ,300 35,400 55, :261 a In chlorobenzene, [PEG-X] 0 :[AIBN] 0 ¼ 5:1, [NVK] 0 ¼ 1.74 mol/l. 60 C, 12 h. b Measured by gravimetry. c M n,theory ¼ [NVK] 0 /[PEG-X] 0 M NVK conversion þ M n,peg. d GPC data were based on polystyrene standard calibration. e Calculated by 1 H NMR in CDCl 3. f DP(EG)/DP(NVK), calculated by 1 H NMR in CDCl 3. Macro-CTA, macromolecular chain transfer agent; DP, degree of polymerization; PDI, polydispersity index, M w,gpc /M n,gpc. the M n,theory of This result confirms that xanthate X1 can mediate the RAFT polymerization of NVK very well even at high conversions, and it further implies that the measured MW of PVK by GPC is obviously under estimated. Synthesis of PEG-b-PVK To prepare PEG-b-PVK, we first synthesized PEG-X according to the literature method, and then used it as a macromolecular chain transfer agent (macro-cta) to mediate the solution RAFT polymerization of NVK at different [NVK] 0 /[PEG-X] 0 ratios (from 25 to 325) at a constant NVK concentration ([NVK] ¼ 1.74 mol/l) and [PEG-X] 0 :[AIBN] 0 ¼ 5:1. In all cases, the conversions of NVK are high (>75%) and the GPC traces of the obtained PEG-b-PVK are unimodal with no obvious residual of PEG-X as shown in Figure 2. The characterization data are summarized in Table 2. The 1 H NMR spectrum of one PEG-b-PVK sample (entry 3, Table 2) is shown in Figure 3. The peaks corresponding to PVK (a, b, c) and PEG (d) are clearly observed. The DP of PVK (n) in the diblock copolymer was calculated according to the relationship of I b /I a,1 ¼ (113 4 þ n)/n, where 113 is the DP of PEG and I is the integration value of the corresponding peaks. As can be seen in Table 2, the MWs and compositions determined by 1 H NMR are quite close to the theoretical values at specific conversion, suggesting a good control of the NVK polymerization with PEG-X, and a series of PEG-b-PVK with low polydispersities can be obtained by changing the [NVK] 0 /[PEG-X] 0 ratio. Synthesis of PVK-b-PVAc Since xanthates are also good chain transfer agents to mediate the RAFT polymerization of VAc, 20,30 32 PVK-b-PVAc can in principle be prepared by successive RAFT polymerization of the two monomers under similar conditions (Scheme 1). There are two approaches: either VAc or NVK can be polymerized first, followed by the polymerization of another monomer mediated with the macro-cta obtained from the first step. We initially tried to use the PVAc-X obtained by RAFT as a macro-cta to prepare the block copolymer. A PVAc-X (M n,theory ¼ 5600, PDI ¼ 1.23) was obtained by the RAFT polymerization of VAc mediated by X1 ([AIBN]:[X1]: [VAc] ¼ 1:10:100, 2 h, 60 C, conversion of VAc ¼ 64%). Then it was used to mediate the RAFT polymerization of NVK. Our results indicated that this approach was FIGURE 3 1 H NMR spectrum of PEG-b-PVK (entry 3, Table 2) prepared by RAFT polymerization of NVK with PEG-X as a macro-cta. FIGURE 4 GPC traces of PVK-b-PVAc prepared by RAFT polymerization of VAc with PVK-X (M n,maldi ¼ 5500, PDI ¼ 1.20) as a macro-cta. Conditions are the same as in Table WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

5 NOTE TABLE 3 Synthesis of PVK-b-PVAc by RAFT Polymerization of VAc Mediated by PVK-X (M n,maldi , PDI ) a Entry [VAc] 0 /[PVK-X] 0 Conv (%) b M n,theory c M n,gpc d M n,nmr e PDI d Composition NVK:VAc f ,000 6,000 5, : ,200 7,100 6, : ,300 8,900 7, : ,400 9,500 10, :46 a [PVK-X] 0 :[AIBN] 0 ¼ 5:1, [VAc] 0 ¼ 4.85 mol/l, 60 C, 24 h, in 1, 4- dioxane. b Measured by gravimetry. c M n,theory ¼ [VAc] 0 /[PVK-X] 0 M VAc conversion þ M n,pvk-x. d GPC data were based on polystyrene standard calibration. e Calculated by 1 H NMR in CDCl 3. f DP(NVK)/DP(VAc), calculated by 1 H NMR in CDCl 3. DP, degree of polymerization; PDI, polydispersity index, M w,gpc /M n,gpc. unsuccessful. As shown in Supporting Information Figure S3, the GPC trace of the product showed a bimodal distribution, which was attributed to the residue of the starting PVAc-X, demonstrating an insufficient blocking efficiency. The poor blocking efficiency can be attributed to the poor fragmentation of the PVAc-X relative to the growing PVK. This result is consistent with the general tendency of the order of blocking for the synthesis of well-defined block copolymer by the RAFT process. 33,34 We then used PVK-X (M n,maldi ¼ 5500, DP NMR ¼ 27, PDI ¼ 1.20) as a macro-cta to mediate the RAFT polymerization of VAc. Figure 4 presents the GPC traces of the starting PVK-X and block polymers at different molar ratio of [VAc] 0 /[PVK- X] 0. A shift toward higher MW in all the GPC traces is observed and the PDI of the block copolymers remained narrow, demonstrating the efficient block copolymer formation. The block copolymers obtained by this approach are summarized in Table 3. A PVK-b-PVAc sample (entry 2, Table 3) was further characterized by 1 H NMR as shown in Figure 5. The typical signals corresponding to PVK (a, c, b) and PVAc (e, f, d) are clearly observed. The DP of PVAc (m) in the diblock copolymer can be calculated using the relationship of I c /I eþa1 ¼ 27/(27 þ m), where I is the integration value of the corresponding peaks and 27 is DP of the PVK block. A series of PVK 27 -b- FIGURE 5 1 H NMR spectrum of PVK-b-PVAc (entry 2, Table 3) prepared by RAFT polymerization of VAc with PVK-X as a macro-cta. PVAc m were obtained by adjusting the [VAc] 0 /[macro-cta] 0 ratio. PVK-X with M n,theory ¼ 14,000, 24,000 were also used and the corresponding well-defined PVK-b-PVAc were obtained (data not shown). Removal of Xanthate End Groups by Tributylstannane Polymers obtained by the xanthate-mediated RAFT polymerization have xanthate groups at the chain ends, which should be removed for further property studies. There are three general methods for end-group removal/transformation: aminolysis, thermal elimination, and radical-induced reduction We have used radical-induced reactions with tributylstannane as the most effective H atom donor to remove the xanthate end groups of PVAc. 35,38 The molar ratio of [polymer]/[bu 3 SnH] was kept at 1:5 to guarantee a quantitative removal. 39 Here, we used the same approach to remove the end xanthate groups of PVK and PEG-b-PVK. However, due to the aforementioned reasons, it was difficult to get direct evidence for complete removal of xanthate groups with 1 H NMR and MALDI-TOF-MS. Fortunately, we confirmed complete removal of the xanthate groups of PVK-X (M n,theory ¼ 3700) by elemental analysis. After being reacted with tributylstannane, the sulfur element in PVK-X cannot be detected, in spite of that the original sample contains sulfur element of 1.17%. So we are quite sure that complete removal of xanthate groups in PVK and PEG-b-PVK is successful. Synthesis of PVK-b-PVA The above prepared PVK-b-PVAc was hydrolyzed under basic conditions to prepare a new amphiphilic diblock copolymer PVK-b-PVA. 38 Both NMR (Supporting Information Fig. S4) and FTIR (Supporting Information Fig. S5) measurements confirmed the complete hydrolysis of the PVAc block. The resulting PVK-b-PVA samples were reacetylated into PVK-b- PVAc, and the GPC trace of the product was overlapped with that of the precursor PVK-b-PVAc sample (Supporting Information Fig. S6), implying that there is no change of the polymer backbone during the hydrolysis process. In summary, we demonstrated that three new types of block copolymers containing PVK, PEG-b-PVK, PVK-b-PVAc and PVK-b-PVA, could be obtained by RAFT polymerization of NVK. By using a xanthate-terminated PEG as a macro-cta, a series of PEG-b-PVK with a wide range of PVK chain lengths was obtained. Preparation of well-defined diblock copolymer based on PVK and PVAc, however, needs to consider the SYNTHESIS OF DIBLOCK COPOLYMERS CONTAINING PVK, HU ET AL. 4625

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