- Email: [email protected]
Novel 3-O-propargyl cellulose as a precursor for regioselective functionalization of cellulose
Reactive & Functional Polymers 69 (2009) 347–352
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Novel 3-O-propargyl cellulose as a precursor for regioselective functionalization of cellulose Dominik Fenn a, Matthias Pohl a,b, Thomas Heinze a,b,* a b
Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany Thuringian Institute of Textile- and Plastics Research, Breitscheidstraße 97, D-07407 Rudolstadt, Germany
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 February 2009 Accepted 21 February 2009 Available online 28 February 2009 Keywords: 3-O-Propargyl cellulose Regioselective synthesis NMR spectroscopy Dendritic cellulose Click-chemistry Huisgen reaction
a b s t r a c t For the first time, the synthesis of 3-O-propargyl cellulose could be realized using the thexyldimethylsilyl moiety as protecting group. The treatment of 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose with tetrabutylammonium fluoride trihydrate leads to the complete removal of the silicon containing groups. The structure and the high regioselectivity were confirmed by NMR spectroscopy. Preliminary studies have shown that a subsequent cycloaddition reaction of the triple bond of the propargyl group with azido-propyl-polyamidoamine (PAMAM) dendrons (some type of click-chemistry) yields dendritic structures based on cellulose. Characterization was carried out by 1D and 2D NMR spectroscopy that gives evidence for the unconventional structure. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose is one of the most promising biopolymers for the design of highly-engineered functional products. The b-1,4-polyglucan contains three hydroxyl groups per repeating unit that are accessible to the typical reactions like etherification, esterification, carbanilation and nucleophilic displacement reactions [1,2]. Cellulose ethers are important commercially produced functional polymers and are applied in large amounts in various fields. It is well known that not only the structure but also the distribution of the functional groups influences the properties of such cellulose products [3]. The structure–property relationships are not completely understood up to now. Thus, there is an intensive research in the field of cellulose derivatives with a controlled substitution pattern. Another important research field is the synthesis and characterization of cellulose derivatives with unconventional functional groups [4,5]. It was found that dendritic structures bond to the cellulose backbone can provide interesting properties. These structures can be introduced using different approaches as recently shown [6–9]. From these studies the issue appeared to synthesize regioselectively functionalized cellulose derivatives bearing reactive groups that are suitable for the introduction of dendritic
structures. Very recently, propargyl cellulose with regioselective functionalization pattern was synthesized by nucleophilic displacement reaction of 6-O-toluenesulfonyl ester of cellulose with propargyl amine. The novel 6-deoxy-6-aminopropargyl cellulose provides an excellent starting material for the selective dendronization of cellulose at position 6 via the copper-catalyzed Huisgen reaction yielding 6-deoxy-6-amino-(4-methyl-[1,2,3-triazolo]-1propyl-polyamidoamine) cellulose derivatives of first- and second generation, which are soluble in polar aprotic solvents [10]. Thus, new functional cellulose derivatives with special properties were obtained. A different approach to such interesting biopolymer derivatives would be the preparation of 2-, 3-mono-O or 2,3-di-Opropargyl cellulose derivatives that is not known up to now. In the present paper the synthesis and characterization of 3-O-propargyl cellulose via 2,6-di-O-thexyldimethylsilyl cellulose is discussed. The thexyldimethylsilyl group was chosen as protective group due to its high selectivity towards a functionalization of position 2 and 6 applying homogeneous conditions with DMA/LiCl as reaction media. Furthermore, the reactivity of the triple bond regarding the copper-catalyzed Huisgen reaction to get dendronized cellulose products was evaluated. 2. Experimental 2.1. Materials and methods
* Corresponding author. Address: Center of Excellence for Polysaccharide Research, at the Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany. Tel.: +49 3641 948270; fax: +40 3641 948272. E-mail address: [email protected] (T. Heinze). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.02.007
Microcrystalline cellulose (AvicelÒ, Fluka, DP = 280) was dried over KOH in vacuum at 105 °C for 24 h. LiCl (Fluka) was dried in
348
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
vacuum at 130 °C over KOH for 24 h. N,N-Dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), CuSO4 5H2O, sodium ascorbate, ethylenediamine, methylacrylate, 1-amino-3-bromopropyl hydrochloride and methanol were obtained from Acros Organics. Thexyldimethylchlorosilane (TDMS-Cl, ABCR Karlsruhe), tetrabutylammonium fluoride trihydrate (TBAF, Fluka), sodium azide (Riedel de Haen) and all other chemicals were used as received. Sodium hydride (60% suspension in mineral oil, Fluka) was washed with n-hexane and n-pentane and dried in vacuum at room temperature. Anhydrous tetrahydrofuran (THF) over molecular sieve was purchased from Fluka. FTIR spectra were recorded with a Nicolet Avatar 370 DTGS spectrometer using the KBr technique. NMR spectra were acquired with Brucker Avance 250 (250 MHz) and Avance 400 (400 MHz) spectrometers in CDCl3 or DMSO-d6 (concentration of the sample: 5–10%) at a temperature up to 60 °C using standard pulse sequences for 1H-, 13C- and DEPT135 NMR spectra. The number of scans accumulated was 16 (1H NMR spectra) and up to 16,000 (13C NMR spectra). Elemental analysis (EA) was performed with a Vario EL III from Elementar Analysensysteme, Hanau, Germany. 2.1.1. Synthesis of 2,6-di-O-thexyldimethylsilyl cellulose (1) Microcrystalline cellulose (10 g, 61.73 mmol) was suspended in 150 ml DMA and heated to 120 °C. After stirring for 2 h the reaction mixture was cooled to 100 °C and 35 g LiCl were added. Stirring was continued without further heating until a clear solution was obtained. To the viscous solution, 19.7 g (0.29 mmol) imidazole and 48.5 ml (44.14 g, 0.25 mmol) TDMS-Cl were added. The reaction mixture was allowed to react at 100 °C for 24 h under stirring. After cooling to room temperature, the polymer was filtered off, washed with water and ethanol and dried in the vacuum at 60 °C. Degree of substitution (DS): 2.0 based on the silicon content of 12.59%. FTIR (KBr, cm 1): 3499 (OH), 2959, 2872 (CH), 1466 (CH, CH2), 1252 (SiC), 1152, 1119, 1078, 1038 (COC), 833, 778 cm 1 (SiC). 2.1.2. Synthesis of 3-O-propargyl-2,6-di-O-thexyldimethylsilylcellulose (2) Ten grams (22.4 mmol) of 2,6-di-O-thexyldimethylsilyl cellulose (1) was dissolved in 300 ml anhydrous THF under exclusion of moisture. Followed by the addition of 5.38 g NaH (224 mmol; 10 mol/mol AGU), a gel is formed. After 30 min, 12.46 ml of an 80 wt% solution of propargylbromide in toluene (13.32 g, 112 mmol; 5 mol/mol AGU) was added. While stirring for 24 h at room temperature, the mixture liquefied and another 12.46 ml of propargylbromide solution were added. The mixture was stirred for additional 24 h at room temperature and 48 h at 50 °C. After cooling to room temperature, 50 ml of 2-propanol followed by 50 ml of water were added slowly to destroy the excess of NaH. The mixture was poured in water, the precipitate was filtered off and washed with water until neutral reaction. After washing again with ethanol, the product was dried in vacuum at 60 °C. FTIR (KBr, cm 1): 3512 (weak, OH), 3314 (C„CH), 2960, 2872 (CH), 1467 (CH, CH2), 1255 (SiC), 1121, 1087, 1038 (COC), 832, 777 cm 1 (SiC). 2.1.3. Synthesis of 3-O-propargyl cellulose (3) 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose (2, 8 g, 16.5 mmol) was dissolved in 250 ml THF. Followed by the addition of 52.1 g (165 mmol) TBAF, the mixture was stirred for 24 h at 50 °C. The polymer was precipitated with 400 ml of water, filtered off and washed with water and ethanol. After drying the crude product was redissolved in 200 ml DMSO and 26 g TBAF were added. After 24 h at 50 °C, the polymer was precipitated with
300 ml water, filtered off, washed with water and methanol and dried in vacuum at 60 °C. FTIR (KBr, cm 1): 3437 (OH), 3284 (C„CH), 2957 (CH), 1437 (CH, CH2), 1161, 1047 (COC). The preparation of the azidopropyl-polyamidoamine (PAMAM) dendrons of first and second generation was carried out based on the procedures described in references [11,12]. 2.1.4. Synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose (1st generation) (4) To a solution of 0.8 g 3-O-propargyl cellulose (3, 4.1 mmol) in 100 ml DMSO, CuSO4 pentahydrate (0.2 g, 0.8 mmol, in 3 ml of water), sodium ascorbate (0.4 g, 2 mmol, in 3 ml of water) and first generation of azido-propyl-PAMAM dendron (3.3 g, 12.4 mmol) were added. The mixture was stirred at 25 °C for 24 h and subsequently, 5 g of sodium diethyldithiocarbamate trihydrate was added in order to remove the copper-catalyst. Isolation took place by precipitation in 800 ml of acetone and washing three times with 350 ml of acetone. After reprecipitation from DMSO (50 ml) into 600 ml of acetone and drying in vacuum at 50 °C, first generation 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose (4) was obtained. DSDend: 0.25 (calculated from N-content determined by EA). FTIR (KBr, cm 1): 3395 (OH), 3288 (C„CH, shoulder), 2929– 2831 (CH3 and CH2), slight 1998 (C„C), 1731 (C@O, methylester), 1022 (COC). 13 C NMR (DMSO-d6): d (ppm) = 172.81 (C@O, ester), 145.61 (C-8, triazole), 124.05 (C-9, triazole), 103.23 (C-1, AGU), slight 83.66–83.15 (C-18 and C-19, C„C) 80.40–73.73 (C-2–C-5, AGU), 65.70 (C-7), 61.04 (C-6OH, AGU), 60.63 (C-17), 51.51 (CH3, ester), 50.70–28.17 (CH2, dendron). 1 H NMR (DMSO-d6): d (ppm) = 8.03 (H, triazole), 5.17–3.39 (H, AGU), 3.59 (CH3, ester), 3.75–2.40 (CH2), 1.94 (NH). 2.1.5. Synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose (2nd generation) (5) 3-O-Propargyl cellulose (3, 0.8 g, 4.1 mmol) was dissolved in 120 ml of DMSO and was converted with 5.38 g (8.2 mmol) of second generation of azido-propyl-PAMAM dendron in the presence of CuSO4 pentahydrate (0.2 g, 0.8 mmol, in 3 ml of water) and sodium ascorbate (0.4 g, 2 mmol, in 3 ml of water). After stirring the mixture for 48 h at 25 °C, 5 g of sodium diethyldithiocarbamate trihydrate was added for complete removal of the catalyst and the polymer was precipitated in 800 ml of acetone and collected by filtration. Washing three times with acetone (400 ml) and reprecipitation from DMSO (50 ml) into 700 ml acetone and drying in vacuum at 50 °C yielded product (5). DSDend: 0.13 (calculated from N-content determined by EA). FTIR (KBr, cm 1): 3395 (OH), 3273 (C„CH, shoulder), 2955– 2841 (CH3 and CH2), slight 1994 (C„C), 1732 (C@O, methylester), 1636 and 1541 (C@O, amide), 1034 (COC). 13 C NMR (DMSO-d6): d (ppm) = 172.83 (C@O, ester), 171.77 (C@O, amide), 124.04 (C-9, triazole), 103.22 (C-1, AGU), 80.38– 73.72 (C2–C5, AGU), 83.34–81.84 (C-23 and C-24, C„C), 65.83 (C-7), 61.02 (C-6OH, AGU), 59.55 (C-22), 51.51 (CH3, ester), 53.02– 28.20 (CH2, dendron). 1 H NMR (DMSO-d6): d (ppm) = 8.09 (NH, amide), 7.45 (H, triazole), 5.07–3.14 (H, AGU), 3.58 (CH3, ester), 4.45–2.02 (CH2, dendron), 1.99 (NH). 2.1.6. Peracetylation of 3-O-propargyl cellulose 200 mg of 3-O-propargyl cellulose (3) was suspended in 10 ml of pyridine and subsequently 10 ml Ac2O were added. After reacting for 24 h at 80 °C, the mixture was cooled to room temperature and the polymer was precipitated with 100 ml metha-
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
nol, filtered off, washed with methanol and dried in the vacuum at 60 °C. 3. Results and discussion The homogenous reaction of cellulose (AvicelÒ) in N,N-dimethylacetamide (DMA)/LiCl with thexyldimethylchlorosilane (TDMSCl) in presence of imidazole yields a 2,6-di-O-thexyldimethylsilyl cellulose (Fig. 1). This product is simply isolated by filtration of the reaction mixture because it gets insoluble in DMA and precipitates within the reaction. The resulting polymer is soluble in n-hexane, toluene, tetrahydrofuran (THF) and chloroform. The determined degree of substitution (DSSi) was 2.0. 3.1. Synthesis of 3-O-propargyl cellulose The 2,6-O-protected cellulose was allowed to react with propargyl bromide in anhydrous THF in the presence of sodium hydride as base. After isolating the product by a typical work-up procedure, a polymer was obtained that is soluble in n-hexane, toluene, THF and chloroform. The FTIR spectrum shows the typical absorption bands of 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose at 3512 (weak, OH), 3314 (C„CH), 2960, 2872 (CH2 and CH3), 1467 (CH, CH2), 1255 (SiC), 1121, 1087, 1038 (COC), 832 and 777 cm 1 (SiC). A weak signal at 3512 cm 1 indicates some OH moieties in the sample. The small and broad signal around 1600 cm 1 shows that a certain amount of water is present, which is strongly bond to the polymer backbone. Thus, an almost completely functionalized cellulose derivative was obtained.
349
The deprotection of the 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose was carried out with tetrabutylammonium fluoride trihydrate (TBAF). It was necessary to treat the polymer twice in order to remove the silyl moieties completely. Thus, the polymer was allowed to react for 24 h at 50 °C in THF and subsequently, a treatment with TBAF in DMSO under comparable conditions was carried out. No signals indicating silicon moieties can be found neither in the FTIR spectrum (no absorption bands at 777, 832 and 1252 cm 1) nor in the 1H NMR spectrum (Fig. 2). Structure characterization of the sample was carried out by means of NMR spectroscopy. In order to get well resolved spectra, the 3-O-propargyl cellulose was allowed to react with acetic acid anhydride in presence of pyridine to yield the 3-O-propargyl-2,6O-acetyl cellulose that is soluble in DMSO. The characteristic resonances can be assigned in the 13C NMR spectrum. The carbon atoms of the modified AGU are found between 76 and 65 ppm. A signal at 99.9 ppm belongs to a C-1 that is influenced by an acetylated hydroxyl group at position 2 only. The signal belonging to the carbon in position 6 is found at 62.8 ppm. The three resonances for the propargyl moiety are visible at 59.6 ppm (CH2) and at 81.2 ppm (quart. C and CH, Fig. 3). Furthermore, in the spectrum two peaks around 20 ppm can be assigned due to the CH3-groups of the acetyl substituents at positions 2 and 6. The signals for the related carbonyl carbon atoms are found at 170.6 and 169.9 ppm, respectively. Whereas the 13C NMR spectrum has a rather good resolution, the 1H NMR spectrum shows very broad peaks (Fig. 3). This comparably poor resolution of the spectrum might be due to an interaction of the acidic proton of the propargyl substituent with other atoms in the molecule.
Fig. 1. Reaction scheme for the synthesis of 3-O-propargyl cellulose using thexyldimethylsilyl moieties as protecting groups.
Fig. 2. 1H NMR spectrum of 3-O-propargyl cellulose recorded in DMSO-d6.
350
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
Fig. 3. 1H- (left) and 13C NMR spectra (right) of 3-O-propargyl-2,6-O-acetyl cellulose recorded in DMSO-d6. The signals of the acetyl groups at 21.0 ppm (CH3) and 169.7 and 170.6 ppm (C@O) are not shown.
The C–C triple bond introduced at position 3 offers various possibilities for the subsequent synthesis of regioselectively cellulose derivatives functionalized with unconventional functional groups. In this regard, the introduction of dendritic substituents applying the copper-catalyzed Huisgen reaction was studied.
sessing a detection limit concerning copper of 5 ppm. Traces of 9 ppm of remaining copper were found only. In addition, the 3O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of first generation is soluble in polar aprotic solvents including DMSO, DMA and DMF.
3.2. 3-O-(4-Methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose 3-O-Propargyl cellulose and the azido-propyl-polyamidoamine (PAMAM) dendron of first generation were dissolved and allowed to react in the presence of copper(II)sulfate pentahydrate and sodium ascorbate for 24 h at ambient temperature in DMSO (Fig. 4). After a usual work-up procedure, a product was obtained that shows 25% conversion of the triple bonds. Decomposition of PAMAM dendrimers due to the presences of different copper species was shown to occur after several months upon standing [13–15]. Thus, the removal of the copper-catalyst after the completed reaction should avoid fragmentation of the PAMAM dendron. Therefore, to remove the copper ions sodium diethyldithiocarbamate trihydrate was applied that is a very efficient complexing agent. The removal of the copper based catalyst was proven by ICP-OES analyses (Perkin Elmer, Optima 2000 DV) posFig. 5. 13C NMR spectrum of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose of first generation, recorded in DMSO-d6 at 60 °C.
Fig. 4. Reaction scheme of the synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose of first generation.
Fig. 6. Structure of the second generation of azido-propyl-polyamidoamine dendron bond to 3-O-propargyl cellulose using the triple bond as reactive group.
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
351
Fig. 7. DEPT135 NMR spectrum of second generation of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose of second generation, recorded in DMSO-d6 at 60 °C.
Structure elucidation was carried out by FTIR- and 13C NMR spectroscopy. New characteristic bands appear due to the introduced dendritic structure at 1731 cm 1 for the carbonyl moiety of the methylester. The methyl groups of the peripheric ester are visible at 2929 cm 1. Fig. 5 shows the 13C NMR spectrum of 3-O(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of first generation recorded in DMSO-d6 at 60 °C. A complete assignment of all carbon atoms is possible. The characteristic signals of the AGU carbon atoms 1–5 are visible between 103.2 and 73.7 ppm. The resonance for the peripheric methyl moieties C-16 occurs at 51.5 ppm and for the carbon atom of the corresponding carbonyl function at 172.2 ppm (C-15). The signals of the CH2 moieties of the dendron (C-10–C-14) are assigned to the signals between 50.7 to 28.2 ppm. The peaks for the carbon atoms of the 1,4-disubstituted 1,2,3-triazole linker can be observed at 145.6 ppm (C-8) and at 124.0 ppm (C-9). The signal of the carbon atom of C7 is visible at 65.7 ppm. Furthermore, the peaks at 83.7 and 83.2 ppm are due to carbon-carbon triple bond (C-18 and C-19) and the signal at 60.6 ppm is attributed for the CH2 moiety (C-17) of remaining propargyl moiety. It should be pointed out that this type of dendronization is completely chemoselective, i.e., the conversion occurred at position 3 only (Fig. 5). Furthermore, no fragmentation of the PAMAM-substituent or impurities could be observed in the NMR spectrum possibly initiated by Cu(II) or upon standing. Applying a comparable procedure, the homogeneous, dendronization of 3-O-propargyl cellulose with the second generation of azido-propyl-PAMAM dendron was carried out in the presence of copper(II) sulfate pentahydrate and sodium ascorbate catalyst in DMSO at ambient temperature (Fig. 6). After the usual work-up procedure, a pure second generation of 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose derivative was obtained. The conversion of 13% of the C–C triple bond was determined by elemental analysis regarding the nitrogen content. Again, sodium diethyldithiocarbamate trihydrate was added for complete removal of the copper-catalyst in order to avoid decomposition of the PAMAM substituent. 11 ppm of copper was determined by ICP-OES analysis, only. The structure could be confirmed by IR-signals characteristic for propyl-PAMAM dendron at 2955–2841 cm 1 due to the CH2 and CH3 moieties and at 1732, 1636 and 1541 cm 1 for the carbonyl functions of the methylester and the amide bond. Furthermore, the remaining C–C triple bond is visible at 3273 (shoulder of signal of the OH moieties) and at 1994 cm 1.
The peaks at 172.83 and 171.77 ppm in the 13C NMR (spectrum not shown) are attributed to the carbonyl functions of the amide and the ester moieties. Fig. 7 shows the DEPT135 NMR spectrum of 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of second generation (DS 0.13) recorded in DMSO-d6 at 60 °C. The resonance (positive amplitude) of the CH3 group of the ester is visible at 51.51 ppm. Furthermore, the peak according to the formed 1,4-disubstituted 1,2,3-triazole linker can be observed at 124.08 ppm (C-9). Beside the signals of the carbon atoms C-2–C6 of the AGU (103.22–61.02 ppm) peaks for remaining propargyl groups occur between 83.34 and 81.84 ppm (C-23 and C-24) and at 59.55 ppm for the CH2 moiety (C-22). Moreover, the CH2 groups of the carbon atoms of the propyl group between the formed 1,2,3triazole and the tertiary amine of the dendron give signals at 48.18 (C-10), 28.20 (C-11) and at 50.54 ppm (C-12). The signals of the CH2 moieties of the dendron (C-13–C-19) are assigned to the peaks with negative amplitude between 53.0 and 32.8 ppm. Regarding the NMR spectra, no structural defragmentation of the dendritic PAMAM substituents could be observed. That is in good agreement with results found in earlier studies about dendronization of cellulose [10]. 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of second generation is soluble in polar aprotic solvents like DMSO, DMF and DMA. 4. Conclusion 3-O-Propargylcellulose with a DS of 1 could be synthesized for the first time by using 2,6-di-O-thexyldimethylsilyl cellulose. The structure could be clearly confirmed by means of NMR spectroscopy of the acetylated samples. It was found that strong interactions occur, which made it impossible to use two-dimensional NMR techniques. Furthermore, it could be shown that the triple bonds are reactive regarding the introduction of dendritic structures of first and second generations PAMAM dendrons into the polymer at position 3 of the repeating unit only. Further studies will be carried out regarding the amount of dendron bound to the cellulose backbone and regarding structure property relationships of the novel cellulose derivatives. Acknowledgements The financial support of the Fachagentur Nachwachsende Rohstoffe of Germany is gratefully acknowledged (project 22003504).
352
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
References [1] D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht, Comprehensive Cellulose Chemistry, vols. 1–2, Wiley-VCH, Weinheim, 1998. [2] T. Heinze, T. Liebert, A. Koschella, Esterification of Polysaccharides, Springer Laboratory, Berlin Heidelberg, 2006. [3] Th. Heinze, in: S. Dumitriu (Ed.), Polysaccharide, second ed., Marcel Dekker, New York, Basel, Hong Kong, 2004, pp. 551–590. [4] A. Koschella, D. Fenn, Th. Heinze, Polym. Bull. 57 (2006) 33. [5] Th. Heinze, A. Koschella, Carbohyd. Res. 343 (2008) 668. [6] Th. Heinze, M. Pohl, J. Schaller, F. Meister, Macromol. Biosci. 7 (2007) 1225. [7] M. Pohl, J. Schaller, F. Meister, Th. Heinze, Macromol. Symp. 262 (2008) 119.
[8] M.L. Hassan, C.N. Moorefield, G.R. Newkome, Macromol. Rapid Comm. 25 (2004) 1999. [9] M.L. Hassan, C.N. Moorefield, K.K. Kotta, G.R. Newkome, Polymer 46 (2005) 8947. [10] M. Pohl, Th. Heinze, Macromol. Rapid Comm. 29 (2008) 1739. [11] B. Carboni, A. Benalil, M. Vaultier, J. Org. Chem. 58 (1993) 3736. [12] J.W. Lee, J.H. Kim, B.-K. Kim, Tetrahedron Lett. 47 (2006) 2683. [13] M.F. Ottiviani, F. Montalti, N.J. Turro, D.A. Tomalia, J. Phys. Chem. B 101 (1997) 158. [14] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc. 120 (1998) 7355 (and references cited herein). [15] G.R. Newkome, C.D. Shreiner, Polymer 49 (2008) 1 (and references cited herein).
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Novel 3-O-propargyl cellulose as a precursor for regioselective functionalization of cellulose Dominik Fenn a, Matthias Pohl a,b, Thomas Heinze a,b,* a b
Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany Thuringian Institute of Textile- and Plastics Research, Breitscheidstraße 97, D-07407 Rudolstadt, Germany
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 16 February 2009 Accepted 21 February 2009 Available online 28 February 2009 Keywords: 3-O-Propargyl cellulose Regioselective synthesis NMR spectroscopy Dendritic cellulose Click-chemistry Huisgen reaction
a b s t r a c t For the first time, the synthesis of 3-O-propargyl cellulose could be realized using the thexyldimethylsilyl moiety as protecting group. The treatment of 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose with tetrabutylammonium fluoride trihydrate leads to the complete removal of the silicon containing groups. The structure and the high regioselectivity were confirmed by NMR spectroscopy. Preliminary studies have shown that a subsequent cycloaddition reaction of the triple bond of the propargyl group with azido-propyl-polyamidoamine (PAMAM) dendrons (some type of click-chemistry) yields dendritic structures based on cellulose. Characterization was carried out by 1D and 2D NMR spectroscopy that gives evidence for the unconventional structure. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose is one of the most promising biopolymers for the design of highly-engineered functional products. The b-1,4-polyglucan contains three hydroxyl groups per repeating unit that are accessible to the typical reactions like etherification, esterification, carbanilation and nucleophilic displacement reactions [1,2]. Cellulose ethers are important commercially produced functional polymers and are applied in large amounts in various fields. It is well known that not only the structure but also the distribution of the functional groups influences the properties of such cellulose products [3]. The structure–property relationships are not completely understood up to now. Thus, there is an intensive research in the field of cellulose derivatives with a controlled substitution pattern. Another important research field is the synthesis and characterization of cellulose derivatives with unconventional functional groups [4,5]. It was found that dendritic structures bond to the cellulose backbone can provide interesting properties. These structures can be introduced using different approaches as recently shown [6–9]. From these studies the issue appeared to synthesize regioselectively functionalized cellulose derivatives bearing reactive groups that are suitable for the introduction of dendritic
structures. Very recently, propargyl cellulose with regioselective functionalization pattern was synthesized by nucleophilic displacement reaction of 6-O-toluenesulfonyl ester of cellulose with propargyl amine. The novel 6-deoxy-6-aminopropargyl cellulose provides an excellent starting material for the selective dendronization of cellulose at position 6 via the copper-catalyzed Huisgen reaction yielding 6-deoxy-6-amino-(4-methyl-[1,2,3-triazolo]-1propyl-polyamidoamine) cellulose derivatives of first- and second generation, which are soluble in polar aprotic solvents [10]. Thus, new functional cellulose derivatives with special properties were obtained. A different approach to such interesting biopolymer derivatives would be the preparation of 2-, 3-mono-O or 2,3-di-Opropargyl cellulose derivatives that is not known up to now. In the present paper the synthesis and characterization of 3-O-propargyl cellulose via 2,6-di-O-thexyldimethylsilyl cellulose is discussed. The thexyldimethylsilyl group was chosen as protective group due to its high selectivity towards a functionalization of position 2 and 6 applying homogeneous conditions with DMA/LiCl as reaction media. Furthermore, the reactivity of the triple bond regarding the copper-catalyzed Huisgen reaction to get dendronized cellulose products was evaluated. 2. Experimental 2.1. Materials and methods
* Corresponding author. Address: Center of Excellence for Polysaccharide Research, at the Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany. Tel.: +49 3641 948270; fax: +40 3641 948272. E-mail address: [email protected] (T. Heinze). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.02.007
Microcrystalline cellulose (AvicelÒ, Fluka, DP = 280) was dried over KOH in vacuum at 105 °C for 24 h. LiCl (Fluka) was dried in
348
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
vacuum at 130 °C over KOH for 24 h. N,N-Dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), CuSO4 5H2O, sodium ascorbate, ethylenediamine, methylacrylate, 1-amino-3-bromopropyl hydrochloride and methanol were obtained from Acros Organics. Thexyldimethylchlorosilane (TDMS-Cl, ABCR Karlsruhe), tetrabutylammonium fluoride trihydrate (TBAF, Fluka), sodium azide (Riedel de Haen) and all other chemicals were used as received. Sodium hydride (60% suspension in mineral oil, Fluka) was washed with n-hexane and n-pentane and dried in vacuum at room temperature. Anhydrous tetrahydrofuran (THF) over molecular sieve was purchased from Fluka. FTIR spectra were recorded with a Nicolet Avatar 370 DTGS spectrometer using the KBr technique. NMR spectra were acquired with Brucker Avance 250 (250 MHz) and Avance 400 (400 MHz) spectrometers in CDCl3 or DMSO-d6 (concentration of the sample: 5–10%) at a temperature up to 60 °C using standard pulse sequences for 1H-, 13C- and DEPT135 NMR spectra. The number of scans accumulated was 16 (1H NMR spectra) and up to 16,000 (13C NMR spectra). Elemental analysis (EA) was performed with a Vario EL III from Elementar Analysensysteme, Hanau, Germany. 2.1.1. Synthesis of 2,6-di-O-thexyldimethylsilyl cellulose (1) Microcrystalline cellulose (10 g, 61.73 mmol) was suspended in 150 ml DMA and heated to 120 °C. After stirring for 2 h the reaction mixture was cooled to 100 °C and 35 g LiCl were added. Stirring was continued without further heating until a clear solution was obtained. To the viscous solution, 19.7 g (0.29 mmol) imidazole and 48.5 ml (44.14 g, 0.25 mmol) TDMS-Cl were added. The reaction mixture was allowed to react at 100 °C for 24 h under stirring. After cooling to room temperature, the polymer was filtered off, washed with water and ethanol and dried in the vacuum at 60 °C. Degree of substitution (DS): 2.0 based on the silicon content of 12.59%. FTIR (KBr, cm 1): 3499 (OH), 2959, 2872 (CH), 1466 (CH, CH2), 1252 (SiC), 1152, 1119, 1078, 1038 (COC), 833, 778 cm 1 (SiC). 2.1.2. Synthesis of 3-O-propargyl-2,6-di-O-thexyldimethylsilylcellulose (2) Ten grams (22.4 mmol) of 2,6-di-O-thexyldimethylsilyl cellulose (1) was dissolved in 300 ml anhydrous THF under exclusion of moisture. Followed by the addition of 5.38 g NaH (224 mmol; 10 mol/mol AGU), a gel is formed. After 30 min, 12.46 ml of an 80 wt% solution of propargylbromide in toluene (13.32 g, 112 mmol; 5 mol/mol AGU) was added. While stirring for 24 h at room temperature, the mixture liquefied and another 12.46 ml of propargylbromide solution were added. The mixture was stirred for additional 24 h at room temperature and 48 h at 50 °C. After cooling to room temperature, 50 ml of 2-propanol followed by 50 ml of water were added slowly to destroy the excess of NaH. The mixture was poured in water, the precipitate was filtered off and washed with water until neutral reaction. After washing again with ethanol, the product was dried in vacuum at 60 °C. FTIR (KBr, cm 1): 3512 (weak, OH), 3314 (C„CH), 2960, 2872 (CH), 1467 (CH, CH2), 1255 (SiC), 1121, 1087, 1038 (COC), 832, 777 cm 1 (SiC). 2.1.3. Synthesis of 3-O-propargyl cellulose (3) 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose (2, 8 g, 16.5 mmol) was dissolved in 250 ml THF. Followed by the addition of 52.1 g (165 mmol) TBAF, the mixture was stirred for 24 h at 50 °C. The polymer was precipitated with 400 ml of water, filtered off and washed with water and ethanol. After drying the crude product was redissolved in 200 ml DMSO and 26 g TBAF were added. After 24 h at 50 °C, the polymer was precipitated with
300 ml water, filtered off, washed with water and methanol and dried in vacuum at 60 °C. FTIR (KBr, cm 1): 3437 (OH), 3284 (C„CH), 2957 (CH), 1437 (CH, CH2), 1161, 1047 (COC). The preparation of the azidopropyl-polyamidoamine (PAMAM) dendrons of first and second generation was carried out based on the procedures described in references [11,12]. 2.1.4. Synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose (1st generation) (4) To a solution of 0.8 g 3-O-propargyl cellulose (3, 4.1 mmol) in 100 ml DMSO, CuSO4 pentahydrate (0.2 g, 0.8 mmol, in 3 ml of water), sodium ascorbate (0.4 g, 2 mmol, in 3 ml of water) and first generation of azido-propyl-PAMAM dendron (3.3 g, 12.4 mmol) were added. The mixture was stirred at 25 °C for 24 h and subsequently, 5 g of sodium diethyldithiocarbamate trihydrate was added in order to remove the copper-catalyst. Isolation took place by precipitation in 800 ml of acetone and washing three times with 350 ml of acetone. After reprecipitation from DMSO (50 ml) into 600 ml of acetone and drying in vacuum at 50 °C, first generation 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose (4) was obtained. DSDend: 0.25 (calculated from N-content determined by EA). FTIR (KBr, cm 1): 3395 (OH), 3288 (C„CH, shoulder), 2929– 2831 (CH3 and CH2), slight 1998 (C„C), 1731 (C@O, methylester), 1022 (COC). 13 C NMR (DMSO-d6): d (ppm) = 172.81 (C@O, ester), 145.61 (C-8, triazole), 124.05 (C-9, triazole), 103.23 (C-1, AGU), slight 83.66–83.15 (C-18 and C-19, C„C) 80.40–73.73 (C-2–C-5, AGU), 65.70 (C-7), 61.04 (C-6OH, AGU), 60.63 (C-17), 51.51 (CH3, ester), 50.70–28.17 (CH2, dendron). 1 H NMR (DMSO-d6): d (ppm) = 8.03 (H, triazole), 5.17–3.39 (H, AGU), 3.59 (CH3, ester), 3.75–2.40 (CH2), 1.94 (NH). 2.1.5. Synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose (2nd generation) (5) 3-O-Propargyl cellulose (3, 0.8 g, 4.1 mmol) was dissolved in 120 ml of DMSO and was converted with 5.38 g (8.2 mmol) of second generation of azido-propyl-PAMAM dendron in the presence of CuSO4 pentahydrate (0.2 g, 0.8 mmol, in 3 ml of water) and sodium ascorbate (0.4 g, 2 mmol, in 3 ml of water). After stirring the mixture for 48 h at 25 °C, 5 g of sodium diethyldithiocarbamate trihydrate was added for complete removal of the catalyst and the polymer was precipitated in 800 ml of acetone and collected by filtration. Washing three times with acetone (400 ml) and reprecipitation from DMSO (50 ml) into 700 ml acetone and drying in vacuum at 50 °C yielded product (5). DSDend: 0.13 (calculated from N-content determined by EA). FTIR (KBr, cm 1): 3395 (OH), 3273 (C„CH, shoulder), 2955– 2841 (CH3 and CH2), slight 1994 (C„C), 1732 (C@O, methylester), 1636 and 1541 (C@O, amide), 1034 (COC). 13 C NMR (DMSO-d6): d (ppm) = 172.83 (C@O, ester), 171.77 (C@O, amide), 124.04 (C-9, triazole), 103.22 (C-1, AGU), 80.38– 73.72 (C2–C5, AGU), 83.34–81.84 (C-23 and C-24, C„C), 65.83 (C-7), 61.02 (C-6OH, AGU), 59.55 (C-22), 51.51 (CH3, ester), 53.02– 28.20 (CH2, dendron). 1 H NMR (DMSO-d6): d (ppm) = 8.09 (NH, amide), 7.45 (H, triazole), 5.07–3.14 (H, AGU), 3.58 (CH3, ester), 4.45–2.02 (CH2, dendron), 1.99 (NH). 2.1.6. Peracetylation of 3-O-propargyl cellulose 200 mg of 3-O-propargyl cellulose (3) was suspended in 10 ml of pyridine and subsequently 10 ml Ac2O were added. After reacting for 24 h at 80 °C, the mixture was cooled to room temperature and the polymer was precipitated with 100 ml metha-
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
nol, filtered off, washed with methanol and dried in the vacuum at 60 °C. 3. Results and discussion The homogenous reaction of cellulose (AvicelÒ) in N,N-dimethylacetamide (DMA)/LiCl with thexyldimethylchlorosilane (TDMSCl) in presence of imidazole yields a 2,6-di-O-thexyldimethylsilyl cellulose (Fig. 1). This product is simply isolated by filtration of the reaction mixture because it gets insoluble in DMA and precipitates within the reaction. The resulting polymer is soluble in n-hexane, toluene, tetrahydrofuran (THF) and chloroform. The determined degree of substitution (DSSi) was 2.0. 3.1. Synthesis of 3-O-propargyl cellulose The 2,6-O-protected cellulose was allowed to react with propargyl bromide in anhydrous THF in the presence of sodium hydride as base. After isolating the product by a typical work-up procedure, a polymer was obtained that is soluble in n-hexane, toluene, THF and chloroform. The FTIR spectrum shows the typical absorption bands of 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose at 3512 (weak, OH), 3314 (C„CH), 2960, 2872 (CH2 and CH3), 1467 (CH, CH2), 1255 (SiC), 1121, 1087, 1038 (COC), 832 and 777 cm 1 (SiC). A weak signal at 3512 cm 1 indicates some OH moieties in the sample. The small and broad signal around 1600 cm 1 shows that a certain amount of water is present, which is strongly bond to the polymer backbone. Thus, an almost completely functionalized cellulose derivative was obtained.
349
The deprotection of the 3-O-propargyl-2,6-di-O-thexyldimethylsilyl cellulose was carried out with tetrabutylammonium fluoride trihydrate (TBAF). It was necessary to treat the polymer twice in order to remove the silyl moieties completely. Thus, the polymer was allowed to react for 24 h at 50 °C in THF and subsequently, a treatment with TBAF in DMSO under comparable conditions was carried out. No signals indicating silicon moieties can be found neither in the FTIR spectrum (no absorption bands at 777, 832 and 1252 cm 1) nor in the 1H NMR spectrum (Fig. 2). Structure characterization of the sample was carried out by means of NMR spectroscopy. In order to get well resolved spectra, the 3-O-propargyl cellulose was allowed to react with acetic acid anhydride in presence of pyridine to yield the 3-O-propargyl-2,6O-acetyl cellulose that is soluble in DMSO. The characteristic resonances can be assigned in the 13C NMR spectrum. The carbon atoms of the modified AGU are found between 76 and 65 ppm. A signal at 99.9 ppm belongs to a C-1 that is influenced by an acetylated hydroxyl group at position 2 only. The signal belonging to the carbon in position 6 is found at 62.8 ppm. The three resonances for the propargyl moiety are visible at 59.6 ppm (CH2) and at 81.2 ppm (quart. C and CH, Fig. 3). Furthermore, in the spectrum two peaks around 20 ppm can be assigned due to the CH3-groups of the acetyl substituents at positions 2 and 6. The signals for the related carbonyl carbon atoms are found at 170.6 and 169.9 ppm, respectively. Whereas the 13C NMR spectrum has a rather good resolution, the 1H NMR spectrum shows very broad peaks (Fig. 3). This comparably poor resolution of the spectrum might be due to an interaction of the acidic proton of the propargyl substituent with other atoms in the molecule.
Fig. 1. Reaction scheme for the synthesis of 3-O-propargyl cellulose using thexyldimethylsilyl moieties as protecting groups.
Fig. 2. 1H NMR spectrum of 3-O-propargyl cellulose recorded in DMSO-d6.
350
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
Fig. 3. 1H- (left) and 13C NMR spectra (right) of 3-O-propargyl-2,6-O-acetyl cellulose recorded in DMSO-d6. The signals of the acetyl groups at 21.0 ppm (CH3) and 169.7 and 170.6 ppm (C@O) are not shown.
The C–C triple bond introduced at position 3 offers various possibilities for the subsequent synthesis of regioselectively cellulose derivatives functionalized with unconventional functional groups. In this regard, the introduction of dendritic substituents applying the copper-catalyzed Huisgen reaction was studied.
sessing a detection limit concerning copper of 5 ppm. Traces of 9 ppm of remaining copper were found only. In addition, the 3O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of first generation is soluble in polar aprotic solvents including DMSO, DMA and DMF.
3.2. 3-O-(4-Methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose 3-O-Propargyl cellulose and the azido-propyl-polyamidoamine (PAMAM) dendron of first generation were dissolved and allowed to react in the presence of copper(II)sulfate pentahydrate and sodium ascorbate for 24 h at ambient temperature in DMSO (Fig. 4). After a usual work-up procedure, a product was obtained that shows 25% conversion of the triple bonds. Decomposition of PAMAM dendrimers due to the presences of different copper species was shown to occur after several months upon standing [13–15]. Thus, the removal of the copper-catalyst after the completed reaction should avoid fragmentation of the PAMAM dendron. Therefore, to remove the copper ions sodium diethyldithiocarbamate trihydrate was applied that is a very efficient complexing agent. The removal of the copper based catalyst was proven by ICP-OES analyses (Perkin Elmer, Optima 2000 DV) posFig. 5. 13C NMR spectrum of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3triazole]) cellulose of first generation, recorded in DMSO-d6 at 60 °C.
Fig. 4. Reaction scheme of the synthesis of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose of first generation.
Fig. 6. Structure of the second generation of azido-propyl-polyamidoamine dendron bond to 3-O-propargyl cellulose using the triple bond as reactive group.
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
351
Fig. 7. DEPT135 NMR spectrum of second generation of 3-O-(4-methyl-1-N-propyl-polyamidoamine-[1,2,3-triazole]) cellulose of second generation, recorded in DMSO-d6 at 60 °C.
Structure elucidation was carried out by FTIR- and 13C NMR spectroscopy. New characteristic bands appear due to the introduced dendritic structure at 1731 cm 1 for the carbonyl moiety of the methylester. The methyl groups of the peripheric ester are visible at 2929 cm 1. Fig. 5 shows the 13C NMR spectrum of 3-O(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of first generation recorded in DMSO-d6 at 60 °C. A complete assignment of all carbon atoms is possible. The characteristic signals of the AGU carbon atoms 1–5 are visible between 103.2 and 73.7 ppm. The resonance for the peripheric methyl moieties C-16 occurs at 51.5 ppm and for the carbon atom of the corresponding carbonyl function at 172.2 ppm (C-15). The signals of the CH2 moieties of the dendron (C-10–C-14) are assigned to the signals between 50.7 to 28.2 ppm. The peaks for the carbon atoms of the 1,4-disubstituted 1,2,3-triazole linker can be observed at 145.6 ppm (C-8) and at 124.0 ppm (C-9). The signal of the carbon atom of C7 is visible at 65.7 ppm. Furthermore, the peaks at 83.7 and 83.2 ppm are due to carbon-carbon triple bond (C-18 and C-19) and the signal at 60.6 ppm is attributed for the CH2 moiety (C-17) of remaining propargyl moiety. It should be pointed out that this type of dendronization is completely chemoselective, i.e., the conversion occurred at position 3 only (Fig. 5). Furthermore, no fragmentation of the PAMAM-substituent or impurities could be observed in the NMR spectrum possibly initiated by Cu(II) or upon standing. Applying a comparable procedure, the homogeneous, dendronization of 3-O-propargyl cellulose with the second generation of azido-propyl-PAMAM dendron was carried out in the presence of copper(II) sulfate pentahydrate and sodium ascorbate catalyst in DMSO at ambient temperature (Fig. 6). After the usual work-up procedure, a pure second generation of 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose derivative was obtained. The conversion of 13% of the C–C triple bond was determined by elemental analysis regarding the nitrogen content. Again, sodium diethyldithiocarbamate trihydrate was added for complete removal of the copper-catalyst in order to avoid decomposition of the PAMAM substituent. 11 ppm of copper was determined by ICP-OES analysis, only. The structure could be confirmed by IR-signals characteristic for propyl-PAMAM dendron at 2955–2841 cm 1 due to the CH2 and CH3 moieties and at 1732, 1636 and 1541 cm 1 for the carbonyl functions of the methylester and the amide bond. Furthermore, the remaining C–C triple bond is visible at 3273 (shoulder of signal of the OH moieties) and at 1994 cm 1.
The peaks at 172.83 and 171.77 ppm in the 13C NMR (spectrum not shown) are attributed to the carbonyl functions of the amide and the ester moieties. Fig. 7 shows the DEPT135 NMR spectrum of 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of second generation (DS 0.13) recorded in DMSO-d6 at 60 °C. The resonance (positive amplitude) of the CH3 group of the ester is visible at 51.51 ppm. Furthermore, the peak according to the formed 1,4-disubstituted 1,2,3-triazole linker can be observed at 124.08 ppm (C-9). Beside the signals of the carbon atoms C-2–C6 of the AGU (103.22–61.02 ppm) peaks for remaining propargyl groups occur between 83.34 and 81.84 ppm (C-23 and C-24) and at 59.55 ppm for the CH2 moiety (C-22). Moreover, the CH2 groups of the carbon atoms of the propyl group between the formed 1,2,3triazole and the tertiary amine of the dendron give signals at 48.18 (C-10), 28.20 (C-11) and at 50.54 ppm (C-12). The signals of the CH2 moieties of the dendron (C-13–C-19) are assigned to the peaks with negative amplitude between 53.0 and 32.8 ppm. Regarding the NMR spectra, no structural defragmentation of the dendritic PAMAM substituents could be observed. That is in good agreement with results found in earlier studies about dendronization of cellulose [10]. 3-O-(4-methyl-1-N-propyl-PAMAM-[1,2,3-triazole]) cellulose of second generation is soluble in polar aprotic solvents like DMSO, DMF and DMA. 4. Conclusion 3-O-Propargylcellulose with a DS of 1 could be synthesized for the first time by using 2,6-di-O-thexyldimethylsilyl cellulose. The structure could be clearly confirmed by means of NMR spectroscopy of the acetylated samples. It was found that strong interactions occur, which made it impossible to use two-dimensional NMR techniques. Furthermore, it could be shown that the triple bonds are reactive regarding the introduction of dendritic structures of first and second generations PAMAM dendrons into the polymer at position 3 of the repeating unit only. Further studies will be carried out regarding the amount of dendron bound to the cellulose backbone and regarding structure property relationships of the novel cellulose derivatives. Acknowledgements The financial support of the Fachagentur Nachwachsende Rohstoffe of Germany is gratefully acknowledged (project 22003504).
352
D. Fenn et al. / Reactive & Functional Polymers 69 (2009) 347–352
References [1] D. Klemm, B. Philipp, T. Heinze, U. Heinze, W. Wagenknecht, Comprehensive Cellulose Chemistry, vols. 1–2, Wiley-VCH, Weinheim, 1998. [2] T. Heinze, T. Liebert, A. Koschella, Esterification of Polysaccharides, Springer Laboratory, Berlin Heidelberg, 2006. [3] Th. Heinze, in: S. Dumitriu (Ed.), Polysaccharide, second ed., Marcel Dekker, New York, Basel, Hong Kong, 2004, pp. 551–590. [4] A. Koschella, D. Fenn, Th. Heinze, Polym. Bull. 57 (2006) 33. [5] Th. Heinze, A. Koschella, Carbohyd. Res. 343 (2008) 668. [6] Th. Heinze, M. Pohl, J. Schaller, F. Meister, Macromol. Biosci. 7 (2007) 1225. [7] M. Pohl, J. Schaller, F. Meister, Th. Heinze, Macromol. Symp. 262 (2008) 119.
[8] M.L. Hassan, C.N. Moorefield, G.R. Newkome, Macromol. Rapid Comm. 25 (2004) 1999. [9] M.L. Hassan, C.N. Moorefield, K.K. Kotta, G.R. Newkome, Polymer 46 (2005) 8947. [10] M. Pohl, Th. Heinze, Macromol. Rapid Comm. 29 (2008) 1739. [11] B. Carboni, A. Benalil, M. Vaultier, J. Org. Chem. 58 (1993) 3736. [12] J.W. Lee, J.H. Kim, B.-K. Kim, Tetrahedron Lett. 47 (2006) 2683. [13] M.F. Ottiviani, F. Montalti, N.J. Turro, D.A. Tomalia, J. Phys. Chem. B 101 (1997) 158. [14] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc. 120 (1998) 7355 (and references cited herein). [15] G.R. Newkome, C.D. Shreiner, Polymer 49 (2008) 1 (and references cited herein).