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Diblock Copolymers as Scaffolds for Efficient Functionalization via
Click Chemistry
Sven Fleischmann, Hartmut Komber, and Brigitte Voit*
Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany Recei V ed April 3, 2008; Re V ised Manuscript Recei V ed May 20, 2008
ABSTRACT: A set of different alkyne containing diblock copolymers based on 4-hydroxystyrene was synthesized by nitroxide mediated radical polymerization (NMRP), all with excellent control over the molecular composition and narrow molar mass distribution. The diblock copolymers consist of labile protected 4-hydroxystyrene motifs in one block and bear alkyne functionalities in each repeating unit of the second block, thus making the materials candidates for polymer analogous modification reactions by a very efficient cycloaddition reaction. The use of 4-(trimethylsilylpropargyloxy)styrene as monomer proved highly advantageous compared to 4-(trimethylsilyl- ethynyl) styrene, first because high control was kept in the NMRP process and second because there was higher accessibility in the postmodification reaction. In fact, quantitative postmodification through Cu(I)-catalyzed cycloaddition reaction of the pending propargyloxy groups with bulky adamantane azide of the diblock copolymers was achieved, yielding microphase-separated materials with a rigid block.
Introduction
During the past decade diblock copolymers have drawn high attention in the scientific world. Microphase separation of incompatible blocks leads to the formation of nanodimensioned features in the range 10-100 nm.^1 The size and ordering of these nanodomains can be controlled by varying the molecular weight, the chemical structure and the molecular architecture.^2 Although the mechanistic principles of the self-assembly of diblock copolymers in thin films are complex and not yet fully understood, scientists expect them to have a striking impact on nanotechnology.^3 The influence of surface effects in such thin films results in nanodomains that can be more complex and different from those being observed in bulk, i.e. spheres, cylinders, lamellae, etc.^4 State-of-the-art research is able to prepare highly oriented diblock copolymer thin films by different approaches.^5 Apart from their application in thin films, am- phiphilic diblock copolymer micelles are used, e.g., as scaffolds for the preparation of cross-linked nanoparticles.^6 Although, the polymers considered in the above-mentioned applications exhibit a high control over the molecular architec- ture, they are usually lacking defined functionalities, a fact that is directly related to ionic polymerization procedures by which most of the block copolymers investigated are currently synthesized. In order to satisfy the demand for complex and smart, further miniaturized devices in nanotechnology, new defined functional materials are required. This need can only be faced with novel methodologies. In this regard, controlled radical polymerizations (CRP) as NMRP,^7 ATRP,^8 and RAFT^9 are excellent tools for the preparation of such highly defined, functional block copolymers. They tolerate less rigorous reaction conditions and are compatible with a variety of functional groups so that these are nowadays preferred over ionic polymerization processes. Recently, Sharpless’ click chemistry,^10 a Cu(I)- catalyzed version of Huisgen’s 1,3-dipolar cycloaddition of azides and alkynes,^11 has been introduced in polymer science by various groups and recent comprehensive review articles can be found in literature.^12 Click chemistry adopted to polymer analogous reactions enables to efficiently and selectively tune the functionality and morphology of a polymeric material. Generally, suitable macromolecules can be considered as scaffolds and a large variety of polymers can be created from
a construction kit of a plethora of reactive substrates. Remark- ably, this concept had been suggested for polymer analogous modifications via pendant active esters some time ago.^13 Specifically, segmented and “clickable” diblock copolymers would allow to manufacture nanodevices in which the functional domains could be selectively addressed in a click reaction and, thus, further modified. Only some examples for combinations of CRP and click chemistry should be depicted here from a research field which is rapidly expanding: Matjyaszewski’s group^14 described the synthesis of poly(3-azidopropylmethacry- late- b - N , N -dimethylaminoethylmethacrylate) by ATRP and its polymer analogous modification with low molecular alkyne substrates. The obtained azido-functional polymers exhibited slightly broader SEC traces and, especially for high conversion, the polydispersity rose above 1.5, a fact that might be attributed to an in situ 1,3-dipolar cycloaddition reaction of the azide and the vinyl double bond of the monomer. Haddelton and co- workers^15 reverted to a well defined polymer based on propargyl methacrylate which they used as a scaffold for the preparation of synthetic glycopolymers by grafting azido-sugar derivatives along the polymer backbone. In their seminal work Hawker et al.^16 demonstrated the efficiency of click chemistry in polymer analogous modifications. They were able to orthogonally functionalize macromolecules in an one-pot synthesis using the strength of Cu(I)-catalyzed 1,3-dipolar cycloadditions and a couple of selected esterification as well as amidation reactions. In this paper, we would like to report on the synthesis of reactive, phase-separated diblock copolymers that are prone to click chemistry as well as on first results on their effective polymer analogous modification with a bulky azide. On one hand, these macromolecules represent an ideal starting point for the modular synthesis of novel polymeric materials by effective side group modifications. On the other hand, potential applications of such diblock copolymers can be found in the fast developing field of nanostructured functional thin films. Recently, we could show that block copolymers based on 4-hydroxystyrene derivatives can be prepared in high molar masses and narrow polydispersity by nitroxide mediated radical polymerization (NMRP).17,18^ In particular, when one of the block consists of unprotected 4-hydroxystyrene, these block copolymers show a high tendency for phase separation, and thin films exhibiting a highly ordered nanostructure could be
- Corresponding author. Fax: +49 351 4658565. E-mail: [email protected]. prepared.^19 By combining the phase separation tendency of that
Macromolecules 2008 , 41 , 5255- 5264 5255
10.1021/ma8007493 CCC: $40.75 2008 American Chemical Society Published on Web 06/25/
type of block copolymers with effective side group modification, it should be possible to create thin microphase-separated films where nanoscopic domains might be exclusively addressed by orthogonal reaction techniques.
Experimental Part
Chemicals. THF (99%, Fluka) and acetone (99.5%, Merck) were used as received, and dichloromethane (DCM, 99%, Fluka), triethylamine (99.5%, Fluka), diisopropylethylamine (DIPEA, 99%, Aldrich), diazabicycloundecene (DBU, 97.5%, Aldrich), and di- ethylene glycol dimethylether (diglyme, 99%, Aldrich) were dried over CaH 2 and purified by fractionated distillation. The synthesis of 4-( tert -butyldimethylsilyloxy)styrene^21 ( 1 ) was described else- whereandthemonomers4-( tert -butyloxy)styrene( 2 ,TBU-oxystyrene, 99%, Aldrich) and 4-acetoxystyrene ( 3 , 96%, Aldrich) were distilled before use. All other reagents and solvents were purchased from Aldrich and used as received. N - tert -Butyl-R-isopropyl-R-phenylni- troxide ( TIPNO ), 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3- azahexane (initiator, TIPNO-Sty ), and Cu(PPh 3 ) 3 Br were synthe- sized as described elsewhere.^20 Tetrabutylammonium fluoride [(TBA)F] was available from Aldrich as a 1 M solution in THF containing 5% water. Adamantane azide (97%) was used as received from Aldrich. Measurements. Molar masses and polydispersities of polymer samples were determined by gel permeation chromatography (GPC) using a 10 μm MIXED-B column (Polymer Laboratories) with polystyrene standards (Polyscience) and chloroform as eluent. 1 H and 13 C measurements were performed with a Bruker DRX 500 spectrometer. CDCl 3 and acetone- d 6 were used as solvents and internal standard (δ(^1 H) ) 7.26 ppm, δ(^13 C) ) 77.0 ppm and δ(^1 H) ) 2.05 ppm, δ(^13 C) ) 30.5 ppm, respectively). Signal assignments were verified by 2D NMR experiments. The pulse sequences included in the Bruker software package were used.
All FT-IR spectra were recorded from prepared films in transmission mode on a Bruker IFS 66 V/S. Differential scanning calorimetry (DSC) was preformed with a DSC 7 from Perkin-Elmer as well as a DSC Q 1000 by TA Instruments with a heating rate of 20 K/min. The TGA analyses were conducted with a TGA 7 by Perkin-Elmer under nitrogen atmosphere with a heating rate of 10 K/min.
Monomer Synthesis. 4-Hydroxystyrene ( 4 ). The synthesis and characterization can be found in the Supporting Information. 4-(Propargyloxy)styrene ( 5 ). A 250 mL round-bottom flask, equipped with a condenser and a stirrer, was fed with 6.90 g ( mmol) of freshly prepared 4-hydroxystyrene ( 4 ), dissolved in 50 mL of acetone. Then, 26.06 g (116 mmol) of potassium carbonate and 1.87 g (11 mmol) of 18-crown-6 were dispersed. The mixture was heated to reflux and 13.38 g (89 mmol) of propargyl bromide (80% in toluene) was added. The reaction was kept under nitrogen atmosphere. After 20 h the product was precipitated into 300 mL of deionized water and 80 mL of chloroform was added. The organic phase was separated, and the water phase was extracted three times with 80 mL of chloroform used each time. The organic layers were combined, dried over sodium sulfate, and evaporated. The crude product was purified by flash chromatography using n -hexane and ethyl acetate (100:1) as solvent resulting in 5.54 g (61%) of a colorless liquid. (^1) H NMR (CDCl 3 ): δ ) 7.38 (d, H (^) b ), 6.96 (d, H (^) c ), 6.69 (dd, C H dCH 2 ), 5.65 (d, CHdC H 2,trans ), 5.17 (d, CHdC H 2,cis ), 4.70 (d, He ), 2.54 ppm (t, Hg ). 13 C NMR (CDCl 3 ): δ ) 157.25 (C (^) d ), 136. ( C HdCH 2 ), 131.31 (C (^) a ), 127.34 (C (^) b ), 114.91 (C (^) c ), 112. (CHd C H 2 ), 78.49 (Cf ), 75.52 (Cg ), 55.80 ppm (Ce ). 4-(3 ′ -Trimethylsilylpropargyloxy)styrene ( 6 ). A 100 mL 2-neck round-bottom protection flask, equipped with a condenser and a dropping funnel, was dried in a high vacuum at 350 °C. Under a nitrogen atmosphere, 538 mg (4 mmol) of silver chloride was dispersed in 30 mL of dry dichloromethane and 5.54 g (35 mmol) of 4-(propargyloxy)styrene ( 5 ) and 6.50 g (43 mmol) DBU were added. The suspension was heated under reflux and 4.90 g ( mmol) trimethylsilyl chloride were dropped slowly. After 18 h, complete conversion of the free alkyne was indicated by TLC so that the reaction mixture was cooled to room temperature and diluted with 60 mL hexane. The crude product was washed with a semiconcentrated sodium hydrogen carbonate solution and with 1% HCl. The organic layer was removed, dried over sodium sulfate and evaporated. The product was isolated by column chromatog- raphy over silica gel with n -hexane and ethyl acetate (100:1) as solvent resulting in 3.38 g (42%) of a colorless oil. (^1) H NMR (CDCl 3 ): δ ) 7.35 (d, H (^) b ), 6.93 (d, H (^) c ), 6.67 (dd, C H dCH 2 ), 5.62 (d, CHdC H 2,trans ), 5.14 (d, CHdC H 2,cis ), 4.68 (s, He), 0.18 ppm (s, Hh). 13 C NMR (CDCl 3 ): δ ) 157.59 (Cd), 136. ( C HdCH 2 ), 131.16 (C (^) a ), 127.28 (C (^) b ), 115.01 (C (^) c ), 111.
Table 1. Experimental Parameters and Analytical Data for the Macroinitiators MI- x protective group n ini [μmol] m monomer [g] conversion [%] M n,cal. [g · mol-^1 ] M n,exp. [g · mol-^1 ] M w,exp. [g · mol-^1 ] PDI MI-1a TBDMS 92.0 1.010 88 9700 7200 8400 1. MI-1b TBDMS 280.0 5.005 70 12500 8600 9400 1. MI-2a TBU 209 5.023 79 19800 15300 17500 1. MI-2b TBU 88 6.125 85 50400 43700 52600 1. MI-2c TBU 84 6.001 62 46400 41700 49600 1. MI-3 acetyl 139 4.960 58 20500 15800 20100 1.
Table 2. Experimental Parameters for the Synthesis of Protected Precursor Diblock Copolymers polymer MI n (^) ini [μmol] m 6 [g] conversion [%] M n,cal. [g · mol-^1 ] M n,exp. [g · mol-^1 ] M w,exp. [g · mol-^1 ] PDI pBC-1a MI-1a 58 0.962 38 16 000 16 000 19 300 1. pBC-2a MI-2a 25 1.011 48 36 000 26 300 31 900 1. pBC-2b MI-2b 46 1.238 33 59 300 63 400 81 200 1. pBC-3 MI-3 49 1.060 49 31 100 24 400 33 700 1.
Table 3. Experimental Parameters for the Synthesis of Precursor Diblock Copolymers with Random Distribution of the Alkyne Monomer 7 in the Second Block polymer MI n ini [μmol] m 7 [g] m styrene [g] conversion [%] M n,cal. [g · mol-^1 ] w 7 a^ [%] M n,exp. [g · mol-^1 ] PDI pBC-1r MI-1b 18 0.225 0.906 68 36 000 9 26 300 1. pBC-2r MI-2c 60 0.392 2.002 48 66 400 13 62 400 1. a (^) Amount of monomer 7 in the second block in mol %.
Table 4. Kinetic Experiment for the Cu(I)-Catalyzed Addition of 1-Adamantane Azide to HP-6 and HP- polymer m polymer [mg] n alkin [μmol] n azide [μmol] m Cu-cat. [mg] m DIPEA [mg] conversion [%] A-HP-6 99.9 631 672 51.6 240.3 100 A-HP-7 77.5 605 648 54.7 242.9 75
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155.8 (Cd), 139.5-137.0 (Ca), 128.3 (Cb), 114.4 (Cc), 100.6 (Cf), 92.2 (Cg ), 56.9 (Ce ), 47.0-41.0 (CH 2 ), 39.4 (CH), - 0.22 ppm (Ch ). GPC (CHCl 3 ): M n ) 5200 g/mol, M w ) 6400 g/mol, PDI ) 1.23. DSC: T g ) 48 °C. FT-IR (ATR): ν ) 3026 (Car-H), 2912 (Cali-H), 2324, 2178, 2051, 1609 (Car-Car ), 1507, 1449, 1363, 1303, 1214, 1176, 1114, 1037, 833 cm-^1. Poly(propargyloxystyrene) ( HP-6 ). A solution of 1.1652 g of MI-6 in THF was reacted with 5 mL of 1 M (TBA)F solution at 0 °C for 30 min. The product was recovered by precipitation from ethanol in 80% yield. (^1) H NMR (CDCl 3 ): δ ) 7.0-6.2 (H (^) b,c ), 4.63 (H (^) e ), 2.50 (H (^) g ), 2.2-1.6 (CH), 1.6-1.1 ppm (CH 2 ). 13 C NMR (CDCl 3 ): δ ) 155. (Cd), 139.5-137.0 (Ca), 128.3 (Cb), 114.3 (Cc), 78.8 (Cf), 75.4 (Cg), 55.8 (Ce ), 47.0-41.01 (CH 2 ), 39.5 ppm (CH). GPC (CHCl 3 ): M n ) 3100 g/mol, M w ) 4000 g/mol, PDI ) 1.29. DSC: T g ) 54 °C. FT-IR (ATR): ν ) 3282 (Calkyne-H), 3031 (Car-H), 2920 (Cali-H), 2324, 2178, 2051, 1749 (CdO), 1608 (Car-Car), 1504, 1446, 1366, 1187, 1014, 911, 829 cm-^1. Poly(ethynylstyrene) ( HP-7 ). This was synthesized according to a procedure developed previously in our laboratory.^24 More details on synthesis and characterization can be found in the Supporting Information. GPC (CHCl 3 ): M n ) 2200 g/mol, M w ) 2800 g/mol, PDI ) 1.27, M n (^1 H NMR) ) 2600 g/mol, M n (cal) ) 2600 g/mol. DSC: T g ) 153 °C. General Procedure for the Synthesis of the Precursor Diblock Copolymers pBC-x. The macroinitiators MI- x were dissolved in a minimum amount of diglyme and comonomer 6 was added to the solution. The mol number of initiator needed to obtain the diblock copolymer of a desired block ratio was calculated as described above based on Mn,cal. of the macroinitiator (Table 2). As an example, in a vial, 566.1 mg (58 μmol) of MI-1a was completely dissolved in 962.2 mg (4.2 mmol) of 6 and 0.5 mL of diglyme. Thereafter, the solution was transferred in a Schlenk tube, degassed, and polymerized at 120 °C under nitrogen atmosphere. After 18 h, the reaction was stopped by cooling, and 0.535 g (35%) of polymer was isolated by repeated precipitation in methanol. In case of the syntheses of precursor diblock copolymers in which the alkyne monomer 7 is randomly distributed in the second block with styrene, the same methodology was followed (Tables 3 and 4). The 1 H NMR, 13 C NMR, IR, DSC, and TGA data can be found in the Supporting Information. General Procedure for the Synthesis of the Target Diblock Copolymers BC-x. The silyl-protected precursor diblock copolymers pBC- x were selectively desilylated by an excess tetrabutylammo-
nium fluoride [(TBA)F] at 0 °C using THF as solvent. As an example, 502.1 mg of silyl-protected diblock copolymer pBC-1a was dissolved in 10 mL of THF and cooled at 0 °C, and 2.2 mL of a 1 M solution of (TBA)F in THF was slowly added by a syringe. After 1 h of stirring at 0 °C, the solution was reduced in a vacuum, and the residue was precipitated in ethanol twice. After drying in a high vacuum at 50 °C overnight, 201 mg (67%) of a solid product was recovered. Be aware that by that procedure the TBDMS protecting group was also removed in the respective block copolymers (e.g., BC-1 ). The 1 H NMR, 13 C NMR, IR, DSC, and TGA data can be found in the Supporting Information. General Procedure for the Polymer Analogous Click Reac- tions. All click reactions were performed in Schlenk tubes at room temperature using THF or CDCl 3 , respectively, as solvent. No precautions were undertaken to exclude oxygen or moisture. As catalyst Cu(PPh 3 ) 3 Br was used in 0.1 equivalents and DIPEA in 3-fold excess, with respect to pendent alkyne groups. The reactions were stopped after full conversion for HP-6 was achieved as determined by 1 H NMR spectroscopy. Poly(4-(1 ′ -adamantanyltriazolylmethyloxy)styrene) ( A-HP-6 ) and Poly(4-(1 ′ -adamantanyltriazolyl)styrene) ( A-HP-7 ). The reac- tions were carried out in CDCl 3 at 40 °C. Samples were taken and the conversion was monitored by 1 H NMR spectroscopy. The final products were worked up by repeated precipitation from acetonitrile. A-HP-6.^1 H NMR (CDCl 3 ): δ ) 7.9-7.7 (Hg′), 7.0-6.2 (Hb′,c′), 5.1 (H (^) e′), 2.21 (H8,9 ), 2.2-1.0 (CH and CH 2 ), 1.76 ppm (H 10 ). 13 C NMR (CDCl 3 ): δ ) 156.4 (Cd′), 143.1 (Cf′), 140.0-137.0 (Ca′), 128.6 (Cb′), 119.8 (Cg′), 114.2 (Cc′), 62.2 (Ce′), 59.7 (C 7 ), 46- 38 (CH and CH 2 ), 42.9 (C 8 ), 39.7 ( C H), 35.9 (C 10 ), 29.4 ppm (C 9 ). DSC: T g ) 168 °C. A-HP-7.^1 H NMR (CDCl 3 ): δ ) 8.2-6.2 (H (^) b′,c′,b,c,g′), 2.99 (Hf ), 2.28 (H (^) 8,9 ), 1.18 (H 10 ), 2.5-1.1 ppm (CH and CH 2 ). 13 C NMR (CDCl 3 ): δ ) 146.8 (Cf′), 146-143 (Ca′,a ), 132.5 - 131.0 (Cc ), 130 - 126 (Cb′,d′,b), 126.0-124.5 (Cc′), 120.0-118.5 (Cd), 117.5-115. (C (^) g′), 84.0 (Cf ), 77.6 (Cg ), 59.5 (C 7 ), 46-38 (CH and CH 2 ), 42. (C 8 ), 36.0 (C 10 ), 29.5 ppm (C 9 ). Signals of ethynylstryrene were observed because of only ∼75% final conversion. Except for signals due to the adamantanyl moiety (7-10 ppm), all signals were strongly broadened by restricted mobility of the polymer backbone. DSC: T g could not be observed below the decomposition temper- ature. Poly(TBU-oxystyrene- b -4-(1 ′ -adamantanyltriazolylmethyloxy)- styrene) (A-BC-2b). A solution of 204.5 mg (155 μmol) of poly( tert -butyloxystyrene)- block -poly(propargyloxystyrene) ( BC- 2b ) in 10 mL of THF was prepared and 19.0 mg (20 μmol) of Cu(PPh 3 ) 3 Br and 55.5 mg (429 μmol) of DIPEA as well as 94. mg (530 μmol) of 1-adamantane azide were added. The reaction mixture was stirred for 48 h at 50 °C. The product was worked up by repeated precipitation from acetonitrile in 67% yield. (^1) H NMR (CDCl 3 ): δ ) 7.81 (H (^) g′), 6.8-6.5 (H3,b′,c′), 6.5-6. (H 2 ), 5.07 (He′), 2.21 (H8,9 ), 2.2 - 1.2 (CH and CH2), 1.76 (H 10 ), 1.26 ppm (H 6 ). 13 C NMR (CDCl 3 ): δ ) 156.4 (Cd′), 152.9 (C 4 ), 143.1 (Cf′), 141.5-139.0 (C 1 ), 139.0-137.0 (Ca′), 129.0-127. (C2,b′), 123.6 (C 3 ), 119.6 (Cg′), 114.1 (Cc′), 77.7 (C 5 ), 62.2 (Ce′), 59.5 (C 7 ), 47.5-42.0 (CH 2 ), 42.9 (C 8 ), 39.7 (CH), 35.9 (C 10 ), 29. (C 9 ), 28.9 ppm (C 6 ). GPC (CHCl 3 ): M n ) 54 100 g/mol, Mw ) 64 900 g/mol, PDI ) 1.20. DSC: T g ) 105 °C. FT-IR (ATR): ν ) 3026 (Car-H), 2974, 2911 (Cali-H), 2324, 2164, 2086, 1981, 1607 (Car-Car ), 1504, 1451, 1388, 1256, 1160, 1086, 1013, 924, 849 cm-^1.
Table 5. Experimental Parameters and Analytical Data for the Silyl-Protected Model Polymers MI-6 and MI- polymer no. n ini [μmol] n TIPNO [μmol] m monomer [g] conversion [%] M n,cal [g · mol-^1 ] M n,exp [g · mol-^1 ] PDI MI-6a 29 0 461.1 81 12400 12900 1. MI-6b 284 0 3.545 46 6600 6200 1. MI-7a 334 408 2.854 67 5700 3400 1. MI-7b 439 0 1.812 88 36200 28100 1.
Figure 2. SEC traces of the homopolymers MI-7b and MI-6a.
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Results and Discussion
Monomer Syntheses. For this work two different alkyne containing monomers based on styrene were used and synthesized, i.e., 4-(trimethylsilylpropargyloxy)styrene ( 6 ) and 4-(trimethylsilylethynyl)styrene ( 7 ). Whereas a synthetic
procedure for 7 is known in the literature, 23 the method for the preparation of a monomer carrying a more flexible alkyne group needed to be developed. Starting with the commercially available 4-acetoxystyrene ( 3 ), 4-hydroxystyrene ( 4 ) was obtained by hydrolysis and subsequently reacted in a straightforward Williamson ether type reaction to yield 4-(propargyloxy)styrene ( 5 ) (Schemes 1 and 2). Since it is known for alkyne protons to undergo chain transfer in radical polymerizations,^25 the functional group had to be protected. This was best done following a methodology similar as described elsewhere,^26 using chlorotrimethylsilane and DBU in the pres- ence of catalytic amounts of silver chloride. So, 6 could be isolated in acceptable yields of 50-60% overall.
Figure 3. Kinetic plots for the polymerization of 6 : (left) the kinetic plot, (right) M n,experimental ( 0 ) and PDI ( 2 ), respectively, vs conversion.
Scheme 3. Synthetic Pathway toward Alkyne-Containing Diblock Copolymers for Polymer Analogous Modification via Click Chemistry and Depiction of Desilylated Target Diblock Copolymers
Table 6. Comparison of Calculated and Experimentally (^1 H NMR) Determined Block Ratios (by Mass) of the Precursor Diblock Copolymers pBC- x Synthesized mass polymer block ratio pBC-1a pBC-1r pBC-2a pBC-2b pBC-2r pBC- calculated 1.5:1 1:2.6 1.2:1 5.7:1 2.3:1 2.0: NMR 1.6:1 1:3.0 1.4:1 6.4:1 2.5:1 1.8:
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( pBC- x ) (Scheme 3). For this purpose, 6 or a mixture of 7 and styrene, respectively, were sequentially added to the MI and, if needed, a minimum amount of diglyme was used additionally. Again, the polymerization temperatures were kept at 120 °C and the reactions were stopped after 20 h. This time, acetic anhydride could not be added since this would lead to a complete removal of the very labile TMS protective group of 6 and 7 under elevated temperatures. The isolated precursor diblock
copolymers were entirely protected in both blocks, as indicated by NMR analysis. Block ratios, i.e., M n ( MI- x )/ M n ( Poly-6 / 7-r- sty ) were experimentally derived from appropriate signal integrals in the 1 H NMR spectra (Table 6). They correspond to the molar ratio of the monomers in the copolymer multiplied with the ratio of their molar masses. This ratio was compared with the value obtained from M n,cal( MI- x )/ M n,cal( Poly-6 / 7-r-sty ). The molecular weights M n,cal ( Poly-6 / 7-r-sty ) for the second block could be concluded from M n,cal ( Poly-6 / 7-r-sty ) ) Rνm6/(7+sty) /nMI with R is the degree of conversion of monomer 6 and of the mixture of 7 and styrene, respectively. The efficiency of the reinitiation by the MI s can also be evaluated when comparing the SEC traces of the precursor diblock copolymers with its corresponding macroinitiator. As it can be concluded from Figure 4, the reinitiation of 6 with
Scheme 4. Synthesis of Diblock Copolymers with Random Distribution of the Alkyne Monomer 7 and Styrene in the Second Block (PG ) TBDMS, MI-1b and pBC-1r; PG ) TBU, MI-2c and pBC-2r)
Table 7. Analytical Data of the Target Diblock Copolymers BC- x Obtained by Selective Desilylation with (TBA)F mass block ratio [B1:B2] a polymer pBC- x M n,cal. [g · mol-^1 ] M n,exp. [g · mol-^1 ] M w,exp. [g · mol-^1 ] PDI calculated NMR BC-2a pBC-2a 30800 23400 29500 1.26 1.8:1 1.7: BC-2b pBC-2b 56400 52500 63100 1.20 8.3:1 8.5: BC-2r pBC-2c 65700 60900 82900 1.36 2.2:1 2.5: BC-3 pBC-3 27700 21000 29100 1.39 2.8:1 2.5: BC-4a pBC-1a 9300 1.2:1 1.1: BC-4r pBC-1b 6400 1:5:5 1:5: a (^) With B1 being the macroinitiator fragment and B2 being the alkyne block.
Figure 5. Reaction of the homopolymers HP-6 and HP-7 with 1-adamantane azide to form the 1-adamantyl-2,3-triazole substituted polymers A-HP-6 and A-HP-7 and 13 C NMR spectra of HP-7 (a) and A-HP-7 (b) (solvent: CDCl 3 ). Signals of low intensity in the spectrum of HP-7 are caused by the initiating and terminating group derived from the NMRP initiator.
Figure 6. Modification of diblock copolymer BC-2b with 1-adamantane azide to form diblock copolymer A-BC-2b. Reaction scheme and 13 C NMR spectra of BC-2b (a) and A-BC-2b (b) in CDCl 3.
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MI-1a was complete and products were characterized by a narrow molar mass distribution. Neither a residual MI peak nor a shoulder especially toward lower molar masses could be detected in the SEC trace. The same positive findings could be also denoted for all other diblock copolymers when crude material, i.e., prior to the precipitation process, was analyzed. In the same manner protected diblock copolymers were developed in which the alkyne functionality of the second block is randomly distributed along a styrene backbone ( pBC-1r and pBC-2r ). In this case, one has to keep in mind that 4-(trim- ethylsilylethynyl)styrene ( 7 ) cannot be incorporated with higher loads than approximately 20% (without using TIPNO ) in order to preserve the controlled character of the polymerization. Therefore, poly( tert -butyloxystyrene) and poly( tert- butyldim- ethylsilyloxystyrene) were used to initiate the copolymerization of styrene and 7 (Scheme 4) with feed compositions of roughly 10:1. Once more, the reliability of the MI -system was proven and macromolecules of a very defined architecture, both, in terms of chemical composition and uniformity were achieved (Table 6). Click chemistry is the Cu(I)-catalyzed 1,3-dipolar cycload- dition reaction between alkynes and azides.^10 The Cu(I) catalysis further provokes an acceleration of an already straightforward cycloaddition, an effect that was first published by Meldal et al.^30 Furthermore, the advantage of the Cu(I) catalysis clearly
is the regioselective formation of the 1,4-triazole adduct.30, Since the transition state is assumed to proceed via a copper acetylide species, only terminal alkyne can be efficiently catalyzed.^32 Therefore, the alkyne needs to be deprotected prior to click reaction. Thereby, it is essential to quantitatively and selectively remove all silyl protection groups, while simulta- neously leaving the labile protected hydroxystyrene block untouched. The reagents of choice for this purpose are based on fluoride salts and were introduced by Corey et al.^33 Meanwhile, this concept of selective and versatile protection/ deprotection has been applied to polymer analogous reac- tions.21,34^ In fact, referring to tetrabutylammonium fluoride we were able to simultaneously induce the complete removal of all silyl protection groups (i.e., TMS and TBDMS) as well as to preserve all other labile protective groups as confirmed by absence of signals of phenolic moieties in the NMR spectra of BC-2 and BC-3. In case of pBC-1a,b also the TMS protecting group of the MI-1 was removed resulting in poly(hydroxysty- rene- b -propargyloxystyrene) ( BC-4a ) and poly(hydroxystyrene- b -ethynylstyrene- r -styrene) ( BC-4r ). If the deprotection was conducted with acids, the TBU and acetoxy groups could not be retained. Therefore, in all experiments the deprotection reactions were carried out in THF at 0 °C with (TBA)F and were shown to have gone to completion after 30 min. Comparing the desilylated target diblock copolymers ( BC- x ) and its precursor macromolecules the molar mass distributions did not alter. Naturally, however, due to the considerable mass loss, the SEC traces were shifted toward lower molecular weights and mass block ratios changed accordingly; i.e., the contribution of the block originating from the macroinitiator increased since that did not experience degradation upon selective deprotection. Thus, the mass loss obtained in the BC-1 family led to vast decrease of the B1:B2 ratio (Table 7) with respect to the molar masses. SEC traces of BC-4a and BC-4r could not be recorded due to adsorption of the material on the column. As the desilylation was shown to be complete in the NMR spectra and since desilylation did not alter the quality of the product for all other diblock copolymers we can assume the same for BC-4a and BC-4r. Most of the desilylated block copolymers are phase-separated as indicated by two distinct glass transition temperatures ( T g ) in differential scanning calorimetry. In particular for polymers with deblocked hydroxystyrene segments the phase separation seems to be very pronounced due to a high incompatibility of the polar and nonpolar blocks. These diblock copolymers are therefore prime candidates for the preparation of nanostructured surfaces and their phase separation in thin films is currently investigated in our laboratories. Polymer Analogous Modifications. In the final step, the deprotected target diblock copolymers ( BC - x ) were reacted with bulky 1-adamantane azide to prove our underlying concept that they are scaffolds for specifically tailoring of macromolecular architecture as well as functionality. Because 1-adamantane azide is a sterically demanding substrate we started with model experiments to evaluate the reactivity of the propargyl and ethynyl group, respectively, in a polymer analogous reaction aiming for an attach-to approach for bulky substituents. In their attentive work Thomsen et al.^35 demonstrated the effective polymer analogous modification of homopolymers as well as random copolymers based on 4-propargyloxystyrene with azido functionalized carboxylic acids. In this case, however, the alkyne polymers originated from polymer analogous reactions of polyhydroxystyrene prepared by free radical polymerization and propargyl bromide, a method which did not allow to prepared defined macromolecular architectures. Poly(propargyloxystyrene) ( HP-6 ) and poly(ethynylstyrene) homopolymer ( HP-7 ) with comparable low molecular weights,
Figure 7. SEC traces of BC-2 (--) and A-BC-2b (s) recorded in chloroform and calibrated versus PS standards.
Figure 8. DSC traces of BC-2b and its reaction product with 1-adamantane azide, A-BC-2b.
5262 Fleischmann et al. Macromolecules, Vol. 41, No. 14, 2008
Once more, the combination of controlled polymerization techniques and click chemistry demonstrated to be a powerful tool for the manipulation and functionalization of macromol- ecules. The presented alkyne containing block copolymers can be considered as macromolecular scaffolds and a further impact in the emerging field of nanoscience can be expected. “Clicking” functionalization onto nanostructured films of such kind of diblock copolymers with high spatial resolution is certainly of great scientific interest. This issue is currently being investigated in our institute.
Acknowledgment. We gratefully thank L. Ha¨ussler for DSC and TGA measurements, and we very much appreciate the constructive collaboration with J. Stadermann in our institute. For the financial support in the frame of SFB 287, we want to acknowledge the Deutsche Forschungsgemeinschaft.
Supporting Information Available: Text giving a detailed structure elucidation by NMR as well as IR spectroscopy for all macroinitiators and diblock copolymers. This information is avail- able free of charge via the Internet at http://pubs.acs.org.
References and Notes
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