Synthesis of Dendronized Polymers via Macromonomer Approach by

Article pubs.acs.org/Macromolecules

Synthesis of Dendronized Polymers via Macromonomer Approach by Living ROMP and Their Characterization: From Rod-Like Homopolymers to Block and Gradient Copolymers Kyung Oh Kim and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul, 151-747, Korea S Supporting Information *

ABSTRACT: Dendronized polymers were synthesized from various endo-tricycle[4.2.2.0]deca-3,9-diene (TD) monomers, using a macromonomer approach, via ring-opening metathesis polymerization. The polymerization was achieved in a living manner to give dendronized polymers with high molecularweights and narrow polydispersity index. The resulting polymers showed highly extended conformations both in solution and in the solid state, even with relatively small thirdgeneration ester dendrons, because of the rigid backbone of TD monomers. Using their rigidity, block and gradient copolymers containing TD and norbornene moieties were synthesized. Diblock copolymers were produced by the sequential addition of two dendronized macromonomers, and gradient copolymers were produced in a simple batch reaction, because large differences in reactivity allowed for the simultaneous one-pot polymerization, favoring TD over norbornene. Polymer conformations were confirmed by both scattering analysis in solution and atomic force microscopy (AFM) imaging in thin films. The high-resolution AFM images of both copolymers enabled us to distinguish the structural differences between the two classes of copolymers because the block copolymer showed clear boundary for each block while the gradient copolymer showed gradual changes in height and thickness. Furthermore, a comparison of the rigidities of these two copolymers was performed by scattering analysis in solution.

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INTRODUCTION Dendronized polymers composed of dendrons attached to a polymer backbone are new materials with a wide range of potential applications in the fields of biomaterials, electronics, and energy harvesting, because one can control the polymer properties by changing either the dendron or polymer backbone independently.1 Also, the conformations of these polymers can be controlled from entangled to extended conformation by varying the steric effect of the dendrons, and the single chains of these polymers can be easily visualized using atomic force microscopy (AFM) imaging techniques.2 There are three synthetic strategies for preparing dendronized polymers: graft-to,3 graft-from,4 and the macromonomer approach.5 Although the graft-to approach is used most frequently, the macromonomer approach has many advantages such as synthesis of defect-free dendronized polymers and, more importantly, the direct synthesis of block copolymers by the sequential addition of monomers.6 However, their synthesis is very challenging in terms of preparing long-chain polymers from macromonomers containing high-generation dendrons because their severe steric hindrance prevents efficient propagation. Recently, ring-opening metathesis polymerization (ROMP)7 has been used as a highly efficient method that provides a solution to the synthetic hurdles of the macromonomer approach.6 For instance, our group reported the © 2013 American Chemical Society

living polymerization of high-generation dendronized macromonomers up to the fifth generation (G5). These polynorbornenes showed rod-like conformations, as confirmed by analysis of solution properties and visualization in the solid state.5f However, simpler method to more rigid polymers is still desired. The endo-tricycle[4.2.2.0]deca-3,9-diene (TD) obtained by a series of pericyclic reactions of cyclooctatetraene has an interesting structure that contains two olefins, a cyclobutene fused to a bicyclo[2,2,2]oct-2-ene. This TD derivative was reported to undergo living ROMP with first-generation Grubbs catalyst.8 Of the two olefins, only the cyclobutene moiety, with higher ring strain, underwent ROMP, whereas the sterically hindered bicyclo[2,2,2]oct-2-ene did not react at all. This TD monomer has rarely been used for ROMP because the reaction is much slower (more than 12 h)8 than that with norbornene derivatives, which are the most widely used monomers for ROMP. However, poly(TD) could easily form rod-like polymers because of its rigid backbone.8c Thus, with a more powerful catalyst, TD derivatives could become more useful monomers for dendronized polymer synthesis. Also, reactivity Received: May 31, 2013 Revised: July 2, 2013 Published: July 24, 2013 5905

dx.doi.org/10.1021/ma401132u | Macromolecules 2013, 46, 5905−5914

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mode using noncontact mode tip from Nanoworld (Pointprobe tip, NCHR type) with spring constant of 42 N m−1 and tip radius of ≤8 nm. General Procedure for the Synthesis of Macromonomers. Macromonomer Containing Ethylene Linker (1, 2). The ethyl alcohol functionalized endo-tricyclo-[4.2.2.02.5]deca-3,9-diene (0.66 g, 1 equiv) solution in dichloromethane (10 mL, 0.3 M) and triethylamine (1.2 mL, 3 equiv) was prepared. Then isopropylidene2,2-bis(oxymethyl)propionic anhydride (1.4 g, 1.5 equiv) and 4dimethylaminopyridine (DMAP) (17.2 mg, 5 mol %) were added at room temperature, and the reaction mixture was stirred for 12 h. After completion of the reaction, saturated NaHCO3 (30 mL) aqueous solution was added and the reaction stirred for 1 h. The mixture was washed with saturated NH4Cl (50 mL) solution and NaHCO3 (50 mL) solution. The organic layer was extracted with ethyl acetate and dried with anhydrous MgSO4 and the solvent was removed on a rotary evaporator. The product was purified by column chromatography with ethyl acetate−hexane mixture. The separated product solutions were collected and concentrated to yield final product. The product from the coupling reaction was deprotected in excess methanol (38 mL, 0.04 M) with catalytic amount of p-toluenesulfonic acid (13 mg, 5 mol %). After completion of the reaction, acid was quenched with triethylamine, and the solvents were dried by rotary evaporator and vacuum. These coupling and deprotection reaction was repeated to prepare final G3 and G4 macromonomer. Macromonomer Containing Biphenyl Linker (3, 4). Macromonomer 3 and 4 were synthesized follow the same process with macromonomer containing ethylene linker. The biphenol functionalized endo-tricyclo-[4.2.2.02.5]deca-3,9-diene (0.50 g, 1 equiv) solution in dichloromethane (5.0 mL, 0.3 M) and triethylamine (0.65 mL, 3 equiv) was prepared. Then isopropylidene-2,2-bis(oxymethyl)propionic anhydride (0.75 g, 1.5 equiv) and 4-dimethylaminopyridine (DMAP) (9.0 mg, 5 mol %) were added at room temperature, and the reaction mixture was stirred for 12 h. Macromonomer 1. Yield of final step: 0.52 g, 89%. 1H NMR (500 MHz, CDCl3): δ 1.15 (s, 12 H), 1.24 (s, 3 H), 1.26 (s, 6 H), 1.35 (s, 12 H), 1.41 (s, 12 H), 2.81 (d, J = 0.7 Hz, 4 H), 3.16 (s, 2 H), 3.63 (d, J = 1.5 Hz, 8 H), 3.70 (t, J = 0.9 Hz, 2 H), 4.11−4.34 (m, 22 H), 5.89 (t, J = 1.3 Hz, 4 H). 13C NMR (125 MHz, CDCl3): δ 17.34, 17.63, 18.46, 22.08, 25.08, 27.59, 36.56, 37.38, 41.98, 43.30, 44.07, 46.49, 46.80, 53.45, 62.39, 64.87, 65.74, 65.87, 65.91, 67.05, 98.04, 128.22, 137.95, 171.75, 171.84, 173.44, 178.41. MS (MALDI−TOF): [M + Na]+ calcd for C61H87NO24, 1241.339; found, 1241.798. Macromonomer 2. Yield of final step: 0.43 g, 83%. 1H NMR (500 MHz, CDCl3): δ 1.14 (s, 24 H), 1.24 (s, 6 H), 1.26 (s, 12 H), 1.35 (s, 24 H), 1.41 (s, 24 H), 2.82 (d, J = 0.7 Hz, 4 H), 3.16 (s, 2 H), 3.62 (d, J = 1.5 Hz, 16 H), 3.68 (t, J = 1.2 Hz, 2 H), 4.12−4.33 (m, 46 H), 5.89 (m, 4 H). 13C NMR (125 MHz, CDCl3): δ 17.27, 17.47, 17.67, 18.50, 22.08, 25.15, 36.61, 37.43, 42.01, 43.32, 44.10, 46.54, 46.70, 46.81, 62.36, 64.81, 65.52, 65.90, 65.95, 98.09, 128.26, 137.99, 171.45, 171.60, 171.822, 173.47, 178.43. MS (MALDI−TOF): [M + Na]+ calcd for C113H167NO48, 2329.055; found, 2329.478. Macromonomer 3. Yield of final step: 0.42 g, 85%. 1H NMR (500 MHz, CDCl3): δ 1.12 (s, 12 H), 1.26 (s, 3 H), 1.31 (s, 6 H), 1.34 (s, 12 H), 1.40 (s, 12 H), 2.89 (s, 2 H), 2.99 (s, 2 H), 3.30 (s, 2 H), 3.62 (d, J = 1.4 Hz, 8 H), 4.12 (m, 10 H), 4.39 (m, 10 H) 5.93 (s, 2 H), 6.04 (t, J = 1.3 Hz, 2 H) 7.14 (d, J = 1.0 Hz, 2 H), 7.29 (d, J = 1.0 Hz, 2 H), 7.56 (d, J = 1.0 Hz, 2 H), 7.61 (d, J = 1.0 Hz, 2 H). 13C NMR (125 MHz, CDCl3): δ 17.75, 18.46, 21.93, 25.30, 37.06, 42.05, 43.43, 44.10, 46.93, 47.09, 64.97, 65.93, 65.98, 98.11, 121.68, 126.85, 127.85, 128.48, 131.211, 138.02, 138.47, 149.94, 170.81, 171.92, 173.53, 177.89. MS (MALDI−TOF): [M + Na]+ calcd for C71H91NO24, 1364.583; found, 1364.251. Macromonomer 4. Yield of final step: 0.34 g, 79%. 1H NMR (500 MHz, CDCl3): δ 1.13 (s, 24 H), 1.25 (s, 12 H), 1.30 (s, 6 H), 1.34 (s, 24 H), 1.40 (s, 24 H), 2.89 (s, 2 H), 2.99 (s, 2 H), 3.30 (s, 2 H), 3.60 (d, J = 1.4 Hz, 16 H), 4.13 (d, J = 1.4 Hz, 18 H), 4.38 (m, 20 H), 4.40 (d, J = 1.3 Hz, 2 H), 4.51 (d, J = 1.3 Hz, 2 H), 5.93 (s, 2 H), 6.03 (t, J = 1.3 Hz, 2 H) 7.16 (d, J = 1.0 Hz, 2 H), 7.29 (d, J = 1.0 Hz, 2 H), 7.57 (d, J = 1.0 Hz, 2 H), 7.62 (d, J = 1.0 Hz, 2 H). 13C NMR (125

differences between TD and norbornene derivatives might provide a simple route to the synthesis of gradient copolymers. Gradient copolymers are a unique class of polymers that have gradual changes in monomer composition along their backbones.9 Gradient copolymers are prepared by two approaches: changing the monomer feed rate by continuous addition of comonomers during polymerization (semibatch),10 or using two monomers having large differences in reactivity in one pot (batch).11 Many researchers have synthesized gradient copolymers by powerful controlled radical polymerization. For example, Matyjaszewski’s group reported gradient copolymerization under batch conditions via atom transfer radical polymerization (ATRP) exploiting the reactivity differences between methyl methacrylate (MMA) and n-butyl acrylate.11a Furthermore, postfunctionalization, or another ATRP from this gradient copolymer backbone, produced gradient-brush copolymers and the conformation of the final polymer was observed by AFM imaging. Gradient copolymers were also prepared by ROMP using the semibatch approach, by controlling the feed rate of the monomers.10a Although the batch approach is synthetically much easier, it is hard to find monomer combinations with different reactivities. Herein, we report living ROMP for the synthesis of dendronized polymers from various TD monomers via a macromonomer approach. Polymers with high-generation dendrons (up to G4) were prepared with controlled molecular-weights and narrow polydispersity index (PDIs). Because these polymers contained highly rigid backbones, the resulting polymers, with relatively small dendrons (even G3), also showed highly extended conformations, as confirmed by both light scattering analysis in solution and AFM imaging in the solid state. Using these properties, diblock and gradient copolymers containing the TD moiety were easily prepared and their direct visualization by high-resolution AFM allowed us to clearly distinguish the structural differences between the two copolymers.

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EXPERIMENTAL SECTION

Materials and Characterization. All commercially available reagents were used without further purification. Solvents for monomer synthesis were also obtained: tetrahydrofuran (THF) and toluene was anhydrous (≥99.8%) grade from Sigma-Aldrich. For polymerization, THF was distilled from sodium and benzophenone. The solvents were degassed with argon gas for 10 min before polymerization. Thin-layer chromatography (TLC) was carried out on MERCK TLC silica gel 60 F254 and the flash column chromatography was performed using MERCK silica gel 60 (0.040−0.063 mm). 1H NMR and 13C NMR were recorded by Varian/Oxford As-500 (500 MHz for 1H and 125 MHz for 13C) and Bruker (300 MHz for 1H and 75 MHz for 13C) spectrometers. High resolution mass spectroscopy (HRMS) analyses were performed by the National Center for Inter-University Research Facility. Gel permeation chromatography (GPC) for polymer molecular weight analysis was carried out with Waters system (1515 pump) and Shodex GPC LF-804 column eluted with THF (GPC grade, Honeywell Burdick & Jackson). Samples in 0.001−0.003 wt % THF were filtered with a 0.45 μm PTFE filter before injection. Flow rate was 1.0 mL/min and temperature of system was maintained at 35 °C. For the determination of molecular weights, and Wyatt triple detector, Dawn 8+/Viscostar II/Optilab T-rEX were used for the MALLS-VIS-RI analysis. The molecular mass of the macromonomers was measured by Matrix-assisted laser desorption/ionization time-offlight mass spectrometer (MALDI−TOF), Bruker Daltonics autoflex II TOF/TOF. Dithranol in ethyl acetate was used as a matrix. Multimode head and Nanoscope V controller (Veeco Instrument) were used for AFM imaging. All images were obtained on tapping 5906


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Scheme 1. Synthesis of Macromonomers and Dendronized Polymers

MHz, CDCl3): δ 17.56, 17.63, 18.46, 21.95, 24.94, 25.26, 25.60, 33.92, 37.05, 42.01, 43.42, 44.08, 46.76, 46.81, 47.14, 64.81, 65.56, 65.89, 65.94, 66.34, 66.61. 98.08, 121.66, 126.88, 127.79, 128.40, 128.46, 131.25, 138.00, 138.39, 140.41, 149.97, 170.63, 171.48, 171.83, 173.46, 177.84. MS (MALDI−TOF): [M + Na]+ calcd for C123H171NO48, 2453.087, found, 2452.835. General Procedure for Dendronized Homopolymers. Macromonomer (0.021 mmol) was weighed in a 5-ml vial with septum and purged with argon. Anhydrous and degassed solvent (0.20 mL) was added to the vial. The solution of initiator (0.05 mL) was added at once under vigorous stirring. After complete consumption of the macromonomers, the reaction was quenched by excess ethyl vinyl ether. The concentrated reaction mixture was precipitated into methanol or isopropyl alcohol, and the obtained white solid was dried in vacuo. Poly(1). Yield: 23 mg, 89%. 1H NMR (500 MHz, CDCl3): δ 1.13 (br, 12 H), 1.26 (br m, 18 H), 1.34 (br, 12 H), 1.40 (br, 3 H), 3.07 (br, 6 H), 3.61 (br d, 8 H), 4.12 (br m, 12 H), 4.27−4.33 (br m, 12 H), 5.50 (br, 2 H), 6.21 (br, 2H). 13C NMR (125 MHz, CDCl3): δ 16.49, 17.49, 18.51, 22.13, 25.19, 42.02, 44.04, 46.43, 46.49, 46.82, 64.76, 64.77, 65.47, 65.89, 65.93, 98.03, 129.01, 132.04, 171.81, 173.42, 177.66. Poly(2). Yield: 41 mg, 85%. 1H NMR (500 MHz, CDCl3): δ 1.13 (br, 24 H), 1.27 (br, 18 H), 1.33 (br, 48 H), 1.40 (br, 3 H), 3.07 (br, 6 H), 3.62 (br d, 16 H), 4.12 (br d, 24 H), 4.31 (br, 24 H), 5.50 (br, 2 H), 6.21 (br, 2H). 13C NMR (125 MHz, CDCl3): δ 17.72, 18.51, 22.16, 25.20, 41.98, 46.80, 64.62, 65.89, 97.99, 171.84, 173.89. Poly(3). Yield: 25 mg, 87%. 1H NMR (500 MHz, CDCl3): δ 1.09 (br, 12 H), 1.29 (br m, 18 H), 1.31 (br, 12 H), 1.38 (br, 3 H), 3.07 (br, 6 H), 3.57 (br d, 8 H), 4.15 (br d, 10 H), 4.33 (br m, 10 H), 5.30 (br, 2 H), 6.42 (br, 2H), 7.16 (br, 4H), 7.51 (br, 4H). 13C NMR (125 MHz, CDCl3): δ 17.77, 18.47, 21.99, 25.29, 42.04, 46.91, 64.90, 65.95, 98.07, 121.75, 128.35, 171.89, 173.48. Poly(4). Yield: 42 mg, 82%. 1H NMR (500 MHz, CDCl3): δ 1.10 (br, 24 H), 1.24 (br m, 18 H), 1.31 (br, 48 H), 1.37 (br, 3 H), 3.07 (br, 6 H), 3.60 (br d, 16 H), 4.16 (br d, 20 H), 4.30 (br, 24 H), 5.30 (br, 2 H), 6.42 (br, 2H), 7.15 (br, 4H), 7.51 (br, 4H). 13C NMR (125 MHz, CDCl3): δ 17.59, 18.46, 21.90, 27.22, 41.99, 46.81, 61.01, 62.38, 98.33, 173.46, 176.40. General Procedure for Dendronized Block Copolymers. Monomer for the first block (0.021 mmol) was weighted in a 5-mL vial with septum and purged with argon. Anhydrous and degassed solvent (0.20 mL) was added to the vial and the reaction mixture was stabilized at appropriate temperature. The solution of initiator (0.05 mL) was added at once under vigorous stirring. After the complete consumption of the first monomer, the solution of second monomer (0.021 mmol) was added. After complete conversion of monomer, the

reaction was quenched by excess ethyl vinyl ether. The concentrated reaction mixture was precipitated into methanol or isopropyl alcohol, and the obtained white solid was dried in vacuo. poly(5)-block-poly(4). Yield: 56 mg, 87%. 1H NMR (500 MHz, CDCl3): δ 1.11 (br, 30 H), 1.18−1.49 (br m, 92 H), 2.64−3.41 (br m, 10 H), 3.61 (br d, 20 H), 3.73 (br, 2 H), 4.14 (br d, 20 H), 4.19−4.51 (br m, 34 H), 5.66 (br m, 4 H), 6.40 (br, 2 H), 7.16 (br, 2 H), 7.36 (br, 2 H), 7.61 (br, 2 H). poly(3)-block-poly(5). Yield: 40 mg, 91%. 1H NMR (500 MHz, CDCl3): δ 1.03−1.18 (br m, 18 H), 1.21−1.48 (br m, 50 H), 2.74 (br, 2 H), 2.97−3.36 (br m, 6 H), 3.52−3.79 (br m, 14 H), 4.05−4.51 (br m, 30 H), 5.50 (br, 2 H), 5.76 (br, 2 H), 6.46 (br, 2 H), 7.13 (br, 4 H), 7.45 (br, 4 H). General Procedure for Dendronized Gradient Copolymers. Two monomers (0.02 mmol) were added into screw-cap NMR tube and purged with argon gas. THF-d8 was added and stabilized at appropriate temperature in NMR instrument. Then, the solution of initiator (0.00013 mmol, 0.05 mL) was added. For the experiment of hexyl monomers, a 5 times larger amount of reagents was used. The conversion of the two monomers was monitored with time until the complete consumption of both monomers. poly(4)-grad-poly(5). Yield: 50 mg, 82%. 1H NMR (500 MHz, CDCl3): δ 1.12 (br, 30 H), 1.23−1.48 (br m, 92 H), 3.12 (br, 10 H), 3.59 (br d, 20 H), 4.10 (br d, 20 H), 4.21−4.50 (br m, 36 H), 5.51 (br, 4 H), 6.34 (br, 2 H), 7.15 (br, 4 H), 7.56 (br, 4 H). poly(6)-grad-poly(7). Yield: 49 mg, 93%. 1H NMR (500 MHz, CDCl3): δ 0.84 (br d, 6 H), 1.33−1.68 (br d, 14 H), 2.13 (br, 2H), 2.60−3.47 (br m, 16 H), 5.01−5.58 (br m, 3 H), 5. 79 (br t, 1 H), 6.25 (br, 2 H).

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RESULTS AND DISCUSSION The syntheses of various TD macromonomers having G3 and G4 ester dendrons and their polymers are summarized in Scheme 1. The TD anhydride moiety was generated by electrocyclization of cyclooctatetraene to give a fused bicyclic intermediate, which then underwent a Diels−Alder reaction with maleic anhydride to yield a tetracyclic compound containing a cyclobutene moiety.12 The final product had only one stereoisomer because of the endo rule. To this TD anhydride, two types of linker, a flexible ethylene linker and a rigid biphenyl linker, were attached, and dendrons were incorporated by divergent routes, following previous work.5f Each macromonomer was purified by flash column chromatography and characterized by NMR spectroscopy and matrix5907


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Table 1. ROMP of Various Macromonomersa entry

monomer

[M]/[I]

T (°C)

Mn(theo)c

Mn(obs)d

PDId

νe

Lp (nm)f

1 2b 3 4 5 6 7

1 2 3 4 4 4 4

300 300 300 300 200 100 50

50 70 50 50 50 50 50

365K 692K 402K 729K 486K 243K 122K

272K 640K 300K 680K 550K 252K 119K

1.06 1.22 1.02 1.02 1.04 1.03 1.02

0.72 0.92 0.87 0.99 − − −

16.8 27.7 20.2 31.2 − − −

a

The polymerization was performed in THF at various temperatures and the full conversion was observed within 20 hours. bToluene was used. Theoretical molecular weight. dThe molecular weight and PDI was measured by MALLS detector. eCalculated from conformation plot measured by MALLS. fPersistence length calculated by eq 1 using the theoretical unit length, l0 = 3.7 Å . c

conducted using size-exclusion chromatography (SEC). The Flory exponent ν, which is calculated from the slope of the conformation plot (log Rg vs log Mw) was extracted from this analysis (Table 1). This shape parameter predicts the conformation of polymer chains in solution; ν larger than 0.6 indicates a rod-like conformation and ν = 1 corresponds to a perfect rigid rod. In our previous work, larger ν values were obtained with polymers containing higher generations of dendrons and also with those containing a rigid biphenyl linker rather than a flexible ethylene linker.5f Similarly, the new polymers containing higher-generation ester dendrons gave larger ν values (2 > 1 and 4 > 3), and polymers containing the rigid biphenyl linker [poly(3) and poly(4)] showed more rodlike conformations than polymers containing the flexible ethylene linker [poly(1) and poly(2)]. Notably, the ν value of poly(4) was 0.99, showing a highly rigid conformation approaching the value for perfect rigid rods (ν = 1.0).14 Furthermore, it is worthwhile to recap that highly rigid polymers with ν = 0.87 were obtained only from polynorbornene containing an extremely large G5 ester dendron whereas in this case, the same shape factor (ν = 0.87) was obtained from the poly(TD) containing only G3 ester dendrons (poly(3), Table 1, entry 3).

assisted laser desorption/ionization mass spectrometry (see Supporting Information). ROMP of these macromonomers was carried out in either tetrahydrofuran or toluene at various temperatures, using thirdgeneration Grubbs catalyst I to ensure fast initiation, and complete conversion of all the macromonomers (1−4) within 20 h. First, the ROMP of TD macromonomers containing G4 dendrons without any linker was tested, but it did not give any polymer because of the severe steric bulkiness of the large G4 dendron. To enhance the reactivity, two linker moieties were introduced between the TD and the dendrons, similar to in our previous work on norbornene derivatives.5f A macromonomer (1) containing a flexible ethylene linker and G3 ester dendron with monomer to initiator (M/I) ratio of 300, was polymerized at 50 °C to give complete conversion and a narrow PDI (Table 1, entry 1). ROMP was conducted at 70 °C for the analogous macromonomer 2, containing larger G4 dendrons, and produced poly(2) with a slightly broader PDI (Table 1, entry 2), presumably due to the reduced stability of the catalyst at 70 °C.5f When a rigid biphenyl linker was incorporated, much better results with narrower PDIs were obtained for macromonomers containing G3 (3) and G4 (4) dendrons (Table 1, entries 3 and 4). Even with large G4 dendrons, 4 could undergo ROMP with complete conversion at 50 °C. To test for living polymerization, we varied the M/I ratio from 50 to 300 and obtained a linear relationship between the degree of polymerization (DP) and the number-average molecular-weight (Mn), and in all cases, the PDIs were well below 1.05 (Table 1, entries 4−7, Figure 1). It was surprising to observe efficient ROMP of these bulky macromonomers containing the intrinsically less reactive endo-TD moiety, because ROMP of norbornene derivatives containing the endo-isomer was highly challenging because of steric hindrance during propagation.13 To examine the conformation of these polymers in solution, multiangle laser light scattering (MALLS) analysis was

F (X ) = −

X=

1 R g2 4 1 1 X + X 3 − X 2 + X + e −X − 1 2 2 L 6 2

(1)

L LP

To further support the rigidity of the poly(TD)s, another representative parameter for estimating the polymer chain stiffness, the persistence lengths, was calculated by the following reported method.15 Using eq 1, the persistence length (Lp) was calculated from the contour length (L) and the radius of gyration (Rg). L was calculated by multiplying the DP and the unit length (3.7 Å), and Rg was measured by MALLS analysis. As summarized in Table 1, the persistence length was larger for polymers containing larger dendrons, and for those containing a rigid biphenyl linker as well. Poly(4) showed the largest Lp, 31 nm. These results are in excellent agreement with the conformation plot data (ν); the higher the ν values, the higher the persistence lengths of the polymers (Figure 2). Surprisingly, the Lps of the dendronized polymers prepared from TD macromonomers were larger (16.8−31.2 nm) than those of dendronized polymers prepared from norbornene containing a G5 ester dendron (8 nm). These results confirm that poly(TD)s are much more rigid than polynorbornenes. Therefore, the rigidity of the polymer backbone itself exhibited great influence on the conformation, in a similar way as the size

Figure 1. Mn control for poly(4). The inset values are PDI. 5908


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and poly(2)]. In particular, among the biphenyl-containing polymers, even poly(3) containing G3 dendrons showed a highly extended structure (Figure 3c), and poly(4) containing the G4 dendron showed almost rod-like images (Figure 3d). The images of the polymers in the film state showed good agreement with the conclusions obtained from the solution state. Using the living polymerization of TD macromonomers, the synthesis of fully dendronized block copolymers with norbornene derivatives was attempted by sequential addition (Table 2). Initially, the norbornene-based macromonomer Table 2. Results of Block Copolymerizationa

Figure 2. Plot of persistence length vs Flory exponent.

of the dendrons. We attribute this rigidity to the nature of the polymer repeat unit, which has a bulky bridged tricylic 3dimensional structure and more densely grafted dendrons as a result of the fewer carbon atoms between the repeating units (4) than that of norbornene (5). Consequently, a larger stress was imposed on the polymer backbone, thereby increasing the rigidity and grafting density of the poly(TD)s. Thus, access to the rod-like polymers became much easier with relatively small dendrons. Polymers based on norbornenes or other vinyl monomers such as styrene require high-generation dendrons to exhibit highly rod-like conformations, because of their flexible backbones.16 However, with the rigid poly(TD), highly rigid rod-like polymers were obtained with much smaller G3 ester dendrons, whose synthesis and purification were much easier and simpler than those containing larger dendrons. The film-state conformation of all polymers was verified using AFM imaging by spin-coating the dilute polymer solution onto a freshly cleaved mica surface, and single chains of the polymers were imaged to reveal rod-like conformations (Figure 3). As expected from the shape parameters in solution, polymers containing a biphenyl linker [poly(3) and poly(4)] showed more extended conformations than polymers containing the same dendrons but a flexible ethylene linker [poly(1)

trial

monomer

[M]/[I]

time (h)

Mn(theo)b

Mn(obs)c

PDIc

1

5 4 3 5

150 200 150 150

0.5 12 4 1

120K 600K 201K 296K

165K 663K 210K 267K

1.07 1.10 1.03 1.07

2 a

The polymerization was performed in THF at various temperatures. Theoretical molecular weight. cThe values measured by MALLS detector. b

containing a G2 ester dendron (5) was polymerized in a living manner at room temperature to give the first block (Scheme 2). After 30 min, the macromonomer 4 was added as the second block and the reaction temperature was increased to 50 °C to enhance the propagation rate. After 12 h, the ROMP was quenched with ethyl vinyl ether, and poly(5)-block-poly(4) was obtained with an Mn of 663K and a narrow PDI of 1.10. SEC analysis showed a complete shift of the trace to the highermolecular-weight region (Figure 4a). Next, by switching the order of the monomer addition, another diblock copolymer having a TD macromonomer containing the same G3 dendron (3) as the first block and the same norbornene macromonomer containing G2 dendron (5) as the second block, was attempted. The poly(3)-block-poly(5) was successfully synthesized with a narrow PDI of 1.07, and SEC analysis confirmed the “blocky” microstructure (Figure 4b). From the shifts in the SEC traces, a narrow PDI, below 1.10, and an absolute Mn closely matching the theoretical Mn, we concluded that the living polymerization method for preparing diblock copolymers was indeed successful. Because of the bulky side-chains, these fully dendronized diblock copolymers should be easily visualized by AFM imaging. The first AFM imaging of block copolymers was reported by Fréc het’s group, 6 a who prepared diblock copolymers by ROMP of norbornenes containing G2 ester dendron and G3 Fréchet dendron. Unfortunately, these images had a weakness, as the blocky nature was not clearly revealed, mainly because of the large polarity differences between two dendrons, polar ester dendrons and nonpolar benzyl ether dendrons. As a result, the block containing hydrophobic Fréchet-type dendron aggregated on the polar mica surface, while the other polar block showed extended conformations, thereby leading to tadpole-like nano-objects.6a Also, the size difference between the G2 and G3 dendrons might not be big enough to evidently distinguish each block. Our dendronized diblock copolymers should reveal their true blocky structure with higher resolution because both blocks contained polar ester dendrons and they adsorbed well onto the mica surface, giving a clear image of single chains, without any aggregation. Moreover, each block was differentiated by larger G4 dendron

Figure 3. AFM images of dendronized homopolymers (a) poly(1), (b) poly(2), (c) poly(3), and (d) poly(4) on a mica surface.17 5909


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Scheme 2. Synthesis of Diblock Copolymers

This large difference was a result not only of differences in the generation of the dendrons, but also of the intrinsically bulkier backbones of the TD monomers compared with those of norbornenes. The other diblock copolymer, poly(3)-blockpoly(5) was imaged by AFM to reveal a similar blocky morphology with high definition (Figure 5b). We then shifted our attention to another type of polymer architecture: gradient copolymers. The simplest method of synthesizing gradient copolymers (Table 3) is by batch copolymerization of two monomers with very different reactivities because two monomers are added at the same time for polymerization, whereas the semibatch method requires a syringe pump technique for continuous addition of comonomers. From the observation that the rates of ROMP for TD monomers were much slower than for norbornene monomers,8b gradient copolymerization under batch conditions was tested since both monomers undergo living ROMP. In a model study, two monomers of norbornene and TD,

Figure 4. SEC traces of (a) poly(5)-b-poly(4) and (b) poly(3)-bpoly(5).

and smaller G2 dendron. Importantly, the height difference was quite large, by a factor of 2 (1.0 nm vs 0.5 nm), so that clean images of a blocky morphology were visualized (Figure 5a).

Figure 5. AFM image of (a) poly(5)-b-poly(4) and (b) poly(3)-b-poly(5). 5910


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Table 3. Results of Gradient Copolymerization.a trial d

1

2

monomer

[M]/[I]

time (h)

Mn(theo)b

Mn(obs)c

PDIc

6 7 4 5

200 200 150 150

1

106K

101K

1.25

16

607K

613K

1.35

measured by MALLS, matched the theoretical values well and the PDIs were still relatively narrow (Scheme 3, trial 2). To understand this unexpected reactivity ratio, we measured the initiation rates (ki) of catalyst I for macromonomers 4 and 5 by monitoring the disappearance of the benzylidene signal via 1 H NMR. These experiments were conducted at −20 °C to obtain reliable ki values because of the intrinsically fast ki of third-generation Grubbs catalyst. First-order kinetics obviously showed that the ki of 4 was 5-fold larger than ki of 5 (0.0035 s−1 vs 0.0007 s−1). In other words, surprisingly, the catalyst could approach the seemingly more bulky TD macromonomer 4 faster than the norbornene macromonomer 5. This puzzled us at first, but when we considered the local steric hindrance around the active olefins, it became clear that the cyclobutene moiety of the TD monomers was more exposed to coordination by the catalyst than the olefins on the norbornenes, which contained an extra bridging methylene group, thereby hindering coordination. Hence, more accessible TD monomers could more easily coordinate the active site of the catalyst. Although the steric hindrance about the catalyst became larger after one turnover, the coordination preference for the TD monomer was maintained throughout the whole polymerization (Figure 7). From the experimental data, we speculated that the propagation rate of active carbenes containing ring-opened TD monomers was slower because of steric congestion around the propagating species, but coordination to the catalysts or propagating species was always favored for TD monomers over the norbornene derivatives, regardless of the bulkiness of the distal side-chains. Thus, the propagating species of TD and norbornene both preferred to react with TD monomers and, as a result, gradient copolymers were prepared with faster consumption of TD monomers. Prior to this work, there was one report on the visualization of gradient copolymers containing brush-type side-chains, prepared by postfunctionalization or the graft-from method to grow brush side-chains.11a Unfortunately, the resolution of the AFM images of those gradient brush copolymers was not high enough because the gradient copolymer comprised mixtures of brush polymer and flexible and small poly(methyl methacrylate) (PMMA). Although brush polymers showed extended conformations, allowing easy AFM imaging, the flexible PMMA adopted random coil conformations, making it

a

The polymerization was performed in THF at various temperatures in NMR tube. bTheoretical molecular weight. cThe molecular weight and PDI was measured by MALLS detector. dThe reaction was performed at room temperature.

containing the same hexyl side-chain (6 and 7 respectively) were simultaneously polymerized at room temperature for 40 min, and the monomer conversions were monitored by 1H NMR (Figure 6a). The copolymers were analyzed by MALLSSEC, showing Mn close to the theoretical Mn and a fairly narrow PDI. Indeed, the conversion profile showed the expected shape for gradient copolymerization. However, an unexpected result was observed, when the TD monomer (6) was consumed much faster than the norbornene monomer 7. To confirm the unexpected kinetics, the reactivity ratios of monomers 6 and 7 were measured using the Finemann−Ross plot method.18 As a result, the reactivity ratios were estimated to be r1 = 3.9 (6) and r2 = 0.1 (7); these values are in agreement with the commonly accepted values for gradient copolymerization: r1 ≫ 1, r2 ≪ 1. These results confirmed that the TD monomer was ironically more reactive than the norbornene monomer, even though it took a much longer time for ROMP. From our initial success in synthesizing gradient copolymers, the investigation was further expanded to the copolymerization of dendronized macromonomers 4 and 5, which were previously used to prepare diblock copolymers. Copolymerization was conducted at 50 °C and the monomer conversion profile for both 4 and 5 was monitored by 1H NMR. Again, the profiles of gradient microstructures were obtained with the TD macromonomer 4, which underwent ROMP faster than the norbornene macromonomer 5, just as in the previous model study. This was even more surprising because 4 contained much larger endo-G4 dendrons than 5, which contained exo-G2 dendron, so 4 should have been much less reactive. The absolute molecular-weights of the gradient copolymers,

Figure 6. Monomer conversion profiles for (a) poly(6)-grad-poly(7) and, (b) poly(4)-grad-poly(5). 5911


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Scheme 3. Synthesis of Gradient Copolymers

5.5 to 2.8 nm. This fully dendronized gradient copolymer adopted highly extended conformations, making AFM imaging easier. Also, it was prepared from two monomers containing dendrons of vastly different sizes by the macromonomer approach, and defect-free and linear-gradient copolymers were visualized by AFM with high resolution. With these high-resolution images of diblock and gradient copolymers prepared from the same two macromonomers (4 and 5), the structural differences between these two polymers was compared in detail by AFM. The gradient copolymer showed smooth and gradual changes in both height and thickness, without any interfacial boundary, whereas the diblock copolymer contained a clear junction, showing abrupt changes in both height and thickness (Figure 9). In addition, the chains of the gradient copolymer exhibited highly extended conformations without local regions of entangled random coil conformations. However, two distinct conformations were observed for the diblock copolymer because the block composed of small dendron (5) showed more entangled conformation, whereas the other block, composed of large dendron (4), showed extended conformations. With these different conformations of two types of copolymers, observed by AFM, we investigated their behavior in solution by conducting MALLS analysis to obtain the shape factor, ν. The ν value for the gradient copolymer was much

Figure 7. Schematic explanation for the steric on (a) norbornene and (b) TD during initiation.

much harder to get clear AFM images. Furthermore, the gradient brush copolymer was prepared by multiple postfunctionalization reactions and the inevitable defects, such as incomplete deprotection or initiation of ATRP, would prevent clear visualization. Here, we report AFM imaging of more welldefined dendronized gradient copolymers, prepared directly in a single step from larger G4 and smaller G2 dendronized macromonomers. Figure 8 shows one-dimensional nanostructures exhibiting a gradual decrease in not only the heights, from 1 to 0.4 nm, but also a gradual decrease in the thickness, from

Figure 8. AFM images of gradient copolymers. 5912


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Figure 9. Structural difference between single chains of block copolymer and gradient copolymer.

faster because of the more reactive cyclobutene moiety. Detailed microstructures of both copolymers were visualized by high-resolution AFM. Accurate imaging was possible because both dendronized polymers were large enough to show clear AFM images, and the size differences between the two dendrons were also large enough to differentiate their microstructures. Furthermore, an interesting contrast between the structures of diblock and gradient copolymers was observed, as the high-resolution images distinguished both types of polymers from the presence of a boundary for each block or gradual changes in height and thickness. Last, conformational analysis in solution indicated that the gradient copolymers were more rigid than the corresponding diblock copolymers.

greater than that for the diblock copolymer (0.82 vs 0.60, Figure 9) but these values were obviously much lower than the value for the homopolymer (4), 0.99. This confirmed that the dendronized gradient copolymer was much stiffer than the dendronized diblock copolymer in solution as well as in the solid state, because the highly rigid macromonomer 4 was incorporated throughout the whole copolymer, thereby stretching the chains, whereas half of the polymer was relatively flexible for the diblock copolymer, thereby lowering the rigidity of the whole copolymer. This was a noteworthy observation because even though these diblock and gradient copolymers were prepared from the same monomers, their conformations and rigidities were very different as a result of different monomer sequences.

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CONCLUSION We synthesized new dendronized polymers from TD monomers by the macromonomer approach, using ROMP. The polymers had high molecular-weights and narrow PDIs, as expected for living polymerization. Also, these polymers showed rod-like conformations both in solution and in the solid state, as determined by MALLS and AFM analysis. Highly rod-like conformations were observed even for polymers containing relatively small G3 dendron, indicating that these dendronized polymers prepared from TD monomers were much more rigid than the dendronized polymers prepared previously from norbornene derivatives. We attributed this superior rigidity to more densely packed dendrons, fewer carbons between the repeat units, and a rigid tricyclic backbone. Controlling the conformation using low-generation dendrons was advantageous because their synthesis and purification was easier. Also, diblock copolymers and gradient copolymers were easily synthesized from dendronized macromonomers based on norbornene and TD moieties. The sequential addition of the two monomers produced diblock copolymers, and simultaneous reaction produced gradient copolymers as a result of the large differences in reactivity. TD monomers were consumed

ASSOCIATED CONTENT

* Supporting Information S

Experimental details, synthesis, characterization data, and spectra of the compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (T.-L.C.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the financial support from Basic Science Research Program, BRL and Nano-Material Technology Development Program through the National Research Foundation of Korea. We thank NCIRF at SNU for supporting HRMS experiments.

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REFERENCES

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Synthesis of Dendronized Polymers via Macromonomer Approach by

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