Efficient Divergent Synthesis of Dendronized Polymers with Extremely

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Efficient Divergent Synthesis of Dendronized Polymers with Extremely High Molecular Weight: Structural Characterization by SEC-MALLS and SFM and Novel Organic Gelation Behavior Masaru Yoshida,†,‡ Zachary M. Fresco,† Satomi Ohnishi,§,| and Jean M. J. Fre´ chet*,† Department of Chemistry, University of California, Berkeley, California 94720-1460; Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200 Australia; and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higushi, Tsukuba, Ibaraki 305-8565, Japan Received June 25, 2004; Revised Manuscript Received September 23, 2004

ABSTRACT: High molecular weight poly(p-hydroxystyrene) (PHS) was dendronized in high yield by the divergent grafting of aliphatic polyester dendrons using acetal and ketal protected anhydride derivatives of bis(hydroxymethyl)propionic acid. Relatively low polydispersities (PDI ) 1.1-1.3) were maintained throughout the dendronization process. Detailed characterization of a series of these polymers by size exclusion chromatography (SEC) and SEC with multi-angle laser light scattering (SEC-MALLS) clearly shows the effect of dendron steric bulk on the persistence length (ξ) of the polymer main chain in solution. As a result of the fixed degree of polymerization (DP), the main chain rigidification that accompanies lateral dendron growth causes an unusual relationship between Rg,z and molecular weight across a series of generations. The rodlike structure and contour length of the alkylated dendronized polymers was also characterized by scanning force microscopy (SFM) on mica, highly oriented pyrolytic graphite (HOPG), and molybdenum disulfide (MoS2) surfaces. One of the G3 dendronized polymers with an alkylated periphery showed significant physical gelation behavior at concentrations below 1 wt % in several organic solvents, which constitutes additional evidence that the solution phase rigidity of a polymer chain can be enhanced by dendronization.

Introduction Dendronized polymers constitute a unique class of synthetic macromolecules because of the high degree of control that can be exerted over their size and shape as well as the ability to place a wide variety of chemical functionalities at spatially defined locations such as the chain end, backbone, branch points, and periphery.1,2 Because of the significant steric demands of the dendron substituents on the polymer backbone, stiff and rodlike polymers can be produced from random-coil polymer precursors. In the extreme case of a high molecular weight (Mw > 105 Da) polymer dendronized to a high generation, a single polymer molecule can be visualized by scanning force microscopy (SFM).3-7 The advantages of a visible single molecule are only just beginning to be explored. Thus far, SFM has been used not only to unequivocally determine the contour length of a dendronized polymer but also to physically manipulate the conformation of this rodlike molecule.8,9 By incorporating appropriate functionality at the periphery, a photochemical coupling reaction can lead to covalent coupling of two individual dendronized polymers.10 Clearly, dendronized polymers are a promising scaffold for the construction of nanometer scale structures. Therefore, we undertook the preparation and detailed structural * Corresponding author. E-mail: [email protected]. † University of California, Berkeley. § Australian National University. | National Institute of Advanced Industrial Science and Technology. ‡ Permanent address: Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 3058565, Japan.

characterization of a well-defined series of these unique macromolecules. The synthesis of high molecular weight (Mw > 106 Da) dendronized polymers with narrow polydispersities has not previously been accomplished and presents significant synthetic challenges. There are three predominant methods for the synthesis of dendronized polymers. These are macromonomer polymerization,3-7,11-16 convergent coupling of dendrons onto a polymer backbone,17,18 and divergent grafting of dendrons from a polymer chain.8,10,15,19,20 The macromonomer approach necessarily produces structurally perfect materials, but the steric bulk of the dendrons makes controlled polymerization to high molecular weight difficult if not impossible. Preforming the polymer backbone allows for excellent control over molecular weight and polydispersity, but dendronization often produces materials with significant structural imperfections. In most instances, the convergent coupling of large preformed dendrons cannot be driven to completion for steric reasons. However, divergent dendrimer growth from a preformed polymer has the potential to produce materials with few structural defects, if the dendronization chemistry is highly efficient and until steric issues restrict the efficiency of further growth. All dendronized polymers previously reported to have molecular weight on the order of 106 Da have polydispersities that are usually greater than two as measured by size exclusion chromatography (SEC) or SEC with multi-angle laser light scattering (SEC-MALLS).4-6,12 Only ring-opening metathesis polymerization (ROMP) of a macromonomer gave high molecular weight dendronized polymer derivatives with a moderate polydis-

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Macromolecules, Vol. 38, No. 2, 2005

persity (Mw > 106 Da PDI ) 1.3-1.4, as determined by SEC-MALLS).7 While a recent study demonstrated the successful synthesis of dendronized polymers with a very narrow polydispersity (PDI ) ca. 1.1) by atom transfer radical polymerization (ATRP) of a macromonomer, the molecular weight was limited to approximately 104 Da.14-16 Recently, we have reported a novel and highly efficient synthesis of a dendronized polymer via the divergent growth of aliphatic polyester dendrons from a poly(p-hydroxystyrene) (PHS) core.21 Esterification with the benzylidene protected anhydride derivative of bis(hydroxymethyl)propionic acid was utilized for the divergent dendronization due to the technically facile growth and purification procedure as well as the ease of deprotection by palladium-catalyzed hydrogenolysis. The molecular weights of the dendronized polymers as determined by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were very close to the values calculated from the molecular weight of the fully functionalized monomer unit and the degree of the polymerization (DP) of the original PHS polymer. These results clearly highlighted the very high coupling efficiency achieved with this monomer in the divergent grafting-from approach. This dendronization technique has also been used in our group for the preparation of a dendronized cyclopolymer with a radial polarity gradient, providing an appropriate nanoenvironment to catalyze difficult esterifications22 and for the synthesis of bow-tie dendrimer hybrids for drug delivery.23 Herein, we report the divergent synthesis of a series of dendronized polymers with extremely high molecular weight (Mw > 106 Da after dendronization) and moderate polydispersity indices (PDI ) 1.1-1.3). Dendronized polymers bearing G1 through G5 dendrons have been prepared. Experimental Procedures Materials. Poly(p-hydroxystyrene), 1 (Mw ) 153 kDa, PDI ) 1.05 by MALLS) was a gift of the Nippon Soda Company (Chiba, Japan). Benzylidene- and isopropylidene-2,2-bis(oxymethyl)propionic anhydrides, 2a42 and 2b,15,22-25 respectively, were prepared by literature procedures. Pyridine was distilled under nitrogen from calcium hydride immediately prior to use. Tetrahydrofuran (THF) was distilled under nitrogen from sodium/benzophenone ketyl immediately prior to use. Other solvents were reagent grade (99.9%), used from freshly opened bottles. All other reagents were obtained commercially and used without further purification unless otherwise noted. Characterization. All 1H NMR spectra were measured with a Bruker AMX-300 spectrometer at 300 MHz in CDCl3 (containing 0.03% TMS as an internal standard), DMSO-d6, or methanol-d4. The solvent signal was used as the internal standard for the latter solvents. Elemental analyses were performed on a Perkin-Elmer 2400 Series II combustion analyzer. IR spectra were measured with Mattson Genesis II FTIR spectrometer. DSC measurements were conducted on a SEIKO DSC6000 differential scanning calorimeter under a nitrogen atmosphere. For gel samples, the heating and cooling rate was 2 °C/min. For neat polymer samples, the heating and cooling rate was 5 °C/min. Size exclusion chromatography (SEC) in DMF containing 0.2 wt % LiBr as an eluent was carried out using a system composed of a Waters 510 pump with a U6K injector and two mixed-bed C (7.8 × 300 mm) columns thermostated at 70 °C with Waters 410 differential refractometer thermostated at 35 °C. The flow rate was 1.0 mL/min. The SEC data were analyzed using Empower software (Waters) based on poly(methyl methacrylate) (PMMA) standards. SEC with THF eluent was carried out using a

Divergent Synthesis of Dendronized Polymers 335 system composed of a Waters 510 pump with a Waters 717 auto sampler, a series of 105, 103, and 500 Å PLgel (7.8 × 300 mm) columns thermostated at 35 °C, and a Optilab DSP differential refractometer thermostated at 35 °C. The flow rate was 1.0 mL/min. The SEC data were analyzed using Empower software (Waters) based on polystyrene standards. SEC-multiangle laser light scattering (MALLS) measurement with DMF containing 0.2 wt % LiBr as the mobile phase was performed using a system composed of a Waters 510 pump, Rheodyne injector with 50 uL sample loop, two PLgel mixed-bed C (7.8 × 300 mm) columns thermostated at 70 °C, Dawn EOS MALLS, and Optilab DSP differential refractometer thermostated at 35 °C. Flow rate was 0.8 mL/min. Injection volume was calibrated by a known concentration of a polystyrene standard in toluene to obtain a very accurate mass of injection. SEC-MALLS with toluene as a mobile phase was carried out using the same instrumental conditions stated previously, except for two PLgel mixed-bed B (7.8 × 300 mm) columns thermostated at 35 °C. The light source was GaAs laser with 690 nm wavelength and 30 mW output power at 685 nm. Scattering signals were collected at 17 different angles from 20 to 153°. The data were analyzed using ASTRA for Windows software (Ver. 4.90.07, Wyatt Technology). Zimm plot (K*c/R) was used for the detector fitting method. Absolute molecular weight of the dendronized polymer was determined using the MALLS data and two additional parameters, known RI detector calibration constant and an assumption of 100% mass recovery of the injected polymer sample. For both SEC and SEC-MALLS, typically 1-2 mg/mL of polymer solution was pre-filtered through a 0.2 µm pore size PTFE filters (Whatman) before injection. Scanning force microscopy (SFM) images were obtained by Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) operated in tapping mode. Silicon cantilevers with a spring constant of ca. 24 N/m, a tip radius of ca. 5-10 nm, and a resonance frequency of ca. 300 kHz were used. Dendronized polymer was deposited onto the surface by spin coating from a dilute solution of chloroform (c ) 0.01-0.001 mg/mL). Spin speeds were 5000 rpm for mica, 3000 rpm for HOPG, and 3500 rpm for MoS2 surfaces. General Procedure for the Anhydride Esterification of the Hydroxy-Substituted Polymer. As described in a preliminary communication, the polymer sample (0.870 g, 7.25 mmol OH) of poly(p-hydroxystyrene) 1 was added to a roundbottom flask equipped with a magnetic stir bar and dissolved in 20 mL of dry pyridine. To the flask was then added (4.63 g, 10.9 mmol (1.5 equiv per OH)) benzylidene-protected anhydride monomer 2a and (0.177 g, 1.45 mmol (0.2 equiv per OH)) 4-(N,N-dimethylamino)pyridine (DMAP). The flask was stoppered, and the solution was allowed to stir overnight. The crude reaction mixture was precipitated into methanol to afford a white powder. The polymer was recovered by filtration through a glass frit and dried under vacuum to afford G1dendronized polymer 3 as a white powder (2.34 g, 99%). Polymers 5, 7, 9, and 11 were prepared in a similar manner using the appropriate anhydride, 2a or 2b. For the isopropylidene protected dendronized polymers 7, 9, and 11, a solvent mixture of hexane/isopropyl alcohol (10:1) was used for precipitation. General Procedure for the Acid-Catalyzed Deprotection of the Alkylidene Protecting Group. The benzylidene protected polymer sample, 3 (2.24 g, 6.91 mmol of benzylidene), was added to a round-bottom flask equipped with a magnetic stir bar and dissolved in 300 mL of THF/methanol, 4:3 v/v. Concentrated sulfuric acid was added to the flask for a final concentration of 2% v/v (pH 3). The flask was stoppered, and the solution was allowed to stir overnight. The sulfuric acid was neutralized with ammonia in methanol, resulting in the precipitation of ammonium sulfate as a white solid. The precipitate was removed by filtration, and the solution was concentrated in vacuo. Precipitation of the resulting viscous liquid into 500 mL of water resulted in a white powder that was recovered by filtration. Because of the equilibrium between the benzaldehyde (or benzaldehyde dimethyl acetal) generated as a deprotection byproduct and the benzylidene protected species, a small amount (3-4%) of benzylidene

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groups frequently remained after the first deprotection reaction. Therefore, a repetition of the acid-catalyzed deprotection reaction described previously was necessary to achieve complete deprotection of the benzylidene group. Reprecipitation into water after the second deprotection reaction gave polymer 4 as a white solid (1.59 g, 98%). Water-soluble dendronized polymers, 8, 10, and 12, were purified by dialysis against distilled water using Spectra/Por membrane tubing with a MW cutoff of 8 kDa. The resulting aqueous polymer solution was lyophilized to give the product as a white solid. PHS-150k-OH (1). 1H NMR (DMSO-d6, 300 MHz) δ ) 1.12.2 (br, 3H), 6.1-6.9 (br, 4H), 8.9-9.1 (br, 1H) ppm. SEC (DMF eluent, PMMA standard), Mw ) 1.75 × 105 Da, PDI ) 1.14. SEC-MALLS (DMF eluent), Mw ) 1.53 × 105 Da, PDI ) 1.05, dn/dc ) 0.154 cm3g-1. PHS-150k-[G1]-Ph (3). 99% yield; 1H NMR (CDCl3, δ, ppm, TMS) 0.9-1.1 (br, 3H), 1.1-2.2 (br, 3H), 3.4-3.6 (br, 2H), 4.54.7 (br, 2H), 5.3-5.5 (br, 1H), 6.2-6.9 (br, 4H), 7.2-7.3 (br, 3H), 7.3-7.5 (br, 2H). IR (cm-1, NaCl) 1740 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 1.77 × 105 Da, PDI ) 1.14. SEC-MALLS (DMF eluent), Mw ) 3.79 × 105 Da, PDI ) 1.05, dn/dc ) 0.113 cm3g-1. Elemental analysis calcd (%) for (C20H20O4)n C 74.06, H 6.21; found C 74.14, H 6.17. PHS-150k-[G1]-(OH)2 (4). 98% yield; 1H NMR (DMSO-d6, δ, ppm) 1.1-1.2 (br, 3H), 1.2-2.2 (br, 3H), 3.5-3.7 (br, 4H), 4.8-4.9 (br, 2H), 6.2-7.0 (br, 4H). IR (cm-1, NaCl) 3400 (broad, OH stretching), 1740 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 2.63 × 105 Da, PDI ) 1.13. SEC-MALLS (DMF eluent), Mw ) 2.86 × 105 Da, PDI ) 1.06, dn/dc ) 0.108 cm3g-1. Elemental analysis calcd (%) for (C13H16O4)n C 66.09, H 6.83; found C 64.56, H 6.95. PHS-150k-[G2]-Ph2 (5). 97% yield; 1H NMR (CDCl3, δ, ppm, TMS) 0.7-0.9 (br, 3H), 1.1-2.2 (br, 3H), 1.2-1.4 (br, 3H), 3.4-3.6 (br, 4H), 4.2-4.7 (br, 4H), 5.3-5.4 (br, 2H), 6.0-6.9 (br, 4H), 7.1-7.3 (br, 3H), 7.3-7.5 (br, 2H). IR (cm-1, NaCl) 1741 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 2.77 × 105 Da, PDI ) 1.13. SEC-MALLS (DMF eluent), Mw ) 7.52 × 105 Da, PDI ) 1.06, dn/dc ) 0.094 cm3g-1. Elemental analysis calcd (%) for (C37H40O10)n C 68.93, H 6.25; found C 69.11, H 6.40. PHS-150k-[G2]-(OH)4 (6). 92% yield; 1H NMR (DMSO-d6, δ, ppm) 1.0-1.2 (br, 6H), 1.2-2.2 (br, 3H), 1.3-1.4 (br, 3H), 3.4-3.7 (br, 8H), 4.1-4.4 (br, 4H), 4.6-4.8 (br, 4H), 6.2-7.0 (br, 4H). IR (cm-1, NaCl) 3400 (broad, OH stretching), 1731 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 3.49 × 105 Da, PDI ) 1.15. SEC-MALLS (DMF eluent), Mw ) 5.67 × 105 Da, PDI ) 1.06, dn/dc ) 0.082 cm3g-1. Elemental analysis calcd (%) for (C23H32O10)n C 58.97, H 6.88; found C 58.93, H 6.85. PHS-150k-[G3]-Acetonide4 (7). 92% yield; 1H NMR (CDCl3, δ, ppm, TMS) 0.7-2.2 (br, ∼48H), 3.3-3.8 (br, ∼8H), 3.9-4.9 (br, ∼20H), 6.0-6.9 (br, ∼4H), IR (cm-1, NaCl) 1740 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 3.85 × 105 Da, PDI ) 1.17. SEC-MALLS (DMF eluent), Mw ) 1.48 × 106 Da, PDI ) 1.10, dn/dc ) 0.048 cm3g-1. Elemental analysis calcd (%) for (C55H80O22)n C 60.43, H 7.38; found C 60.16, H 7.43. PHS-150k-[G3]-(OH)8 (8). 86% yield; 1H NMR (DMSO-d6, δ, ppm) 0.7-1.7 (br, ∼24H), 3.4-3.7 (br, ∼16H), 3.8-4.4 (br, ∼12H), 4.4-4.7 (br, ∼8H), 6.2-7.1 (br, ∼4H). IR (cm-1, NaCl) 3400 (broad, OH stretching), 1732 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 4.54 × 105 Da, PDI ) 1.14. SEC-MALLS (DMF eluent), Mw ) 1.12 × 106 Da, PDI ) 1.06, dn/dc ) 0.066 cm3g-1. Elemental analysis calcd (%) for (C43H64O22)n C 55.36, H 6.91; found C 55.14, H 7.13. PHS-150k-[G4]-Acetonide8 (9). 80% yield; 1H NMR (CDCl3, δ, ppm, TMS) 0.2-2.0 (br, ∼96H), 2.6-5.5 (br, ∼60H), 6.26.9 (br, ∼4H), IR (cm-1, NaCl) 3420 (broad, weak peak, OH stretching), 1737 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 3.82 × 105 Da, PDI ) 1.18. SEC-MALLS (DMF eluent), Mw ) 2.60 × 106 Da, PDI ) 1.06, dn/dc ) 0.043 cm3g-1. Elemental analysis calcd (%) for (C107H160O46)n C 55.36, H 6.91; found C 55.14, H 7.13. PHS-150k-[G4]-(OH)16 (10). 83% yield; 1H NMR (DMSOd6, δ, ppm) 0.7-2.1 (br, ∼48H), 3.3-3.8 (br, ∼32H), 3.8-5.1 (br, ∼28H), 6.2-7.0 (br, ∼4H). IR (cm-1, NaCl) 3400 (broad,

Macromolecules, Vol. 38, No. 2, 2005 OH stretching), 1730 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 5.58 × 105 Da, PDI ) 1.24. SEC-MALLS (DMF eluent), Mw ) 2.36 × 106 Da, PDI ) 1.06, dn/dc ) 0.056 cm3g-1. Elemental analysis calcd (%) for (C83H128O46)n C 53.54, H 6.93; found C 51.71, H 7.12. PHS-150k-[G5]-Acetonide16 (11). 86% yield; 1H NMR (CDCl3, δ, ppm, TMS) 0.2-2.0 (br, ∼192H), 2.6-5.5 (br, ∼124H), 6.2-6.9 (br, ∼4H). IR (cm-1, NaCl) 3420 (broad, weak peak, OH stretching), 1737 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 5.76 × 105 Da, PDI ) 1.19. SECMALLS (DMF eluent), Mw ) 5.01 × 106 Da, PDI ) 1.04, dn/dc ) 0.037 cm3g-1. Elemental analysis calcd (%) for (C211H320O94)n C 58.12, H 7.40; found C 57.78, H 7.43. PHS-150k-[G5]-(OH)32 (12). 79% yield; 1H NMR (MeOHd4, δ, ppm) 0.7-2.1 (br, ∼96H), 3.3-3.8 (br, ∼64H), 3.8-5.1 (br, ∼60H), 6.2-7.0 (br, ∼4H). IR (cm-1, NaCl) 3400 (broad, OH stretching), 1731 (carbonyl). SEC (DMF eluent, PMMA standard), Mw ) 6.66 × 105 Da, PDI ) 1.30. SEC-MALLS (DMF eluent), Mw ) 4.63 × 106 Da, PDI ) 1.04, dn/dc ) 0.048 cm3g-1. Elemental analysis calcd (%) for (C163H256O94)n C 52.63, H 6.94; found C 50.86, H 6.97. PHS-150k-[G3]-Stearoyl8 (13). This polymer was prepared by esterification of 8 with stearic anhydride. 89% yield. 1H NMR (CDCl3, δ, ppm, TMS) 0.4-2.5 (br, ∼280H), 3.9-4.4 (br, ∼28H), 6.0-7.0 (br, ∼4H), IR (cm-1, NaCl) 2918, 2850, 1740 (carbonyl). SEC-MALLS (toluene eluent), Mw ) 4.04 × 106 Da, PDI ) 1.21, dn/dc ) -0.009 cm3g-1. Elemental analysis calcd (%) for (C187H336O30)n C 73.29, H 11.05; found C 73.31, H 11.14. PHS-150k-[G3]-Myristoyl8 (14). This polymer was prepared by esterification of 8 with myristic anhydride. 83% yield. 1 H NMR (CDCl3, δ, ppm, TMS) 0.8-0.9 (br, ∼24H), 1.0-1.8 (br, ∼176H), 2.1-2.4 (br, ∼16H), 3.9-4.4 (br, ∼28H), 6.1-6.9 (br, ∼4H), IR (cm-1, NaCl) 2924, 2853, 1743 (carbonyl). SECMALLS (toluene eluent), Mw ) 3.28 × 106 Da, PDI ) 1.08, dn/dc ) -0.011 cm3g-1. Elemental analysis calcd (%) for (C155H272O30)n C 71.17, H 10.48; found C 70.40, H 10.91. PHS-150k-[G2]-Stearoyl4 (15). This polymer was prepared by esterification of 6 with stearic anhydride. 95% yield. 1H NMR (CDCl3, δ, ppm, TMS) 0.8-0.9 (br, ∼12H), 1.0-2.4 (br, ∼130H), 3.9-4.7 (br, ∼12H), 6.0-6.9 (br, ∼4H), IR (cm-1, NaCl) 2918, 2850, 1740 (carbonyl). SEC (THF eluent, polystyrene standard), Mw ) 6.70 × 105 Da, PDI ) 1.23. SECMALLS (THF eluent), Mw ) 2.08 × 106 Da, PDI ) 1.11, dn/dc ) 0.059 cm3g-1. Elemental analysis calcd (%) for (C95H168O14)n C 74.37, H 11.04; found C 74.89, H 11.37.

Results and Discussion Synthesis of Dendronized Polymer. As described in our preliminary communication,21 poly(p-hydroxystyrene) was used as a linear polymer precursor due to the convenient presence of phenolic hydroxyl groups for the attachment of dendron substituents. In this study, the commercially available PHS precursor we selected had a much higher molecular weight (Mw ) 153 kDa by MALLS) than the material used in our previous study. While this commercial precursor originally contained a small and inseparable high molecular weight fraction, a narrow PDI of 1.14 was still observed by SEC with PMMA standards. The high molecular weight of this polymer facilitates a more detailed analysis of the effect of dendron steric bulk on the long-range stiffness of the main chain. Dendronization from the PHS core to the G5 polymer is shown in Scheme 1. Although the basic strategy of repetitive esterification and deprotection is the same as that described in our preliminary report, important modifications were made in the current synthetic approach. One key difference is the deprotection method used for the benzylidene acetals. While palladium-catalyzed hydrogenolysis cleanly deprotects the benzylidene groups of the low molecular weight dendronized polymers, we have found that the


Divergent Synthesis of Dendronized Polymers 337

Scheme 1. Divergent Dendronization from Poly(p-hydroxystyrene) to G5 Alcohol

reaction leads to a bimodal molecular weight distribution and significant molecular weight broadening if the polymer has high molecular weight and contains a large number of benzylidene moieties. We assume that this is caused by a small amount of trans-acetalization and hydrogenation that occurs during hydrogenolysis. Therefore, homogeneous acid-catalyzed benzylidene deprotection was used in place of hydrogenolysis for the present work. A weakly acidic (pH 3) mixture of methanol and THF containing a small amount of sulfuric acid was used for the acid-catalyzed deprotection reaction. Using these deprotection conditions, a small amount (3-4%) of the benzylidene groups tend to remain even after long reaction times (2-3 days). Therefore, repeating the acid-catalyzed deprotection after removal of benzaldehyde and benzaldehyde dimethyl acetal derivatives from the reaction solution proved critical for achieving complete deprotection. The products, 4 and 6, can be isolated after deprotection in nearly quantitative yields by simple precipitation into water followed by filtration.

The second key modification in our synthetic strategy involved the use of the less crystalline and less hydrophobic isopropylidene ketal protecting group for higher generation materials.15,22-25 Although the high crystallinity of the benzylidene acetal protecting group helped to facilitate isolation of the G1 and G2 dendronized polymers, 3 and 5, by simple precipitation, it leads to poorly soluble products at higher dendron generations, especially for the G4 polymer. The synthesis of G3, G4, and G5 polymers, 7, 9, and 11, using isopropylidene ketal protected bis(hydroxymethyl)propionic anhydride, 2b, proceeded very smoothly. However, the IR spectra of G4 and G5 polymers, 9 and 11, showed a weak absorption in the 3400 cm-1 region, recognizable as an OH-stretching band. In our previous study, we observed that quantitative esterification of a low molecular weight G3-OH polymer with a 5 kDa core using a benzylidene protected anhydride was fairly difficult.21 Despite these difficulties, the structural imperfections of the polymers are relatively insignificant based on the molecular weight determinations described in the next

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Figure 1. SEC profiles of hydroxylated dendronized polymers (G1-G5: 4, 6, 8, 10, and 12 including PHS precursor, 1). DMF eluent. Normalized by peak area. A small amount of inseparable high molecular weight material was observed in the SEC of the commercial PHS sample. Because of the nonlinear relationship between molecular weight and retention time, this high molecular weight material is less apparent in the SEC profile of polymers with larger dendrons.

section. Polyhydroxylated polymers, 8, 10, and 12, are sparingly soluble in water and therefore can be purified by dialysis of a dilute aqueous solution against distilled water. All of the dendronized polymers discussed in this work were isolated by precipitation or dialysis without the need for any chromatographic procedures. The overall yield for the 10 steps from PHS (1) to 12 was 31%, which demonstrates the high efficiency of the present divergent dendronization method. Molecular Weight Determination by SEC and SEC-MALLS. In this study, SEC-MALLS was used for absolute molecular weight determination because of the extremely high molecular weights (105∼106 Da) of these polymers. Because of their intrinsically large molecular size (Rg,z > 10 nm), we have found that the present dendronized polymers, including PHS precursor, are suitable for analysis by laser light scattering. SEC using THF as an eluent works well for benzylidene protected G1 and G2 polymers, 3 and 5. However, it was found that protected polymers with dendrons above G3 showed a significant broadening of the SEC profile under the same measurement conditions. Similar broadening of the SEC profile has been observed for several high molecular weight dendronized polymers4 and densely grafted polymer brushes.26 This anomalous broadening of the SEC profile for extremely high molecular weight dendronized polymers indicates the retardation of these polymers by the SEC columns and may result from affinity of these polymers for the column packing material. Furthermore, none of the hydroxylated polymers could be characterized by SEC using THF as the solvent due to their poor solubility. However, when SEC of these materials was performed in DMF, reasonably narrow peaks were obtained as shown in Figure 1. The observed weight average molecular weights (Mw, PMMA standard) and polydispersity indices (PDI) obtained by SEC are summarized in Table 1. In a series


of hydroxyl-substituted dendronized polymers of increasing generation number (1, 4, 6, 8, 10, and 12), a continuous increase in molecular weight is observed after each dendronization step as shown in Figure 1. Up to G3 (polymer 8) PDI was not affected by the dendronization and remained essentially constant. A slight increase of PDI up to an ultimate value of 1.3 due to a tailing of the SEC peaks was observed for 10 and 12 even when using DMF as an eluent. We attribute this broadening to affinity of the packing material for the large dendrons as observed in our own SEC using THF as an eluent and in previous studies.4,26 The effect is attenuated relative to THF but it is still visible. The large discrepancy between the SEC derived molecular weight values and the theoretical values (Table 1) shows the limitations of SEC for the characterization of these materials with their distinctive and unusual architecture. This is especially true for the high molecular weight, high generation derivatives where the observed molecular weights were significantly smaller than the expected values. For example, the Mw of 12 calculated from SEC data was 6.66 × 105 Da, which is almost 7 times smaller than the theoretical value of 4.76 × 106 Da. This molecular weight underestimation observed by SEC is quite large even when the influence of the poly(methyl methacrylate) (PMMA) standard is taken into account. In addition, SEC could not reproduce the real mass gain and loss behavior caused by the esterification and deprotection cycles. The apparent molecular weight of the hydroxylated polymers estimated by SEC always increased after the deprotection reaction, due to an increase in the hydrodynamic volume. The apparent increase in hydrodynamic volume with deprotection is attributed to better solvation of the OH terminated polymers by the DMF eluent used for SEC measurements. As expected, SEC-MALLS in DMF provided molecular weight data that was much more consistent with the theoretical values, as shown in Table 1. The real gain and loss of molecular weight during dendronization was accurately reflected by the MALLS data. The average DPw value calculated from MALLS data for polymers 3-12 was 1220, an error of only 5% when compared to that of the original PHS core. The largest deviation was observed for G5 polymer 11, which has a 10% mass shortage based on the theoretical value for 11. The mass discrepancy of 11 is consistent with the analysis of its IR spectrum described in a previous section. This deviation was comparable to the 8% deviation observed in our previous study for a G4 polymer with a 5 kDa core bearing benzylidene protecting groups.21 Taking into account the smaller, 3% deviation observed for hydroxylated G5 polymer 12, which was prepared from 11, and the unavoidable experimental error in MALLS measurement, we conclude that the present divergent dendronization method is quite effective up to the fifth generation for the high molecular weight PHS used in this study. Solution Structure of the Dendronized Polymers. There have been several studies on the conformation of dendronized polymers in solution using MALLS and small angle neutron scattering (SANS).4,14,19 Although these studies provide evidence for the stiffening effect of dendronization on polymer conformation, there have been no systematic studies on the structures of a series of dendronized polymers containing different dendron generations and identical backbone lengths. All



Table 1. Characterization and Conformational Parameters of Dendronized Polymers Determined by Size Exclusion Chromatography (SEC) and Multi-angle Laser Light Scattering (MALLS) Measurement SEC (DMF)a

SEC-MALLS (DMF)

compound

theor. Mw (DPw ) 1280)

Mw

PDI

Mw

PDI

DPwc

Rg,z (nm)

ξd (nm)

PHS-150K-O H (1) G1-Ph (3) G1-OH2 (4) G2-Ph2 (5) G2-OH4 (6) G3-acetonide4 (7) G3-OH8 (8) G4-acetonide8 (9) G4-OH16 (10) G5-acetonide16 (11) G5-OH32 (12)

153 000 414 000 302 000 823 000 598 000 1 400 000 1 190 000 2 790 000 2 380 000 5 580 000 4 760 000

175 000 177 000 263 000 277 000 349 000 385 000 454 000 382 000 558 000 576 000 666 000

1.14 1.14 1.13 1.13 1.15 1.17 1.14 1.18 1.24 1.19 1.30

153 000 379 000 286 000 752 000 567 000 1 480 000 1 120 000 2 600 000 2 360 000 5 010 000 4 630 000

1.05 1.05 1.06 1.06 1.08 1.10 1.06 1.06 1.06 1.04 1.04

1 280 1 170 1 210 1 170 1 210 1 360 1 200 1 190 1 260 1 150 1 240

15.1 16.7 18.3 21.0 23.6 26.0 27.6 27.6 32.0 32.0 36.0

2.1 2.6 3.1 4.1 5.2 6.3 7.1 7.1 9.6 9.6 12.2

b

a PMMA standard was used. b Based on the SEC-MALLS data of 1. c Weight-averaged degree of polymerization. calculated by eq 4 using the theoretical contour length, Lw ) 320 nm.

Figure 2. Plot of log Rg,z vs log Mw for dendronized polymers. Hydroxylated polymers (1, 4, 6, 8, 10, and 12); protected polymers (3, 5, 7, 9, and 11). Errors were calculated from a combination of baseline noise assessment and the quality of the fit in the Debye plot.

dendronized polymers used in this study, including the PHS precursor, are large enough to be observed by MALLS, as described previously. Therefore, they are excellent candidates for such a systematic study. The data obtained from MALLS provides statistical information about the conformation of dendronized polymers in solution. In general, the polymer conformation is correlated to the exponent ν in the power dependence of the following equation relating z-average root-mean-square (RMS) radius (Rg,z) to the weightaverage molecular weight (Mw)

Rg,z ∝ Mwν

(1)

An increase in Rg,z values with dendron generation is clearly observed in the MALLS data shown in Table 1. For example, the Rg,z values gradually increase from 15.1 nm for 1 to 36.0 nm for 12 in a series of hydroxylated polymers. This 2.4-fold increase in Rg,z results from the enhanced stiffness of the polymer backbone. The log Rg,z vs log Mw plots for both the polar (hydroxylsubstituted) and less polar (protecting group-substituted) dendronized polymers are shown in Figure 2. The slopes on this graph, which correspond to the power component ν in eq 1, were found to be 0.31 for the polar polymers 1, 4, 6, and 8 and 0.33 for the less polar polymers 3, 5, and 7. The other polymers with larger

d

Persistence length

G4 and G5 dendrons showed much smaller slope values (ν ) 0.17 for 9 and 11; ν ) 0.19 for 10 and 12), indicating that very small gains in the RMS radius were induced by the latter dendronization steps. We believe that the close correspondence between the experimental slopes of 0.31 and 0.33 and the theoretical slope of 1/3 for a spherical molecule is strictly coincidental as it is extremely unlikely that these linear dendronized polymers would in fact adopt a spherical shape in solution. The significant deviation of these experimental values from the theoretical value for a rigid rod polymer (ν ) 1) can be explained by considering that the classical model used to analyze ν assumes that the form of the polymer remains constant throughout the mass increase. In the case of a classical rigid rod polymer (one in which the backbone in inherently rigid and growth occurred from the chain ends (Figure 3a)), mass increases occurs one-dimensionally, so there is a linear increase in Rg,z relative to the mass increase of the polymer, resulting in a slope of unity (ν ) 1). In the hypothetical case of a flat disk growing outward in two dimensions (Figure 3b), one would expect Rg,z to increase with the square root of mass (ν ) 1/2). A spherical molecule in which mass increases symmetrically in three dimensions (Figure 3c) has a cube root relationship between Rg,z and mass (ν ) 1/3). The present dendronization process causes a gradual change in form from a random coil to a rigid rod as mass increases. The lack of a constant form factor prevents rigorous interpretations of ν; therefore, none of these models are appropriate. It is also not surprising that the slope of Rg,z versus Mw is not constant as shown by the significant change in slope in the vicinity of the G3 material. It is probable that once the polymer reaches G3 or G4 (depending on the steric bulk of the peripheral groups), it is essentially in its fully extended conformation. If this is the case, then the higher generation material may be modeled as a rigid rod of constant length subjected to radial mass increase along its backbone (Figure 3d). In the case where the backbone length is much greater than the dendron length, one would expect the Rg,z to be minimally affected by mass increase and so ν should approach zero. It is possible that the data shown in Figure 2 would be better represented by a curve than by two linear trend lines, in which case the reported ν values can be considered tangent lines for comparative purposes. A better demonstration of the effect of dendronization on the stiffness of the polymer backbone comes from an evaluation of the persistence length (ξ) instead of the ν

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Figure 3. Comparison of the direction of mass increase vs Rg,z for several different geometries. (a) An inherently rigid rod growing length wise experiences a linear increase in Rg,z with increasing mass. (b) A flat disk (r . h) that grows radially should experience a square root increase in Rg,z with increasing mass. (c) A sphere grows in all directions at once and therefore experiences a cube root increase in Rg,z with increasing mass. (d) In the case of a very long, narrow cylinder (l . r), radial mass increase has a minimal effect on Rg,z.

value, in a manner similar to that of previous studies.4,19 In such cases, the authors have examined the ξ value of dendronized polymers using the equation of Benoıˆt and Doty for a linear Gaussian polymer (eq 2):27

Rg2 2

ξ

)

2 2 x - 1 + - 2 (1 - e-x) 3 x x

(2)

where Rg2 represents the mean square radius of the polymer, and the parameter x is defined by x ) Lw/ξ. The parameter Lw represents the contour (fully extended) length of the polymer chain and is calculated from the length of the monomer unit (lm) and DPw by the following equation:

Lw ) lmDPw

(3)

In this study, lm for a vinyl-type monomer repeat unit was assumed to be 0.25 nm.1,4 Since DPw of PHS precursor was determined to be 1280, Lw ) 320 nm was obtained from eq 3. In our case, Lw is much greater than ξ, and so x becomes extremely large and eq 2 can be simplified to eq 4:28

Rg,z2 )

Lwξ 3

(4)

where Rg2 was approximated as the square of the z-averaged RMS radius, Rg,z. The solutions for ξ obtained from eq 4 are shown in Table 1. The calculated persistence length gradually increased from 2.1 nm for PHS to 12.2 nm for G5 polymer, 12, representing a 6-fold increase in ξ. The persistence length of 12 is comparable to values estimated in previous studies on dendronized polymers by Percec and co-workers (ξ ) 11.1 nm, for mono-dendron jacketed polystyrene)4 and by Me´ry and co-workers (ξ > 20 nm)19 using the Benoıˆt-Doty equation. The ξ value of 12 is also comparable to those of typical π-conjugated

polymers (i.e., poly(p-phenylene-vinylene) derivatives (ξ ) 6, 11, and 40 nm)28 and poly(p-phenylene-ethynylene) derivatives (ξ ) 13.5 and 16 nm)).29 Therefore, it is clear that the present dendronization process can significantly enhance the rigidity of the PHS main chain. Scanning Force Microscopy (SFM) Visualization of Dendronized Polymers. The rodlike structures of dendronized polymers can be directly observed using SFM.4-10 The SFM images thus provide additional evidence for a cylindrical structure as well as an independent method for measuring the contour length. It has been reported that attachment of long alkyl chains to the periphery of a dendronized polymer assists its alignment on the lattice structure of graphite.7 For the purpose of facilitating the use of SFM visualization to measure contour length, alkylated dendronized polymers 13 and 15 were prepared from 6 and 8 using stearic anhydride (Scheme 2). Figure 3 shows representative SFM images of the dendronized polymers spin-casted onto various substrates. The cylindrical structure of the G3 polymer, 13, is shown on a mica surface (Figure 4a). On the basis of this image, the contour length was estimated to be 200 nm; about 35% shorter than the calculated contour length (Lw ) 320 nm) based on an all-trans conformation. It is plausible that when placed on a surface the polymer adopts a secondary structure that is slightly more compact than the ideal all-trans conformation. When the acetonide or benzylidene terminated polymers were visualized on mica, they exhibited structures equivalent to the stearoyl terminated materials. However, when highly ordered pyrolytic graphite (HOPG) was used to more accurately measure the contour length of the acetal-terminated polymers, the polymers displayed significant aggregation, preventing their analysis. However, 13 could be visualized on HOPG and showed a straighter and more rodlike structure than on mica, reflecting the templating effect of a graphite surface (Figure 4b). The best determination of contour length came from visualization of the semiflexible G2 polymer 15 on a



Scheme 2. Esterification of Dendronized Polymer with Aliphatic Acid Anhydride for SFM Visualization

MoS2 surface. The structure was strongly influenced by the underlying substrate, resulting in a characteristic zigzag morphology on the surface (Figure 4c). It is wellknown that a MoS2 surface exerts a templating effect on the structure of organic monolayers similar to that of HOPG.30 Rod structures with sharp kinks of 60° and 120° were clearly observed in a manner similar to related dendronized polymers on an HOPG surface.4-7,9 The regularly kinked structure of 15 suggests that the backbone retains its flexibility despite the presence of small G2 dendrons. Higher generation stearoyl polymers did not template as well because their inherent stiffness prevented them from following the MoS2 lattice, and measurements of their contour lengths were difficult. Using the wide area (800 × 800 nm) image of 15 on MoS2 shown in Figure 4d, the statistical distribution of the contour length among 60 molecules of 15 was calculated. The number-average and weight-average contour lengths were estimated to be 189 and 195 nm, respectively. The polydispersity index calculated from these two contour lengths is very small (PDI ) 1.03), in line with the narrow molecular weight distribution measured for the PHS precursor. Physical Gelation of the Dendronized Polymer Solutions. One of the most interesting properties of rigid rodlike polymers is their extensively studied thermo-reversible physical gelation behavior in a variety of solvents.31 Representative rodlike polymers that have been investigated as typical polymeric gelators include poly(alkyl-isocyanate)s,32 poly(alkyl-glutamate)s,33 and π-conjugated polymer derivatives.34 This gelation is not dependent on any of the forces commonly associated with intermolecular interactions (i.e., hydrogen bonding or electronic donor-acceptor interactions). Rather, the cause of gelation is attributed to significant backbone stiffness, usually derived from a linear or helical main chain. In other words, only a limited numbers of polymers with an inherently stiff backbone have shown physical gelation behavior.

In theory, if dendronization can produce a significant increase in backbone stiffness, even very common polymers could display such gelation behavior after dendronization. However, the physical gelation of solutions of dendronized polymers has not been previously reported. In contrast, numerous low molecular weight dendritic gelators have been reported,35-41 but they all possess multi-functional structures suitable for the formation of a three-dimensional networks via strong intermolecular interactions. We found that one of the G3 polymers, 13 bearing stearoyl groups on its periphery showed significant physical gelation behavior in several organic solvents. To the best of our knowledge, polymer 13 is the first dendronized material to act as a gelator despite the lack of polar or hydrogen bonding groups at its periphery. When polymer 13 is gently heated in THF (9.6 mg/ mL, ca. 1 wt %), it affords a homogeneous and transparent solution. However, when the hot solution of 13 is allowed to stand at room temperature, it solidifies upon cooling, resulting in a slightly turbid physical gel (Figure 5). Hexanes, cyclohexane, benzene, toluene, chloroform, and THF were all found to be good solvents for enabling this physical gelation behavior. Because of poor solubility, physical gelation in more polar solvents, such as alcohols, acetonitrile, and ethyl acetate, was not observable. A 1-2 wt % solution is the critical minimum concentration for the gelation of 13. The exact concentration depends slightly on the solvent, and the lowest concentration (1 wt %) was observed in THF. It is noteworthy that structurally similar alkylated G3 dendronized polymers, 14 and 15, show physical gelation behavior only at low temperatures (< 0 °C) as the gels melt immediately at room temperature. These polymers do not show physical gelation at room temperature even under much more concentrated conditions (c > 10 wt %). The subtle differences between these alkylated polymers, an alkyl chain shorter by four methylenes for 14 and a lower dendron generation for 15, demonstrate

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Figure 4. SFM images in tapping mode of dendronized polymer. (a) 13 on mica (phase image), (b) 13 on HOPG (height image), and (c and d) 15 on MoS2 (height images).

Figure 5. Physical gel in upturned flask, formed with G3dendronized polymer, 13, bearing stearoyl (C18) substituents (1 wt % in THF, at room temperature).

the striking sensitivity of physical gelation to these structural parameters. The thermal behavior of the physical gel of 13 was examined by differential scanning calorimetry (DSC) of a 5-6 wt % solution in THF, as shown in Figure 6. All DSC experiments were performed in sealed aluminum pans, and weight comparisons before and after each

Figure 6. DSC profiles of dendronized polymers in the solid state. (a) Gel form of 13 (in THF), (b) 13 (neat), (c) 14 (neat), and (d) 15 (neat).

experiment indicated less than 5% solvent loss during the measurement. Two transitions were observed in both heating and cooling cycles (Figure 6a). The transition temperatures were 38 °C (7 mJ/mg) and 43 °C (3 mJ/mg) for the heating cycle and 30 °C (7.5 mJ/mg) and 38.5 °C (4.5 mJ/mg) for the cooling cycle.


These phase transitions were completely reversible and reproducible. Since the higher transition temperature corresponds to melting point of the polymer gel, there must be an additional mesomorphic phase between the gel form and isotropic polymer solution. In addition, the transition temperatures were not the same as those observed in the dry state. Between 0 and 150 °C, a dry sample of 13 showed only one broad transition at 49 °C (26 mJ/mg) in the heating cycle and 41 °C (26 mJ/mg) in the cooling cycle. These values are slightly higher than the transition temperatures observed in the gel state. A similar dry state analysis of nongelator polymer 14 with myristoyl groups on periphery showed a broad thermal transition at 17 °C (7.5 mJ/mg) for heating cycle and 10 °C (7.5 mJ/mg) for cooling cycle. The fact that the low temperature transition of 14 occurs below room temperature explains why this material does not display physical gelation behavior at room temperature. The G2 polymer, 15, in the dry state showed totally different thermal behavior from 13. There are two transitions observed in both the heating cycle (T1 ) 43 °C (13 mJ/mg), T2 ) 69 °C (20 mJ/mg)) and the cooling cycle (T1 ) 54 °C (14 mJ/mg), T2 ) 39 °C (14 mJ/mg)). This is clear evidence for the dependence of the gelation effect on the thermal properties of the dendronized polymer. The significant difference between the thermal behavior of G2 and G3 polymers also explains the difference in gelation ability of these polymers. Conclusion We have successfully prepared high molecular weight dendronized polymers up to the fifth generation using the divergent grafting-from method. These extremely high molecular weight materials were characterized by SEC-MALLS and SFM. The observed increase of the persistence length proves the stiffening effect of dendronization on the polymer backbone in solution and SFM measurements show the rodlike structure of the dendronized polymers on a surface. To our knowledge, the thermo-reversible physical gelation observed for polymer 13 constitutes the first report of physical gelation by a dendronized polymer. Despite the absence of polar or hydrogen bonding groups at the polymer periphery, the thermo-reversible gelation occurs in various common organic solvents at ca. 1-2 wt %. Since physical gelation is known as a typical property of rigid, rodlike polymers, this gelation behavior provides additional support for the stiffening effect brought about by the steric demands of the bulky dendron substituents. These observations confirm the effectiveness of the divergent approach we have used for producing extremely high molecular weight dendronized polymers with minimal structural defects. Acknowledgment. We thank the Department of Energy (Basic Energy Sciences, LBNL Polymer Program) and the National Science Foundation (DMR) for support of this research. We also acknowledge Nippon Soda Company (Chiba, Japan) for the gift of PHS samples. M.Y. also thanks the Japan Society for the Promotion of Science (JSPS) for Japan Overseas Fellowship for Young Scientists. S.O. thanks the Queen Elizabeth II Fellowship supported by the Australian Research Council.


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MA048722O

Efficient Divergent Synthesis of Dendronized Polymers with Extremely

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