Surface-Initiated, Ring-Opening Metathesis Polymerization: Formation

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Surface-Initiated, Ring-Opening Metathesis Polymerization: Formation of Diblock Copolymer Brushes and Solvent-Dependent Morphological Changes Bokyung Kong,† Jungkyu K. Lee,‡ and Insung S. Choi*,† Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, KAIST, Daejeon 305-701, Korea, and Department of Chemistry, Stanford UniVersity, Stanford, California 94305 ReceiVed February 27, 2007 In this article, we report the formation of diblock copolymer brushes on a gold surface by surface-initiated, ringopening metathesis polymerization (SI-ROMP) with the newly developed ruthenium catalyst [(H2IMes)(3-Br-py)2(Cl)2RudCHPh]. Taking advantage of the highly improved activity of the ruthenium catalyst and the rapid initiation step of ROMP, we successfully formed thin films of well-defined block copolymers with 5-norbornene-2-endo,3endo-dimethanol and norbornene carboxylic acid methyl esters (44:56 endo/exo). The catalyst was found to be active enough to polymerize endo isomers of norbonene derivatives from the surface as well as to form diblock copolymer brushes. SI-ROMP of diblock copolymers from the surface was confirmed by ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy. After the formation, the polymer-grafted substrates were immersed in various solvents, and the selective swelling characteristics of polymer brushes were investigated by atomic force microscopy.

Introduction The formation of polymer films on surfaces, such as glasses, silicon wafers, and gold, has widely been used for tailoring/ controlling surface properties such as wettability,1 friction,2 and protein/cell adhesion.3 In addition, the deposition of polymer films onto solid substrates is an important process in the fabrication of optoelectronic devices,4-6 sensors,7-11 micromachines,12 and cell engineering systems.13-15 Although polymer films have been applied successfully to the control of surface properties, recent advances in materials science imposed progress in the fabrication of more sophisticated systems that possess multiple surface properties. As a result of these increasing demands for more functionalized surfaces, intensive research has been devoted to mixed polymer brushes and block copolymers that are grafted onto surfaces.16 * Corresponding author. E-mail: [email protected]. † KAIST. ‡ Stanford University. (1) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (2) Raviv, U.; Glasson, S.; Kampf, N.; Gohy, J.-F.; Je´roˆme, R.; Klein, J. Nature 2003, 425, 163. (3) Ma, H.; Hyun, J.; Stiller, P.; Chilkoti, A. AdV. Mater. 2004, 16, 338. (4) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (5) Dodabalapur, A.; Bao, Z.; Makhija, A.; Laquindanum, J. G.; Raju, V. R.; Feng, Y.; Katz, H. E.; Rogers, J. Appl. Phys. Lett. 1998, 73, 142. (6) Kim, E.; Whitesides, G. M.; Lee, L. K.; Smith, S. P.; Prentiss, M. AdV. Mater. 1996, 8, 139. (7) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (8) Bubb, D. M.; McGill, R. A.; Horwitz, J. S.; Fitz-Gerald, J. M.; Houser, E. J.; Wu, P. W.; Ringeisen, B. R.; Pique´, A.; Chrisey, D. B. J. Appl. Phys. 2001, 89, 5739. (9) Kim, J.; McQuade, D. T.; McHugh, S. K.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 3868. (10) Liu, Y.; Zhao, M.; Bergbreiter, D. E.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 8720. (11) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739. (12) Jager, E. W. H.; Smela, E.; Ingana¨s, O. Science 2000, 290, 1540. (13) Lu, L.; Yaszemski, M. J.; Mikos, A. G. Biomaterials 2001, 22, 3345. (14) Ostuni, E.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 2828. (15) Jung, K. J.; Ahn, K. D.; Han, D. K.; Ahn, D. J. Macromol. Res. 2006, 13, 446. (16) Russell, T. P. Science 2002, 297, 964.

Polymer brushes have been generated by several methods, including the physisorption/chemisorption of polymer brushes onto surfaces, the “grafting-onto” method, and the “graftingfrom” method.17 Compared with the grafting-onto method, the grafting-from approach, such as surface-initiated, TEMPOmediated radical,18 living anionic,19,20 living carbocationic,21 atom-transfer radical polymerization (ATRP),22-25 and ringopening metathesis polymerization (ROMP),26-32 has been known as an effective way to control the surface density and thickness of grafted polymer brushes. Among the polymerization methods used for surface-initiated polymerization, only ATRP has been widely utilized to generate block copolymer brushes from surfaces.33-37 (17) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (18) Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (19) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016. (20) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607. (21) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243. (22) Shipp, D. A.; Wang, J.-L.; Matyjaszewski, K. Macromolecules 1998, 31, 8005. (23) Muhlebach, A.; Gaynor, S. G.; Matyjaszewski, K. Macromolecules 1998, 31, 6046. (24) Gao, B.; Chen, X.; Iva´n, B.; Kops, J.; Batsberg, W. Polym. Bull. 1997, 39, 559. (25) Lee, Y.-W.; Kang, S. M.; Yoon, K. R; Chi, Y. S.; Choi, I. S. Macromol. Res. 2005, 13, 356. (26) Husemann, M.; Mecerreys, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38, 647. (27) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723. (28) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. Macromolecules 2000, 33, 32. (29) Wieringa, R. H.; Schouten, A. J. Macromolecules 1996, 29, 3032. (30) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088. (31) Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G.; Whitesides, G. M.; Laibinis, P. E. Macromolecules 2000, 33, 2793. (32) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321. (33) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921.

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ROMP has become a prevailing method for synthesizing sidechain-functionalized polymers and copolymers in solution-phase polymerization.38-40 With the development of well-defined metalalkylidine olefin-metathesis catalysts, controlled/living ROMP became possible. In particular, the newly developed N-heterocyclic ruthenium catalyst, [(H2IMes)(3-Br-py)2(Cl)2RudCHPh] (1), enables controlled/living ROMP with high activity and a rapid initiation step and generates both homopolymers and block copolymers with notably improved polydispersities in solutionphase polymerization.41 On the basis of these recent advances in catalyst development, we reasoned that ROMP would be a strong candidate for accomplishing the surface-initiated polymerization of diblock copolymers as well as overcoming the difficulties of polymerization from surfaces. In this work, we utilized ruthenium catalyst 1 to form diblock copolymer brushes on gold by surface-initiated ROMP (SI-ROMP) and demonstrated the undisturbed SI-ROMP of endo isomers of norbornene derivatives. The solvent-dependent morphological switchability of the brushes was also investigated after the formation of diblock copolymer brushes. Experimental Section Materials. An endo/exo isomeric mixture of 5-norbornene-2methanol (Nb-MeOH), 5-norbornene-2-endo,3-endo-dimethanol (Nb-diMeOH), methyl acrylate, dicyclopentadiene, Grubbs catalyst second generation, hydroquinone, tert-butylchlorodimethylsilane, tetrabutyl ammonium fluoride (TBAF), ethyl vinyl ether, and anhydrous dichloromethane (CH2Cl2) were all purchased from Aldrich. Unde-10-ene-1-thiol and the newly invented Grubbs catalyst, [(H2IMes)(3-Br-py)2(Cl)2RudCHPh] (1), were synthesized by following literature methods.42,43 The endo/exo isomeric mixture (44: 56) of norbornene carboxylic acid methyl ester (Nb-COOMe) was prepared by the Diels-Alder reaction of cyclopentadiene and the corresponding dienophile (methyl acrylate). The methanol groups of Nb-MeOH and Nb-diMeOH were protected by tert-butylchlorodimethylsilane before polymerization. The resulting norbornene tert-butyl dimethylsiloxane (Nb-MeOTBS) and 5-norbornene-2endo,3-endo-di(tert-butyldimethylsiloxane) (Nb-diMeOTBS) were used for the polymerization. All of the solvents were purchased from Merck. The gold substrates were prepared by a thermal evaporation of titanium (5 nm) and gold (100 nm) onto silicon wafers. All polymerization and manipulations of air-sensitive compounds were performed under a nitrogen atmosphere in a glovebox. SI-ROMP: Poly(5-norbornene-2-(tert-butyldimethylsiloxane)), p(Nb-MeOTBS). The gold substrates were cut into 1 × 1 cm2 slides and thoroughly washed with CH2Cl2, acetone, and ethanol and then dried under a stream of argon prior to use. The mixed SAMs were formed by immersing the gold substrates in an ethanol solution of unde-10-ene-1-thiol and octanethiol at room temperature for 12 h. The ratios of unde-10-ene-1-thiol and octanethiol were varied to 1:0, 1:1, and 2:3. After the formation of the mixed SAMs, the substrates were rinsed with ethanol several times and then dried under a stream of argon. In the glovebox, the SAM-coated gold substrates were immersed in a CH2Cl2 solution of 1 for 1 h. After the catalyst-attached substrates were thoroughly washed with anhydrous CH2Cl2, they were exposed to the CH2Cl2 solution of (34) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. (35) Huang, X.; Wirth, M. J. Macromolecules 1999, 32, 1694. (36) Boyes, S. G.; Brittain, W. J.; Weng, X.; Cheng, S. Z. D. Macromolecules 2002, 35, 4960. (37) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837. (38) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012. (39) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (40) Pollino, J. M.; Stubbs, L. P.; Weck, M. J. Am. Chem. Soc. 2004, 126, 563. (41) Choi, T.-L.; Grubbs, R. H. Angew. Chem., Int. Ed. 2003, 42, 1743. (42) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141. (43) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 4035.

Kong et al. Nb-MeOTBS (0.5 M) for 30 min. The polymerization was terminated by the addition of a small amount of ethyl vinyl ether, leading to the formation of p(Nb-MeOTBS) films. SI-ROMP: Poly(5-norbornene-2-endo,3-endo-dimethanol)b-poly(norbornene carboxylic acid methyl esters), p(Nb-diMeOH)-b-p(Nb-COOMe). The gold substrates were cut into 1 × 1 cm2 slides and thoroughly washed with CH2Cl2, acetone, and ethanol and then dried under a stream of argon prior to use. The formation of SAMs was achieved by immersing the gold substrate in a 1 mM ethanol solution of unde-10-ene-1-thiol at room temperature for 12 h. After the formation of the vinyl-terminated SAMs, the gold substrates were rinsed with ethanol several times and then dried under a stream of argon. In the glovebox, the SAM-coated gold substrate was immersed in a CH2Cl2 solution of 1 for 1 h. After the catalyst-attached substrate was thoroughly washed with anhydrous CH2Cl2, it was exposed to the first monomer (Nb-diMeOTBS, 0.5 M) in CH2Cl2. After 15 min, the substrate was rinsed with anhydrous CH2Cl2 and then exposed to the second monomer (Nb-COOMe, 0.5 M) in CH2Cl2 for 15 min. The polymerization was terminated by the addition of a small amount of ethyl vinyl ether, leading to the formation of a p(Nb-diMeOTBS)-b-p(Nb-COOMe) film. The deprotecting step of the tert-butyldimethylsilyl (TBS) group in the first block of the copolymer was then performed with TBAF to introduce the hydroxyl group, and finally the p(Nb-diMeOH)-bp(Nb-COOMe) diblock copolymer brush was successfully obtained on the gold substrate. Solvent-Dependent Change in Surface Morphology. The gold substrate presenting p(Nb-diMeOH)-b-p(Nb-COOMe) diblock copolymer brushes was immersed in various solvents such as ethanol, hexane, THF, and CH2Cl2 at room temperature for 3 h and then dried under a flow of argon. The solvent-dependent change in surface morphology was characterized with atomic force microscopy (AFM). Characterization. The thicknesses of the layers after each step were measured with a Gaertner L116s ellipsometer (Gaertner Scientific Corp., IL) equipped with a He-Ne laser (632.8 nm) at a 70° angle of incidence. Polarized infrared external reflectance spectroscopy (PIERS) spectra were obtained in single reflection mode using a dry N2-purged Thermo Nicolet Nexus FT-IR spectrophotometer equipped with the smart SAGA (smart apertured grazing angle) accessory. The p-polarized light was incident at 80° relative to the surface normal of the substrate. The spectra were taken by adding approximately 2000 scans for the background and 800-2000 scans for the samples at a resolution of 4 cm-1, and all spectra were reported in absorption mode relative to a clean gold surface. AFM images were acquired using a Nanoscope IIIa multimode scanning probe microscope (Veeco, USA) in tapping mode. The X-ray photoelectron spectroscopy (XPS) study was performed with a VG-Scientific ESCALAB 250 spectrometer (U.K.) with a monochromatized Al KR X-ray source. Emitted photoelectrons were detected by a multichannel detector at a takeoff angle of 90° relative to the surface. During the measurements, the base pressure was 10-9-10-10 Torr. Survey spectra were obtained at a resolution of 1 eV from three scans, and high-resolution spectra were acquired at a resolution of 0.05 eV. Contact angle measurements were performed using a Phoenix 300 goniometer (Surface Electro Optics Co., Ltd., Korea).

Results and Discussion Surface-Initiated, Ring-Opening Metathesis Polymerization (SI-ROMP). SI-ROMP has previously been investigated for generating polymer brushes at surfaces. For example, Weck et al. generated polymer chains on a gold surface by attaching a ROMP initiator onto defect sites of the self-assembled monolayers (SAMs).29 The SI-ROMP of polymer films on silicon and silicon oxide surfaces was also successfully accomplished

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Figure 1. Schematic description of the formation of p(Nb-diMeOH)-b-p(Nb-COOMe) diblock copolymer brushes on a gold surface.

by several groups.31,32,44-46 However, the conventional olefinmetathesis catalysts suffered from decreased activity and the lack of functional-group tolerance.47,48 Therefore, reports on the SI-ROMP of diblock copolymer brushes have been limited: Mirkin et al. reported the formation of diblock copolymers on gold nanoparticles49 and flat silicon surfaces.50 However, in these cases, only the exo isomer of norbornene derivatives was used as a monomer because of the limited activity of the catalyst toward SI-ROMP. In this article, we studied the catalytic activity of [(H2IMes)(3-Br-py)2(Cl)2RudCHPh] (1) for SI-ROMP with the aim of generating diblock copolymer brushes at surfaces in a controlled way. Figure 1 shows the general procedure of SI-ROMP employed in this work. For the attachment of Ru catalyst 1 onto the gold surface, we formed vinyl-terminated SAMs of unde-10-ene-1thiol. Vinyl-terminated SAMs have previously been used in SIROMP because they offer alkylidene ligands for the attachment of ruthenium catalysts.32,45 For example, Nuzzo et al. studied the effect of the initial concentration of the Grubbs catalyst first generation, RuCl2(PCy3)2(dCHPh), on the growth of polymer brushes on Si/SiO2 surfaces by adjusting the relative surface density of the catalyst-anchoring group, 7-octenyltrichlorosilane, and obtained the thickest polymeric film with 20-40% surface density of 7-octenyltrichlorosilane. We also investigated the optimal surface density of vinyl-terminated SAMs with catalyst 1 for the formation of thicker polymer brushes and better control over the grafting density. The mixed SAMs were formed with unde-10-ene-1-thiol and an olefin-free compound, octanethiol. After the immobilization of 1 onto the SAMs-coated substrate, the catalyst-attached substrate was immersed in the CH2Cl2 solution of 5-norbornene-2-(tert-butyldimethylsiloxane) (NbdiMeOTBS) (0.5 M) for 30 min. The ratio of unde-10-ene-1thiol and octanethiol was varied to 1:0, 1:1, and 2:3, and the ellipsometric thicknesses of the resulting polymer films were (44) Jeon, N. L.; Choi, I. S.; Whitesides, G. M.; Kim, N. Y.; Laibinis, P. E.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G. Appl. Phys. Lett. 1999, 75, 4201. (45) Liu, X.; Guo, S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2003, 42, 4785. (46) Harada, Y.; Girolami, G. S.; Nuzzo, R. G. Langmuir 2003, 19, 5104. (47) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953. (48) Chatterjee, A. K.; Morgan, J. P.; Scholl, M.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 3783. (49) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (50) Liu, X.; Guo, S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2003, 42, 4285.

Figure 2. PIERS spectra of (a) SAMs of undec-10-ene-1-thiol, (b) p(Nb-diMeOTBS), (c) p(Nb-diMeOTBS)-b-p(Nb-COOMe), and (d) p(Nb-diMeOH)-b-p(Nb-COOMe).

266 ( 32, 149 ( 4, and 101 ( 31 nm, respectively. The results indicated that the catalytic activity of 1 was not affected by the steric congestion in our system and the thickest polymer film was obtained with 100% vinyl-terminated SAMs. Therefore, for further investigation, we used the gold surface fully covered with vinyl-terminated SAMs. The well-defined SAMs of unde-10-ene-1-thiol were analyzed by ellipsometry and FT-IR spectroscopy. The thickness of the SAMs was measured to be 10.8 Å, which was in agreement with the reported value.42 The IR spectrum (Figure 2a) showed peaks associated with the ω-olefin group: 1643 cm-1 (CdC stretch), 2983 cm-1 (dCH2 symmetric), 3007 cm-1 (β C-H stretch), and 3084 cm-1 (dCH2 antisymmetric). After the formation of the vinyl-terminated SAMs, catalyst 1 was attached to the surface by immersing the gold substrate in the CH2Cl2 solution of 1. For SI-ROMP, the catalyst-attached gold substrate was then immersed in the CH2Cl2 solution of norbornene derivatives. As monomers, we used 5-norbornene-2-endo,3-endo-dimethanol (Nb-diMeOH)

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Figure 3. XPS spectra of diblock copolymers (a) before deprotection, p(Nb-diMeOTBS)-b-p(Nb-COOMe), and (b) after deprotection, p(Nb-diMeOH)-b-p(Nb-COOMe).

and norbornene carboxylic acid methyl esters (Nb-COOMe) (44: 56 endo/exo). Nb-diMeOH was chosen as a monomer to investigate the SI-ROMP activity of 1 toward endo compounds. Our previous attempt indicated that other ruthenium catalysts failed to polymerize endo compounds from surfaces (unpublished results). Therefore, the successful SI-ROMP of endo compounds would lead to a broader selection of monomers and more sophisticated control of the resulting polymeric surfaces. SI-ROMP: p(Nb-diMeOTBS)-b-p(Nb-COOMe) and p(NbdiMeOH)-b-p(Nb-COOMe). We formed a diblock copolymer brush composed of p(Nb-diMeOH) and p(Nb-COOMe) because the synthesis of a block copolymer is a decisive factor in the determination of living/controlled polymerization.51 Examples of the formation of diblock copolymers by SI-ROMP are limited, presumably because ruthenium catalysts anchored onto surfaces lose their activity after some period of time. For example, Whitesides and Laibinis reported that polymer growth at the surface stopped after ∼30 min in SI-ROMP.31 Nuzzo also reported that the Ru catalyst lost its activity after several hours in his system.46 After the formation of the catalyst-attached gold surface, the surface was first exposed to the CH2Cl2 solution of Nb-diMeOTBS (0.5 M) for 15 min to form the first block of the diblock copolymer. For SI-ROMP, the hydroxyl groups of Nb-diMeOH were protected with tert-butyldimethylsilyl (TBS) to prevent the hydroxyl groups from damaging the activity of the ruthenium catalyst. After polymerization, the ellipsometric measurements showed a thickness of 174 ( 18 nm. In the IR spectrum, the peaks from the ω-olefin group disappeared, and new peaks appeared at 2956-2858 cm-1 (sp3 C-H stretch), 1390 and 1258 cm-1 (Si-CH3 stretch), and 1473 and 1092 cm-1 (Si-CH2 stretch) (Figure 2b). It is noteworthy that the result was the first example of SI-ROMP of endo compounds of norbonene derivatives. (51) Webster, O. W. Science 1991, 251, 887.

Kong et al.

The second block was formed with Nb-COOMe (0.5 M). After the 15-min polymerization of the second block, we observed the strong appearance of the CdO stretching peak at 1736 cm-1 and the C-O stretching peak at 1199 and 1171 cm-1 in the IR spectrum (Figure 2c). The thickness was increased to 292 ( 26 nm (from 174 ( 18 nm), which indicates that the thickness of the second block was 118 nm. On the basis of these results, we concluded that the diblock copolymer was finely formed on the gold surface by using Ru catalyst 1. Furthermore, we also could verify that the activity of the catalyst at the surface was not affected by the stereochemistry of monomers (endo or exo) during polymerization. After the diblock copolymer was successfully formed by SIROMP, the TBS group of the first block was deprotected to introduce the hydroxyl functional group. The deprotection reaction was performed by immersing the diblock copolymer-grafted substrate in a THF solution of TBAF (0.5 M) for 3 h. In the IR spectrum (Figure 2d), the peaks corresponding to the TBS group disappeared, and the broad O-H stretching peak newly appeared around 3400 cm-1. After the deprotection, Si 2s (at 148.1 eV) and Si 2p (at 98.1 eV) peaks disappeared in the XPS spectrum (Figure 3). In addition, the water contact angle was decreased from 111 to 56° because the hydroxyl groups were exposed to the surface. Solvent-Dependent Change in Surface Morphology. For the analysis of morphological changes in the diblock copolymer brush, various solvents were tested, and the surfaces were characterized by AFM. Diblock copolymer brushes tethered onto a planar surface have attracted a lot of interest because they have unique characteristics that respond to changes in the surroundings.52 Because of the conflicting properties between two blocks, diblock copolymers produce variable and switchable surface morphologies through the selective swelling and collapsing of polymer brushes.53-55 These morphological changes could be utilized in the controlled change in wettability56-58and the fabrication of nanostructures.59-61 Numerous theoretical62-64 and experimental53,54 studies have been reported on the morphological switching of surface-tethered diblock copolymers depending on environmental conditions, especially solvents. Figure 4 shows AFM images of p(Nb-diMeOH)-b-p(NbCOOMe) after the solvent treatment. Figure 4a,b shows the AFM images of the surface after 3 h of immersion in ethanol and hexane at room temperature, respectively. The aggregated polymer brushes formed round, humplike structures when the sample was treated with ethanol and hexane. In contrast, when the sample was immersed in THF and CH2Cl2, the brushes were more stretched and formed sharp structures (Figure 4c,d). Because both p(Nb-diMeOH) and p(Nb-COOMe) were relatively insoluble in ethanol and hexane, we thought that the polymers chains tended (52) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (53) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407. (54) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. Macromolecules 2000, 33, 8821. (55) Tomlinson, M. R.; Genzer, J. Langmuir 2005, 21, 11552. (56) Peng, J.; Xuan, Y.; Wang, H.; Yang, Y.; Li, B.; Han, Y. J. Chem. Phys. 2004, 120, 11163. (57) Lu, X.; Peng, J.; Li, B.; Zhang, C.; Han, Y. Macromol. Rapid Commun. 2006, 27, 136. (58) Xu, C.; Wayland, B. B.; Fryd, M.; Winey, K. I.; Composto, R. J. Macromolecules 2006, 39, 6063. (59) Hamley, I. W. Nanotechnology 2003, 14, R39. (60) Jinnai, H.; Sawa, K.; Nishi, T. Macromolecules 2006, 39, 5815. (61) Tang, C.; Tracz, A.; Kruk, M.; Zhang, R.; Smilgies, D.-M.; Matyjaszewski, K.; Kowalewski, T. J. Am. Chem. Soc. 2005, 127, 6918. (62) Zhulina, E. B.; Singh, C.; Balazs, A. C. Macromolecules 1996, 29, 6338. (63) Singh, C.; Balazs, A. C. Macromolecules 1996, 29, 8904. (64) Zhulina, E. B.; Balazs, A. C. Macromolecules 1996, 29, 2667.

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Figure 4. AFM images of p(Nb-diMeOH)-b-p(Nb-COOMe) films after treatment with various solvents at room temperature: (a) ethanol, (b) hexane, (c) THF, and (d) CH2Cl2.

to avoid contact with bad solvents and formed aggregates with neighboring polymer brushes, leading to the generation of globular structures. However, the polymer chains were stretched to avoid contact with neighboring chains in good solvents (THF and CH2Cl2).65

Conclusions In this article, we demonstrated that surface-initiated, ringopening metathesis polymerization (SI-ROMP) could be utilized (65) By changing the addition order of monomers, we also succeeded in generating p(Nb-COOMe)-b-p(Nb-diMeOH) diblock films. The data is not shown in this report because its solvent-dependent morphological changes were similar to those of p(Nb-diMeOH)-b-p(Nb-COOMe). We also investigated the temperature effect on the morphological changes. When the swelling temperature was increased to 40 °C, we observed more apparent changes in morphology. (See Supporting Information.)

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for the formation of diblock copolymer brushes from surfaces with a recently invented ruthenium catalyst, [(H2IMes)(3-Brpy)2(Cl)2RudCHPh]. Taking advantage of the highly improved activity of the ruthenium catalyst and the rapid initiation step of SI-ROMP, we successfully formed thin films of well-defined diblock copolymers with 5-norbornene-2-endo,3-endo-dimethanol (Nb-diMeOH) and an endo/exo isomeric mixture (44:56) of norbornene carboxylic acid methyl ester (Nb-COOMe). The catalyst was found to be active enough to polymerize endo isomers of norbonene derivatives from the surface as well as to form p(Nb-diMeOH)-b-p(Nb-COOMe) diblock copolymer brushes. We believe that successful SI-ROMP with endo compounds would lead to a broader selection of monomers and consequently more sophisticated control of the resulting polymeric surfaces. In addition, amphiphilic diblock copolymers grafted onto surfaces have attracted a lot of interest because they show multiple responses toward external stimuli.54 Moreover, the characteristic morphological switching of amphiphilic block copolymer brushes, induced by the conflicting response of each block toward environmental changes, is applicable to various fields.58,59 Our approach reported here could be applied to the formation of amphiphilic block copolymers at surfaces by SI-ROMP, which is our next research thrust. Acknowledgment. This work was supported by a Korea Science and Engineering Foundation grant (no. 2005-215-C0007). The FT-IR spectrophotometer and ellipsometer were purchased with research funds from the Center for Molecular Design and Synthesis. We appreciate Dr. Gwangsu Byun for support with respect to the synthesis of norbornene derivatives. Supporting Information Available: AFM images of p(NbCOOMe)-b-p(Nb-diMeOH) after immersion in various solvents at 40 °C. This material is available free of charge via the Internet at http://pubs.acs.org. LA700568J