Precise Synthesis of Bottlebrush Block Copolymers from ω-End

Jan 9, 2018 - A facile and efficient synthetic grafting-through strategy for preparing well-defined bottlebrush block copolymers (BBCPs) was developed...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Precise Synthesis of Bottlebrush Block Copolymers from ω‑End-Norbornyl Polystyrene and Poly(4-tert-butoxystyrene) via Living Anionic Polymerization and Ring-Opening Metathesis Polymerization Yong-Guen Yu,†,‡ Chang-Geun Chae,†,‡ Myung-Jin Kim,†,‡ Ho-Bin Seo,†,‡ Robert H. Grubbs,‡,§ and Jae-Suk Lee*,†,‡ †

School of Materials Science and Engineering and ‡Grubbs Center for Polymers and Catalysis, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea § Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: A facile and efficient synthetic grafting-through strategy for preparing well-defined bottlebrush block copolymers (BBCPs) was developed through a combination of living anionic polymerization (LAP) and ring-opening metathesis polymerization (ROMP). ω-End-norbornyl polystyrene (NPSt) and poly(4-tert-butoxystyrene) (NPtBOS) were synthesized by LAP using terminator of chlorine moiety containing silane-protecting amine and coupled with a subsequent amidation using norbornyl activated ester. Bottlebrush homopolymers of NPSt were obtained by ROMP with ultrahigh molecular weights (MWs, Mw = 2928 kDa) and narrow molecular weight distributions (MWDs, Đ = 1.07) at high degree of polymerizations (DPw = 1084). Welldefined BBCPs with ultrahigh MWs (Mw ∼ 3055 kDa) and narrow MWDs (Đ ∼ 1.13) were synthesized through sequential ROMP of NPSt with NPtBOS. The effect of ultrahigh MWs was investigated by self-assembly of the BBCPs in which the phaseseparated BBCPs presented periodic lamellar structures and exhibited structural colors from blue to pink.

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INTRODUCTION Bottlebrush polymers, a type of branched polymer, are characterized by a linear backbone with densely packed polymeric side chains. When comparing the conformational properties of linear polymers to those of bottlebrush polymers, the architecture of the bottlebrush polymers causes not only a significant reduction in chain entanglement but also an extension of the backbone because of steric congestion between the polymeric side chains.1−3 The unique properties make bottlebrush polymers attractive for the preparation of photonic crystals, which are fabricated by the self-assembly of bottlebrush block copolymers (BBCPs) with ultrahigh molecular weights.4−7 Generally, the synthesis of bottlebrush polymers is accomplished by one of three methodologies, namely, grafting-from, -onto, or -through.1,8−10 Grafting-through using the polymerization of macromonomers (MMs) provides a perfectly uniform grafting density along each repeat unit throughout the material.1 The use of ring-opening metathesis polymerization © XXXX American Chemical Society

(ROMP) for MMs has received a substantial amount of attention because conversions and degree of polymerizations (DPs) of the backbone with this method are quite high. Graft polymerization of MMs containing norbornene moieties was successfully achieved with good control of the molecular weights (MWs) and molecular weight distributions (MWDs) using fastinitiating third-generation Grubbs catalyst (G3).11 However, despite the successes achieved in the precise synthesis of bottlebrush polymers with well-controlled MWs and MWDs via ROMP, few polymers with ultrahigh MWs and DPs have been reported.12−14 To overcome the limitations regarding ultrahigh-MW polymers with narrow MWDs, the rapid propagation rate from less sterically hindered MMs during ROMP11,13 and synthesis of Received: November 20, 2017 Revised: December 19, 2017

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DOI: 10.1021/acs.macromol.7b02447 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules MMs with absence of α,ω-dinorbornyltelechelic polymer as cross-linker14 have been developed. The successful synthesis of bottlebrush polymers that satisfy the reported requirements was heavily impacted by efforts to synthesize elaborately designed MMs. MMs were successfully prepared by atomic transfer radical polymerization (ATRP),7,15 reversible addition−fragmentation chain-transfer polymerization (RAFT),16−19 and other polymerizations.20,21 These methodologies were used to suppressing the complicated side reaction such as radical recombination. Living anionic polymerization (LAP) is a promising approach for preparing high-quality MMs. LAP is the most effective polymerization method for the synthesis of well-defined polymers with predictable MWs and narrow MWDs. Furthermore, this technique provides a versatile method for preparing mono-inchain or α,ω-end-functionalized polymers with amines,22,23 hydroxy,24 and other functional groups.25−28 A few strategies have been developed for the synthesis of MMs containing norbornene moieties at the ω-end position. For example, MMs of various epoxide were synthesized by anionic ringopening polymerization with the norbornyl initiator of oxyanion.29 Norbornene-substituted diphenylhexyllithium have also been used to initiate methacrylate moiety to yield a MM.30 The synthesis of MMs containing polystyrene was achieved by direct end-capping of living polymer with norbornyl terminators.31 Furthermore, for the preparation of polystyrene-based MMs, hydroxyl-terminated prepolymers have been coupled with norbornene-containing carboxylic acid via esterification.32,33 Nevertheless, due to the lack of end-functionalization efficiency and kinetic barrier of the steric hindrance during propagation, MMs based on LAP were hard to be synthesized to the BBCPs with ultrahigh MWs and narrow MWDs, which are sufficient for relevant applications in photonic crystals. The newly designed MMs for developing BBCPs with ultrahigh MWs and narrow dispersity are strongly recommended. LAP of styrenes with various para substituents has been widely studied for practical applications.34 We have been studied on living anionic polymerization of pyridine-, oxadiazole-, tertbutyltriphenylamine-, and carbazole-based monomer.35−37 These materials showed hole transport, CO2 capture, and unimolecular self-assembly behaviors. The incorporation of various functional groups into bottlebrush polymers broadens the applicability of these polymers. In this work, we present an efficient synthetic route to welldefined bottlebrush block copolymers (BBCPs) with ultrahigh MWs by living anionic polymerization (LAP) and ring-opening metathesis polymerization (ROMP). Syntheses of macromomers, ω-end-norbornyl polystyrene (NPSt), and poly(4-tert-butoxystyrene) (PtBOS) are carried out in two steps: (i) primary amine-terminated polystyrene (PSt-NH2) and poly(4-tertbutoxystyrene) (PtBOS-NH2) were synthesized by LAP and termination with a chlorine moiety containing silane-protecting amine and (ii) amidation of PSt-NH2 and PtBOS-NH2 with norbornyl activated ester. Using the NPSt and NPtBOS, bottlebrush homopolymers and BBCPs with ultrahigh MWs and narrow MWDs are prepared by ROMP. The BBCPs of NPSt and NPtBOS are utilized to investigate the phase-separation behavior to generate ultrahigh MWs.

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prepared according to the literature procedures.23,38 6-Aminohexanoic acid (Merck, >98.5%), pentafluorophenol (Thermo Fisher Scientific, 99%), 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDC; Acros Organics, >98%), 4-(dimethylamino)pyridine (DMAP; Merck, >99%), trimethylamine (Merck, >99%), 1,8diazabicyclo[5.4.0]undec-7-ene (DBU; Merck, 98%), 1,2-dichlorobenzene (Merck, 99%), toluene (Merck, 99.9%), anhydrous dichloromethane (Merck, 99.8%), anhydrous tetrahydrofuran (Merck, 99.9%), and sec-BuLi (Merck, 1.4 M in cyclohexane) were used as received and without further purification. Styrene (Merck, 99%) and 4-tertbutoxystyrene (Merck, 99%) were passed through alumina columns, dried for 24 h over anhydrous CaH2, and distilled under reduced pressure. These compounds were then further distilled from dibutylmagnesium (Merck, 1 M in heptane) under high-vacuum system (∼10−6 mmHg). THF was distilled in the presence of sodium naphthalenide under vacuum. All initiators and monomers were stored at −30 °C in ampules equipped with break-seals prior to polymerization. Characterization. Both 1H and 13C NMR spectra were acquired on a JEOL JNM-ECX 400 instrument using CDCl3 as the solvent. Chemical shifts were referenced to tetramethylsilane (TMS) at 0 ppm. Absolute molecular weights and molecular weight distributions (MWDs) determinations were obtained from using a size exclusion chromatograph−multiangle laser light scattering (SEC-MALLS) system equipped with a set of four columns (HR 0.5, HR 1, HR 3, and HR 4; Waters Styragel columns run in series with column pore sizes of 50, 100, 500, and 1000 Å, respectively), an Optilab DSP interferometric refractometer (Wyatt Technology), and a DAWN EOS laser photometer (Wyatt Technology). SEC with refractive index (RI) detection was used to determine the relative molecular weights and MWDs after calibration with polystyrene standards (American Polymer Standards Corp.). THF containing 2% triethylamine (TEA, (C2H5)3N) was used as the eluent at a flow rate of 1.0 mL/min at 40 °C. The dn/dc value for each polymer in THF at 40 °C was calculated by assuming 100% mass elution of the injected polymer samples. Field-emission scanning electron microscopy (FE-SEM) was performed on an Hitachi S-4700 instrument. Samples of all films for crosssection analysis were fractured to expose a polymer surface perpendicular to the coated silicon wafer. The samples were then sputter coated with a layer of platinum a few nanometers thick prior to SEM imaging. Domain sizes of the samples were measured from the average of the fitted distance in the SEM images. The reflection spectra of the photonic films were measured by a LAMBDA 750 UV/vis/NIR spectrophotometer (PerkinElmer). Synthesis of N-(Carboxylhexanoyl)-cis-norbornene-exo-2,3dicarboxiimide. This compound was prepared using a modified version of a previously reported procedure.38 cis-5-Norbornene-exo2,3-dicarboxylic anhydride (10 g, 60.9 mmol) and 6-aminohexanoic acid (8.79 g, 67 mmol) in toluene (50 mL) were added to a roundbottom flask equipped with a Dean−Stark trap and a magnetic stirrer. The mixture was reacted under a nitrogen atmosphere at 125 °C for 24 h. Once the reaction was complete, it was cooled and then concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with hexane/ethyl acetate = (5/5 v/v) as an eluent. The product was obtained as white solid (10.8 g, 93%). 1 H NMR (400 MHz, CDCl3, δ): 10.6 (1H, s), 6.20 (2H, s), 3.38 (2H, s), 3.18 (2H, s), 2.60 (2H, s), 2.25 (2H, s), 1.80−0.83 (8H, m). 13C NMR (100 MHz, CDCl3, δ): 179.22, 178.29, 137.85, 47.82, 45.17, 42.75, 38.45, 33.78, 27.42, 26.35, 24.16 (see Figures S3 and S4). Synthesis of N-(Perfluorophenylhexanoate)-cis-norborneneexo-2,3-dicarboxiimide (T2). N-(Carboxylhexanoyl)-cis-norborneneexo-2,3-dicarboxiimide (4.54 g, 16.4 mmol), EDC (4.08 g, 21.3 mmol), DMAP (0.680 g, 5.55 mmol), and dichloromethane (50 mL) were added to a round-bottom flask equipped with a magnetic stirrer under a nitrogen atmosphere. Pentafluorophenol (3.61 g, 19.6 mmol) dissolved in dichloromethane (10 mL) was added dropwise to the reaction mixture at 0 °C. The mixture was stirred at room temperature for 48 h. The organic layer was washed with saturated aqueous NaHCO3 three times, dried over anhydrous Na2SO4, and filtered. After removal of the solvent under reduced pressure, the crude product was purified

EXPERIMENTAL SECTION

Materials. (H2IMes)(pyr)2(Cl)2RuCHPh (G3), cis-5-norborneneexo-2,3-dicarboxylic anhydride, and 2,2,5,5-tetramethyl-1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane (T1; see Figures S1 and S2) were B

DOI: 10.1021/acs.macromol.7b02447 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of (a) Macromonomers (NPSt and NPtBOS), (b) Bottlebrush Homopolymers (P(NB-g-St) and P(NB-g-tBOS)), and (c) Bottlebrush Block Copolymers (P(NB-g-St)-b-P(NB-g-tBOS))

by silica gel column chromatography with hexane/ethyl acetate = 7/3 (v/v) as the eluent. The product was obtained as a white solid (8.54 g, 85%). 1H NMR (400 MHz, CDCl3, δ): 6.25 (2H, s), 3.45 (2H, t), 3.23 (2H, s), 2.62 (4H, t), 1.80−0.83 (8H, m). 13C NMR (100 MHz, CDCl3, δ): 178.14, 169.27, 137.86, 47.86, 45.21, 42.76, 38.33, 33.09, 27.37, 26.19, 24.24. 19F NMR (376 MHz, CDCl3, δ): −152.75 (2F, d), −158.15 (1F, t), −162.40 (2F, t) (see Figures S5 and S6). Synthesis of ω-End-Norbornyl Polystyrene (NPSt). The synthesis of NPSt proceeded in two steps, in which the first steps entailed LAP and termination, followed by amidation (Scheme 1a). Synthesis of Primary Amino-Terminated Polystyrene by LAP and Termination (PSt-NH2). PSt-NH2 was synthesized according to a previous procedure.22,23 Anionic polymerization was performed in a sealed, all-glass system equipped with break-seals under high vacuum conditions (∼10−6 mmHg). sec-BuLi solution as initiator was transferred into the prewashed glass reactor and then cooled to −78 °C. A solution of styrene (32.3 mmol) in THF (20.7 mL) were poured to the sec-BuLi (1.40 mmol) in hexane (4.89 mL), and the styrene polymerized. After 1 h of polymerization, the reaction was terminated with 2,2,5,5-tetramethyl-1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane (T1, 4.20 mmol) in THF (10 mL) at −78 °C, and the mixture was reacted at room temperature for 3 h. The reaction solution was poured into excess cold methanol and then filtered. The obtained polymers were further purified by reprecipitation from excess cold methanol, filtered, and dried under vacuum. The product was deprotected during the precipitation step. The crude product was purified by silica gel column chromatography with gradient elution from pure

dichloromethane to pure tetrahydrofuran to remove unfunctionalized polystyrene. The desired PSt-NH2 was collected, concentrated under reduced pressure, dissolved in benzene, and then freeze-dried prior to characterization. The polymer was collected as a white solid (3.09 g, 89%). 1H NMR (400 MHz, CDCl3, δ): 7.23−6.26 (br), 2.53−0.77 (br), 0.75−0.52 (br) (see Figure S8). Synthesis of ω-End-Norbornyl Polystyrene by Amidation (NPSt). To a stirred solution of PSt-NH2 (2.01 g, 667 μmol) and N-(perfluorophenyl hexanoate)-cis-norbornene-exo-2,3-dicarboxiimide (T2, 0.890 g, 2 mmol) in dry THF (20 mL) under a nitrogen atmosphere was added DBU (100 μL, 667 μmol). The reaction mixture was stirred continuously at room temperature under a nitrogen atmosphere. After 72 h of reaction, the resulting solution was precipitated in excess cold methanol and then filtered. The obtained polymers were further purified by precipitation in excess cold methanol, filtered, dried in a vacuum oven at 40 °C, and collected (2.14 g, ∼100%, entry 1 in Table S1). The reaction conversion was quantitative based on 1H NMR analysis. Mn,NMR = 2.76 kDa, Mn,SEC = 2.69 kDa, Đ = 1.14. 1H NMR (400 MHz, CDCl3, δ): 7.23−6.17 (br), 6.27 (2H, s), 3.43 (2H, t), 3.26 (2H, s), 3.01 (2H, s), 2.66 (2H, s), 2.49−0.80 (br), 0.78−0.53 (br) (see Figure S10). Synthesis of ω-End-Norbornyl Poly(4-tert-butoxystyrene) (NPtBOS). The synthesis of NPtBOS followed same procedure as was used for NPSt, in which the first and second steps were proceeded by LAP, termination, and amidation, respectively (Scheme 1a). Synthesis of Primary Amino-Terminated Poly(4-tert-butoxystyrene) by LAP and Termination (PtBOS-NH2). 4-tert-Butoxystyrene (19.2 mmol) in THF (17 mL), sec-BuLi (1.29 mmol) in hexane (4.50 mL), and C

DOI: 10.1021/acs.macromol.7b02447 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 2,2,5,5-tetramethyl-1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane (T1, 3.87 mmol) in THF (9.21 mL) were used and collected as a white solid (2.84 g, 83%). 1H NMR (400 MHz, CDCl3, δ): 6.83−6.17 (br), 2.52−0.75 (br), 0.74−0.42 (br) (see Figure S9). Synthesis of ω-End-Norbornyl Poly(4-tert-butoxystyrene) by Amidation (NPtBOS). PtBOS-NH2 (2.24 g, 831 μmol), N-(perfluorophenylhexanoate)-cis-norbornene-exo-2,3-dicarboxiimide (T2, 1.11 g, 2.49 mmol), THF (22 mL), and DBU (124 μL, 837 μmol) were used used and collected as a white solid (2.38 g, ∼100%, entry 4 in Table S1). The conversion of the reaction was quantitative based on 1H NMR analysis. Mn,NMR = 3.13 kDa, Mn,SEC = 3.01 kDa, Đ = 1.06. 1H NMR (400 MHz, CDCl3, δ): 6.83−6.17 (br), 6.27 (s), 3.42 (2H, t), 3.24 (2H, d), 2.96 (2H, s), 2.64 (2H, t), 2.36−0.74 (br), 0.73−0.44 (br) (see Figure S11). Bottlebrush Homopolymerization of NPSt or NPtBOS. The ROMP of macromonomers was performed according to a previous report.11 A dry 5 mL vial was charged with macromonomers and a magnetic bar in a glovebox with a nitrogen atmosphere. the macromonomer was dissolved in THF ([MM]0 = 0.05 M), and then the solution was stirred until the mixture was homogeneous. A stock solution of G3 (9.63 mM in THF) was rapidly added to initiate polymerization. The desired amount of G3 was determined based on mole ratios ([MM]0/[G3]0). Aliquots of 20 μL were extracted from polymerization solution at regular intervals and immediately injected into solutions of a few drops of ethyl vinyl ether in 1 mL of THF to quench the reaction. The conversion, MW, and Đ for the kinetic studies were determined by SEC analysis of the samples. After the maximum conversion was reached, the polymerization was terminated by adding a few drops of ethyl vinyl ether. The mixture was purified by precipitation from excess cold methanol, filtered, dried in a vacuum oven at 40 °C, and collected. Synthesis of P(NB-g-St) ([NPSt]/[G3] = 650). NPSt (0.173 g, 62.6 μmol), THF (1252 μL), and a stock solution of G3 (10 μL, 0.0963 μmol, 9.63 mM in THF) were used and collected as a white solid (0.172 g, ∼100%). The conversion of the reaction was quantitative based on GPC. dn/dc = 0.184 mL/g. 1H NMR (400 MHz, CDCl3, δ): 7.23−6.27 (br), 5.86−5.25 (br), 3.32−0.78 (br), 0.77−0.51 (br) (see Figure S12). Synthesis of P(NB-g-tBOS) ([NPtBOS]/[G3] = 150). NPtBOS (0.0866 g, 28.8 μmol), THF (576 μL), and a stock solution of G3 (20 μL, 0.193 μmol, 9.63 mM in THF) were used and collected as a white solid (0.186 g, ∼100%). The conversion of the reaction was quantitative based on GPC. dn/dc = 0.122 mL/g. 1H NMR (400 MHz, CDCl3, δ): 6.83−6.17 (br), 5.84−5.17 (br), 3.85−0.74 (br), 0.73−0.42 (br) (see Figure S13). Bottlebrush Block Copolymerization of NPSt and NPtBOS (P(NB-g-St)-b-P(NB-g-tBOS)) ([NPSt]0/[G3]0 = [NPtBOS]0/[G3]0 = 150). In a glovebox, two dry 5 mL vials were separately charged with NPSt (0.0397 g, 14.4 μmol) and NPtBOS (0.0433 g, 14.4 μmol). The desired amount of THF (288 μL, [NPSt]0 = 0.05 M) was transferred to the reaction vessel via syringe, and then mixture was stirred. The first-stage polymerization of NPSt was carried out with the rapid addition of G3 stock solution (10 μL, 0.0963 μmol, 9.63 mM in THF) at room temperature for 3−6 min. An aliquot of the living P(NB-g-St) was withdrawn from the first block solution to characterize the homopolymer. Then, NPtBOS in THF (288 μL, [NPtBOS]0 = 0.05 M) was added to the living P(NB-g-St) solution via syringe, and the block copolymerization was continued at room temperature for 2 h. After termination by ethyl vinyl ether, the resulting polymer was purified by precipitation from a large excess of cold methanol. Bottlebrush block copolymers were obtained as a white solid (0.0822 g, ∼100%). The conversion of the reaction was quantitative based on GPC (Scheme 1c). dn/dc = 0.147 mL/g. 1H NMR (400 MHz, CDCl3, δ): 7.23−6.22 (br), 5.86−5.19 (br), 3.77−0.78 (br), 0.77−0.42 (br) (see Figure S14).

polystyrene (NPSt) and poly(4-tert-butoxystyrene) (NPtBOS) were designed to introduce large alkyl chain spacers and were used to quantitatively polymerize the end-capped norbornene as described in Scheme 1a,39,40 and the resulting macromonomers are summarized in Table S1. Before synthesizing the macromonomers, the PSt-NH2 and PtBOS-NH2 prepolymers were successfully prepared by LAP, and their reactions were quenched with 2,2,5,5-tetramethyl-1(3-chloropropyl)-1-aza-2,5-disilacyclopentane (T1). The conversion was not quantitative. Wurtz-type coupling reactions may explain the lower conversions despite the use of an alkyl chloride to avoid the side reaction; the susceptibility of the alkyl halides to Wurtz-type coupling follows the order RI > RBr > RCl.23,41 The pure ω-end-amino polystyrene were easily obtained from column chromatography, indicating the yield of the endfunctionalization reaction reached nearly 100%. The chemical structures of PSt-NH2 and PtBOS-NH2 were confirmed by 1 H NMR spectroscopy as shown in Figures S8 and S9. The ω-end-amino functionalization in polystyrene was confirmed by the appearance of the new peak at 2.43 ppm from the methylene protons adjacent to the primary amine. Subsequently, the amidation reaction between the primary amino-terminated prepolymer and the norbornene-containing pentafluorophenyl ester was performed; the reaction of the ester activated by the pentafluorophenyl group provided a highly efficient way to quantitatively incorporate functionalities into the α- or ω-ends and the side chains of polymers in commonly available solvents.39 The chemical structures of NPSt and NPtBOS were also confirmed by 1H NMR spectroscopy as shown in Figures S10 and S11. The peak at 2.43 ppm from the methylene protons near the amine had disappeared, but the new peaks at 6.28 and 3.02 ppm were indicative of the cyclic olefin of the norbornene and the methine protons adjacent to the amide bond, respectively. The quantitative conversion was confirmed by calculations based on 1H NMR analysis. By comparing the signals at 0.71−0.57 ppm from the two methyl groups in sec-BuLi and the peaks at 3.43, 3.26, and 2.66 ppm from the methylene in the norbornene moiety, the well-matched integration ratio of 3:1 was obtained, which supported the complete amidation between the polymers and the norbornene moiety. The results indicated that the combination of LAP and subsequent amidation is a powerful method for the synthesis of well-defined polymers as well as almost quantitative end-norbornene-functionalized polystyrene and poly(4tert-butoxystyrene). The information on macromonomers was summarized. Synthesis of Bottlebrush Homopolymers of NPSt or NPtBOS (P(NB-g-St) or P(NB-g-tBOS)). The bottlebrush homopolymers, P(NB-g-St) ([NPSt]0/[G3]0 = 150, 350, 650, 1050) and P(NB-g-tBOS) ([NPtBOS]0/[G3]0 = 150, 650), were successfully synthesized via ROMP as summarized in Table 1. All polymers shown were prepared quantitatively and with a high degree of control. While maintaining narrow MWDs (Đ ∼ 1.15), the absolute MWs (Mw ∼ 2928 kDa) were in good agreement with the theoretical values. The MWs of P(NB-g-St) and P(NBg-tBOS) were controlled by increasing the ratio of NPSt and NPtBOS to G3. The chemical structures of the bottlebrush homopolymers were confirmed by 1H NMR spectroscopy as shown in Figures S12 and S13. After polymerization, new peaks from the double bonds of the cis- and trans-conformers were obtained, and the cyclic olefin peak from the macromonomer had clearly disappeared. The efficient syntheses of P(NB-g-St)

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RESULTS AND DISCUSSION Synthesis of Norbornyl Polystyrene or Poly(4-tertbutoxystyrene) (NPSt or NPtBOS). The ω-end-norbornyl D

DOI: 10.1021/acs.macromol.7b02447 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Characteristics of Bottlebrush Homopolymers of NPSt or NPtBOS (P(NB-g-St) or P(NB-g-tBOS))a entry

MM

[MM]0/[G3]0

Mn,theob (kDa)

Mn,obsdc (kDa)

Mw,obsdc (kDa)

DPw

Đc

convd (%)

1 2 3 4 5 6

NPSt NPSt NPSt NPSt NPtBOS NPtBOS

150 350 650 1050 150 650

405 945 1755 2835 450 1950

399 910 1636 2736 458 1811

407 937 1701 2928 463 2081

151 347 630 1084 154 694

1.02 1.03 1.04 1.07 1.01 1.15

>99 >99 >99 >99 >99 >99

a Polymerization performed in THF at room temperature ([MM]0 = 0.05 M). bMn,theo = [MM]0/[G3]0 × conv/100% × MW of MMs. cDetermined from SEC-MALLS. dConversion from MM to bottlebrush homopolymer is determined by comparing the peak areas of bottlebrush polymer with residual MM from GPC measurement of the crude product.

Figure 1. GPC curves of the bottlebrush homopolymers of (a) NPSt or (b) NPtBOS.

and P(NB-g-tBOS) were confirmed by GPC (Figure 1). The chromatograms showed significantly narrow MWDs and unimodal curves. The SEC profile was clearly shifted from high to ultrahigh MWs depending on the mole ratio of NPSt to G3. Almost 100% conversion to the polymer was confirmed by the shift in the peak and the absence of residual low MW peaks after ROMP, which supported that the norbornene moiety was quantitatively incorporated into the ω-ends of the polystyrene. Kinetic Studies of P(NB-g-St). To enable the synthesis of well-defined bottlebrush block copolymers, the living nature of the propagation sites of first block during ROMP should be maintained without quenching the polymerization, which will allow them to fully initiate the reaction with the second monomers.5 The living character of the bottlebrush homopolymerization of NPSt was confirmed by kinetic studies as shown in Figure 2. The changes in the MWs, conversions, and MWDs were monitored by analyzing the extracted aliquots via SEC-MALLS. The SEC traces for P(NB-g-St) continuously developed with a clear shift to higher MWs, which is in contrast with the concomitant depletion of [NPSt]0 during polymerization (Figure S17). Complete conversion from NPSt to bottlebrush homopolymer was rapidly achieved within 240 s for [NPSt]0/ [G3]0 = 150 and 350 and within 480 s for [NPSt]0/[G3]0 = 650 (Figure 2b). On the other hand, the conversion for [NPSt]0/[G3]0 = 1050 is higher than 83% within 480 s, which showed the reaction slowed because of steric hindrance between the propagating active sites and monomers.20 The MWs of P(NB-g-St) were linearly correlated with increasing conver-

Figure 2. Kinetic profiles on ROMP of NPSt in THF at room temperature ([NPSt]0 = 0.05 M). (a) Plots of ln([NPSt]0/[G3]0) versus time. (b) Plots of Mw and Đ versus conversion. (c) Plots of Mw and Đ versus [NPSt]0/[G3]0.

sion in the polymerization. As shown in Figure 2a, the relationship between ln([NPSt]0/[NPSt]t) and polymerization time within the range of [NPSt]0/[G3]0 = 150, 350, 650, and 1050 followed first-order kinetics. The correlation between the MW and the feed ratios of NPSt to G3 was linear, which further supported the living nature of the P(NB-g-St) during polymerization (Figure 2c). These results indicate that the long alkyl spacers might significantly reduce steric hindrance during E

DOI: 10.1021/acs.macromol.7b02447 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Characteristics of Bottlebrush Block Copolymers of NPSt and NPtBOS (P(NB-g-St)-b-P(NB-g-tBOS))a [NPSt]0/[G3]0:

Mn,theob

Mn,obsdc

Mw,obsdc

entry

[NPtBOS]0/[G3]0

(kDa)

(kDa)

(kDa)

Đ

1 2 3 4 5 6 7 8

25:25 50:50 150:150 230:230 310:310 380:380 440:440 560:560

143 285 855 1311 1767 2166 2508 3192

136 264 806 1221 1583 1886 2115 2704

148 296 830 1331 1694 2056 2348 3055

1.09 1.12 1.03 1.09 1.07 1.09 1.11 1.13

c

f wt,NPStd

DPwd

conv (%)e

(%)

NPSt:NPtBOS

NPSt

NPtBOS

51 50 49 49 49 50 51 50

28:24 55:49 151:140 230:237 307:287 381:341 444:382 566:507

>99 >99 >99 >99 >99 >99 >99 >99

>99 >99 >99 >99 >99 >99 >99 >99

Polymerizations performed in THF at room temperature ([NPSt]0 = [NPtBOS]0 = 0.05 M). bMn,theo = [NPSt]0/[G3]0 × conv (%)/100 × MW of NPSt + [NPtBOS]0/[G3]0 × conv (%)/100 × MW of NPtBOS. cDetermined from SEC-MALLS. df wt,NPSt: weight fraction of NPSt in feeding ratios of NPSt and NPtBOS. eConversion from NPSt and NPtBOS to bottlebrush block copolymers is determined by comparing the peak areas of bottlebrush polymers with residual NPSt and NPtBOS from GPC measurement of the crude product. a

ROMP and resulted in well-defined bottlebrush homopolymers with living characteristics at high DPs (DPw = 1084). Synthesis of Bottlebrush Block Copolymers of NPSt and NPtBOS (P(NB-g-St)-b-P(NB-g-tBOS)). When considering the living nature of P(NB-g-St) polymerization, the block copolymerization is one of the most important pieces of evidence. The BBCPs were synthesized by sequential ROMP of NPSt with PtBOS, as shown in Scheme 1c. Table 2 shows the quantitative conversion and precise synthesis of BBCPs. The MWDs values of the resulting polymers were less than 1.13, and the MWs were predictable. The results of absolute MWs were in good agreement with the expected values. The MWs could be increased by changing the mole ratio of NPSt and NPtBOS to G3; MWs ranging from high (Mw = 148 kDa, Đ = 1.09) to ultrahigh MW (Mw = 3055 kDa, Đ = 1.13) could be obtained. Each block was designed to be equal in size to produce a lamellar morphology. Although it was difficult to determine each block ratio from 1H NMR analysis due to overlapping signals, the 100% conversion facilitated the calculation of each block ratio; the ratios could be determined from the feeding ratio of NPSt and NPtBOS. The 1H NMR spectrum of P(NB-g-St)-b-P(NB-g-tBOS) proved the chemical structure of BBCPs (Figure S14). The chemical structures of P(NB-g-St)-b-P(NB-g-tBOS) were confirmed by the appearance of a new peak at 1.43 ppm from the tert-butyl moiety of the PtBOS, and this peak was absent in the spectrum of the P(NB-g-St) bottlebrush homopolymer. The signals from the cyclic vinyl group of NPSt and NPtBOS clearly disappeared after ROMP, while the new peaks of the double bonds in the main backbone were observed. GPC characterization confirmed the successful and precise synthesis of BBCPs (Figure 3). The sequential living ROMP was confirmed by the clear shift in the GPC curves from the starting macromonomer (NPSt) to the first block stage of the bottlebrush homopolymer (P(NB-g-St)) and then toward BBCPs (P(NB-gSt)-b-P(NB-g-tBOS)) in the higher MW region (Figure 3b). The curves of the resulting polymers lacked the residual low MW peaks, which indicated the polymerization of NPSt and NPtBOS exclusively proceeded with high conversion. The SEC profiles given in Figure 3a of the BBCPs also exhibited clear shifts from the high to ultrahigh MW regions as the feeding ratio of NPSt and NPtBOS to G3 changed while simultaneously maintaining narrow unimodal curves. The results of the bottlebrush block copolymerization indicated the living nature of the first block plays an important role in the successful synthesis of well-defined BBCPs.

Figure 3. SEC profile of (a) bottlebrush block copolymers (Table 2, entries 1−4 and 8) and from (b) macromonomer (NPSt) to bottlebrush homopolymer (P(NB-g-St)) and then to bottlebrush block copolymer (P(NB-g-St)-b-P(NB-g-tBOS)) at [NPSt]0/[G3]0 = [NPtBOS]0/[G3]0 = 150 (Table 2, entry 3).

Phase Separation of P(NB-g-St)-b-P(NB-g-tBOS). The influence of the ultrahigh MWs was demonstrated by the investigation of the phase-separated BBCPs, in which their periodic structures resulted in structural colors in the visible range. Thin films were fabricated by direct thermal compression, which was carried out by placing polymer powder between two glass substrates. The resulting self-assembly behaviors were confirmed by FE-SEM of the cross-sectional images, as shown in Figure 4a,c,e,g. The preference for ordered, lamellar structures was observed over a large region of the samples in which they comprised almost equal volume ratios of each block. The entire films showed that the stacked microdomains were parallel to the substrates. The confinement effect involved in thermal compression from the two sandwiched substrates may influence the morphology and orientation of the material.42 Domain sizes were improved from 40 to 200 nm when the MWs of BBCPs were increased from 296 to 3055 kDa. The results suggested that the MWs of the polymers caused large enough domains F

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photonic films corresponded to the reflectance peaks of λmax = 440 nm from Mw = 1694 kDa, 550 nm from Mw = 2056 kDa, 600 nm from Mw = 2348 kDa, and 640 nm from Mw = 3055 kDa as the primary optical bands, whereas the peaks below MW of 1694 kDa were not detected due to overlap with the signal of glass substrate. The observed reflection spectra gradually shifted as the MW of the bottlebrush block copolymers increased. This shift was rationalized by the equation for 1D photonic crystals given by λmax = 2(n1x1 + n2x2), which indicates that the domain spacing (x1 or x2) is related to the MW of the block copolymers.46 In future studies, we will further synthesize a different type of BBCPs for the potential application of this material in photonic crystals with large domain sizes and appropriate contrasts in the refractive index.

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CONCLUSIONS We demonstrated a facile strategy for the synthesis of welldefined bottlebrush block copolymers (BBCPs) of ω-endnorbornyl polystyrene (NPSt) and poly(4-tert-butoxystyrene) (NPtBOS) with ultrahigh MWs via living anionic polymerization (LAP) and ring-opening metathesis polymerization (ROMP). NPSt (Mn = 2.76 kDa, Đ = 1.14) and NPtBOS (Mn = 3.13 kDa, Đ = 1.06) were precisely synthesized by LAP and subsequent amidation due to the living nature of polystyrene derivatives and the high reactivity of norbornyl acvated ester, respectively. From the kinetic study, the bottlebrush homopolymers of NPSt showed predictable ultrahigh molecular weights (MWs, Mw ∼ 2928 kDa), quantitative conversion, and narrow molecular weight distributions (MWDs, Đ ∼ 1.07). The living characteristics of the ROMP of the NPSt were sustainable at the high degree of polymerizations (DPw = 1084) because there was the large alkyl chain spacers and appropriate DPs of side chains of bottlebrush polymer. The BBCPs (P(NB-g-St)-b-P(NBg-tBOS)) synthesized via sequential ROMP were prepared with remarkably well-controlled ultrahigh MWs (Mw ∼ 3055 kDa) and narrow MWDs (Đ ∼ 1.13). The phaseseparated BBCPs were used to make fundamental dielectric mirrors, which confirmed the effect of ultrahigh MWs.

Figure 4. Cross-sectional SEM images and photographs of films of P(NB-g-St)-b-P(NB-g-tBOS) with (a, b) Mw = 1694 kDa (Table 2, entry 5), (c, d) Mw = 2056 kDa (Table 2, entry 6), (e, f) Mw = 2348 kDa (Table 2, entry 7), and (g, h) Mw = 3055 kDa (Table 2, entry 8). (i) Plot of reflectance as a function of wavelength with Mw = 1694 (blue), 2056 (green), 2348 (yellow), and 3055 kDa (pink).

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ASSOCIATED CONTENT

* Supporting Information S

that these polymers can be used to prepare dielectric mirrors.43,44 The morphologies of the BBCP with a low DPw (= 52) (Table 2, entry 1) showed features characteristic of disordered and amorphous structures (Figure S15). Self-assembly of the BBCPs was involved in the phase separation between PS and the PtBOS side chains. When the DPw of backbone is decreased, the reduced degree of incompatibility lessens the motivation for phase separation due to strong segregation theory.45 Furthermore, it took less than 3 h to achieve phase separation of the bottlebrush block copolymers through direct thermal compression above their glass transition temperatures; this observation is considered a feature of the rapid self-assembly behavior of the material. Although conventional linear block copolymers with ultrahigh molecular weights rarely self-assemble, the self-assembly of bottlebrush-type polymers is negligible due to the suppressed chain entanglement characteristics.2,3 After complete phase separation, polymer samples corresponding to Mw of 1694, 2056, 2348, and 3055 kDa appeared blue, green, yellow, and pink to the naked eye (Figure 4). Subsequently, the reflectance of BBCPs was quantitatively measured by a UV/vis/NIR spectrophotometer to confirm the structural colors shown in Figure 4b,d,f,g. The colors of the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02447. Figures S1−S14: NMR analysis of synthesized chemical compounds; Figures S15−S16: the additional characterization of bottlebrush block copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+82)-62-715-2306 (J.-S.L.). ORCID

Robert H. Grubbs: 0000-0002-0057-7817 Jae-Suk Lee: 0000-0002-6611-2801 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future G

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Planning (NRF-2015R1A2A1A01002493) and by the GIST Research Institute (GRI) grant funded by the GIST in 2017.

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