Letter Cite This: ACS Macro Lett. 2018, 7, 65−69
pubs.acs.org/macroletters
Lewis Pair-Mediated Surface-Initiated Polymerization Liman Hou,†,§ Yongjiu Liang,† Qianyi Wang,‡ Yuetao Zhang,*,‡ Dewen Dong,† and Ning Zhang*,† †
Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China § University of the Chinese Academy of Sciences, Beijing 100864, China S Supporting Information *
ABSTRACT: We present the first example of surface-initiated polymerization mediated by Lewis pairs for the synthesis of polymer brushes on planar substrates. The method enables the rapid grafting polymerization from the self-assembled monolayer or surface-attached macroinitiators, furnishing linear polymer brushes and bottle-brush brushes. Both homopolyester and block copolyester brushes can be synthesized via this versatile approach. This work not only opens up new opportunities for the application of Lewis pair-mediated polymerization but also enriches the surface-initiated polymerization on different surfaces. he chemistry of “frustrated Lewis pairs” (FLPs) has entered a rapid growth stage since the FLP concept was uncovered by the seminal work of Stephan and Erker.1−3 Various FLP systems have been developed in view of their ease of preparation, low cost, and diverse Lewis acid (LA) and Lewis base (LB) sources.4−6 The application of the FLP motif ranges from organic synthesis, transition-metal and free-radical chemistry, material science, and enzymatic models to surface chemistry and will undoubtedly continue to evolve especially when reduced costs and elimination of toxic contaminants are considered.7−12 Since the first pioneering attempts in the polymerization of polar monomers using FLPs and classical Lewis pairs (CLPs) by Chen and co-workers, Lewis pairmediated polymerization (LPP) has attracted great attention due to its exceptional reactivity, excellent selectivity, and wide monomer scope.13,14 To date, different types of monomers have been successfully polymerized by FLPs and CLPs to achieve high molecular weight polymers with narrow molecular weight distributions (Đ = Mw/Mn).15 For instance, methyl methacrylate and naturally renewable methylene butyrolactone can be rapidly polymerized by Al(C6F5)3 (Alane)/phosphines or carbene FLPs via zwitterionic phosphonium or imidazolium enolaluminate active species.13−15 Recent advances have expanded the monomer scope to polar N-containing vinyl monomers. For instance, 2vinylpyridine and 2-isopropenyl-2-oxazoline can be efficiently polymerized in a similar fashion.16 It was demonstated by Erker and co-workers that the addition of FLPs to nitric oxide gave a new family of aminoxyl radicals, which could induce the controlled nitroxide-mediated radical polymerization (NMP) of styrene.17 Controlled oligomerization rather than thermodynamically favored cyclization of cyanamides can be achieved in the presence of aluminum−nitrogen-based Lewis pairs.18 N-
T
© XXXX American Chemical Society
Heterocyclic olefin (NHO)/alane pairs were recently demonstrated to be efficient in the polymerization acrylates and ringopening polymerization of lactone.19,20 In a recent report, the combination of phosphorus-containing LB and organoaluminum LA was demonstrated to be effective for the living polymerization of a broad variety of sterically demanding and functionalized monomers with high initiator efficiencies.21 Besides the success achieved in the polymerization of various nonpolar and polar monomers, frustrated Lewis pair-mediated polymerization (FLPP) also exhibited excellent regioselectivity for the polymerization of divinyl polar monomers in which polymerization only occurs at the conjugated methacrylic C C bond with the pendant nonconjugated CC bond unreacted.22 With the development of solution polymerization techniques, surface-initiated polymerization (SIP) allowing the tailoring of the surface with diverse functionalities has become a key method in surface modification and functionalization.23−25 To date, almost all polymerization strategies have been transplanted to SIP for the synthesis of polymer brushes with high grafting density, defined composition and architecture;26−31 however, to our knowledge, SIP mediated by Lewis pairs (LPs) has not been achieved yet, in spite of the successful application of LPs in solution polymerization and the great potential in achieving novel polymer composition and architectures. As a robust polymerization tool, LPP will give access to a series of polar polymer coatings that cannot be efficiently realized by traditional techniques. Early examples of polymerization Received: November 15, 2017 Accepted: December 14, 2017
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DOI: 10.1021/acsmacrolett.7b00903 ACS Macro Lett. 2018, 7, 65−69
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Scheme 1. Schematic Illustration for the Preparation of MMSPII SAMs (a) and Grafted Polymer Chain (b) Bearing NHO Functionalities on Silicon Wafer and Subsequent SI-LPP of δ-VL or ε-CL to Afford Linear Polymer Brushes and BBBs
mediated by CLPs or FLPs are suffering from the low initiator efficiencies and poor controllability, which are unfavorable for a defined and efficient SIP.32,33 Herein, we developed two initiating systems, i.e., the combination of self-assembled monolayers (SAMs) with NHO ends or surface-attached polyethylene bearing NHOs with Alane, which promoted the SIP of lactones to give linear polymer brushes and bottle-brush brushes (BBBs). For the introduction of LPP to a defined SIP, the silanization reaction of 1,2-dimethyl-3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium iodide (MMSPII) on the oxidized silicon substrates was employed to afford the SAMs of MMSPII according to the established procedure, 34 which were subsequently deprotonated by KH to form SAMs with NHO terminals (NHO SAMs) (Scheme 1). The chemical structure of MMSPII was confirmed by 1H NMR, and the formation of MMSPII SAMs was verified by XPS measurement (Figure S1 and S2). The subsequent reaction of MMSPII SAMs with KH resulted in the formation of NHO SAMs,35 which was confirmed by the XPS characterization (Figure S3). Infrared (IR) characterization of the NHO SAMs was attempted, but no ideal spectrum was obtained probably due to the traces of NHO presented on the substrate and the extreme sensitivity of NHO to air and moisture. For LPP using Alane as LA and NHO as LB, preactivation of the monomers by Alane is a prerequisite due to the fact that a highly stable NHO·Alane adduct can be easily formed which can hardly initiate a polymerization.21 For the following surfaceinitiated Lewis pair-mediated polymerization (SI-LPP), the freshly prepared NHO SAMs were added immediately to the premixed δ-valerolactone (δ-VL) monomer and Alane to trigger the surface-initiated polymerization. After the SI-LPP, the substrate was rigorously sonicated in several solvents with different polarities to ensure the removal of any physisorbed salts, small molecules, and polymers. Evidence for the formation of PVL coating was provided by IR spectroscopy. As shown in Figure 1, the appearance of a strong band at around 1732 cm−1 assigned to the CO stretching mode indicates the presence of PVL brushes on the substrate. XPS measurement also confirms the formation of PVL brushes (Figure S5). AFM measurement indicates that the silicon substrate was covered with a 12 nm thick uniform polymer coating after the SI-LPP of δ-VL for 3 h (Figure S7). This layer thickness is
Figure 1. IR spectra of PVL brushes grafted on MMSPII SAMs (black), PNVI brushes (red), PIMI brushes (blue), and P(NHO-bVL) (orange) on silicon substrates.
typical for polymer brushes prepared under strict reaction conditions.36,37 The highly reactive surface-initiating sites may be decomposed upon the addition of monomer, solvent, or catalyst due to the presence of traces of water. Moreover, the dense and rigid SAMs may limit the accessibility of the terminal NHO moieties for the bulky Alane-activated δ-VL.21,34 In any case, the formation of PVL brushes with a low grafting density is expected. To improve the grafting density and further enrich the surface-attached polymer topology via this method, surfaceattached polymer with NHO groups, i.e., poly(NHO) brushes on a silicon wafer, was prepared to perform the SIP: (1) selfinitiated photografting and photopolymerization (SIPGP) of Nvinyl imidazole (NVI) resulted in the formation of poly(Nvinylimidazole) (PNVI) brushes on the H-terminated silicon substrate (Scheme 1) and (2) postfunctionalization reactions of PNVI brushes afforded surface-bonded macroinitiator, i.e., poly(NHO) brushes. Following our reported procedure, 66
DOI: 10.1021/acsmacrolett.7b00903 ACS Macro Lett. 2018, 7, 65−69
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ACS Macro Letters SIPGP of NVI enabled direct photografting and the formation of PNVI brushes on H-terminated silicon substrates via the thermal and chemically stable Si−C bond.34 After SIPGP, the substrate was rigorously cleaned by ultrasonication in several solvents with different polarities to ensure that only chemically grafted polymer remains on the substrate. The presence of grafted PNVI on silicon wafer was confirmed by the appearance of characteristic vibrational modes of CN at 1665 cm−1 and CC at 1640 cm−1 in the IR spectrum (Figure 1). AFM measurements revealed a PNVI layer with a thickness of ∼144 nm was formed on the substrate after the SIPGP of NVI for 24 h (Figure S8a). Successively, we conducted polymer analogue reaction of the as-prepared PNVI brushes with BuLi and subsequent CH3I, resulting in the methylation of an imidazole ring (Scheme 1).38 Further reaction with MeI at elevated temperature gave poly(imidazolium iodine) (PIMI) brushes which were converted to poly(NHO) brushes after the reaction with KH.21 The successful conversion of PNVI to PIMI brushes was confirmed by IR spectroscopy (Figure 1). The appearance of new bands at 1584 and 1558 cm−1 is typical for a substituted imidazolium ring, demonstrating the high conversion from imidazole to methyl-substituted imidazolium moieties.39 The brush thickness was monitored by AFM measurement at the scratched edge of the polymer coating. Polymer layer thickness increased drastically from 144 nm for PNVI to 530 nm for PIMI brushes due to the electrostatic repulsion between the adjacent cationic imidazolium groups and thus the stretched conformation of PIMI. The deprotonation of PIMI brushes by KH resulted in a significant decrease of polymer layer thickness from 530 to 214 nm for poly(NHO) brushes (Figure S8). The massive decrease of thickness is attributed to the absence of charge repulsion after the deprotonation reaction. Furthermore, the formation of poly(NHO) brushes was confirmed by XPS measurement (Figure S6). Poly(NHO) brush layer serving as a Lewis base macroinitiator precursor was combined with Alane to cooperatively promote the polymerization of δ-VL following the above-mentioned procedure. The SI-LPP was expected to result in the formation of PVL side chains to give BBBs. The formation of poly(N-heterocyclic olefin-graft-δ-valerolactone) [P(NHO-g-VL)] was confirmed by the presence of a strong peak at 1730 cm−1 assigned to CO stretching vibrational modes of the ester group from PVL in the IR spectrum (Figure 1), suggesting the successful SI-LPP. BBB structures are intriguing and widely exist in proteoglycans.40 Inspired by nature, researchers have synthesized various BBBs and found that these architectures play a significant role in cell adhesion, motility, proliferation, stem cell differentiation, and tissue morphogenesis.41−43 Our approach provides an efficient alternative for the preparation of BBBs with polyester side chains and shows potential for the realization of BBB with other polymer compositions due to the wide applicable monomer scope of LPP. AFM measurement of P(NHO-g-VL) brushes reveals a homogeneous coverage of the entire substrate and an almost constant layer thickness increase with polymerization time (Figure 2), owing to the living polymerization mechanism promoted by NHO/Alane.21 So far, a detailed picture about the grafting efficiency can not be provided. Nevertheless, the systematic thickness indicates the SI-LPP can be performed in a reproducible and consistent fasion. In order to demonstrate the general applicability and further the livingness of the established strategy, SI-LPP of ε-
Figure 2. Tapping mode AFM height images of a poly(NHO) layer on a silicon wafer and polymer brushes after SI-LPP of δ-VL after different polymerization time. (a) Poly(NHO) layer with a thickness of 214 nm and (b−d) SI-FLPP of δ-VL on the same substrate after 0.5 h, 1.5 h, and 3 h results in 256, 360, and 463 nm thick polymer brush layers, respectively. (e) P(NHO-g-VL) layer thickness as a function of SI-LPP time.
caprolactone (ε-CL) was performed using the poly(NHO) brush as the surface-attached macroinitiator following the above-described procedure, yielding uniform BBB with poly(εcaprolactone) (PCL) side chains. Similarly, the thickness as revealed by AFM increased from 214 nm for poly(NHO) to 435 nm for BBB with PCL side chains [P(NHO-g-CL), Figure S9]. Moreover, sequantial SI-LPP was conducted by using δ-VL as the first block monomer and ε-CL as the second block monomer. After the first sequence polymerization, polymer layer thickness increased from 214 nm for poly(NHO) to 430 nm for P(NHO-g-VL) brushes (Figure S10b). Additional sequence of the monomer ε-CL into the polymerization system resulted in a further increase of polymer layer thickness to 550 nm, indicating the successful block copolymerization (Figure S10c). The surface hydrophilic/hydrophobic character of the prepared polymer coatings was investigated by contact angle (CA) measurements (Figure 3). MMSPII SAM-modified silicon substrate had a static water CA of 21 ± 3°. After SILPP, the value increased to 63 ± 2° for linear PVL brushes or 67
DOI: 10.1021/acsmacrolett.7b00903 ACS Macro Lett. 2018, 7, 65−69
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is greatly acknowledged. N. Z. thanks the support of Youth Innovation Promotion Association CAS (2016207).
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(1) Stephan, D. W. Frustrated Lewis Pairs. J. Am. Chem. Soc. 2015, 137, 10018−10032. (2) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Reversible, Metal-Free Hydrogen Activation. Science 2006, 314, 1124− 1126. (3) Stephan, D. W.; Erker, G. Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More. Angew. Chem., Int. Ed. 2010, 49, 46− 76. (4) Stephan, D. W.; Erker, G. Frustrated Lewis Pair Chemistry: Development and Perspectives. Angew. Chem., Int. Ed. 2015, 54, 6400−6441. (5) Stephan, D. W. The broadening reach of frustrated Lewis pair chemistry. Science 2016, 354, aaf7229. (6) Erker, G. Frustrated Lewis pairs: Some recent Developments. Pure Appl. Chem. 2012, 84, 2203−2217. (7) Forrest, S. J. K.; Clifton, J.; Fey, N.; Pringle, P. G.; Sparkes, H. A.; Wass, D. F. Cooperative Lewis Pairs Based on Late Transition Metals: Activation of Small Molecules by Platinum(0) and B(C6F5)3. Angew. Chem., Int. Ed. 2015, 54, 2223−2227. (8) Kalz, K. F.; Brinkmeier, A.; Dechert, S.; Mata, R. A.; Meyer, F. Functional Model for the [Fe] Hydrogenase Inspired by the Frustrated Lewis Pair Concept. J. Am. Chem. Soc. 2014, 136, 16626−16634. (9) Sumerin, V.; Schulz, F.; Nieger, M.; Leskelä, M.; Repo, T.; Rieger, B. Facile heterolytic H2 activation by amines and B(C6F5)3. Angew. Chem., Int. Ed. 2008, 47, 6001−6003. (10) Ménard, G.; Hatnean, J. A.; Cowley, H. J.; Lough, A. J.; Rawson, J. M.; Stephan, D. W. C−H Bond Activation by Radical Ion Pairs Derived from R3P/Al(C6F5)3 Frustrated Lewis Pairs and N2O. J. Am. Chem. Soc. 2013, 135, 6446−6449. (11) Ghuman, K. K.; Wood, T. E.; Hoch, L. B.; Mims, C. A.; Ozin, G. A.; Singh, C. V. Illuminating CO2 reduction on frustrated Lewis pair surfaces: investigating the role of surface hydroxides and oxygen vacancies onnanocrystalline In2O3‑x(OH)y. Phys. Chem. Chem. Phys. 2015, 17, 14623−14635. (12) Ye, J.; Johnson, J. K. Design of Lewis Pair-Functionalized Metal Organic Frameworks for CO2 Hydrogenation. ACS Catal. 2015, 5, 2921−2928. (13) Xu, T.; Chen, E. Y.-X. Probing Site Cooperativity of Frustrated Phosphine/Borane Lewis Pairs by a Polymerization Study. J. Am. Chem. Soc. 2014, 136, 1774−1777. (14) Zhang, Y.; Miyake, G. M.; Chen, E. Y.-X. Alane-Based Classical and Frustrated Lewis Pairs in Polymer Synthesis: Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High-Molecular-Weight Polymers. Angew. Chem., Int. Ed. 2010, 49, 10158−10162. (15) Zhang, Y.; Miyake, G. M.; John, M. G.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. Alane-Based Classical and Frustrated Lewis Pairs in Polymer Synthesis: Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High-MolecularWeight Polymers. Dalton Trans. 2012, 41, 9119−9134. (16) He, J. H.; Zhang, Y. T.; Chen, E. Y.-X. Synthesis of Pyridineand 2-Oxazoline-Functionalized Vinyl Polymers by Alane-Based Frustrated Lewis Pairs. Synlett 2014, 25, 1534−1538. (17) Sajid, M.; Stute, A.; Cardenas, A. J. P.; Culotta, B. J.; Hepperle, J. A. M.; Warren, T. H.; Schirmer, B.; Grimme, S.; Studer, A.; Daniliuc, C. G.; Fröhlich, R.; Petersen, J. L.; Kehr, G.; Erker, G. N,N-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals. J. Am. Chem. Soc. 2012, 134, 10156− 10168. (18) Holtrichter-Röβmann, T.; Isermann, J.; Rösener, C.; Cramer, B.; Daniliuc, C.-G.; Kösters, J.; Letzel, M.; Würthwein, E.-U.; Uhl, W. An Aluminum-Nitrogen Based Lewis Pair as an Effective Catalyst for the Oligomerization of Cyanamides: Formation of Acyclic C-N Oligomers Instead of Thermodynamically Favored Cyclic Aromatic Trimers. Angew. Chem., Int. Ed. 2013, 52, 7135−7138.
Figure 3. Static water CA of MMSPII SAMs and different polymer coatings on silicon substrates at room temperature.
72 ± 3° for linear PCL bruhes. Similarly, the PNVI brush gave a static CA of 45 ± 2°, which increased to 77 ± 4° for BBB with PVL side chains and 88 ± 5° for BBB with PCL side chains. It is interesting to note that silicon substrate grafted with linear PVL showed similar CA in comparison to PVL thin film that was spin-coated on a silicon substrate.44,45 BBBs with PVL or PCL side chain all showed greater CAs compared to the linear counterparts. We attribute the significant CA increase to the more crowding environment exhibited by BBB architecture. Our finding is consistent to previous reports that the surface wettability of polymer brushes is strongly dependent on the grafting density.46,47 In summary, we have demonstrated the first example of SILPP for the preparation of polymer coatings on planar substrates. SAMs and surface-grafted polymer chains with NHO functionalities in combination with Alane are effective for the catalytic polymerization of lactones, yielding linear polymer brushes and BBBs. The current work is undoubtedly a novel application of LPs and is anticipated to enrich surface-initiated polymerizations, thus providing novel options for surface modification.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00903. Experimental procedures and characterization data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*N. Zhang. E-mail:
[email protected]. *Y. Zhang. E-mail:
[email protected]. ORCID
Yuetao Zhang: 0000-0002-6406-1959 Ning Zhang: 0000-0003-3516-1907 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of this work by the National Natural Science Foundation of China (51673194 and 51373170 to N. Zhang; 21422401, 21374040 to Y. T. Zhang) and Departments of Science and Technology of Jiangsu Province (BK20151189) 68
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