Article pubs.acs.org/Macromolecules
ADMET Polymers Containing Precisely Spaced Pendant Boronic Acids and Esters Chester Simocko, Thomas C. Young, and Kenneth B. Wagener* Center for Macromolecular Science and Engineering, The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Precise aryl boronic ester- and acid-containing polymers have been synthesized via acyclic diene metathesis. High-molecular weight phenyl boronic acid polymers were synthesized. Cross-linked phenyl boronic acid polymers were also synthesized and demonstrate a unique crystallization behavior not usually seen in cross-linked polymers.
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INTRODUCTION Polymers having boron-centered Lewis acids as pendant groups are used for a variety of applications. As catalyst supports, polymers containing tetraarylboronates have been used to protect highly active catalysts, including covalently bound ammonium tetrakis(pentafluorophenyl) borate,1 and [Rh(cod) (dppb)]+(OTf)−.2 Diblock copolymers containing hydrophilic poly(dimethylacrylamide) and a block with a boronic acid repeat unit have been used as glucose sensors and drug delivery vehicles.3−6 Boronate polymers have also been used as films; for example, the Bazan group has fabricated a bilayer p−n junction of poly(fluorene-co-phenylene) containing a pendant cationic electrolyte with a fluoride counterion.7 The neutral layer contains a conjugated polymer with pendant dimesityl borane, which binds fluoride ions. When a charge is applied, the fluoride ions migrate from the charged layer into the neutral boron-containing layer. Devices made from these polymers have displayed superior light-emitting and current rectification performance.7 Many of the properties displayed by these boron-containing Lewis acid polymers rely heavily on polymer morphology. To study these effects, boron-containing polymers with controlled morphologies are needed. Such precise control can be achieved using acyclic diene metathesis (ADMET) polymerization. Creating a precise polymer starts with the synthesis of a symmetric α,ω-diene, with the desired functional group having the same number of carbons as the terminal olefins, usually three, six, or nine carbons, although longer examples have been made.8 Olefin metathesis has been shown not only to be compatible with boron-centered Lewis acids but also to enhance the performance of Grubbs type catalysts.9 These precision polymer systems can lead to unique morphologies10 that can help elucidate the morphology of existing boron-containing polymer systems, as well as potentially enhance their performance. © XXXX American Chemical Society
RESULTS AND DISCUSSION
Precision Boronic Ester Polymers. Synthesis of 4,4,5,5Tetramethyl-2-{4-[(tricosa-1,22-dien-12-yloxy)methyl]phenyl}-1,3,2-dioxaborolane (Figure 1). Synthesis of 3
Figure 1. Synthesis of 4,4,5,5-tetramethyl-2-{4-[(tricosa-1,22-dien-12yloxy)methyl]phenyl}-1,3,2-dioxaborolane (6) via ether synthesis of 3 with 4-bromobenzyl bromide followed by a Grignard reaction to attach the boronic ester.
followed procedures previously reported in the literature.11 Compound 3 was then reacted with excess sodium hydride and 4-benzyl bromide (4) to form ether 5 in 90% yield. The boronic ester was then attached using a Grignard reaction. Compound 5 was reacted with magnesium metal to form the Grignard reagent, which was then reacted with Received: June 27, 2015 Revised: July 25, 2015
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DOI: 10.1021/acs.macromol.5b01410 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane to form the final monomer 6. The yield for this reaction was 60%, but it was decreased in the presence of any hydrolyzed boronic ester reagent. This decrease in yield was not stoichiometric in nature, as would occur in the presence of 2-propanol; instead, even a small amount of hydrolyzed reagent resulted in a considerable reduction in yield, on the order of 40 or 50%. Polymerization and Characterization of Poly(4,4,5,5tetramethyl-2-{4-[(tricosa-1,22-dien-12-yloxy)methyl]phenyl}-1,3,2-dioxaborolane) (UPP6). Monomer 6 was first polymerized using a Grubbs first-generation catalyst under bulk conditions. However, because of the high viscosity of the monomer and subsequent oligomer, only low molecular weights of ∼2500 g/mol were achieved. In an attempt to improve molecular weights, ionic liquids were employed following previously reported techniques.12−14 In this system, 6 was dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6) along with 1 mol % Grubbs first-generation catalyst (Figure 2). After polymerization for 96 h,
Figure 3. DSC of the unsaturated precision boronic ester polymer (top) and the saturated boronic ester polymer (bottom).
Figure 2. Polymerization and hydrogenation of a precise boronic ester-containing polymer.
the polymer was extracted and precipitated in ethanol and analyzed via gel permeation chromatography, resulting in an Mn of 58000 with a polydispersity index of 1.86. This polymer was then analyzed for thermal characteristics via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA of the unsaturated polymer UPP6 (Figure SI1) demonstrates good thermal stability with no real decomposition until 350 °C, followed by complete decomposition at 450 °C. DSC of UPP6 (Figure 3) displays a Tg of approximately −23 °C. The Tg is comparable to that of the identically spaced phenyl sulfonic ester polymer produced by ADMET (−27 °C).15 As expected, no Tm was observed because of the unsaturation in the backbone and the bulky, noninteracting pendant group. The remaining double bonds were then hydrogenated to yield a pure polyethylene backbone (Figure 4). While Pd/C is traditionally used for hydrogenation of the residual double bonds, it was avoided in this case because of its propensity to cleave benzylic ethers. Instead, Wilkinson’s catalyst was first used. However, the catalyst caused cleavage of the benzylic ether pendant group. This was determined via nuclear magnetic resonance (NMR) by observing an increase in the relative number of backbone hydrogens in relation to the benzylic protons. After this was discovered, a more tolerant noncatalytic hydrogenation method was used. The unsaturated polymer was combined with 3 equiv of p-toluenesulfonyl hydrazine (TSH) and tripropylamine (TPA) dissolved in o-xylene. After a bubbler had been attached, the reaction mixture was refluxed until nitrogen was no longer
Figure 4. Hydrogenation of the phenyl boronic ester polymer via Wilkinson’s catalyst (top) and TSH (bottom).
being evolved from the reaction vessel. After addition of more TSH and TPA, the mixture was refluxed until no more nitrogen was released. The solvent was removed, and the polymer was analyzed via 13C and 1H NMR to determine whether complete saturation was achieved and cleavage of the benzylic ether was avoided. The spectra showed that no loss of the pendant group occurred, and that complete hydrogenation was achieved. Quantitative removal of the pinacol ester was not possible, so the free boronic acid could not be studied with this polymer. The hydrogenated polymer SPP6 was then analyzed via DSC (Figure 3). This saturated polymer also displayed no melting temperature; however, the glass transition temperature increased to approximately −11 °C. The increase in Tg is consistent with the thermal behavior of the identically spaced phenyl sulfonic ester polymer [Tg(satd) = 24 °C].15 The endotherm seen at −36 °C is not observed in the DSC results B
DOI: 10.1021/acs.macromol.5b01410 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Synthesis of in-chain boronic acid monomers (16−18). The alkenyl groups are attached via ether synthesis; then the boronic acid is added using a standard lithiation reaction.
of the unsaturated polymer and is believed to be caused by the precision nature of the polymer. This allows the phenyl boronic ester pendant groups to stack, creating their own melting endotherm. This hypothesis is supported by the work of Watson et al.,16 who reported a precision polymer with a phenyl ring on every 19th carbon that displayed similar behavior, presumably from phenyl ring stacking. Precision Boronic Acid Polymers. Synthesis of In-Chain Boronic Acid Monomers (Figure 5). The alkenyl bromides with spacers of three, six, and nine carbons (10, 9, and 2, respectively) were formed from the corresponding alkenyl alcohols (8, 7, and 1, respectively) via previously published procedures.17 The alkenyl bromides were then reacted with 1-bromo-3,5-dihydroxybenzene to form 13−15, respectively, in good yields. Because of the excessive cost of 1-bromo-3,5dihydroxybenzene, it was prepared by reacting the much cheaper 1-bromo-3,5-dimethoxybenzene (11) with boron tribromide in 80% yield. The final step was introduction of the boronic acid via a standard lithiation procedure, which is preferred to use of a Grignard reagent because the side reaction caused by hydrolysis of the triisopropyl borate is avoided. Triisopropyl borate was used as the boron source, yielding free boronic acid via standard aqueous workup procedures. A simple silica plug followed by recrystallization in hexanes resulted in yields of 60% for 16, 77% for 17, and 53% for 18. Polymerization and Characterization of In-Chain Boronic Acid Polymers (UPP16−18). Following the same polymerization procedure as described for monomer 6, the ionic liquid [bmim]PF6 was employed to control the viscosity of the boronic acid polymerization reaction mixture (Figure 6). Because the resulting polymers were insoluble, molecular weights could not be obtained. In an attempt to characterize these polymers, Fourier transform infrared with an ATR attachment was employed. Figure 7 compares the IR spectra of 18 and UPP18. The IR spectrum of UPP18 displays many similarities when compared to that of monomer 18. However, the differences between the monomer and polymer yield insights into the nature of the polymer. The first difference is in the O−H peak
Figure 6. Polymerization of the in-chain phenyl boronic acid polymer via a Grubbs first-generation catalyst in [bmim]PF6.
Figure 7. IR spectra of boronic acid monomer (blue) and polymer (red).
at ∼3450 cm−1; in the monomer, this peak is fairly intense and well-defined, but in the polymer, the intensity and definition of the peak are greatly reduced. This indicates that some of the boronic acids may have formed boron anhydrides, effectively cross-linking the polymer, leading to the observed insolubility. It is important to note that the cross-linking was likely the result of the high-vacuum polymerization conditions, not any reaction with a catalyst. Use of low-vacuum or atmosphericpressure polymerization techniques should result in a soluble polymer. The next difference in the polymer is the disappearance of the peak at ∼800 cm−1 and the formation of the peaks at ∼700 and ∼850 cm−1, indicating the reduction C
DOI: 10.1021/acs.macromol.5b01410 Macromolecules XXXX, XXX, XXX−XXX
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olefins observed. These polymers demonstrate good thermal stability, not degrading until well over 300 °C. Precise boroncontaining polymers with long carbon chains between pendant groups display crystallization behavior, which is unusual for cross-linked polymers.
of the level of the terminal olefin in the monomer and increases in the levels of both the cis (700 cm−1) and trans (850 cm−1) internal olefins, indicative of polymerization. Similar results were obtained for UPP16 and UPP17. The TGA thermograms of UPP16 and UPP18 showed that both polymers have good thermal stability, maintaining 90 wt % until over 313 and 353 °C, respectively (Figure SI2). These decomposition temperatures are consistent with those of many unsaturated ADMET polymers, especially polymers containing acid and ester functionalities.18 Finally, the three polymers were characterized via DSC (Figure 8). UPP16 and UPP17 display no Tm, which is
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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/acs.macromol.5b01410. Representative TGA thermograms and text giving detailed reaction procedures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]fl.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Army Research Laboratory and the U.S. Army Research Office via Grant W911NF-09-1-0290 and is also based upon work supported in part by the National Science Foundation via Grant DMR-1203136.
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Figure 8. DSC of cross-linked boronic acid polymers.
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consistent with the small number of methylene spacers between functional groups and the residual unsaturation in the backbone. The polymer UPP16 displays a Tg of −4 °C. This glass transition temperature is high when compared to those of other ADMET polymers; however, a high percentage of the polymer backbone corresponds to the ridged phenyl ring, which increases the Tg. UPP18 displays a significantly lower Tg (approximately −28 °C). This was expected because the relative number of phenyl rings in the backbone is much lower. A Tg is not observed in UPP17. In contrast to the thermal behavior of UPP16 and UPP17, UPP18 displays a Tm of ∼68 °C. The peak is fairly broad, indicating a variety of crystalline species that melt at slightly different temperatures. The presence of a melting temperature is unusual in an ADMET polymer with unsaturation in the backbone. The crystallinity in this case is presumably caused by the presence of pendant boronic acid functional groups.
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CONCLUSION Both boronic acid and boronic ester polymers have been synthesized via ADMET. The boronic ester polymer was polymerized using both bulk and ionic liquid polymerization techniques. Bulk conditions resulted in very low molecular weights; however, using ionic liquids, high-molecular weight polymers were obtained. Upon saturation of the residual double bonds in the backbone of the polymer, unique thermal behavior was observed. Boronic acid polymers with 20, 14, and 8 carbons between each functional group were also synthesized. These polymers are intractable, presumably because of chemical cross-linking formed via boron anhydrides under vacuum conditions. The IR spectra indicate that the repeat unit remained intact and the terminal olefin of the monomer disappeared with only internal D
DOI: 10.1021/acs.macromol.5b01410 Macromolecules XXXX, XXX, XXX−XXX
