ADMET Polymers Containing Precisely Spaced Pendant Boronic Acids and Esters

Article abstract of DOI:10.1021/acs.macromol.5b01410

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. (Graph Presented).

Full text of DOI:10.1021/acs.macromol.5b01410

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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.
INTRODUCTION
RESULTS AND DISCUSSION
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■
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 con-
trolled morphologies are needed. Such precise control can be
achieved using acyclic diene metathesis (ADMET) polymeri-
zation. 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.
Precision Boronic Ester Polymers. Synthesis of 4,4,5,5-
Tetramethyl-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-12-
yloxy)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
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b01410
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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(4,4,5,5-
tetramethyl-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 decom-
position until 350 °C, followed by complete decomposi-
tion 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.
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
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
B
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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,
Figure 6. Polymerization of the in-chain phenyl boronic acid polymer
via a Grubbs first-generation catalyst in [bmim]PF6.
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-3,5-
dihydroxybenzene, 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
Figure 7. IR spectra of boronic acid monomer (blue) and polymer
(red).
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 polymeri-
at ∼3450 cm−1; in the monomer, this peak is fairly intense and
zation 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
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 atmospheric-
pressure 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
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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
olefins observed. These polymers demonstrate good thermal
stability, not degrading until well over 300 °C. Precise boron-
containing polymers with long carbon chains between pendant
groups display crystallization behavior, which is unusual for
cross-linked polymers.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01410.
■
S
Finally, the three polymers were characterized via DSC
(Figure 8). UPP16 and UPP17 display no Tm, which is
Representative TGA thermograms and text giving
detailed reaction procedures (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: wagener@chem.ufl.edu.
■
Notes
The authors declare no competing financial interest.
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.
Figure 8. DSC of cross-linked boronic acid polymers.
REFERENCES
(1) Roscoe, S. B.; Frec
■
́
het, J. M.; Walzer, F.; Dias, A. J. Science 1998,
280, 270−273.
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.
(2) Cui, C.; Bonder, E. M.; Jakle, F. J. Am. Chem. Soc. 2010, 132,
̈
1810−2.
(3) Cambre, J. N.; Sumerlin, B. S. Polymer 2011, 52, 4631−4643.
(4) Cambre, J. N.; Roy, D.; Gondi, S. R.; Sumerlin, B. S. J. Am. Chem.
Soc. 2007, 129, 10348−10349.
(5) Roy, D.; Cambre, J. N.; Sumerlin, B. S. Chem. Commun. 2009,
2106−8.
(6) Roy, D.; Sumerlin, B. S. ACS Macro Lett. 2012, 1, 529−532.
(7) Hoven, C. V.; Wang, H.; Elbing, M.; Garner, L.; Winkelhaus, D.;
Bazan, G. C. Nat. Mater. 2010, 9, 249−52.
(8) Inci, B.; Wagener, K. B. J. Am. Chem. Soc. 2011, 133, 11872−
11875.
(9) Simocko, C. K.; Wagener, K. B. Organometallics 2013, 32, 2513−
2516.
(10) Baughman, T. W.; Chan, C. D.; Winey, K. I.; Wagener, K. B.
Macromolecules 2007, 40, 6564−6571.
(11) Valenti, D.; Wagener, K. B. Macromolecules 1998, 31, 2764−
2773.
(12) Simocko, C. K.; Yang, Y.; Swager, T. M.; Wagener, K. B. ACS
Macro Lett. 2013, 2, 1061−1064.
CONCLUSION
■
(13) Han, H.; Chen, F.; Yu, J.; Dang, J.; Ma, Z.; Zhang, Y.; Xie, M. J.
Polym. Sci., Part A: Polym. Chem. 2007, 45, 3986−3993.
(14) Vygodskii, Y. S.; Shaplov, A. S.; Lozinskaya, E. I.; Filippov, O.
A.; Shubina, E. S.; Bandari, R.; Buchmeiser, M. R. Macromolecules
2006, 39, 7821−7830.
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.
(15) Opper, K. L.; Wagener, K. B. Macromol. Rapid Commun. 2009,
30, 915−919.
(16) Watson, M. D.; Wagener, K. B. Macromolecules 2000, 33, 8963−
8970.
(17) Baughman, T. W.; Sworen, J. C.; Wagener, K. B. Tetrahedron
2004, 60, 10943−10948.
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
(18) Opper, K. L.; Markova, D.; Klapper, M.; Mullen, K.; Wagener,
̈
K. B. Macromolecules 2010, 43, 3690−3698.
D
DOI: 10.1021/acs.macromol.5b01410
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