Organoboronium-functionalized polystyrenes as a new class of polycations

Article abstract of DOI:10.1039/b902644d

The facile preparation of organoboronium polymers via spontaneous reaction of bromoborylated polystyrene with 2,2′-bipyridine is reported; these novel polycations represent an interesting alternative to commonly used ammonium-based polycations. The Royal

Full text of DOI:10.1039/b902644d

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Organoboronium-functionalized polystyrenes as a new class
of polycationsw
¨
Chengzhong Cui and Frieder Jakle*
Received (in Cambridge, UK) 9th February 2009, Accepted 2nd March 2009
First published as an Advance Article on the web 20th March 2009
DOI: 10.1039/b902644d
The facile preparation of organoboronium polymers via sponta-
neous reaction of bromoborylated polystyrene with 2,20-bipyridine
is reported; these novel polycations represent an interesting
alternative to commonly used ammonium-based polycations.
The attachment of dipyridylboronium cation moieties to a
polymeric framework can ultimately be achieved either by treat-
ment of a suitable bipy-functionalized polymer with an organo-
boron halide or reaction of an organoboron-functionalized
polymer with bipy derivatives. We demonstrate here the facile
formation of boronium-cation functionalized polymeric
materials from boron polymer precursors.
Organoboron cations, as outlined in a recent review by Piers
et al.,1 are a promising class of compounds that are receiving
increasing interest due to their potential use, for example, as
alternatives to ionic liquids based on quaternary ammonium
salts (pyridinium, imidazolium, etc.),2 as multi-step redox-
active materials,3 as disinfectants,4 and as intermediates and
catalysts in organic synthesis.5 While different synthetic routes
for their preparation are available, a straightforward approach
is the reaction of diorganoboron halides R2BX with chelating
ligands such as 2,20-bipyridine. These effectively replace the
halide substituent on boron thereby giving rise to the desired
cations [R2B(bipy)]X, in which boron is tetracoordinated and
thus electronically saturated.3,6,7 The products usually enjoy
exceptionally high stability if chelating ligands are employed
and they are soluble in common polar organic solvents and
even in protic media.7
Poly(4-trimethylsilylstyrene) (P1) was prepared by atom trans-
fer radical polymerization (ATRP) as previously reported,15,16
and subsequent exchange of the trimethylsilyl groups with boron
tribromide results in quantitative formation of the boron-
functionalized polymer PSBBr2 (P2). Using highly selective
organometallic transformations we then first replaced one of
the bromine substituents of each boryl group with an organic
group as illustrated in Scheme 1. Treatment of P2 with one
equivalent of tert-butylphenyl trimethyltin produces the polymer
P3-Ph containing reactive –BBr(t-BuPh) pendant groups.
Similarly, reaction with mesityl copper17 produces a polymer,
P3-Mes, that is decorated with –B(Br)Mes (mesityl = 2,4,6-
trimethylphenyl) functional groups. These polymers are obtained
with excellent selectivity for the monosubstitution product as
shown by multinuclear NMR spectroscopy and further confirmed
by comparison with the results for the respective molecular
system based on t-BuC6H4BBr2. In a second step, the
remaining B–Br functionalities were then replaced with
2,20-bipyridine, giving rise to the cationic polymers P4
(Ar = t-BuPh, Mes), which precipitated from the reaction
solution in CH2Cl2. The products were conveniently purified
Several aspects of the chemistry of boronium cations could
prove highly beneficial for the preparation of functional poly-
meric materials. First, cation formation occurs under very mild
conditions (RT) and the products are typically formed in essen-
tially quantitative yields. In this respect, the formation of boro-
nium cations resembles the broadly applied ‘‘click chemistry’’,8
without the need for any catalyst. Secondly, many derivatives of
2,20-bipyridine and related chelate ligands9 are commercially
available and a plethora of methods allow for further functiona-
lization. Similarly, the organic substituents on boron can easily
be varied thereby providing for additional tunability. Among
polymers with cationic functionalities, ammonium-functionalized
polymers have been widely studied; recent applications include
their use in fuel cell membranes,10 nanofiltration membranes,11
smart surfaces with switchable wettability,12 and antimicrobial
surfaces,13 just to name a few. However, other polycations are
comparatively rare. The development of boronium-functionalized
polymers is therefore expected to lead to new materials with
useful properties such as antibacterial activity4,13 or neutron
shielding behavior based on the well-known large neutron
cross section of the 10B nucleus.14
Rutgers University-Newark, Department of Chemistry, 73 Warren
Street, Newark, NJ 07102, USA. E-mail: fjaekle@rutgers.edu;
Fax: +1 973 353 5064; Tel: +1 973 353 1264
w Electronic supplementary information (ESI) available: Experimental
details for PSBbipyMes, MBbipyMes and MBbipyPh. See DOI:
10.1039/b902644d
Scheme 1 Synthesis of ‘‘boronium-type’’ polyelectrolytes P4 and
molecular model systems. ArM
=
(t-BuPh)SnMe3, MesCu;
Mes = 2,4,6-trimethylphenyl, 2,20-bipy = 2,20-bipyridine, t-BuPh =
4-tert-butylphenyl.
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Fig. 2 Representative GPC-UV traces for the boronium-functionalized
polymers (recorded in DMF–20 mM NH4[PF6] at 60 1C).
Table 1 GPC-UV results and yields for organoboronium polymers
(DMF–20 mM NH4[PF6] at 60 1C, polystyrene standards)
Fig. 1 Comparison of the 1H and 11B NMR spectra of P4-Ph with
those of the molecular species M4-Ph in DMSO-d6 (Brꢁ counterion).
Mn (ꢂ103)a
15.4
43.7
42.2
Mw (ꢂ103)a
17.7
56.1
51.5
PDIa
Yield (%)b
by dialysis in methanol and isolated as light yellow powdery
materials by precipitation into diethyl ether.z
P1
P4-Ph
P4-Mes
1.15
1.28
1.22
87
80
79
79
71
Successful preparation of the boronium salts is evident from
a distinct upfield shift in the 11B NMR spectra as a result of
conversion of the tricoordinate boranes to tetracoordinate
boronium moieties. The 11B NMR spectra of P4 show one
single peak at ca. 4 ppm, which is similar to the chemical shift
of 8 ppm for the molecular models M4 (Fig. 1). The slight
upfield shift and sharpening of the signal of the polymer may
indicate somewhat stronger binding of bipy, but other factors
may also contribute and, indeed, similar shielding effects have
been previously observed for tricoordinate organoboron
polymers.18 In the 1H NMR spectra in DMSO-d6 several
broad resonances are observed in the range from 8.0 to
9.2 ppm, which can be confidently assigned to the boron-bound
bipyridyl moieties by comparison to the molecular compounds
(Fig. 1). Specifically, the signal at ca. 9.2 ppm is characteristic
of the 3,30-protons of the complexed bipy moieties in
DMSO-d6.3 Integration of the aromatic region relative to
the aliphatic resonances (t-Bu, Me groups) is consistent with
a 1 : 1 ratio of bipy : tert-butylphenyl/mesityl, confirming
quantitative attachment of bipy to the boron sites. The
product composition was also examined by elemental analysis
of the [PF6]ꢁ derivatives, which were obtained by precipitation
of the boronium polymers into an aqueous NH4[PF6] solution.
For both polymers elemental analyses are consistent with the
proposed structure.
a
Based on GPC-UV detection (vs. PS) in DMF–20 mM NH4[PF6] at
60 1C and for the silylated precursor polymer P1 in THF at 35 1C; the
[PF6]ꢁ counterions are included in the calculations. Isolated yield of
Brꢁ salt.
b
the PS backbone was confirmed by multinuclear NMR,
elemental analysis, and molecular weight determination by
GPC in DMF–NH4[PF6] mixtures. We anticipate potential
applications as antimicrobial coatings and possibly as neutron
protection materials. Moreover, our approach of combining
controlled polymerization methods with selective polymer
modification procedures makes more complex polymer
architectures such as amphiphilic block copolymers16 accessible
and studies in this regard are now in progress.
Acknowledgement is made to the National Science Foundation
(NSF CAREER award CHE-0346828 to F. J.) and the
Alfred P. Sloan foundation for support of this research. We also
thank the NSF for GPC and LS instrumentation provided
through the NSF-MRI program (MRI 0116066).
Notes and references
z Representative experimental procedure: Synthesis of poly(dipyridyl-
(tert-butylphenyl)styrylboronium bromide) (P4-Ph).
A solution of
poly(4-trimethylsilylstyrene) (0.50 g, 2.84 mmol SiMe3, Mn = 15390,
Mw = 17680, PDI = 1.15) in CH2Cl2 (10 mL) was added to a solution
of BBr3 (0.85 g, 3.39 mmol) in CH2Cl2 (10 mL) in a glove box. The
reaction mixture was stirred at room temperature for 12 h. A solution
of 1-trimethylstannyl-4-tert-butylbenzene (1.30 g, 4.38 mmol) in
CH2Cl2 (30 mL) was slowly added. The reaction mixture was stirred
for 4 h, and then all volatile components were removed under high
vacuum. The white residue was redissolved in CH2Cl2 (50 mL), and a
solution of 2,20-bipyridine (2.00 g, 12.8 mmol) in CH2Cl2 (20 mL) was
added dropwise. After stirring for 5 h, the mixture was dried under
vacuum, redissolved in methanol (35 mL), dialyzed against methanol
(regenerated cellulose membrane with 6000 to 8000 Dalton molecular
weight cut-off), concentrated and precipitated into diethyl ether. The
product was dried under high vacuum at 60 1C to give a light yellow
solid. Yield: 1.08 g (79%). 1H NMR (499.890 MHz, DMSO-d6) d = 9.2,
8.7, 8.0 (v br, 8H, bipy-H), 7.1, 6.9 (br, 8H, styryl-H, Ph-H), 1.0
(br, 9H, t-Bu), backbone n.r. 11B NMR (160.385 MHz, DMSO-d6)
d = 3.4 (w1/2 = 210 Hz). Conversion to [PF6]ꢁ salt. Poly(dipyridyl(tert-
butylphenyl)styrylboronium bromide) (0.30 g, 0.62 mmol boronium
groups) in methanol (5 mL) was added to a solution of NH4[PF6]
(0.30 g, 1.84 mmol) in H2O (100 mL). The resulting suspension was
stirred for 1 h at RT. A white solid was collected on a sheet of filter
paper and then washed extensively with water. The product was dried
The new boronium polyelectrolytes were further studied by
gel permeation chromatography (GPC) in DMF containing
20 mM NH4[PF6]. Both polymers eluted quite well from the
columns under these conditions and the traces showed mono-
modal profiles with only a slight broadening at higher elution
times, which is likely due to a small amount of column
interaction (Fig. 2). Analysis relative to narrow PS standards
gave considerably higher molecular weights in comparison to
the silylated precursor polymer P1 and the calculated degree of
polymerization (DPn) was found to be in the expected range
(Table 1).
These data confirm that the polymer substitution reactions
occur with excellent selectivity for the boronium cation formation
and without any sign of crosslinking or degradation processes.
In conclusion, we have developed a new class of cationic
polyelectrolytes, in which the commonly used quaternary
ammonium functionality was formally replaced with iso-
electronic boronium moieties. Highly selective attachment to
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under high vacuum at 60 1C. Yield: 0.27 g (80%). 1H NMR
(499.890 MHz, acetonitrile-d3) d = 8.6, 8.1, 7.2, 6.9 (v br, 16H, aromatic
H), 1.0 (br, 9H, t-Bu), backbone H n.r. 13C NMR (125.697 MHz,
acetonitrile-d3) d = 152.7, 146.6, 144.2, 140 (br), 133.8, 129 (v br), 126.6,
124.8 (aromatic C), 46–38 (backbone C), 35.2 (Me3C), 31.6 (Me3C).
11B NMR (160.385 MHz, CD3CN) d = 3.6 (w1/2 = 210 Hz). 19F NMR
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(470.367 MHz, CD3CN) d = ꢁ72.4 (d, JPF = 709 Hz, [PF6]ꢁ). 31P
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