Weakly coordinating amphiphilic organoborate block copolymers

Article abstract of DOI:10.1021/ja908996e

(Figure Presented) Selective functionalization of polystyrene with weakly coordinating organoborate functionalities BAr₄^- (Ar = Ph, C₆F₅) has been accomplished by a polymer modification strategy. The first examples of perfluoroarylborate block copolymers have been prepared; light scattering and TEM studies confirm the formation of reverse micelles, in which the weakly coordinating anion sites are covered by a shell of inert polystyrene. The core of the micelles was successfully loaded with a cationic rhodium complex. Copyright

Full text of DOI:10.1021/ja908996e

Published on Web 01/26/2010
Weakly Coordinating Amphiphilic Organoborate Block Copolymers
Chengzhong Cui, Edward M. Bonder, and Frieder Ja¨kle*
Department of Chemistry, Rutgers UniVersitysNewark, 73 Warren Street, Newark, New Jersey 07102, and
Department of Biological Sciences, Rutgers UniVersitysNewark, 195 UniVersity AVenue,
Newark, New Jersey 07102
Received October 21, 2009; E-mail: fjaekle@rutgers.edu
Organoborates are widely used as counterions to stabilize main
group and transition metal complexes.1 They also serve important
roles as electrolytes in lithium ion batteries2 and electrochemical
redox processes,3 as components of ionic liquids,4 and as
membrane materials.5 The protection of highly reactive cationic
species from attack by nucleophiles with subsequent degradation
and/or deactivation is of particular importance in the area of
olefin polymerization, where borates have been extensively used
as counterions to active cationic transition metal complexes.6
Fluorinated derivatives are attractive because of their weakly
coordinating and chemically more inert nature. To incorporate
these borate counterions into polymeric structures is an intriguing
premise, since beneficial effects can be expected from the
improved processability and anticipated stabilizing effect of the
polymer chain.7 One approach that has been successfully
employed involves noncovalent anchoring of molecular orga-
noborate anions to cationic polymers.8 The alternative method
of covalent functionalization of polymers with weakly coordinat-
ing organoborate moieties is less developed, and especially
reports of fluorinated organoborate polymers remain scarce.9,10
We became interested in the incorporation of organoborate
functionalities into amphiphilic block copolymers11 for their well-
known ability to form interesting micellar aggregates. Organome-
tallic block copolymers have received tremendous interest in recent
years.12 However, organoborate block copolymers have to the best
of our knowledge not been reported to date and new controlled
synthesis routes to these types of materials are needed.13-15 We
describe here a versatile new method for the preparation of
organoborate polymers and report the first block copolymers, in
which one of the constituent blocks is functionalized with weakly
coordinating organoborate moieties, while the other block (poly-
styrene, PS) is unfunctionalized and thus provides solubility in
nonpolar solvents. Studies on their self-assembly in solvents that
selectively dissolve one of the constituent copolymer components
are also presented.
the mild reagent Me3SiOMe, and the resulting boronate polymer
was then reacted with PhMgBr at 85 °C in THF for 3 h (Scheme
1). An alternative route involves initial formation of the
triorganoborane intermediate which, if it is readily available,18
can be very selectively and under mild conditions converted to
the organoborate. This method19 was successfully applied to the
preparation of PSBPf3 (Pf ) C6F5). PSBBr2 was first reacted
with the organocopper reagent C6F5Cu in CH2Cl2 to give PSBPf2,
which was in turn converted in situ to the desired borate polymer
by addition of a third equivalent of C6F5Cu in acetonitrile.
Acetonitrile coordinates to copper and, thereby, promotes
formation of the [Cu(CH3CN)x]n[PSBPf3] polymeric complex.
The organoborate polymers were isolated in the form of their
sodium derivatives after addition of the reaction mixture into
an aqueous sodium carbonate solution, extraction into acetone,
and subsequent dialysis against acetone. The amphiphilic block
copolymers PSBR3-b-PS (R ) Ph, Pf) were prepared from
PSBBr2-b-PS using similar methods.
Scheme 1. Synthesis of Organoborate-Functionalized
Homopolymers and Block Copolymers; Cat+ ) Na+, Bu4N+
Poly(4-trimethylsilylstyrene) (PSSiMe3; n ) 93) and the corre-
sponding block copolymers with styrene as a second block
(PSSiMe3-b-PS; n ) 33, m ) 120 or n ) 29, m ) 238) are readily
accessible via atom transfer radical polymerization (ATRP) ac-
cording to published procedures.15,16 Exchange of the SiMe3 groups
with a slight excess of BBr3 results in the selective and essentially
quantitative formation of the boron-functionalized homopolymers
and block copolymers, PSBBr2 and PSBBr2-b-PS, based on
multinuclear NMR studies.15,16
The sodium borate homopolymers are water-soluble, while the
block copolymers are soluble in either polar (e.g., THF, acetone,
MeOH, DMF, DMSO, water) or nonpolar solvents (e.g., toluene).
For dissolution in block-selective solvents such as MeOH, water,
or toluene, the block copolymers were first taken up in a common
solvent and then dialyzed. Conversion to the Bu4N derivatives was
achieved by dissolution in water and subsequent addition into a
There are several possible approaches for the conversion of
these BBr2-functionalized polymers to the desired organoborate
polymers. One method that was applied to the triphenylborate-
modified polymer PSBPh3 is based on the treatment of boronates
with an excess of organolithium or Grignard reagents.17 Hence,
we first converted the BBr2 groups to B(OMe)2 moieties with
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1810 J. AM. CHEM. SOC. 2010, 132, 1810–1812
10.1021/ja908996e  2010 American Chemical Society

C O M M U N I C A T I O N S
solution of [Bu4N]Br. The products were obtained as white solids
that are readily isolated and well soluble in a range of polar organic
solvents.
1.14 to 1.20 (Table 1). The bands are only slightly broadened
relative to those of the silylated precursor polymers, thereby further
confirming the high selectivity of the polymer modification reac-
tions. The molecular weights are generally in the expected range,
but those of the fluorinated polymers are similar to or even lower
than those for the phenylborate polymers. This may be the result
of a relatively more compact conformation of the fluorinated
polymers in the presence of LiBr (more effective shielding of the
charges).
We then carried out studies on the assembly behavior of the
organoborate block copolymers in solvents that selectively dissolve
one of the constituent blocks. Dynamic light scattering (DLS) was
performed on toluene solutions of the sodium and TBA salts of
PSBPh3-b-PS and PSBPf3-b-PS (derived from PSSiMe3-b-PS with
n ) 33; m ) 120), which were obtained by dialysis from acetone
to toluene. The results from regularization analysis (DYNALS
algorithm) suggest the presence of aggregates with an average
apparent hydrodynamic radius, Rh,app, of ca. 15 nm, independent of
the particular aryl substituents on boron or the counterion employed
(Figure 2). It is reasonable to assume that the PS block forms the
corona of the micellar structures in toluene as the solvent.
Conversely, when the ammonium borate block copolymer PSBPf3-
b-PS is dissolved in (hot) MeOH, the polystyrene should form the
core, leaving the organoborate functionalities exposed to the polar
solvent. Slightly larger aggregates with Rh,app of ca. 20 nm and a
somewhat broader distribution were observed in this case.
Table 1. Results for Polymer Modification Procedures
Mn
(×103)a
Mw
(×103)a
Yield
(%)c
PDIa
DPnb
PSSiMe3
PSBPh3
PSBPf3
PSSiMe3-b-PS
PSBPh3-b-PS
PSBPf3-b-PS
16.4
60.1
52.1
18.3
28.9
28.2
18.4
70.3
59.7
19.5
34.5
32.7
1.11
1.17
1.14
1.07
1.20
1.16
93
102
61
33/120
28/120d
18/120d
54
69
70
66
a Relative to PS standards based on GPC-RI detection of the
ammonium salts in DMF/20 mM LiBr at 40 °C and the silylated
precursor polymers in THF at 35 °C; the Bu4N counterions are included
in the calculations. b DPn ) number average degree of polymerization of
the first and second block, respectively, based on GPC analysis.
c Isolated yield of the sodium derivative. d Calculated assuming that the
MW of the PS block remains constant.
Figure 1. Comparison of selected 11B and 19F NMR spectra of homo- and
block copolymers with those of the respective molecular species
Na[(tBuC6H4)BAr3]; cation ) Na+, solvent ) acetone-d6.
Formation of the borate polymers was confirmed by multinuclear
NMR spectroscopy.20 The phenylborate homopolymers and block
copolymers show slightly broadened 11B NMR resonances at ca.
-6 ppm (w1/2 ) 30 Hz), a chemical shift that is identical to that of
the related molecular model compound Na[tBuC6H4BPh3] (w1/2
)
10 Hz) (Figure 1). In comparison, the fluorinated derivatives PSBPf3
and PSBPf3-b-PS feature slightly more upfield shifted signals at
ca. -13 ppm, again consistent with the chemical shift of the
molecular species Na[tBuC6H4B(C6F5)3]. The fluorinated polymers
are also conveniently analyzed by 19F NMR; three broad resonances
at -129.7, -166.6, and -168.7 ppm are found for the ortho, para,
and meta-fluorines of PSBPf3, and essentially identical shifts are
observed for the respective block copolymer (Figure 1). The absence
of any other signals in the 11B and 19F NMR spectra supports
selective borate formation.21
Figure 2. Top: Schematic illustration of the block copolymer assembly of
PSBR3-b-PS in toluene (R ) Ph, Pf; Cat ) countercation (Na+, Bu4N+);
PS ) polystyrene). Bottom: Size distribution histogram of a solution of
the sodium salt of PSBPf3-b-PS in toluene; n ) 33, m ) 120.
To demonstrate the ability of the reverse micelles to accept
cationic transition metal complexes, we treated the sodium salt of
another sample of block copolymer PSBPf3-b-PS (n ) 29; m )
1
While the H NMR data are less informative due to the broad
nature of the signals, 13C NMR spectroscopy proved useful, and
the downfield region of the spectra that shows the ipso-carbon atoms
is most instructive (Figure S3).20 For the molecular model
Na[tBuC6H4BPh3], a large and a small quartet are found for the
boron-bound carbons of the phenyl and tert-butylphenyl groups,
respectively, as a result of coupling to 11B. The same pattern is
found in the spectra for PSBPh3 and PSBPh3-b-PS, except that the
signals are slightly broadened, thus confirming selective attachment
of the tetraarylborate functionalities to the polymers. For the block
copolymer, a second set of resonances that match those of PS is
also apparent.
238) with [Rh(cod)(dppb)]+(OTf)-.22,23 By H NMR integration,
1
the degree of loading was estimated to be >85%. Micellization was
then induced by (a) addition of MeOH as a borate-selective solvent
to a solution of the block copolymer in THF as a common solvent
and (b) dialysis of the block copolymer in THF to toluene as a PS
selective solvent. As expected, formation of regular micelles in
MeOH could be confirmed by DLS and TEM analysis (Supporting
Information). Interestingly, TEM analysis of the unstained samples
prepared in toluene revealed the formation of reverse micelles,
which contained dark cores as a result of the rhodium metal loading
(Figure 3). The size of these micellar structures was in reasonably
good agreement with data derived from DLS (Rh,app ca. 19 nm).
This is slightly larger than what we found for the Na precursor
(Rh,app ) 16 nm) prior to reaction with the Rh complex, suggesting
a modest expansion of the micellar core upon loading with the
transition metal complex.
The molecular weight of the organoborate homopolymers and
block copolymers was estimated by gel permeation chromatography
(GPC-RI) relative to PS standards in DMF using 20 mM LiBr as
an additive. For all polymers well-defined monomodal elution
profiles were observed with polydispersities in the range of PDI )
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J. AM. CHEM. SOC. VOL. 132, NO. 6, 2010 1811

C O M M U N I C A T I O N S
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Figure 3. TEM image of PSBPf3-b-PS (n ) 29; m ) 238) after loading
with [Rh(cod)(dppb)]+ and subsequent micellization in toluene.
In conclusion, we report a new method for the preparation of
organoborate polymers. Both homopolymers and the first examples
of organoborate block copolymers are readily accessible through
selective postmodification procedures. These weakly coordinating
polyanions, and in particular the self-assembled block copolymer
micelles, are promising candidates as supports for reactive cationic
metal complexes, where the “inert” PS corona can not only promote
solubility of charged species [MLn]+[BR4]- in nonpolar solvents
but also possibly act as a protective layer. Further studies in this
regard and on their use in catalysis are in progress.
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2002, 124, 12672–12673. (b) Qin, Y.; Cheng, G.; Parab, K.; Achara, O.;
Ja¨kle, F. Macromolecules 2004, 37, 7123–7131.
Acknowledgment. We gratefully acknowledge support by the
National Science Foundation and the Research Council at Rutgers
University. F.J. thanks the Alfred P. Sloan foundation for a research
fellowship and the Alexander von Humboldt foundation for a
Friedrich Wilhelm Bessel award. We thank Dr. Y. Qin for the
synthesis of PSSiMe3-b-PS.
(17) Muetterties, E. L. The Chemistry of Boron and Its Compounds; John Wiley
& Sons, Inc.: New York, London, Sydney, 1967.
(18) The phenyl derivative PS-BPh2 is not readily accessible from PSBBr2.
(19) Another possible advantage is that mixed-substituted organoborate polymers
PSBR1R2R3 could become available through selective stepwise introduction
of the organic substituents. While not further explored in this work, such
an approach could prove highly useful for the development of other
functional organoborate polymers.
Supporting Information Available: Experimental procedures and
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
(20) Further experimental details are provided in the Supporting Information.
(21) No difference in the efficiency of the polymer modification reactions for
the block copolymers in comparison to the homopolymers was observed.
According to the 11B and 19F NMR data the boron-pendant groups consist
of >95% tetrarylborate functionalities; moreover, the extent of borylation
of the functional block is >95% based on NMR analysis after the initial
Si/B exchange reaction. However, a small amount of deborylation in
subsequent steps cannot be completely ruled out since these defects would
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