Synthesis of hyperbranched poly(m-phenylene)s via suzuki polycondensation of a branched AB2 monomer

Article abstract of DOI:10.1021/ma102023a

An efficient synthesis of hyperbranched poly(m-phenylene)s is achieved by Suzuki polycondensation of a m-terphenyl-derived branched AB₂ monomer. The resulting hyperbranched polymers show high molecular weight and relatively low polydispersity a

Full text of DOI:10.1021/ma102023a

Macromolecules 2010, 43, 9277–9282 9277
DOI: 10.1021/ma102023a
Synthesis of Hyperbranched Poly(m-phenylene)s via Suzuki
Polycondensation of a Branched AB2 Monomer
Zheng Xue, Aaron D. Finke, and Jeffrey S. Moore*
Department of Chemistry & Beckman Institute for Advanced Science and Technology,
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
Received August 31, 2010; Revised Manuscript Received October 12, 2010
ABSTRACT: An efficient synthesis of hyperbranched poly(m-phenylene)s is achieved by Suzuki poly-
condensation of a m-terphenyl-derived branched AB2 monomer. The resulting hyperbranched polymers
show high molecular weight and relatively low polydispersity and are readily soluble in common organic
solvents. A catalyst system composed of Pd(OAc)2 and S-Phos was found to be highly active in the present
Suzuki polycondensation protocol. The molecular weight and polydispersity of the polymer are controllable
by varying the catalyst loading and the starting monomer concentration. Experimental results are consistent
with a pseudo-chain-growth pathway that involves intramolecular catalyst transfer during polymerization.
Furthermore, the triflate end groups of the hyperbranched poly(m-phenylene)s can be efficiently converted to
other functionalities via in situ Suzuki-Miyaura cross-couplings.
Introduction
by using a branched AB2 monomer and a highly active palladium
catalyst to construct hyperbranched poly(m-phenylene)s (HB-
PmPs) with high molecular weights and excellent solubility in
common organic solvents.
In hyperbranched polymeric architectures, high branching
density leads to compact macromolecular objects with multiple
end-functionalities. As a result, hyperbranched polymers (HBPs)
offer higher solubility and lower viscosity when compared with
their linear counterparts, partly due to their globular shape and
decreased degree of interchain entanglement.1-6 HBPs are there-
fore frequently referred to the close analogues to dendrimers,
despite that the latter are perfectly branched macromolecules with
high structural regularity and monodispersity, whereas HBPs
generally have less well-defined structures and broad molecular
weight distributions.7 Among the wide variety of HBPs that have
been synthesized to date, hyperbranched poly(phenylene)s (HB-PPs)
exhibit structural rigidity and extraordinary thermal stability.8-11
Our interest in HB-PPs arose from the pursuit of carbon-rich
polymeric architectures.12,13 In this context, the unique characteristics
of HB-PPs coincide with that of the hyperbranched polymers
developed in our group;hyperbranched aromatic poly(ether
imide)s14-17 and poly(phenyleneacetylene)s.18-20
Three methods have dominated the synthetic approaches of
HB-PPs. The first, and earliest examples, utilize transition-metal-
catalyzed cross-coupling reactions of AB2-type monomers.21-24
Another approach utilizes the Diels-Alder reaction between
phenylethynyl and cyclopentadienone functionalities incorporated
in multifunctionalized monomers (i.e., AB2, A2/B3, etc.), giving
rise to highly branched poly(phenylene)s.25,26 A third strategy
involves catalytic cyclotrimerization of diynes, wherein newly
formed trisubstituted phenyl rings constitute the branch points of
the hyperbranched structures.27 While considering the synthetic
methods, we were particularly interested in SPC due to the high
specificity, good efficiency, and mild reaction conditions that allow
for an accurate configuration of the branch points and tolerance
of a wide range of functionalities.28 In addition, although SPC
was one of the earliest methods utilized to access HB-PPs, there
has since been very limited effort devoted to further exploration
of this methodology. We present here an efficient SPC protocol
Results and Discussion
Monomer Design and Synthesis. One of the most impor-
tant characterization parameters associated with HBPs is the
degree of branching (DB).29 For HBPs synthesized by poly-
merization of AB2 monomers, DB may vary from 0 to 100%
according to Frey’s definition30-32
2D
DB ¼
ð1Þ
2D þ L
where D and L denote the mole fractions of dendritic and
linear units in the HBP structure, respectively. Theoretical
studies predict that the maximum DB for the HBP obtained
from an AB2 monomer is only 50%, assuming that the
reactivities of the terminal groups remain constant through-
out the course of the reaction.32 Hyperbranched polymers
with high DB require increasing the number of dendritic
units (D) and/or reducing the amount of linear units (L).
Examples of HBPs up to 100% DB have been reported but
are limited to only a small number of monomers and
methods.33-36 A more general approach to increase DB is
to incorporate one or more branch points within the mono-
mer struture.37,38 Considering the structural rigidity and
relatively low solubility of unsubstituted poly(phenylene)s,
we designed a branched AB2 monomer M that is built upon a
m-terphenyl scaffold composed of one focal point A, two
terminal units B, and two solubilizing groups R, as shown in
Figure 1, where functional groups A and B are reacting
partners in the polycondensation reactions. In addition to
the built-in branch point, the increased distance between the
reactive termini may facilitate the coupling of monomers.
Upon polymerization, each terminus is converted to a linear
unit that forms the connection between two dendritic units,
*Corresponding author. E-mail: jsmoore@illinois.edu.
r 2010 American Chemical Society
Published on Web 11/01/2010
pubs.acs.org/Macromolecules

9278 Macromolecules, Vol. 43, No. 22, 2010
Xue et al.
Figure 1. Calculation of the degree of branching (DB) for the hyperbranched poly(m-phenylene)s synthesized from m-terphenyl AB2 monomer M.
In the structure, “A” and “B” denote generic reactive sites, R = solubilizing group, and Ar = aryl group.
and two new termini are generated in the meanwhile. This
reaction pattern will eventually lead to n dendritic, (n - 1)
linear, and (n þ 1) terminal units, where n is the degree
of polymerization (DP). The DB of the resulting polymer
is calculated to become essentially constant at 67% when
n>27.
Among the catalyst systems tested, Pd(OAc)2 combined
with Buchwald’s S-Phos ligand was most active.49 The
catalyst was preformed by stirring a THF solution of
0.5 mol % Pd(OAc)2 and 1 mol % S-Phos (molar ratio with
respect to the AB2 monomer) for 30 min under argon. The
reaction mixture containing the monomer (starting mono-
mer concentration [M]0 = 50 mM), the catalyst, and 2 M
K3PO4 solution was then stirred in an argon-filled sealed
container. At 65 °C, the monomer was consumed within
20 min, by which time the molecular weight (Mn) of the
polymer portion reached ca. 35.8 kDa relative to polystyrene
(PS) standards and remained essentially constant thereafter.
The polydispersity index (PDI) was 1.5 as determined by gel
permeation chromatography (GPC). At room temperature,
the reaction also showed high efficiency in that consump-
tion of the monomer and formation of high molecular
weight polymer (Mn=35.1 kDa, PDI=1.8) occurred within
20 min, as shown in Figure 2a. The polymer obtained after
4 h showed only a slight increase in both Mn (36.1 kDa)
and PDI (1.9) compared to the 20 min sample. When the
polymerization was conducted at 0 °C and monitored by
GPC analysis (Figure 2b), polymer formation was observed
after 30 min, and the Mn of the polymer slowly increased to
48 kDa with a PDI of 2.4 after 5 h, by which time the
monomer conversion was about 50% and no obvious low
molecular weight oligomers were observed. Mechanistically,
this result is in sharp contrast to what is expected for a
step-growth polycondensation, wherein high molecular
weight polymers generally form only at a high monomer
conversion.50
The monomer was synthesized by adopting a strategy of
iterative C-H activation/borylation followed by Suzuki-
Miyaura cross-coupling sequence as we recently reported for
the construction of 1,3,5-polyphenylene dendrons.39 As
depicted in Scheme 1, using iridium-catalyzed C-H bond
functionalization, tert-butyldimethylsilyl (TBS)-protected
3-tert-butylphenol 1 was borylated regioselectively at the
meta position with respect to the two substituents.40,41 The
resulting boronate ester 2 was then coupled with 1,3-dibro-
mobenzene in the presence of a catalyst system composed of
Pd(OAc)2 and 2-dicyclohexylphosphino-20,60-dimethoxy-
biphenyl (S-Phos),42 providing m-terphenyl 3 in quantitative
yield.43 The meta position of the internal benzene ring in 3
was then borylated, again by using iridium-catalyzed C-H
activation/borylation to give the m-terphenyl boronate ester
4.44 Deprotection of the TBS groups was carried out in THF
with tetrabutylammonium fluoride (TBAF) to provide bis-
phenol 5.45 Subsequent esterification with triflic anhydride
provided the branched AB2 monomer 6 in good yield.
Polymerization Conditions: Catalyst, Temperature, and
Solvent. The synthesis of polyarylenes via SPC has been
extensively studied.46,47 Most commonly these preparations
involved the use of difunctionalized aryl boronic acids/esters
and aryl halides. Examples with aryl triflates remain rare,48
despite that these starting materials are easily accessed from
a wide range of phenolic precursors. Another advantage of
using aryl triflates is that, unlike halides, phenols are readily
masked by various protecting groups if orthogonal reactiv-
ities need to be realized, resulting in a more facile synthetic
protocol. To test the performance of the branched AB2
monomer in SPC, we subjected 6 to various conditions to a
screening of catalysts and solvents. A relatively mild base,
K3PO4, was applied in each case due to its extensive uses in
Suzuki-Miyaura cross-couplings and to minimize potential
hydrolysis of the triflate end groups.
Furthermore, the molecular weight distribution of the
polymer was relatively narrow, especially at the initial stage
of the reaction. Therefore, a pseudo-chain-propagation pro-
cess seems more plausible than a random condensation of
low molecular weight oligomers. Examples of chain-growth
Suzuki polycondensation have been recently reported,34,51,52
wherein an intramolecular catalyst transfer process was
believed to be operative leading to polymers with relatively
narrow molecular weight distributions. In our case, during
polymerization, the Pd0 center may migrate from the grow-
ing chain end to a vicinal aryl triflate group that will

Article
Macromolecules, Vol. 43, No. 22, 2010 9279
Figure 2. GPC chromatograms of Suzuki polycondensation of 6 at (a) room temperature and (b) 0 °C. Reaction conditions: 0.5 mol % Pd(OAc)2;
1 mol % S-Phos; 6 equiv K3PO4 (2 M aq); THF; [M]0 = 50 mM.
Scheme 1. Synthesis of Branched AB2 Monomer 6
become the new chain end and generate the branch points in
hyperbranched polymers. This explanation also correlates
well with the catalyst-dependent preferential oxidative addi-
tion process (vide infra). On the other hand, however, a
gradual increase in PDI was observed in our case as the
reaction progressed, suggesting that coupling of oligomers
may also have occurred to some extent.
Other phosphine-based catalysts, such as Pd(PPh3)4,
Pd(OAc)2/dppf, and Pd(OAc)2/P(o-tolyl)3, or a commercial
N-heterocyclic carbene-based catalyst, PEPPSI,53 were
tested, but all appeared to be less efficient than the combina-
tion of Pd(OAc)2 and S-Phos, as evidenced in each case by
the relatively lower molecular weight and higher PDI of the
product (Figure S1 and Table S1).
It is worth noting that the polymer remained completely
soluble during the entire course of the reaction, and the
products were isolated by simple extraction. The polymer
was further purified by dissolving it in a minimum amount of
THF or DCM and then precipitating it from methanol,
resulting in yields ranging from 83% to 95% for different
runs. The white powdery product is readily soluble in a wide
range of solvents such as THF, DCM, chloroform, ethyl
acetate, toluene, and benzene. The polymers are partially
soluble in cyclohexane, diethyl ether, acetone,1,4-dioxane,
and DMF but insoluble in methanol, ethanol, acetonitrile,
DMSO, and water.
In comparison to the HB-PmPs obtained from branched
monomer 6, HB-PmPs obtained from simple AB2-type

9280 Macromolecules, Vol. 43, No. 22, 2010
Xue et al.
monomers based on trisubstituted benzene only polymerizes
to relatively low molecular weights in most of the cases.22 For
example, in great contrast to monomer 6, when 3,5-dibro-
mophenyl boronate ester 7 was subjected to the optimized
polymerization conditions, precipitation occurred almost
instantly after the reaction started, whereas the soluble part
only comprised a small amount of low molecular weight
polymers. The high molecular weight and excellent solubility
of our HB-PmPs may be attributed to the more active
catalyst used and the structural characteristics of the
branched monomer M. Moreover, the presence of regularly
spaced branch points and tert-butyl-substituted linear and
terminal units in the polymer may contribute to the increased
solubility and hence the high molecular weights that can be
achieved. The SPC conditions we employed also contribute
to the successful polymerization of 6. For example, when
Pd(PPh3)4 or Pd(OAc)2/dppf was used with 6, a bimodal
distribution of the molecular weight was obtained in both
cases, which may be attributed to the decreased rate of
the Pd(0) oxidative addition step, as seen in other SPC
reactions34,54,55 and related studies.56,57
Table 1. Effect of Catalyst Loading and Starting Monomer Concen-
tration [M]0 on the Molecular Weight and PDI of the HB-PmPs
entry
catalyst (mol %)
[M]0 (mM) Mn (kDa)a
DPb
PDIa
1
2
3
4
5
6
7
8
9
4
2
50
50
50
50
50
100
20
10
100
20
12.0
16.0
35.8
54.0
57.3
46.2
31.9
27.1
59.6
48.5
44.7
24
32
73
110
117
94
65
55
1.3
1.4
1.5
1.9
1.9
3.0
1.3
1.3
2.1
1.6
1.6
0.5
0.05
0.01
0.5
0.5
0.5
0.05
0.05
0.05
122
99
91
10
11
10
a Molecular weights and PDI were determined by GPC with respect to
polystyrene standards. b Degree of polymerization (DP) was calculated
based on the Mn values determined by GPC.
For the solvents screened, THF was found to be optimal.
Other solvents such as DCM, toluene, cyclohexane, and 1,
4-dioxane resulted in decreased molecular weights and
relatively broad PDIs in most of the cases (Figure S2 and
Table S2). Precipitation of the products from the reac-
tion mixture occurred in the case when cyclohexane or 1,
4-dioxane was used.
Figure 3. Selected regions of (a) 1H NMR (CDCl3) and (b) 13C NMR
(CDCl3) spectra of hyperbranched poly(m-phenylene) with a molecular
weight (Mn) of 35.8 kDa.
loading (0.05 mol %), wherein the molecular weight and PDI
decreased upon dilution. Thus, these reaction parameters
can be readily altered to allow for easy control over the
molecular weight of the polymer.
Effects of Catalyst Loading and Starting Monomer Con-
centration. The amount of catalyst used was found to have a
drastic effect on the molecular weight and PDI of the poly-
mer (entries 1-5 in Table 1, Figure S3). First, the standard
polymerization conditions were chosen (65 °C, Pd(OAc)2/
S-Phos, THF, and 2 M K3PO4 at a starting monomer
concentration of 50 mM). Use of 4 and 2 mol % catalyst
(entries 1 and 2, Table 1) resulted in polymers with a much
lower molecular weight (Mn=12 kDa and 16 kDa, respec-
tively) compared to the case with 0.5 mol % catalyst (entry 3,
Table 1), although the PDI of the corresponding polymer
was lower. In contrast, decreasing the catalyst loading by an
order of magnitude to 0.05 mol % (entry 4, Table 1) led to an
increase in molecular weight (Mn = 54 kDa) without any
significant decrease in efficiency. More strikingly, use of only
0.01 mol % catalyst (entry 5, Table 1) led to nearly complete
monomer conversion and a high molecular weight polymer
(Mn = 57.3 kDa). The molecular weight dependence upon
catalyst loading also reveals that SPC of this AB2 monomer
is more consistent with a pseudo-chain-growth rather than a
step-growth mechanism. On the other hand, as the catalyst
loading decreased from 4 to 0.01 mol %, the corresponding
PDI increased from 1.3 to 1.9.
Polymer Characterization. In the 1H NMR spectra of the
HB-PmP with a molecular weight (Mn) of 35.8 kDa, the
aromatic region showed complex overlapping signals,
whereas the upfield region showed three broad signals from
δ 1.58 to δ 0.25, corresponding to the tert-butyl groups of
different types. In Figure 3a, the broad signals from δ 1.58 to
δ 1.25 (region 1) are assigned to the tert-butyl groups
attached to the peripheral terminal units, as the tert-butyl
groups in the monomer resonate at δ 1.34. The other two
broad signals spanning from δ 1.25 to δ 0.25 (region 2) are
attributed to the tert-butyl groups attached to the linear
units, for which the difference in chemical shift may indicate
the presence of different environments for the linear units
nearer to the peripheral units compared to those located in
the inner tiers of the HB-PmP. When DP is sufficiently high,
the ratio of the terminal versus linear units is expected to
be ca. 1:1 based on their relationship, that is, T = L þ 2.
However, because of the broadness of the signals, it was
not possible to obtain an accurate integration ratio of
“region 1” versus “region 2”. In the 13C NMR spectra, the
tert-butyl -CH3 groups show two relatively narrow lines at
δ 31.5 and δ 31.1 that overlap with a broad signal centered at
δ 30.9 (Figure 3b). The 19F NMR spectra (not shown) of the
HB-PmP only display one broad signal centered at δ -73.3,
indicating that hydrolysis of the triflate termini does not
appear to occur to any great extent.
The starting monomer concentration, [M]0, also has a
significant effect on the molecular weight and PDI of the
polymer (entries 6-11 in Table 1, Figure S4). For instance,
under otherwise identical conditions, as the monomer con-
centration increased, both the molecular weight and PDI
increased. The same trend was observed with a lower catalyst

Article
The HB-PmPs were also examined by MALDI-TOF
Macromolecules, Vol. 43, No. 22, 2010 9281
spectroscopy. For a polymer sample with a Mn (GPC) of
16 kDa, a series of oligomers up to 18mer were detected
(Figure S6).58 For each of these oligomers, the presence of
one pinacol boronate ester and (n þ 1) triflate end groups
was confirmed by comparing the results with the calculated
values. This observation suggests that cyclization at the focal
point is not a significant reaction pathway.
The UV-vis absorption spectra for HB-PmPs with dif-
ferent molecular weights (Mn = 27.0, 35.8, and 57.3 kDa)
show very similar profiles with a strong absorption at λmax
=
255 nm, exhibiting a slight red shift as compared to that
of the monomer (λmax=249 nm). As expected, the long-range
conjugation in the 1,3,5-terphenyl-based polymer backbone
is disrupted and confined within discrete biphenyl units. As a
result, these three polymer samples with the same weight-
volume percentage in THF solution showed essentially the
same magnitude of absorbance per mole of repeat unit
(Figure S7).
Figure 4. Normalized absorption and emission spectra of triflate
(λexc = 255 nm) and pyrenyl (λexc = 346 nm) end-capped HB-PmPs
measured in THF. Molecular weight: Mn(HB-PmP-OTf) = 35.8 kDa,
Mn(HB-PmP-pyr)=39.3 kDa.
The thermal stability of the HB-PmPs was examined by
thermal gravimetric analysis (TGA) under N2 from room
temperature to 900 °C at a constant increment rate of 10 °C/
min (Figure S8). Two mass loss steps were present with the
onset temperature of 330 and 490 °C. Each step corresponds
to approximately 22-24% loss of the original polymer
mass, for which we attribute the first stage to the loss of
triflate groups and the second to the tert-butyl groups
according to our previous results obtained for the tert-
butyl-substituted 1,3,5-polyphenylene dendrons.39 Differen-
tial scanning calorimetry (DSC) experiments carried out
under N2 from room temperature to 400 °C did not show
detectable phase transition before decomposition of the
polymer (Figure S10).
Given the high efficiency of the SPC process, we were
interested in the feasibility of end group modification of the
HB-PmPs via direct Suzuki-Miyaura cross-coupling. Poly-
condensation of 6 was first conducted under standard reac-
tion conditions in the presence of 0.5 mol % Pd(OAc)2/
S-Phos for 1 h, and phenylboronic acid or pyrene-1-boronic
acid was then introduced to the reaction mixture without
adding more catalyst. When compared to their triflate pre-
cursors, the resulting phenyl- and pyrenyl-capped HB-PmPs
only showed slightly increased molecular weights and PDIs
(Figure S5 and Table S3). However, a significant effect on the
solubility is clearly seen for these end-capped HB-PmPs,
which showed limited solubility in ethyl acetate, one of the
best solvents for triflate-terminated HB-PmPs. TGA curves
of the phenyl- and pyrenyl-capped HB-PmPs display less
than 4% mass loss before 450 °C, indicating an over 80%
efficiency for the end-capping process (Figure S9). The
UV-vis absorption and fluorescence spectra of the pyrenyl
end-capped (HB-PmP-pyr) and triflate-terminated (HB-
PmP-OTf) HB-PmPs are shown in Figure 4. The absorption
profile of HB-PmP-pyr displays a strong band peaking at
250 nm along with a small shoulder peak at 281 nm and a
broad band centered around 346 nm, indicating the presence
of pyrenyl groups. In the fluorescence spectra (λexc=346 nm)
of HB-PmP-pyr, a strong emission at 460 nm is indicative
of pyrene excimer formation.59,60 The fluorescence quantum
yield measured for HB-PmP-pyr (Mn = 39.3 kDa) in
THF (Φ = 0.74) is much higher than that for the triflate-
terminated HB-PmPs (Φ=0.09) with a comparable molec-
ular weight (Mn=35.8 kDa). Furthermore, it is noteworthy
that the strong excimer emission in HB-PmP-pyr was even
observed at a submicromolar concentration (0.1 μM with
respect to the pyrene unit),61 suggesting an intramolecular
rather than intermolecular excimer formation as resulted
from the confined spatial arrangement of the pyrenyl groups
within the hyperbranched polymer.
Conclusions
In summary, we have developed a Suzuki polycondensation
(SPC) protocol of a m-terphenyl-based branched AB2 monomer
that efficiently delivers the hyperbranched poly(m-phenylene)s
(HB-PmPs) with high molecular weights and excellent solubility
in common organic solvents. The highly active catalyst system
used and the structural characteristics of the branched monomer
are believed to be the two main factors that lead to these
improvements with respect to the cases where simple trisubsti-
tuted benzene-based AB2 monomers were used. The molecular
weight and PDI of the HB-PmPs could be controlled by varying
the catalyst loading and the starting monomer concentration. In
addition, the degree of branching was expected to be constant
(67%) for HB-PmPs obtained from this branched AB2 monomer.
With the strategy described, one should be able to modify the
monomer structure by replacing the tert-butyl groups with other
functionalities, so that different functional groups could be
incorporated within the HB-PmP scaffold. A wide range of
available phenolic compounds are therefore potential precursors
to the corresponding triflate-based monomer components. We
have demonstrated that the triflate end groups of the HB-PmPs
can be efficiently replaced in situ by other functional groups via
Suzuki-Miyaura cross-coupling. Our current work is focused on
these variations that would enable a systematic investigation
toward the properties, functions, and implications of functiona-
lized HB-PmPs.
Acknowledgment. This material is based upon work sup-
ported as part of the The Center for Electrochemical Energy
Storage-Tailored Interfaces, an Energy Frontier Research Cen-
ter funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Award DE-AC02-
06CH11357 9F-31921. This work was supported (in part) by
the National Science Foundation (CHE-1010680).
Supporting Information Available: Experimental details of
the monomer and polymer synthesis, characterization data,
GPC chromatograms, MALDI-TOF data, UV-vis spectra, thermal
analysis data, and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Voit, B. I.; Lederer, A. Chem. Rev. 2009, 109, 5924–5973.
(2) Yates, C. R.; Hayes, W. Eur. Polym. J. 2004, 40, 1257–1281.

9282 Macromolecules, Vol. 43, No. 22, 2010
Xue et al.
(3) Gao, C.; Yan, D. Prog. Polym. Sci. 2004, 29, 183–275.
(4) Jikei, M.; Kakimoto, M.-a. Prog. Polym. Sci. 2001, 26, 1233–1285.
(5) Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2505–2525.
(6) Kim, Y. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685–
1698.
(36) Maier, G.; Zech, C.; Voit, B.; Komber, H. Macromol. Chem. Phys.
1998, 199, 2655–2664.
(37) Ishida, Y.; Sun, A. C. F.; Jikei, M.; Kakimoto, M.-a. Macromole-
cules 2000, 33, 2832–2838.
(38) Hawker, C. J.; Chu, F. Macromolecules 1996, 29, 4370–4380.
(39) Finke, A. D.; Moore, J. S. Org. Lett. 2008, 10, 4851–4854.
(40) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura,
N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263–14278.
(41) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith,
M. R., III Science 2002, 295, 305–308.
€
(7) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. In Dendrimers
and Dendrons: Concepts, Syntheses, Applications; Wiley-VCH:
Weinheim, Germany, 2001; p 331.
(8) Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2009, 109, 5799–
5867.
(9) Martin, S. J.; Godschalx, J. P.; Mills, M. E.; Shaffer, E. O., II;
(42) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L.
J. Am. Chem. Soc. 2005, 127, 4685–4696.
Townsend, P. H. Adv. Mater. 2000, 12, 1769–1778.
€
€
(10) Berresheim, A. J.; Muller, M.; Mullen, K. Chem. Rev. 1999, 99,
1747–1785.
(43) K3PO4 was used as the base and THF-H2O as the solvent. Use of
strong bases such as NaOH led to quantitative deprotection of the
TBS groups.
(11) Tour, J. M. Adv. Mater. 1994, 6, 190–198.
(12) Schultz, M. J.; Zhang, X.; Unarunotai, S.; Khang, D.-Y.; Cao, Q.;
Wang, C.; Lei, C.; MacLaren, S.; Soares, J. A. N. T.; Petrov, I.;
Moore, J. S.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2008, 105,
7353–7358.
(13) Moore, J. S. Acc. Chem. Res. 1997, 30, 402–413.
(14) Orlicki, J. A.; Thompson, J. L.; Markoski, L. J.; Sill, K. N.; Moore,
J. S. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 936–946.
(15) Thompson, D. S.; Markoski, L. J.; Moore, J. S.; Sendijarevic, I.;
Lee, A.; McHugh, A. J. Macromolecules 2000, 33, 6412–6415.
(16) Markoski, L. J.; Thompson, J. L.; Moore, J. S. Macromolecules
2000, 33, 5315–5317.
(44) To achieve complete conversion of 3, 3 mol % [Ir(COD)(OMe)]2
and 6 mol % 4,40-di-tert-butyl-2,20-bipyridine (dtbpy) needed to be
added in two portions. See Supporting Information for details.
(45) Hydrolysis of the boronate ester was observed in this process that
resulted in decreased yield. All the pinacol boronate esters, i.e., 2, 4,
5, and 6, are sufficiently stable to the silica gel flash chromatogra-
phy conditions that we used, although small amounts of decom-
position may occur if a prolonged column chromatography process
was applied.
€
(46) Sakamoto, J.; Rehahn, M.; Wegner, G.; Schluter, A. D. Macromol.
Rapid Commun. 2009, 30, 653–687.
€
(17) Thompson, D. S.; Markoski, L. J.; Moore, J. S. Macromolecules
1999, 32, 4764–4768.
(18) Suh, H.-J.; Bharathi, P.; Beebe, D. J.; Moore, J. S. J. Microelec-
tromechan. Syst. 2000, 9, 198–205.
(47) Schluter, A. D. J. Polym. Sci., Part A: Polym. Chem. 2001, 39,
1533–1556.
(48) Sangvikar, Y.; Fischer, K.; Schmidt, M.; Schluter, A. D.; Sakamoto,
€
J. Org. Lett. 2009, 11, 4112–4115.
€
(19) Bharathi, P.; Moore, J. S. Macromolecules 2000, 33, 3212–3218.
(20) Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1997, 119, 3391–3392.
(21) Kim, Y. H.; Beckerbauer, R. Macromolecules 1994, 27, 1968–1971.
(22) Kim, Y. H.; Webster, O. W. Macromolecules 1992, 25, 5561–5572.
(23) Kim, Y. H.; Webster, O. W. J. Am. Chem. Soc. 1990, 112, 4592–
4593.
(49) Kandre, R.; Schluter, A. D. Macromol. Rapid Commun. 2008, 29,
1661–1665.
(50) Carraher, C. E., Jr. Seymour/Carraher’s Polymer Chemistry, 5th
ed.; Marcel Dekker: New York, 2000; p 230.
(51) Beryozkina, T.; Boyko, K.; Khanduyeva, N.; Senkovskyy, V.;
Horecha, M.; Oertel, U.; Simon, F.; Stamm, M.; Kiriy, A. Angew.
Chem., Int. Ed. 2009, 48, 2695–2698.
(52) Yokoyama, A.; Suzuki, H.; Kubota, Y.; Ohuchi, K.; Higashimura,
H.; Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236–7237.
(53) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass,
G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem.;Eur. J.
2006, 12, 4743–4748.
(24) Kim, Y. H.; Webster, O. W. Polym. Prepr. (Am. Chem. Soc., Div.
Polym. Chem.) 1988, 29, 310.
€
(25) Morgenroth, F.; Mullen, K. Tetrahedron 1997, 53, 15349–15366.
(26) Stumpe, K.; Komber, H.; Voit, B. I. Macromol. Chem. Phys. 2006,
207, 1825–1833.
(27) Xu, K.; Peng, H.; Sun, Q.; Dong, Y.; Salhi, F.; Luo, J.; Chen, J.;
Huang, Y.; Zhang, D.; Xu, Z.; Tang, B. Z. Macromolecules 2002,
35, 5821–5834.
€
(54) Bo, Z.; Schluter, A. D. Chem. Commun. 2003, 2354–2355.
(55) Murage, J.; Eddy, J. W.; Zimbalist, J. R.; McIntyre, T. B.; Wagner,
Z. R.; Goodson, F. E. Macromolecules 2008, 41, 7330–7338.
(56) Weber, S. K.; Galbrecht, F.; Scherf, U. Org. Lett. 2006, 8, 4039–
4041.
(28) Ohe, T.; Miyaura, N.; Suzuki, A. J. Org. Chem. 1993, 58, 2201–2208.
ꢀ
(29) Hawker, C. J.; Lee, R.; Frechet, J. M. J. J. Am. Chem. Soc. 1991,
113, 4583–4588.
€
(30) Frey, H.; Holter, D. Acta Polym. 1999, 50, 67–76.
(57) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127, 10006–10007.
(58) The GPC Mn value (16 kDa) corresponds to a DP of 32, despite
that the molecular weight of hyperbranched polymers is often
underestimated by simply comparing to polystyrene standards in
GPC analysis.
€
€
(31) Holter, D.; Frey, H. Acta Polym. 1997, 48, 298–309.
(32) Holter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 30–35.
(33) Segawa, Y.; Higashihara, T.; Ueda, M. J. Am. Chem. Soc. 2010,
132, 11000–11001.
(34) Huang, W.; Su, L.; Bo, Z. J. Am. Chem. Soc. 2009, 131, 10348–
10349.
(35) Fu, Y.; Van Oosterwijck, C.; Vandendriessche, A.; Kowalczuk-Bleja,
A.; Zhang, X.; Dworak, A.; Dehaen, W.; Smet, M. Macromolecules
2008, 41, 2388–2393.
(59) Baker, L. A.; Crooks, R. M. Macromolecules 2000, 33, 9034–9039.
(60) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354–
4360.
(61) The concentration with respect to the pyrene unit was calculated
based on a 100% pyrenyl end-capped HB-PmP with a DP of 50.