"AB2 + AC2" approach to hyperbranched polymers with a high degree of branching

Article abstract of DOI:10.1039/b306601k

A novel one-pot "AB₂ + AC₂" approach based on palladium catalyzed Suzuki polycondensation was developed to prepare hyperbranched aryl/alkyl polymers with a high degree of branching.

Full text of DOI:10.1039/b306601k

“AB2 + AC2” approach to hyperbranched polymers with a high degree
of branching†
Zhishan Bo*a and A. D. Schlüterb
a State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and
Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China.
E-mail: zsbo@iccas.ac.cn; Fax: +86(10)62559373; Tel: +86(10)82618587
b Institut für Chemie/Organische Chemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin,
Germany. E-mail: adschlue@chemie.fu-berlin.de; Fax: +49(30)838 53357; Tel: +49(30)838 53358
Received (in Cambridge, UK) 10th June 2003, Accepted 29th July 2003
First published as an Advance Article on the web 11th August 2003
A novel one-pot “AB2 + AC2” approach based on palladium
catalyzed Suzuki polycondensation was developed to pre-
pare hyperbranched aryl/alkyl polymers with a high degree
of branching.
especially mild conditions.10 Ideally at 45 °C, only iodoaro-
matics undergo reaction. At this stage the predominant
intermediates formed are oligomers, mostly composed of
dendritic and terminal units as shown in Scheme 1. If the
reaction temperature is further increased to reflux (about 65 °C),
bromoaromatic compounds start to react and form higher molar
mass polymers. Using the two monomers’ rate difference, the
Dendrimers and hyperbranched polymers have gained consider-
able scientific attention due to their unusual molecular struc-
tures and properties.1 Dendrimers are mono-dispersed, well-
defined, and highly branched macromolecules prepared by
multistep reactions,1 whereas hyperbranched polymers are
irregularly branched and polydisperse macromolecules, which
can be prepared by one-pot reaction of ABn monomers.2 Due to
their structural similarity, dendrimers and hyperbranched
macromolecules have some similar features such as good
solubility, low viscosity when compared to their linear analogs,
and large numbers of end groups. The main advantage of
hyperbranched polymers over dendrimers is their easy prepara-
tion, which makes large-scale synthesis possible at a reasonable
cost. The syntheses of many kinds of hyperbranched polymers
have been well documented by excellent reviews.2 A challeng-
ing goal in synthesis of hyperbranched polymers is control over
their degree of branching (DB). Normally the DB varies from
0% for linear polymers and 100% for dendrimers, and for most
hyperbranched polymers prepared from AB2 monomers it is
around 50%. In the literature some special techniques or
reactions have been used to obtain high DB polymers. For
example, Hawker et al. developed an AB4 monomer route,
which gave a hyperbranched polymer with DB up to 71%.3
Ishida et al. synthesized a series of monomers AB2, AB4, and
AB8 and examined their polymerization results. They found that
the DB increases with increasingly branched monomers. AB2
monomers gave DB of only 32%, AB4 ones gave 72%, and AB8
gave 84%.4 Another route for increasing DB was developed by
Hult’s5 and Moore’s groups.6 They added a core molecule and
adopted a pseudo-one-step reaction to prepare hyperbranched
macromolecules with DB nearly 80%. There are only three
examples reported with DBs close to 100%.7
Scheme 1 Reagents and conditions: Pd(PPh3)4, aq. NaHCO3, THF, 45 °C
for 1 d, reflux for 1 d. (For detailed structures of the monomers see Scheme
2.)
Hyperbranched polymers prepared by polymerization of AB2
monomers are composed of dendritic (D), linear (L), and
terminal (T) units. According to the definition given by
Fréchet,8 the DB is calculated by the following equation:
DB = (D + T)/(D + T + L)
Here we describe a novel “AB2 + AC2” approach to prepare
hyperbranched polymers with a high DB (Scheme 1). The
strategy was to minimize the fraction of L units and should lead
to chemically relatively resistant aryl/alkyl-based structures.
Monomers 1 and 2 were designed accordingly (Scheme 2).
Suzuki polycondensation (SPC) was used for polymerization of
the two monomers (Scheme 1).9 It is well known that the
reaction of arylboronic acid (or ester) proceeds at a higher rate
with iodoaromatics than with bromoaromatics and also under
Scheme 2 Reagents and conditions: a) BBr3, CH2Cl2, r.t. b) Pinacol,
CH2Cl2, reflux. c) n-BuLi, ether, 278 °C, 1 h. d) Trimethylsilyl chloride,
278 °C to r.t. e) PdCl2, THF, allyl chloride, r.t. f) 9-BBN, THF, r.t., 2 d. g)
4-Bromoiodobenzene, Pd(PPh3)4, aq. NaOH, THF, r.t. h) n-BuLi, THF/
ether, 278 °C to 0 °C. i) B(O-i-Pr)3, 278 °C to r.t. j) ICl, CH2Cl2, r.t.
† Electronic Supplementary Information (ESI) available: experimental
procedures and characterization of compounds. See http://www.rsc.org/
suppdata/cc/b3/b306601k/
2354
CHEM. COMMUN., 2003, 2354–2355
This journal is © The Royal Society of Chemistry 2003

View Online
whole polymerization process could be controlled, which will
lead to a hyperbranched polymer with a high DB.
followed by reaction with allyl bromide using PdCl2 as a
catalyst precursor affording 5 in 92% yield. Reaction of 5 with
a 0.5 M solution of 9-BBN in THF at room temperature
furnished the corresponding adduct 6, which was used for
coupling without further purification. The reaction of alkyl
borane 6 with 4-bromoiodobenzene gave 7 in a 86% yield.
Treatment of 7 in ether with n-BuLi at 278 °C and quenching
with triisopropyl borate gave the corresponding boronic acid in
83% yield. Reflux of this boronic acid with pinacol in CH2Cl2
gave the corresponding ester 8. Iodination of 8 with ICl in
CH2Cl2 at room temperature afforded 2 as a white solid in 92%
yield. Monomers 1 and 2 were unambiguously characterized
with 1H and 13C NMR spectroscopy and elemental analysis.
In conclusion, hyperbranched aryl/alkyl polymers with high
DB were prepared by using a novel one-pot AB2 + AC2
approach. The reaction temperature controlled the reaction
process. At lower temperature boronic esters reacted with
iodoaromatics much faster than with bromoaromatics, and
formed an ABn type hyperbranched intermediate with a high
DB. After further temperature increase, bromoaromatics started
to react and resulted in a higher molar mass polymer. The DB
for hyperbranched polymers from 1 + 2 was 86 ± 10%, which is
much higher than that for hyperbranched polymers synthesized
from 1 only. For the former no iodo endgroups could be
detected in their 13C NMR spectra; the main difference between
these two hyperbranched polymers seems to be their DB.
Financial support from The Chinese Academy of Sciences
(Hundred Talents Program), the National Natural Science
Foundation of China (the Outstanding Youth Fund No.
20225415), the Deutsche Forschungsgemeinschaft (Sfb 448, TP
A1), and the Fonds der Chemischen Industrie is greatly
acknowledged.
An alkylene spacer was introduced to make the polymer
backbone more flexible and easier for determining the DB by
13C NMR integration (vide infra). One-pot SPC of 1 and 2 was
performed in a biphasic system THF/aqueous NaHCO3 with
Pd(PPh3)4 as catalyst precursor.9–11 The reaction procedure was
set as follows: first stirred at 45 °C for 1 d, and then refluxed for
1 d. A bromo-terminated hyperbranched polymer was obtained
as a pale white solid material in 81% yield after precipitating
from ether and freeze-drying from benzene. Weight-average
molar mass (Mw) determined by gel permeation chromatog-
raphy (GPC) calibrated with polystyrene standard was 13500
and polydispersity (PD) 2.22 (bimodal). A DB of 86 ± 10% was
calculated from 13C NMR integration (vide infra). As a
contrasting reaction, SPC of monomer 1 went well and gave
bromo-terminated hyperbranched polymers in a 69% yield with
DB of 56 ± 10% (vide infra). Mw was up to 26500 and
polydispersity (PD) was 1.41. The GPC shows a much smaller
almost baseline separated peak at lower masses. When SPC of
only the diiodo monomer was done, the polymers precipitated
from the reaction mixture and could not be redissolved in any
solvent. Limited solubility hampered sufficient characterization
of iodo-terminated hyperbranched polymers.
1H NMR spectra of polymers prepared from 1 + 2 and 1 are
completely identical, and it was impossible to calculate the DB
from proton integrations. In their 13C NMR spectra, most
signals in the aromatic region were difficult to identify due to
overlapping and could not be used for integration. Fortunately
the signals belonging to the middle alkylene carbons (circled in
Fig. 1) were well separated for L, D, and T units. The 13C NMR
spectra used for integration were carefully recorded by using a
longer pulse delay time (2 s) in order to account for the nuclei’s
different relaxation times. Some model compounds12 were used
to assign these carbons and the results are shown in Fig. 1. It is
obvious that the fraction of L units decreased dramatically for
polymers prepared from 1 + 2 (Fig. 1b) compared to the ones
prepared from 1 only (Fig. 1a). It was also concluded from these
spectra that all iodo groups had been consumed. The combus-
tion results of both polymers also support the above conclu-
sion.‡
The synthesis of 1 and 2 is outlined in Scheme 2. Compound
313 was converted to boronic acid by treatment with BBr3 in
CH2Cl2, subsequent esterification with pinacol gave 1 as a
white solid in a total yield of 65%. Starting from the
commercially available 1,3,5-tribromobenzene, its treatment
with n-BuLi at 278 °C and then quenching with TMSCl gave
compound 4 in 99% yield.14 4 was converted to the correspond-
ing Grignard reagent by treatment with Mg powder in THF,
Notes and references
‡ Polymers prepared from 1: Anal. Calcd. for (C15H13Br)n: C, 65.95; H,
4.80. Found: C, 65.70; H, 5.12. Polymers prepared from 1 + 2: Anal. Calcd.
for (C15H13Br)n: C, 65.95; H, 4.80. Found: C, 66.08; H, 4.94%.
1 (a) G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendrimers and
Dendrons, Wiley-VCH, Weinheim, 2001; (b) J. M. J. Fréchet and D. A.
Tomalia, Dendrimers and other Dendritic Polymers, Wiley-VCH,
Weinheim, 2001; (c) H. T. Chang and J. M. J. Fréchet, J. Am. Chem.
Soc., 1999, 121, 2313; (d) C. Gao and D. Y. Yan, Macromolecules,
2003, 36, 613; (e) C. Gao and D. Y. Yan, Chem. Commun., 2001, 107;
(f) A. Kumar and E. W. Meijer, Chem. Commun., 1998, 1629; (g) W. J.
Feast, A. J. Keeney, A. M. Kenwright and D. Parker, Chem. Commun.,
1997, 1749.
2 (a) Y. H. Kim, J. Polym. Sci. Part A: Polym. Chem., 1998, 36, 1685; (b)
B. Voit, J. Polym. Sci. Part A: Polym. Chem., 2000, 38, 2505.
3 C. J. Hawker and F. K. Chu, Macromolecules, 1996, 29, 4370.
4 Y. Ishida, A. C. F. Sun, M. Jikei and M. Kakimoto, Macromolecules,
2000, 33, 2832.
5 E. Malmström, M. Johansson and A. Hult, Macromolecules, 1995, 28,
1698.
6 P. Bharathi and J. S. Moore, Macromolecules, 2000, 33, 3212.
7 (a) L. J. Hobson, A. M. Kenwright and W. J. Feast, Chem. Commun.,
1997, 1877; (b) M. Smet, E. Schacht and W. Dehaen, Angew. Chem.,
2002, 114, 4729; (c) G. Maier, C. Zech, B. Voit and H. Komber,
Macromol. Chem. Phys., 1998, 199, 2655.
8 (a) C. J. Hawker, R. Lee and J. M. J. Fréchet, J. Am. Chem. Soc., 1991,
113, 4583.
9 (a) A. D. Schlüter and Z. S. Bo, Synthesis of conjugated polymers for
materials science, in Handbook of organopalladium chemistry for
organic synthesis, ed. E. Negishi, Wiley-VCH, New York, 2002; (b) A.
D. Schlüter, J. Polym. Sci. Part A: Polym. Chem., 2001, 39, 1533.
10 S. Schlüter, J. Frahn and A. D. Schlüter, Macromol. Chem. Phys., 2000,
201, 139.
11 (a) Z. S. Bo, J. P. Rabe and A. D. Schlüter, Angew. Chem., Int. Ed.,
1999, 38, 2370; (b) Z. S. Bo and A. D. Schlüter, Chem. Eur. J., 2000, 6,
3235; (c) Z. S. Bo, C. M. Zhang, N. Severin, J. P. Rabe and A. D.
Schlüter, Macromolecules, 2000, 33, 2688.
12 See electronic supplementary information.
13 Z. S. Bo and A. D. Schlüter, J. Org. Chem., 2002, 67, 53.
14 M. Beinhoff, B. Karakaya and A. D. Schlüter, Synthesis, 2003, 79.
Fig. 1 13C NMR peak assignments for middle alkylene carbons of (a)
polymers prepared from 1 and (b) polymers prepared from 1 + 2.
CHEM. COMMUN., 2003, 2354–2355
2355