Hyperbranched polymers with a degree of branching of 100% prepared by catalyst transfer Suzuki-Miyaura polycondensation

Article abstract of DOI:10.1021/ja9033846

(Chemical Equation Presented) Hyperbranched polymers with a degree of branching of 100% were prepared by catalyst transfer Suzuki-Miyaura polymerization of AB₂ monomers carrying one boronic acid and two aromatic bromo functional groups; in contrast, Suzuki-Miyaura polymerization of the same AB₂ monomers using a traditional catalyst afforded hyperbranched polymers with a branching degree of only ～56%. This is a nice example of controlling the topology of hyperbranched polymers via the catalyst.

Full text of DOI:10.1021/ja9033846

Published on Web 07/08/2009
Hyperbranched Polymers with a Degree of Branching of 100% Prepared by
Catalyst Transfer Suzuki-Miyaura Polycondensation
Weiguo Huang, Linjie Su, and Zhishan Bo*
Beijing National Laboratory for Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received April 27, 2009; E-mail: zsbo@iccas.ac.cn
Hyperbranched polymers have attracted considerable interest
because of their unique molecular structures and properties, such
as good solubility, low viscosity, and large numbers of functional
end groups.1 A degree of branching (DB) of 100% is a typical
feature for perfectly branched dendrimers, whereas hyperbranched
polymers prepared by one-pot reaction are usually not perfectly
branched and have a DB of ∼50%.1 Many attempts have been made
to improve the degree of branching of hyperbranched polymers.
Pioneering works have been reported by several groups. For
example, Hawker and Chu2 developed an AB4 monomer route,
which gave a hyperbranched polymer with a DB of up to 71%.
Ishida et al.3 synthesized a series of monomers (AB2, AB4, and
AB8) and found that the DB increases with increasingly branched
monomers; AB2 monomers gave a DB of only 32%, AB4 monomers
gave 72%, and AB8 gave 84%. Moore, McHugh, and co-workers4
reported a method of synthesis of hyperbranched polymers with
varying degrees of branching from a single monomer. Hult and
co-workers5 developed a pseudo-one-step reaction for preparing
hyperbranched macromolecules with DBs of nearly 80%. Ho¨lter
and Frey6 reported that the maximum DB obtainable from a slow
addition process is 67% for AB2 systems. We reported that the use
of an AB2 + AC2 approach could afford hyperbranched polymers
with a DB of 86%.7 These modification methods only enhance the
DB, but none of them can achieve a hyperbranched polymer with
a DB of 100%. Hyperbranched polymers with a DB of 100% have
been recently reported by several groups but are limited to few
monomers.8
amount, and its synthesis is depicted in Scheme S1 in the Supporting
Information. Bromination of 9-bromohexylcarbazole (1) with NBS
afforded compound 2 in a yield of 98%. Williamson etherification
of 2 and 4-hydroxybenzene boronic pinacol ester furnished M2 in
65% yield.
Chart 1. Chemical Structures of AB2-Type Monomers
The synthesis of hyperbranched polymers is shown in Scheme
1. Hyperbranched polymers with a DB of 100% were first prepared
by catalyst transfer Suzuki-Miyaura polymerization of AB2
monomers M1 and M2. The polymerization was carried out in a
biphasic mixture of THF and aqueous NaHCO3 with P(t-Bu)313 as
the ligand and Pd2(dba)3CHCl3 as the source of zero-valent
palladium. The reaction was stirred and refluxed for 4 days under
N2 to afford hyperbranched polymers P1 and P2 in yields of 63
and 68%, respectively. During the polymerization, no polymer
precipitated from the reaction mixture. The weight-average mo-
lecular weights and polydispersities of these hyperbranched poly-
mers, as determined by GPC with THF as an eluent calibrated with
polystyrene standards, were 7400 and 1.16 for P1 and 14 900 and
1.53 for P2, respectively. The polymerization of M1 and M2 with
Pd(PPh3)4 as the catalyst precursor afforded hyperbranched poly-
mers P1′ and P2′ in yields of 51 and 61%, respectively, and the
corresponding weight-average molecular weights were 14 100 and
2900, respectively. The weight-average molecular weights of the
four hyperbranched polymers were also measured by static laser
light scattering to be 1.35 × 105 for P1, 1.44 × 105 for P1′, 3.7 ×
105 for P2, and 6.4 × 104 for P2′. During the polymerization, P2′
precipitated from the reaction mixture. The lower molecular weight
of P2′ is probably caused by its poor solubility, which may be due
to its more disordered topology. As studied by differential scanning
calorimetry (DSC), no melting point or glass transition was observed
in the range 50-200 °C for these four hyperbranched polymers.
According to the definition given by Fre´chet and co-workers,14
the DB of hyperbranched polymers was calculated as DB ) (D +
T)/(D + L + T), where D denotes the dendritic units, T the terminal
units, and L the linear units. The content of each unit was
Recently, Sinclair and Sherburn9 reported that Suzuki-Miyaura
coupling of diiodoaryls and arylboronic acid in a feed ratio of 10:1
with Pd(PPh3)4 as the catalyst precursor and Ag2CO3 as the base
preferentially afforded the product of double coupling. Dong and
Hu10 found that Suzuki coupling of dibromobenzenes with 1 equiv
of arylboronic acid using Pd2(dba)3 and P(t-Bu)3 as catalyst
precursors formed exclusively diaryl-substituted benzenes. Later,
Scherf and co-workers11 also reported that the coupling of 2,7-
dibromofluorene with 1 equiv of arylboronic acid under conditions
similar to those of Dong and Hu preferentially afforded diaryl-
substituted fluorenes. Yokozawa and co-workers12 successfully
demonstrated the chain-growth Suzuki-Miyaura polymerization of
bromoarylboronic acid using P(t-Bu)3Pd(Ph)Br as an arylpalladium
halide catalyst. Herein, we report the preparation of hyperbranched
polymers with a DB of up to 100% starting from AB2-type
monomers by catalyst transfer Suzuki-Miyaura coupling.
Scheme 1. Synthesis of Hyperbranched Polymers
The AB2-type monomers M1 and M2, whose chemical structures
are shown in Chart 1, were used to verify this concept. These two
AB2-type monomers contain one aromatic boronic pinacol ester
and two aromatic bromo atoms linked by an alkyl chain spacer.
The alkyl chain spacer was introduced to enhance the solubility of
the hyperbranched polymers. The synthesis of M1 was described
in our previous publication.7 M2 was facilely synthesized in a large
9
10348 J. AM. CHEM. SOC. 2009, 131, 10348–10349
10.1021/ja9033846 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
determined by NMR integration. To obtain hyperbranched polymers
with a DB of 100%, the linear chain propagation should be
completely suppressed.
A possible mechanism for the formation of the fully branched
hyperbranched polymers can be described as follows (Scheme 2):
(1) oxidative addition of the palladium atom to the aryl-bromo
bond; (2) transmetalation and reductive elimination to form a
carbon-carbon bond between two aryls; (3) formation of a complex
between the bulk Pd(0)/P(t-Bu)3 catalyst and the aromatic ring,
which helps to prevent the palladium catalyst from diffusing to the
reaction mixture; (4) oxidative addition of the catalyst to the
neighboring aryl-bromo bond by catalyst transfer via the π system;
(5) transmetalation, reductive elimination, and diffusion of the
catalyst to the reaction mixture. Consequently, fully hyperbranched
polymers are formed.
In conclusion, hyperbranched polymers with a degree of branch-
ing of 100% were prepared by catalyst transfer Suzuki-Miyaura
polymerization of AB2-type monomers carrying one arylboronic
acid and two aryl bromides. The catalyst transfer Suzuki-Miyaura
polymerization was carried out with Pd2(dba)3 as the zero-valent
palladium source and P(t-Bu)3 as the bulky ligand. In comparison,
AB2-type monomers polymerized by using the traditional Pd(PPh3)4
as the catalyst precursor afforded hyperbranched polymers with a
DB of only ∼56%. Our results clearly demonstrate that the chemical
structures of hyperbranched polymers can be controlled by the
choice of catalyst and also open a convenient way to prepare fully
branched hyperbranched polymers from facilely achievable AB2-
type monomers. This is the first report of controlling the topology
of hyperbranched polymers via the catalyst precursor.
The DB of P1 prepared by catalyst transfer Suzuki polymeri-
zation was ∼100%, which was determined by 13C NMR integration
methods as described in our previous paper.7 In a control experi-
ment, Suzuki-Miyaura polymerization of M1 under standard
conditions with a traditional catalyst, Pd(PPh3)4, afforded P1′ with
a DB of only ∼56%.7 The assigned peaks used for the integration
are shown in Figure 1.
For P2, most of the signals in their alkyl regions were difficult
to identify because of overlap and could not be used for integration,
but the aromatic signals of the carbazole unit (as shown in Figure
1) were well-separated for the L, D, and T units. In order to assign
these signals, several model compounds were synthesized (see the
1
Supporting Information). For comparison, the H NMR spectrum
of the hyperbranched polymer prepared by using the traditional
catalyst, Pd(PPh3)4, is also shown in Figure 1. For the hyperbranched
polymers prepared by catalyst transfer Suzuki-Miyaura polymer-
ization, only signals from dendritic and terminal units can be
observed in the spectrum, whereas the signals from linear units
are completely absent. The DB of P2 is close to 100%. In contrast,
for P2′ prepared using Pd(PPh3)4 as the catalyst precursor, the
signals from linear units could be clearly seen, and its DB was
also only ∼56%.
Acknowledgment. Financial support by the NSF of China
(20834006 and 20774099) is gratefully acknowledged.
Supporting Information Available: Detailed experimental proce-
dures, characterization of all compounds, and their spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Fre´chet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.;
Grubbs, R. B. Science 1995, 269, 1080–1083. (b) Guan, Z. B.; Cotts, P. M.;
McCord, E. F.; McLain, S. J. Science 1999, 283, 2059–2062. (c) Voit, B.
J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2505–2525. (d) Kim, Y. H.
J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685–1698. (e) Inoue, K.
Prog. Polym. Sci. 2000, 25, 453–571. (f) Jikei, M.; Kakimoto, M. Prog.
Polym. Sci. 2001, 26, 1233–1285. (g) Gao, C.; Yan, D. Prog. Polym. Sci.
2004, 29, 183–275. (h) Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2005,
43, 2679–2699.
Figure 1. (a) 13C NMR peak assignments for the middle alkylene carbons
of (bottom) P1 and (top) P1′. (b) 1H NMR peak assignments for the
carbazole protons of (bottom) P2 and (top) P2′.
(2) Hawker, C. J.; Chu, F. K. Macromolecules 1996, 29, 4370–4380.
(3) Ishida, Y.; Sun, A. C. F.; Jikei, M.; Kakimoto, M. Macromolecules 2000,
33, 2832–2838.
Scheme 2. Possible Mechanism for the Formation of
Hyperbranched Polymers with a Branching Degree of 100%
(4) Thompson, D. S.; Markoski, L. J.; Moore, J. S.; Sendijarevic, I.; Lee, A.;
McHugh, A. J. Macromolecules 2000, 33, 6412–6415.
(5) Malmstro¨m, E.; Johansson, M.; Hult, A. Macromolecules 1995, 28, 1698–
1703.
(6) Holter, D.; Frey, H. Acta Polym. 1997, 48, 298–309.
(7) Bo, Z. S.; Schlu¨ter, A. D. Chem. Commun. 2003, 2354–2355.
(8) (a) Hobson, L. J.; Kenwright, A. M.; Feast, W. J. Chem. Commun. 1997,
1877–1878. (b) Smet, M.; Schacht, E.; Dehaen, W. Angew. Chem., Int.
Ed. 2002, 41, 4547–4550. (c) Maier, G.; Zech, C.; Voit, B.; Komber, H.
Macromol. Chem. Phys. 1998, 199, 2655–2664. (d) Sinananwanich, W.;
Ueda, M. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2689–2700. (e)
Fu, Y.; Vandendriessche, A.; Dehaen, W.; Smet, M. Macromolecules 2006,
39, 5183–5186. (f) Fu, Y.; Van Oosterwijck, C.; Vandendriessche, A.;
Kowalczuk-Bleja, A.; Zhang, X.; Dworak, A.; Dehaen, W.; Smet, M.
Macromolecules 2008, 41, 2388–2393.
(9) Sinclair, D. J.; Sherburn, M. S. J. Org. Chem. 2005, 70, 3730–3733.
(10) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127, 10006–10007.
(11) Weber, S. K.; Galbrecht, F.; Scherf, U. Org. Lett. 2006, 8, 4039–4041.
(12) Yokoyama, A.; Suzuki, H.; Kubaota, Y.; Ohuchi, K.; Higashimura, H.;
Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236–7237.
(13) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176–4211.
(14) Hawker, C. J.; Lee, R.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1991, 113,
4583–4588.
JA9033846
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10349