Hyperbranched polymers with controlled degree of branching from 0 to 100%

Article abstract of DOI:10.1021/ja105213r

A linear polymer, hyperbranched polymers with various degrees of branching, and 100% hyperbranched polymers were successfully synthesized by self-polycondensation of 2,2,2-trifluoro-1-[4-(4-phenoxyphenoxy)phenyl]ethanone by using different amounts of trifluoromethanesulfonic acid from the same AB₂ monomer.

Full text of DOI:10.1021/ja105213r

Published on Web 07/28/2010
Hyperbranched Polymers with Controlled Degree of Branching from
0 to 100%
Yukari Segawa, Tomoya Higashihara, and Mitsuru Ueda*
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of
Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo, Japan, 152-8552
Received June 15, 2010; E-mail: ueda.m.ad@m.titech.ac.jp
The products were isolated in quantitative yields, and the results
are summarized in Figure S5 (see SI). Only the diarylated product,
2,2,2-trifluoro-1,1,1-tri(4-methoxyphenyl)ethane (3), was selectively
obtained in an equimolar ratio of [TFSA]/[1], whereas a monoary-
lated product, 2,2,2-trifluoro-1,1-bis(4-methoxyphenyl)-1-methoxy-
ethane (4), was formed quantitatively in molar ratios of [TFSA]/
[1] over 7. When molar ratios of [TFSA]/[1] between 2 and 6 are
Abstract: A linear polymer, hyperbranched polymers with various
degrees of branching, and 100% hyperbranched polymers were
successfully synthesized by self-polycondensation of 2,2,2-
trifluoro-1-[4-(4-phenoxyphenoxy)phenyl]ethanone by using dif-
ferent amounts of trifluoromethanesulfonic acid from the same
AB2 monomer.
used, a mixture of two products 3 and 4 is isolated in all cases
(Scheme 1), where the product ratio of 4 to 3 increases by increasing
the ratio of [TFSA]/[1], which means that the ratios of the rate
constants, k1/k2, for the monoarylation (k1) and diarylation steps
(k2) are between 0 and 100.16
Hyperbranched polymers (HBPs) have attracted considerable
attention during the past decade1-5 because of their intrinsic
globular structures and unique properties, such as low viscosity,
high solubility, and a high degree of functionality, compared to
linear analogues. The majority of hyperbranched polymer syntheses
involves a step-growth polycondensation reaction employing mono-
mers of the ABx type, where A and B represent two different
functional groups. In this respect, the degree of branching (DB)6
has been discussed as one of the key parameters for the charac-
terization of hyperbranched polymers, which is controlled by the
reactivity of the functional groups involved in the synthesis.
Theoretically, for polymers derived from an AB2 monomer, the
DB is determined by statistics and only reaches around 50%,
assuming it is based on equal reactivity of the two B functional
groups of the AB2 monomer.
Scheme 1. Model Reactions of 2,2,2-Trifluoro-4′-methoxy-
acetophenone (1) with Anisole (2) in Various Molar Ratios of
[TFSA]/[1]
Recently, several groups have reported the synthesis of 100%
HBPs.7-14 Such 100% HBPs can be obtained when the first reaction
step of an AB2 monomer activates the second reaction. Quite
recently, we briefly reported that an AB2 monomer, 2,2,2-trifluoro-
1-[4-(4-phenoxyphenoxy)phenyl]ethanone (7), produced a linear
poly(phenylene ether) in trifluoromethanesulfonic acid (TFSA).15
The resulting poly(phenylene ether)s with various functional groups
in every single repeating unit were formed simply by pouring the
polymer solutions into nucleophilic solvents. This finding prompted
us to develop the synthesis of HBPs with arbitrarily controlled
degrees of branching (DBs).
Herein, we report the first synthesis of HBPs with controlled
DBs by one-pot polycondensation of an AB2 monomer 7, in various
molar ratios of TFSA to monomer. We found that a 100% HBP is
produced in an equimolar ratio of [TFSA]/[monomer] in chloroform.
On the other hand, a linear polymer, that is, a 0% HBP, is obtained
by using TFSA as a solvent. In addition, HBPs with a DB of any
percent between 0 and 100% are formed by changing the ratio of
[TFSA]/[monomer]. To date no other examples have been reported
for the synthesis of HBPs with controlled DB from 0 to 100%
derived from the same AB2 monomer.
These results suggest that the condensation of 1 with 2 in an
equimolar ratio of [TFSA]/[1] proceeds via intermediate carbocation
5 (Scheme S1 in the SI), whose reactivity is much higher than that
of starting compound 1. Therefore, when 5 is formed in a mixture,
it will be readily attacked by 2 to provide diarylated product 3,
where k2 is larger than k1. A detailed kinetic study is presented in
SI (Scheme S2, Figures S2-S4). On the other hand, in molar ratios
of [TFSA]/[1] over 7, k2 becomes considerably smaller than k1.
Olah et al. reported that TFSA is a highly ionizing and low-
nucleophilicity medium.17 Unexpectedly, the electrophiles primarily
formed were activated by further superelectrophilic solvation, which
is an interaction between the electrophile and a Lewis/Brønsted
superacid. Such interaction of these cations can exist only in a
superacid medium, resulting in extremely reactive dication species
in the condensed state. According to Olah’s report, we speculate
that diprotonated carbonyl compound 6 (Scheme S3 in the SI) in
the superacid condenses with 2 to give intermediate 5, whose
reactivity is lower than that of dication 6; that is, k2 becomes smaller
than k1.
We investigated the model reactions of an equimolar amount of
2,2,2-trifluoro-4′-methoxyacetophenone (1) with anisole (2) by
changing the ratios of [TFSA] to [1] at room temperature, being
Based on these model reactions, self-polycondensation of AB2
monomer 7 was carried out with various molar ratios of [TFSA]/
[7] in CHCl3 at room temperature (Scheme 2). The polymerizations
proceeded in a homogeneous state and were completed within
several hours. The polymers were isolated by pouring the polymer
1
monitored by H NMR (Scheme 1, Figure S1 in the Supporting
Information (SI)). After the reaction, the solution was poured into
methanol.
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11000 J. AM. CHEM. SOC. 2010, 132, 11000–11001
10.1021/ja105213r  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of Hyperbranched Polymers with Controlled
Degree of Branching from 0 to 100%
solutions into methanol. The results are summarized in Figure S6
and Table S1 (see SI). A 100% HBP is produced in an equimolar
ratio of [TFSA]/[7], whereas a linear polymer (DB ) 0%) is
obtained by using TFSA as a solvent. Moreover, HBPs with a DB
of any percent between 0 and 100% are formed by changing the
ratio of [TFSA]/[7].
A linear relationship between the molar ratio of [TFSA]/[1] and
DB of HBPs was observed when the molar ratio of [TFSA]/[1]
was adjusted from 1 to 6. We expect that a linear polymer (DB )
0) would be obtained when the molar ratio of [TFSA]/[1] is over
8.6 by extrapolation of the fitted straight line. However, the polymer
precipitated during the polymerization with [TFSA]/[1] ) 8.6,
because of low solubility of the linear polymer in CHCl3. Therefore,
only TFSA, which showed better solubility than CHCl3, was used
as a solvent.
Figure 1. 1H NMR spectra of HBPs with various DBs.
obtained were confirmed by FT-IR (Figure S7-S9 in the SI) and
13C NMR spectroscopy (Figure S12-S14 in the SI).
In conclusion, we demonstrated the first synthesis of a linear
polymer, HBPs with various DBs, and 100% HBP by self-
polycondensation of 7 in proper molar ratios of [TFSA]/[7]. These
polymerizations relied on the change of the rate determination steps
in the mono- and diarylation of 2,2,2-trifluoroacetophenone by
changing the molar ratios of [TFSA]/[7]. These new findings will
promote the synthesis of various series of HBPs with controlled
DBs and their application to functional materials.
The structures of these HBPs were characterized by FT-IR and
NMR spectroscopy. The FT-IR spectra of HBPs (Figures S7 and
S8 in the SI) showed characteristic absorptions at 1712 cm-1 due
to the stretching of the CdO group of terminal units. A very weak
absorption due to the CdO group of terminal units is observed for
1
the linear polymer (Figure S9 in the SI). The H NMR spectra of
Acknowledgment. Y.S. thanks Yoshida Scholarship Foundation
the 100% HBP and the linear polymer together with those of 7
(terminal unit), 3 (dendritic unit), and 4 (linear unit) are presented
in Figure S10 (see SI). The doublet peak for the o-phenyl proton
(7g) next to the trifluoroacetyl group is observed at 8.1 ppm for 7,
and the characteristic methoxy protons (3a and 4d) for 3 and 4
appear at 3.8 and 3.3 ppm (Figures S10c and S10b), respectively.
In the 100% HBP, the o-phenyl protons (HBP1a) are observed at
8.1 ppm, and the signal corresponding to the methoxy protons (4d)
for the linear unit cannot be found at 3.3 ppm (Figure S10d). On
the other hand, a linear polymer exhibits characteristic methoxy
protons (LP1d) at 3.3 ppm, and a very small peak assignable to
the end trifluoroacetyl group is observed at 8.1 ppm (Figure S10e).
1H NMR spectra of HBPs with various DBs were shown in
Figure 1. The DB of HBPs was determined by the ratio of the
o-phenyl proton (HBP2a) at 8.1 ppm for the terminal unit and the
methoxy protons (HBP2b) at 3.3 ppm for the linear unit. The DB
of HBPs decreases by increasing the molar ratio of [TFSA]/[7], as
expected from the results of the model reactions (Figure S5 in the
SI).
The observed number-average molecular weights (Mn’s), weight-
average molecular weights (Mw’s), and polydispersities of the
polymers determined by SEC using polystyrene standards were in
the range of 11 000-24 000 g/mol, 26 000-189 000 g/mol, and
1.8-10.2, respectively (Table S1 and Figure S11 in the SI). The
Mw and radius of gyration (Rg) of the 100% HBP were measured
by static laser light scattering (SLS) to obtain values of 301 500
g/mol and 13 nm, respectively. The Mw value determined from SEC
(71 400 g/mol for the 100% HBP) was much smaller than that from
SLS.18 This discrepancy is usually observed for hyperbranched
polymers, due to their highly branched and compact structures
compared to linear counterparts.19 The structures of polymers
for the fundamental support.
Supporting Information Available: Full experimental details,
elemental analysis, IR and NMR spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Jikei, M.; Kakimoto, M. Prog. Polym. Sci. 2001, 26, 1233–1285.
(2) Kim, Y. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685–1698.
(3) Gao, C.; Yan, D. Prog. Polym. Sci. 2004, 29, 183–275.
(4) Voit, B. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2679–2699.
(5) Voit, B.; Lederer, A. Chem. ReV. 2009, 109, 5924–5973.
(6) Hawker, C. J.; Lee, R.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1991, 113,
4583–4588.
(7) Maier, G.; Zech, C.; Voit, B.; Komber, H. Macromol. Chem. Phys. 1998,
199, 2655–2664.
(8) Smet, M.; Schacht, E. H.; Dehaen, W. Angew. Chem., Int. Ed. 2002, 41,
4547–4550.
(9) Fu, Y.; Vandendriessche, A.; Dehaen, W.; Smet, M. Macromolecules 2006,
39, 5183–5186.
(10) Fu, Y.; Oosterwijck, C. V.; Vandendriessche, A.; Kowalczuk-Bleja, A.;
Zhang, X.; Dworak, A.; Dehaen, W.; Smet, M. Macromolecules 2008, 41,
2388–2393.
(11) Kono, S.; Sinananwanich, W.; Shibasaki, Y.; Ando, S.; Ueda, M. Polym.
J. 2007, 39, 1150–1156.
(12) Sinananwanich, W.; Ueda, M. J. Polym. Sci., Part A: Polym. Chem. 2008,
46, 2689–2700.
(13) Sinananwanich, W.; Higashihara, T.; Ueda, M. Macromolecules 2009, 42,
994–1001.
(14) Sinananwanich, W.; Segawa, Y.; Higashihara, T.; Ueda, M. Macromolecules
2009, 42, 8718–8724.
(15) Segawa, Y.; Sinananwanich, W.; Higashihara, T.; Ueda, M. Macromolecules
2008, 41, 8309–8311.
(16) Holter, D.; Frey, H. Acta Polym. 1997, 48, 298–309.
(17) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767–788.
(18) Weimer, M. W.; Fre´chet, J. M. J.; Gitsov, I. J. Polym. Sci., Part A: Polym.
Chem. 1998, 36, 955–970.
(19) Peleshanko, S.; Gunawidjaja, R.; Petrash, S.; Tsukruk, V. V. Macromol-
ecules 2006, 39, 4756–4766.
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