Synthesis of well-defined hyperbranched polyamides by condensation polymerization of AB2 monomer through changed substituent effects

Article abstract of DOI:10.1002/anie.200901714

Branching out: Condensation polymerization of AB₂ monomer proceeds selectively from a core molecule owing to the change of substituent effects between the monomer and the polymer (see scheme). Hyperbranched polyamides are obtained with very low

Full text of DOI:10.1002/anie.200901714

Communications
DOI: 10.1002/anie.200901714
Hyperbranched Polymers
Synthesis of Well-Defined Hyperbranched Polyamides by
Condensation Polymerization of AB2 Monomer through Changed
Substituent Effects**
Yoshihiro Ohta, Shuichi Fujii, Akihiro Yokoyama, Taniyuki Furuyama, Masanobu Uchiyama,
and Tsutomu Yokozawa*
Hyperbranched polymers have received considerable atten-
tion in recent years because of their unusual properties that
arise from their unique molecular architecture and conven-
ient synthesis by one-step polymerization of ABm (m ꢀ 2)
type monomers.[1] The physical properties of hyperbranched
polymers depend on the chemical composition, molecular
weight, molecular weight distribution, and degree of branch-
ing (DB), so a challenging goal is the development of general
AB2 polymerization methods that achieve controlled molec-
ular weight and narrow molecular weight distribution. The
basic approach to these controlled polymerizations is selec-
tive reaction of the monomer with a core molecule and the
polymer ends in a manner similar to chain-growth polymer-
ization. Several attempts have been reported that use chain-
growth ring-opening isomerization polymerization of cyclic
latent AB2 monomers from a core molecule,[2] polymerization
from a core molecule on an insoluble solid support,[3] slow
addition of monomers to a core molecule,[4] and reactive core
molecules.[5] Polymers with controlled molecular weight and
low polydispersity (weight-average/number-average molecu-
lar weight, Mw/Mn < 1.3) were generally obtained when the
monomer/core ratio was less than about 100. Above that,
however, the molecular weight distribution became broad[4b]
or the deviation of the observed molecular weight from the
calculated one increased even when polymers with low
polydispersity were obtained.[4c,d] These results probably
arose from self-polymerization of AB2 monomers at high
feed ratio.[4b]
acid ethyl ester (1a) as an AB2 monomer with a base, the
amide anion of 2 deactivates both the ester moieties in 2
through the inductive effect (+ I effect) to suppress the self-
polymerization of 2, and 2 selectively reacts with a core
molecule (initiator) 3 and the terminal ester moieties of
polymer to afford a well-defined hyperbranched polymer
(Scheme 1), in a similar manner to that in which AB-type
monomers undergo chain-growth condensation polymeri-
zation.[6] Here, we report that control over the molecular
weight (Mn = 2370–39300) of hyperbranched polyamide
(HBPA) was achieved by varying the ratio of the monomer
to the core from 7 to 200, while retaining very low
polydispersity (Mw/Mn ꢁ 1.13), in the polymerization of 1a
with core molecule 3. We also utilized this approach to
synthesize a block copolymer of linear and hyperbranched
aromatic polyamides by means of a convenient one-pot
monomer addition method.
Herein, we propose a new approach to controlled
polymerization of AB2 monomers by utilizing the change of
substituent effects between monomer and polymer. In the
condensation polymerization of 5-(methylamino)isophthalic
[*] Y. Ohta, S. Fujii, Prof. Dr. A. Yokoyama, Prof. Dr. T. Yokozawa
Department of Material and Life Chemistry, Kanagawa University
Rokkakubashi, Kanagawa-ku, Yokohama 221-8686 (Japan)
Fax: (+81)45-413-9770
E-mail: yokozt01@kanagawa-u.ac.jp
T. Furuyama, Dr. M. Uchiyama
Advanced Elements Chemistry Laboratory
Institute of Physical and Chemical Research
RIKEN, Hirosawa, Wako-shi, Saitama 351-0198 (Japan)
[**] This work was supported in part by a Grant-in-Aid (Grant 19550128)
for Scientific Research and a Scientific Frontier Research Project
from the Ministry of Education, Culture, Sports, Science, and
Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901714.
Scheme 1. Condensation polymerization of AB2 monomer 1 from core
initiator 3 by making use of changed substituent effects.
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The polymerization of 1a was first carried out by addition
of 1a over 40 min to a mixture of 1.1 equivalent of lithium
1,1,1,3,3,3-hexamethyldisilazide (LiHMDS) and 6.7 mol% 3a
as a core initiator in THF at À308C. Further stirring at À308C
for 1 h afforded HBPA with a relatively narrow molecular
weight distribution (Mw/Mn = 1.28; Supporting Information,
Figure S1). The Mn value of the hyperbranched polymer
estimated by gel-permeation chromatography (GPC) is gen-
erally lower than the real Mn value, and therefore the Mw
value was determined with a multiangle laser light scattering
(MALLS) detector. To facilitate comparison with the theo-
retical value (Mn(calcd)) based on the monomer/core feed
ratio, the Mn value designated as Mn(MALLS) was calculated
by division of the Mw value from MALLS by the Mw/Mn ratio
from GPC. According to this, the Mn(MALLS) of the
obtained HBPA was 3500.
reacted intramolecularly with one of the ester moieties to
form a cyclic structure. This result means that the polymer-
ization of 1a involves not only selective reaction of 1a with 3a
and the polymer ends, but also self-polymerization of 1a. We
speculated that 1a would be too reactive for self-polymeri-
zation to be suppressed, and so the reactivity of the amide
anion 2 was decreased by the use of LiCl.[9] Thus, the
polymerization of 1a with 6.7 mol% 3a was carried out in the
presence of 5 equivalents of LiCl. The Mw/Mn ratio became
1.11 (Supporting Information, Figure S3), and the Mn value
(Mn(MALLS) = 3920) was in fair agreement with the calcu-
lated value, assuming that one core molecule forms one
HBPA molecule (Mn(calcd) = 3570). The DB value of HBPA
was 0.52. The MALDI-TOF mass spectrum contains only one
series of peaks corresponding to the Na+ adduct of HBPA
with the core 3a unit (Figure 1b).[10]
To confirm the branched structure, model compounds of
the dendritic and linear units were synthesized, and the
1H NMR spectrum of HBPA was compared with those of the
model compounds (Supporting Information, Figure S2). The
1H NMR spectrum of HBPA showed separately the signals of
the dendritic (D), linear (L), and terminal (T) units, and the
DB of the HBPA calculated by use of the Frꢀchet formula[7]
was 0.52. This DB value is close to the theoretical value of 0.5
in the general polymerization of an AB2 monomer.[8]
The polymer end group of the HBPA was analyzed by
matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectrometry with dithranol as matrix
in the presence of sodium trifluoroacetate as cationizing salt.
The mass spectrum of HBPA contains two series of peaks
(Figure 1a). One corresponds to the Na+ adducts of HBPA
with the core 3a unit and the other comes from HBPA
without the 3a unit, in which the focal amino group has
Furthermore, when the polymerization of 1a was carried
out with various feed ratios of 1a to 3a ([1a]0/[3a]0), the
observed Mn value of the HBPA increased in proportion to
the [1a]0/[3a]0 ratio up to 200, and a narrow polydispersity
was retained (Figure 2a).[11] The gel-permeation chromato-
gram of HBPA obtained even at a monomer/core feed ratio of
200 showed a very narrow, monomodal peak (Figure 2b).
Consequently, the polymerization of the AB2 monomer 1a
with LiHMDS in the presence of LiCl proceeds in a chain-
growth polymerization manner from the core 3a.
As the AB2 monomer 1a was added to a mixture of the
core 3a and base for 40 min in this polymerization, one might
think that the chain-growth polymerization from the core
would stem not from the changed substituent effects, but from
slow monomer addition (SMA), although it took more than
12 h for controlled synthesis of hyperbranched polymers by
the reported SMA method.[4] Accordingly, the polymerization
of 1a was carried out by the addition of 1a and 3a
([1a]0/[3a]0 = 15) at once to a mixture of LiHMDS and
LiCl. The obtained HBPA also possessed a controlled
molecular weight and narrow molecular weight distri-
bution
(Mn(MALLS) = 3450
(Mn(calcd) = 3570),
Mw/Mn = 1.17; Supporting Information, Figure S6).
This finding indicates that this controlled polymeri-
zation of AB2 monomer is governed not by SMA, but
by the change of substituent effects between the
monomer and polymer.
In addition, density functional theory (DFT) cal-
culations were performed to estimate the change of
substituent effects. We examined the propagation
reaction of a methyl ester monomer 1b with a dimethyl
ester core 3b and the self-condensation of 1b as model
reactions (Scheme 1) by using the DFT (B3LYP/6-
31G*) method. Although the effect of LiCl is important
and remains to be elucidated, in this instance we
calculated only the substituent effects for the sake of
simplicity. The activation energies for the propagation
and self-condensation are 25.7 and 29.2 kcalmolÀ1
,
respectively. On the basis of the geometries, energies,
and vibrational frequencies obtained, the theoretical
rate constants were then evaluated at 243 K and 1 atm.
The reaction rate constant for the propagation (3.8 ꢁ
10À11 sÀ1) is 1.4 ꢁ 103-fold greater than that for the self-
Figure 1. MALDI-TOF mass spectra of HBPA obtained by the polymerization of
1a with 3a ([1a]0/[3a]0 =15): a) in the absence of LiCl; b) in the presence of
LiCl.
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polymers. Furthermore, this established method enables the
synthesis of block copolymers of linear and hyperbranched
polymers by a convenient monomer addition method in one
pot, and other architectures including hyperbranched poly-
mers should also be accessible in the same way. Studies of
other controlled condensation polymerizations of AB2 mono-
mers on the basis of changed substituent effects are under
way.
Figure 2. a) Mn and Mw/Mn values of HBPA as a function of the feed
ratio of 1a to 3a. b) GPC profile of HBPA obtained by the polymeri-
zation of 1a with 3a ([1a]0/[3a]0 =200).
Experimental Section
Typical polymerization procedure: LiCl (0.152 g, 3.59 mmol) was
placed in a flask equipped with a three-way stopcock, and dried at
2508C under reduced pressure. The flask was cooled to room
temperature under an argon atmosphere, and then charged with
1.0m LiHMDS in THF (0.720 mL, 0.720 mmol). The flask was cooled
to À308C under an argon atmosphere with stirring. A solution of 3a
(0.0053 g, 0.011 mmol) and naphthalene (internal standard, 0.0096 g,
0.075 mmol) in dry THF (1.0 mL) was added to the flask under dry
nitrogen, followed by a solution of 1a (0.163 g, 0.649 mmol) in dry
THF (4.0 mL) dropwise over about 40 min at À308C with stirring
under dry nitrogen. The mixture was stirred at À308C for 1 h, and
then the reaction was quenched with sat. NH4Cl. A small portion of
the THF layer was withdrawn into a syringe and analyzed by GC to
determine the conversion of 3a and 1a (conversion = 100%). After
that, the whole was extracted with CH2Cl2. The organic layer was
washed with water, dried over anhydrous MgSO4, and concentrated
under reduced pressure. The residue was dissolved in CH2Cl2
(2.0 mL), and the solution was added to hexane (150 mL). After
filtration, the insoluble material was washed with hexane and dried in
desiccator to give HBPA as a white solid (0.113 g, 82%, Mn-
(MALLS) = 13500, Mw/Mn = 1.11). 1H NMR (600 MHz, CDCl3):
d = 8.55–8.41 (aromatic), 8.11–7.89 (aromatic), 7.87–7.57 (aromatic),
7.53–7.30 (aromatic), 7.25–6.90 (aromatic), 4.46–4.11 (COOCH2CH3),
3.51–2.81 (N-CH3), 1.43–1.12 ppm (COOCH2CH3).
condensation (2.7 ꢁ 10À14 sÀ1) and, hence, is consistent with
the experimental tendency that propagation was favored over
self-condensation by virtue of the substituent effects.
Finally, with this established polymerization method, we
tried to synthesize a well-defined diblock copolymer of linear
and hyperbranched polymer in one pot.[12] Thus, methyl 3-(4-
octyloxybenzylamino)benzoate (4)[13] was polymerized in the
presence of an initiator 5 ([4]0/[5]0 = 20), 2.2 equivalents of
LiHMDS, and 11 equivalents of LiCl to give a prepolymer
(Figure 3a; Mn = 6230 (Mn(calcd) = 6900), Mw/Mn = 1.10). A
fresh feed of 1a ([1a]0/[5]0 = 20) was added to the prepolymer
Synthesis of linear–hyperbranched block copolymer: LiCl
(0.305 g, 7.19 mmol) was placed in a flask equipped with a three-
way stopcock, and dried at 2508C under reduced pressure. The flask
was cooled to room temperature under an argon atmosphere, and
then charged with 1.0m LiHMDS in THF (1.45 mL, 1.45 mmol). The
flask was cooled to 08C under an argon atmosphere with stirring. A
solution of 5 (0.0069 g, 0.033 mmol) and naphthalene (internal
standard, 0.0105 g, 0.819 mmol) in dry THF (1.0 mL) was added to
the flask under dry nitrogen, followed by a solution of 4 (0.240 g,
0.650 mmol) in dry THF (0.6 mL) dropwise over about 20 min at 08C
with stirring under dry nitrogen. The mixture was stirred at 08C for
20 min, then 0.3 mL of the solution was withdrawn and quenched with
sat. NH4Cl to measure the Mn value and Mw/Mn ratio of poly4 (the
GPC trace is shown in Figure 3a: Mn(GPC) = 6230, Mn(NMR) =
7470, Mw/Mn = 1.10). Immediately after the sampling, the mixture
was cooled to À308C, then a solution of 1a (0.163 g, 0.649 mmol) in
dry THF (4.0 mL) was added dropwise to the mixture over about
40 min at À308C with stirring under dry nitrogen. The mixture was
stirred at À308C for 1 h, then the reaction was quenched with sat.
NH4Cl. The whole was extracted with CH2Cl2, and the organic layer
was washed with brine, followed by drying over anhydrous MgSO4.
After concentration under reduced pressure, the residue was
dissolved in CH2Cl2 (1.0 mL), and the solution was added to hexane
(30 mL). After filtration, the insoluble material was washed with
hexane and dried in a desiccator to give HBPA as a white solid
(0.276 g, 77%, Mn(MALLS) = 10900, Mw/Mn = 1.14). 1H NMR
(600 MHz, CDCl3): d = 8.50–8.44 (aromatic), 8.05–7.89 (aromatic),
7.84–7.55 (aromatic), 7.53–7.29 (aromatic), 7.25–6.86 (aromatic),
6.85–6.62 (aromatic), 6.41–6.23 (aromatic), 4.96–4.55 (N-CH2), 4.42–
4.17 (COOCH2CH3), 3.94–3.73 (OCH2CH2C5H10CH3), 3.56–2.82
(N-CH3), 2.21–2.16 (CH3C6H4), 1.73 (OCH2CH2C5H10CH3),
Figure 3. GPC profiles of polymers in the block copolymerization of 4
and 1a: a) poly4 as a prepolymer ([4]0/[5]0 =20, Mn =6230, Mw/Mn
=1.10); b) poly4-b-HBPA ([1a]0/[5]0 =20, Mn(MALLS)=10900, Mw/Mn
=1.14).
in the reaction mixture, and the added 1a was smoothly
polymerized. The gel-permeation chromatogram of the
product (Figure 3b) was clearly shifted toward the higher-
molecular-weight region, while retaining a narrow molecular
weight distribution (Mn(MALLS) = 10900 (Mn(calcd) =
11500), Mw/Mn = 1.14), which indicated successful production
of the linear–hyperbranched block copolymer from 4 and 1a.
In conclusion, hyperbranched aromatic polyamides, with a
degree of polymerization in the range of 7–200 and very low
polydispersity (Mw/Mn ꢁ 1.13), can be synthesized by simple
condensation polymerization of an AB2 monomer. These
results demonstrate that AB2 condensation polymerization
involving a change of substituent effects between the
monomer and polymer to suppress self-polymerization is
very effective for the controlled synthesis of hyperbranched
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1.46–1.20 (OCH2CH2C5H10CH3 and COOCH2CH3), 0.88 ppm
(OCH2CH2C5H10CH3).
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Received: March 30, 2009
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[10] The MALDI-TOF mass spectra of the HBPAs during polymer-
ization also contain only one series of peaks (Supporting
Information, Figure S4).
[11] For GPC profiles of HBPA obtained with various feed ratios of
[1a]0/[3a]0, see Figure S5 in the Supporting Information; the
results are summarized in Table S1.
Published online: June 30, 2009
Keywords: block copolymers · hyperbranched polymers ·
.
polymerization · substituent effects
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