Inductive effect-assisted chain-growth polycondensation. Synthetic development from para- to meta-substituted aromatic polyamides with low polydispersities

Article abstract of DOI:10.1021/ja052566z

The polycondensation of m-(octylamino)benzoic acid esters (1) with base was investigated in order to extend the synthesis of well-defined condensation polymers from para-substituted polymers to meta-substituted ones. We expected that the aminyl anion of 2

Full text of DOI:10.1021/ja052566z

Published on Web 06/30/2005
Inductive Effect-Assisted Chain-Growth Polycondensation. Synthetic
Development from para- to meta-Substituted Aromatic Polyamides with Low
Polydispersities
Ryuji Sugi,†,‡ Akihiro Yokoyama,† Taniyuki Furuyama,§,‡ Masanobu Uchiyama,§,‡ and
Tsutomu Yokozawa*,†,‡
Department of Applied Chemistry, Kanagawa UniVersity, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan,
Graduate School of Pharmaceutical Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan,
and “Synthesis and Control”, PRESTO, Japan Science and Technology Agency (JST)
Received April 20, 2005; E-mail: yokozt01@kanagawa-u.ac.jp
Scheme 1
para-Substituted aromatic condensation polymers have high
mechanical strength and are useful as tough organic fabrication ma-
terials, engineering plastics, and so on. However, these polymers
are generally insoluble in organic solvents because of strong inter-
molecular interaction based on the high symmetry of the polymers.
To improve the solubility while maintaining the excellent mechan-
ical properties, long alkyl chains are introduced into the para-sub-
stituted monomers, or the para-substituted monomers are copoly-
merized with meta-substituted ones. We have succeeded in control-
ling the molecular weight and polydispersity of soluble aromatic
condensation polymers, such as polyamides,1 polyethers,2a and poly-
esters,2b by chain-growth polycondensation of para-substituted AB-
type aromatic monomers having long alkyl chains. In all of these
Table 1. Polymerization of 1 with 3a
polymerizations, the nucleophilic site of the monomer is the anion
formed by deprotonation of an amino or a hydroxyl group, and
this strongly electron-donating anion deactivates the electrophilic
site at the para-position through the resonance effect (+R effect).
This deactivation results in suppression of self-condensation of the
monomer, but selective reaction with an initiator and the propagating
end, leading to chain polymerization. Therefore, para-substituted
monomers are necessary for chain-growth polycondensation.
We now report successful chain-growth polycondensation of
meta-substituted monomers, 3-(octylamino)benzoic acid esters 1,
which yield poly(m-benzamide)s with well-defined molecular
weights and low polydispersities (Mw/Mn e 1.1). This result
demonstrates that the inductive effect (+I effect) of the nucleophilic
site on the reactivity of the electrophilic site at the meta position
of the monomer is as applicable to chain-growth polycondensation
as is the R effect of the para-substituted monomers.3 In general,
the I effect is weaker than the R effect, but a functional group with
a negative charge is expected to show an I effect as strong as the
R effect. Thus, the aminyl anion of 2 would deactivate the acyl
group at the meta position through the strong +I effect, resulting
in suppression of the self-polycondensation of 2. The anion 2 would
then selectively react with an initiator 3 and the polymer chain end,
the acyl group of which is more reactive than that of the monomer
with the aminyl anion, and growth would continue in a chain-
polymerization manner (Scheme 1).
e
e
entry
1
3
base
T (
°
C) [1]0/[3]0 time conv.d (%)
M
n calcd Mn
Mw/Mn
1
2
3
4
5
6
7
1a 3a LDA
LiTMPb
-78 19.6 8 h
19.3 5 h
85
74
87
98
96
96
98
98
3990 3940 1.24
3540 2820 1.11
3780 3250 1.09
4100 3700 1.08
7750 4560 1.10
8930 7210 1.12
10800 9690 1.18
9790 1.25
10800 10000 1.05
10000 1.05
LiN(i-Pr)tBu
LiHMDSc
LiHMDS
LiHMDS
LiHMDS
18.9 5 h
19.1 5 h
34.5 6 h
-30 39.8 1 h
0
47.3 5 min
1 h
8
1b 3b LiHMDS
0
47.1 5 min 100
2 h 100
a Polymerization of 1 with 3 in the presence of 1.1 equiv of base in
THF ([1]0 ) 0.40 M). b Lithium 2,2,6,6-tetramethylpiperidide. c Lithium
hexamethyldisilazide. d Determined by GC. e Determined by GPC based on
polystyrene standards (eluent: THF).
seemed to gradually react with the propagating chain end to
terminate the polymerization because the H NMR spectra of the
1
obtained polymer showed a weaker signal of the terminal methyl
ester moiety. Thus, the polymerization was performed with lithium
amides having bulkier alkyl substituents to give polymers with
narrower molecular weight distributions (entries 2-4). When
lithium hexamethyldisilazide (LiHMDS) was used, the conversion
of 1a was almost 100%, although the Mn values of the polymers
were lower than the calculated values (entries 4, 5). When the
polymerization of 1a was carried out at higher temperature than
-78 °C, the Mn values of the polymers were close to the calculated
values (entries 6, 7). However, prolongation of the polymerization
time at 0 °C after consumption of 1a resulted in a broad molecular
weight distribution due to a small shoulder of the GPC chromato-
gram in the higher molecular weight region (entry 7). This implies
that there was a small amount of self-polycondensation polymer,
which might react with the chain end of the chain-growth polymer
after 1a was consumed.5
The methyl ester monomer 1a was first polymerized with 1.1
equiv of LDA, the basicity of which is higher than that of the base
(N-octyl-N-triethylsilylaniline/CsF) that we used for the polymer-
ization of the para-counterpart,1a in the presence of initiator 3a4 at
-78 °C to yield a polymer with a rather narrow molecular weight
distribution, implying that the polymerization had proceeded in a
chain-polymerization manner (Table 1, entry 1). However, LDA
† Kanagawa University.
To prevent self-polycondensation, the ethyl ester monomer 1b
was used because lower reactivity of the ethyl ester moiety is
§ The University of Tokyo.
‡ Japan Science and Technology Agency (JST).
9
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J. AM. CHEM. SOC. 2005, 127, 10172-10173
10.1021/ja052566z CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
did not react at all with the terminal ester moiety of poly1b, which
was able to initiate the polymerization of 5 as the second step.
Finally, density functional theory (DFT) calculations were
performed to understand the present polymerization. The results
indicate that successful chain-growth polycondensation of the meta-
substituted monomers is based on the different reactivity of the
ester moieties between the polymer end group and monomer 2b,
as we expected; the polymer end group is much more reactive than
2b, which is deactivated by the +I effect of the aminyl anion at
the meta position. To address the different reactivity (electrophi-
licity) in question, we examined the propagation reaction of 4c (R3
) H) with 2c and the self-condensation of 2c as model reac-
tions using the DFT (B3LYP/6-31G*)8 method. The activation
energies for the propagation and self-condensation are 21.6 and
27.0 kcal/mol, respectively. On the basis of the geometries, energies,
and vibrational frequencies obtained, the theoretical rate constants
were then evaluated at 298.15 K and 1 atm. The reaction rate
constant (1.1 × 10-3 s-1) for the propagation is 8.6 × 103-fold
greater than that for the self-condensation (1.3 × 10-7 s-1) and,
hence, is consistent with the experimental finding that propagation
was observed exclusively over self-condensation; that is, successful
chain-growth polycondensation of meta-substituted aminobenzoic
ester monomers proceeded.
Figure 1. (A) 1H NMR spectrum of poly1b obtained by the polymerization
of 1b with 3b in the presence of 1.1 equiv of LiHMDS in THF at 0 °C. (B)
Mn and Mw/Mn values of poly1b, obtained with 1.1 equiv of LiHMDS in
the presence of 3b in THF ([1b]0 ) 0.40 M) at 0 °C, as a function of the
feed ratio of 1b to 3b. The lines indicate the calculated Mn values assuming
that one polymer chain forms per unit of 3b.
In conclusion, our present results demonstrate that not only para-
substituted AB-type aromatic monomers but also meta-substituted
ones undergo chain-growth polycondensation by virtue of different
inductive effects between monomer and polymer. This I effect-
assisted polycondensation should enable us to extend the synthesis
of well-defined condensation polymers from para-substituted
aromatic polymers to a variety of meta-substituted ones, which
possess higher solubility. Furthermore, this polymerization method
should be applicable to chain-growth polycondensation of even
aliphatic monomers, such as amino acid and lactic acid derivatives,
and so on. Experiments along these lines are in progress.
Figure 2. GPC profiles of block copolymerization of 1b and 5 by the
monomer addition method: (a) poly1b as a prepolymer ([1b]0/[3b]0 ) 20,
100% monomer conversion), Mn ) 4600, Mn/Mn ) 1.09; (b) poly1b-b-
poly5 as a postpolymer ([added 5]0/[3b]0 ) 23, 94% monomer conversion),
Mn ) 11 000, Mw/Mn ) 1.12.
expected to suppress the reaction between the deprotonated 1b
monomers (self-condensation). Thus, monomer 1b polymerized with
LiHMDS at 0 °C to yield a polymer with a defined molecular
weight and a very low polydispersity (entry 8). Furthermore, the
Mn value and the Mw/Mn ratio were not changed even 2 h after 1b
Supporting Information Available: Synthesis of monomer 1a and
1b, polymerization procedure, the synthesis of diblock copolymer of
1b and 5, and computational methods (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
1
was consumed. The H NMR spectrum of the poly1b obtained
shows the signal a of the methyl group of the initiator 3b unit and
the signals of the terminal monomer units b, c, and d. The integral
ratio of a:b:c:d is 3:1:1:2, indicating that one initiator molecule
formed one polymer chain (Figure 1A). When the polymerization
of 1b was carried out with various feed ratios of 1b to initiator 3b
([1b]0/[3b]0), the observed Mn values of the polymers increased
linearly in proportion to the [1b]0/[3b]0 ratio, and the polydispersity
was quite narrow, maintaining an Mw/Mn ratio of less than 1.1 over
the entire range of the [1b]0/[3b]0 ratio. Moreover, the ratio of the
initiator unit to the end group was consistently 1.0, irrespective of
the [1b]0/[3b]0 ratio (Figure 1B). This polymerization behavior is
consistent with the features of living polymerization and is
consistent with the chain-growth polycondensation of the para-
substituted counterpart.1a
References
(1) (a) Yokozawa, T.; Asai, T.; Sugi, R.; Ishigooka, S.; Hiraoka, S. J. Am.
Chem. Soc. 2000, 122, 8313. (b) Shibasaki, Y.; Araki, T.; Okazaki, M.;
Ueda, M. Polym. J. 2002, 34, 261.
(2) (a) Yokozawa, T.; Suzuki, Y.; Hiraoka, S. J. Am. Chem. Soc. 2001, 123,
9902. (b) Yokoyama, A.; Iwashita, K.; Hirabayashi, K.; Aiyama, K.;
Yokozawa, T. Macromolecules 2003, 36, 4328. For chain-growth poly-
condensation for other polymers, see the following reviews: (c) Yokozawa,
T.; Yokoyama, A. Polym. J. 2004, 36, 65. (d) Yokozawa, T.; Yokoyama,
A. Chem. Rec. 2005, 5, 47.
(3) The -I effect of nitro and trifluoromethyl groups has been reported to be
applicable to the polycondensation of aromatic halide monomers bearing
these electron-withdrawing groups at the meta position. See: (a) Chung,
I. S.; Kim, S. Y. J. Am. Chem. Soc. 2001, 123, 11071. (b) Lee, H. S.;
Kim, S. Y. Macromol. Rapid Commun. 2002, 23, 665. (c) Kim, Y. J.;
Sik, C. I.; Kim, S. Y. Macromolecules 2003, 36, 3809.
(4) When the polymerization of 1a was carried out in the presence of phenyl
4-nitrobenzoate as an initiator, the aminyl anion of 1a reacted not only
with the methyl ester moiety of the propagating chain end but also with
the amide carbonyl carbon of the initiator unit in the polymer, resulting
in a polymer with a broad molecular weight distribution.
With this established polymerization method, we tried to synthe-
size a well-defined diblock copolymer of meta- and para-substituted
poly(benzamide).6 Thus, 1b was polymerized in the presence of
3b ([1b]0/[3b]0 ) 20) and 2.2 equiv of LiHMDS to give a
prepolymer (Figure 2 (a), Mn ) 4600 (Mn(calcd) ) 4790), Mw/Mn
) 1.09). A fresh feed of methyl 4-(octylamino)benzoate 5 was
added to the prepolymer in the reaction mixture.7 The added 5 feed
was smoothly polymerized. The GPC chromatogram of the product
(Figure 2 (b)) was clearly shifted toward the high molecular weight
region, while retaining a narrow molecular weight distribution (Mn
) 11 000 (Mn(calcd) ) 9700), Mw/Mn ) 1.12), indicating successful
production of the block copolymer of 1b and 5. It should be noted
that excess LiHMDS in the polymerization of 1b as the first step
(5) Yokozawa, T.; Sugi, R.; Asai, T.; Yokoyama, A. Chem. Lett. 2004, 33,
272.
(6) For diblock copolymers of poly(p-benzamide) and poly(m-benzamide) by
the coupling reaction of each step-growth homopolymer, see: (a) Chavan,
N. N.; Ciferri, A.; Dell’Erba, C.; Novi, M.; Renamyor, C. S. Macromol.
Chem. Phys. 1996, 197, 2415. (b) Cavalleri, P.; Ciferri, A.; Dell’Erba,
C.; Gabellini, A.; Novi, M. Macromol. Chem. Phys. 1998, 199, 2087.
(7) For the polymerization of 3 with LiHMDS, see: Yokozawa, T.; Muroya,
D.; Sugi, R.; Yokoyama, A. Macromol. Rapid Commun. 2005, 26, 979.
(8) (a) Becke, A. D. Phys. ReV. 1988, A38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 1372. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(d) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785.
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