Chain-growth polycondensation for nonbiological polyamides of defined architecture [8]

Full text of DOI:10.1021/ja001871b

J. Am. Chem. Soc. 2000, 122, 8313-8314
8313
Scheme 1
Chain-Growth Polycondensation for Nonbiological
Polyamides of Defined Architecture
Tsutomu Yokozawa,* Toshinobu Asai, Ryuji Sugi,
Shingo Ishigooka, and Shuichi Hiraoka
Department of Applied Chemistry
Kanagawa UniVersity, Rokkakubashi
Kanagawa-ku, Yokohama 221-8686, Japan
ReceiVed May 30, 2000
Many natural polymeric materials are perfect monodisperse
macromolecules and are produced by the successive condensation
of monomers with polymer end groups that are activated by
enzymes.1-4 Although these syntheses proceed via many com-
plicated and tightly controlled processes, the overall process could
be regarded as a kind of chain-growth polycondensation. In the
polycondensation of artificial monomers, however, macromol-
ecules with a wide range of molecular weights have been
synthesized, because there is little difference of reactivity between
monomers and polymer end groups, and step-growth polymeri-
zation occurs. If the polymer end groups become more reactive
than monomers and the reaction of monomers with each other is
prevented, chain-growth polycondensation would take place to
yield artificial condensation polymers having well-defined mo-
lecular weights and narrow molecular weight distributions
(MWD). Kinetic studies showed that some polycondensations
involve more reactive polymer end groups than monomers, but
the MWD of polymers were not evaluated.5 The synthesis of poly-
(2,6-dimethyl-1,4-phenylene oxide) by oxidative polymerization
of 2,6-dimethylphenol6 and by phase transfer catalyzed polycon-
densation of 4-bromo-2,6-dimethylphenynol7 also involved the
reactive polymer end groups and did not show the behavior of a
classic polycondensation. Percec conducted this polycondensation
in the presence of chain initiators and obtained well-defined
polyphenylene oxides.7 However, the molecular weight values
were much higher than the calculated values based on the
[monomer]/[initiator] ratios, and polymers having a narrow MWD
were obtained after precipitation; the crude polymerization mixture
had a broad MWD. In the polycondensations of bifunctional
nucleophilic monomers with bifunctional electrophilic monomers,
polymers having a low polydispersity (Mw/Mn < 1.3) were also
prepared by phase transfer catalyst (PTC) techniques when
polymer end groups were more reactive than monomers.8-10 This
type of polycondensation, however, could not control the mo-
lecular weight.
Scheme 2
Our previous work has shown that the Pd-catalyzed polycon-
densation of 4-bromo-2-octylphenol and carbon monoxide un-
derwent chain-growth polycondensation from an initiator in the
initial stage.11 Another approach to chain-growth polycondensation
was the polycondensation of solid monomer with PTC in organic
solvent containing an initiator, where the reaction of monomers
with each other was prevented.12 The molecular weight was
controlled but the MWD was a little broad (Mw/Mn < 1.3). We
now report the successful chain-growth polycondensation of
phenyl 4-aminobenzoate derivatives 1 for aromatic polyamides
having precisely controlled molecular weights and quite narrow
MWD (Mw/Mn < 1.12), where all of the experimental criteria of
a living polymerization are exhibited even in polycondensation.
The expected course of polymerization of silylated 1a with
CsF in the presence of a small amount of reactive initiator 2
bearing an electron-withdrawing group is shown in Scheme 1.
Thus, 1a would react with 2 to yield amide 3 faster than with the
acyl group of desilylated 1a having the strong electron-donating
aminyl anion group. Monomer 1a would now react with 3 to yield
a dimeric amide faster than with 1a itself, because the amide group
of 3 is the much weaker electron-donating group than the aminyl
anion group of the monomer, and the acyl group of 3 would be
more reactive than that of the monomer. Growth would continue
in a chain polymerization manner with the conversion of the
strong electron-donating aminyl anion of 1a to the weak electron-
donating amide group in polymer.
To estimate the reaction selectivity of monomer 1a with the
polymer end group (resulting in “chain-growth polymerization”)
or with 1a itself (resulting in “step-growth polymerization”), the
reaction of monomer amino group model 6 with the 1:1 mixture
of polymer terminal phenyl ester model 4 and monomer phenyl
ester model 5 was carried out in the presence of CsF and 18-
crown-6 in THF at room temperature (Scheme 2). The conversion
ratio 4/5 was 95/5. Furthermore, the reaction of 6 with 4 and that
of 6 with 5 were carried out, respectively. The reaction of 6 with
5 did not proceed when the reaction of 6 with 4 was completed.
The results of model reactions indicate that monomer 1a would
react with the polymer end group with high selectivity and
undergo chain-growth polycondensation.
(1) For the biosynthesis of polypeptides, see: (a) Weissbach, H.; Pestka,
S. Molecular Mechanism of Protein Biosynthesis; Academic Press: New York,
1977. (b) Bermek, E. Mechanism of Protein Synthesis. Structure-Function
Relations, Control Mechanisms, and EVolutionary Aspectss; Springer-Ver-
lag: New York, 1985.
(2) For the biosynthesis of DNA, see: Kornberg, A. Science 1960, 131,
1503.
(3) For the biosynthesis of RNA, see: (a) Travers, A. Nature 1976, 263,
641. (b) Chamberlin, M. In The Enzymes; Boyer, P. D., Ed.; Academic Press:
New York, 1982; Vol. 15, p 61.
(4) For the biosynthesis of cis-polyisoprene rubber, see: (a) Light, D. R.;
Dennis, M. S. J. Biol. Chem. 1989, 264, 18589. (b) Light, D. R.; Lazarus, R.
A.; Dennis, M. S. J. Biol. Chem. 1989, 264, 18598. (c) Dennis, M. S.; Light,
D. R. J. Biol. Chem. 1989, 264, 18608.
(5) (a) Lenz, R. W.; Handlovits, C. E.; Smith, H. A. J. Polym. Sci. 1962,
58, 351. (b) Newton, A. B.; Rose, J. B. Polymer 1972, 13, 465. (c) Hibbert,
D. B.; Sandall, J. P. B. J. Chem. Soc., Perkin Trans. 2 1988, 1739. (d) Robello,
D. R.; Ulman, A.; Urankar, E. J. Macromolecules 1993, 26, 6718.
(6) (a) Koch, W.; Risse, W.; Heitz, W. Makromol. Chem. Suppl. 1985, 12,
105. (b) Risse, W.; Heitz, W. Makromol. Chem. 1985, 186, 1835.
(7) (a) Percec, V.; Wang, J. H. J. Polym. Sci. Part A: Polym. Chem. 1991,
29, 63. (b) Percec, V.; Shaffer, T. D. J. Polym. Sci. Part C: Polym. Lett.
1986, 24, 439. (c) Percec, V.; Wang, J. H. Polym. Bull. 1990, 24, 493.
(8) Boileau, S. In New Methods for Polymer Synthesis; Mijs, W. J., Ed.;
Plenum Press: New York, 1992; p 179.
(9) (a) Percec, V. In ACS Symposium Series; American Chemical Society:
Washington,DC, 1987; Vol. 326, p 96. (b) Percec, V.; Shaffer, T. D.; Nava,
H. J. Polym. Sci. Part C: Polym. Lett. 1984, 22, 637. (c) Shaffer, T. D.;
Percec, V. J. Polym. Sci. Part A: Polym. Chem. 1986, 24, 451.
(10) Shaffer, T. D.; Kramer, M. C. Makromol. Chem. 1990, 191, 71.
(11) Yokozawa, T.; Shimura, H. J. Polym. Sci. Part A: Polym. Chem. 1999,
37, 2607.
(12) Yokozawa, T.; Suzuki, H. J. Am. Chem. Soc. 1999, 121, 11573.
10.1021/ja001871b CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/09/2000

8314 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Communications to the Editor
Figure 1. Mn and Mw/Mn values of poly1b, obtained with 6 and CsF/
18-crown-6 in the presence 2 in THF at 25 °C, as a function of the feed
ratio of 1b to 2.
Figure 2. Demonstration of chain-growth polycondensation. (A) Mn and
Mw/Mn values of poly1b, obtained with 6 and CsF/18-crown-6 in the
presence 2 in THF at 25 °C, as a function of monomer conversion. (B)
GPC profile of the monomer addition experiment in THF at room
temperature: (a) prepolymer ([1b]0/[2]0 ) 20), conversion ) 98%, Mn
) 4500; (b) postpolymer ([added 1b]0/[2]0 ) 24), conversion ) 187%,
Mn ) 10000.
The polymerization of 1a was carried out in the presence of
1.0 equiv of CsF, 2.0 equiv of 18-crown-6, and phenyl 4-nitro-
benzoate 2 as an initiator in THF at room temperature to yield a
polyamide quantitatively, which was soluble in THF, chloroform,
dichloromethane, N,N-dimethylformamide (DMF), and even
toluene. In the polymerization with varying the feed ratio ([1a]0/
[2]0), the MWD of polymers was narrow (Mw/Mn ) 1.16-1.20)
when the [1a]0/[2]0 ratios were 10 or less but gradually increased
up to 1.47 after that. Broad MWD in high [1a]0/[2]0 ratios may
be interpreted in terms of the hydrolysis of the silyl group of
monomer 1a with a very small amount of water coming from
CsF and/or 18-crown-6. Hydrolyzed 1b would not react as a mon-
omer but eventually worked as an initiator, and the amount of
1b could not be neglected in high [1a]0/[2]0 ratios. Accordingly,
we carried out the polymerization of 1b by using N-triethylsilyl-
N-octylaniline 6 as a base generated by CsF. Thus, the aminyl
anion generated from 6 would abstract the proton of the amino
group of 1b, followed by polymerization in a similar manner of
1a. If the reaction mixture was contaminated with a small amount
of water, only 6 would be hydrolyzed and not affect monomer
1b. In addition, the purification of 1b and 6 was much easier
than that of 1a.
The polymerization of 1b was carried out by using 1.1 equiv
of 6 (Figure 1). The MWD of polymers was quite narrow and
close to a monodisperse distribution, maintaining the Mw/Mn ratio
below 1.1 over the whole [1b]0/[2]0 range. The Mn values of
polymers13 were in agreement with the calculated values assuming
that one polymer chain forms per unit 2 until Mn ) 22 000. This
result implies that the polycondensation of 1b proceeded from
initiator 2 in a chain-growth polymerization manner like living
polymerizations of vinyl monomers and cyclic monomers.14 To
elucidate whether chain-growth polymerization takes place in this
polycondensation, the polymerization of 1b was carried out in
the presence of 5 mol % of 2, and the Mn values and the Mw/Mn
ratios were plotted against monomer conversion (Figure 2A). The
Mn values increased in proportion to conversion, and the Mw/Mn
ratios were less than 1.12 over the whole conversion range. This
polymerization behavior agrees with the features of living poly-
merizations.14 In general polycondensations that proceed in a step
polymerization manner, the molecular weight does not increase
much in low conversion of monomer and is accelerated in high
conversion, and the Mw/Mn ratios increase up to 2.0. Consequently,
Figure 2A shows that the polycondensation of 1b proceeds in a
chain-growth polymerization manner. The character of chain-
growth polymerization of this polycondensation was further dem-
onstrated by so-called “monomer-addition” experiments where a
fresh feed of 1b and 6 was added to the prepolymer (Mn ) 4500
(Mn(calcd) ) 4780), Mw/Mn ) 1.09) in the reaction mixture. The
added 1b feed was smoothly polymerized. The GPC chromato-
gram of the product (Figure 2B (b)) clearly shifted toward the
higher molecular weight region, while retaining the narrow
distribution (Mn ) 10 000 (Mn(calcd) ) 9970), Mw/Mn ) 1.12).15
This experiment implies that this polycondensation will enable
us to synthesize block copolyamides having different aminoalkyl
side chains and well-defined sequences. Furthermore, we antici-
pate that this polycondensation will provide new approaches for
the design of nanoarchitectures, which have been achieved by
living polymerizations of vinyl and cyclic monomers14 and by
stepwise synthesis of dendritic macromolecules16 and linear
oligomers.17 Experiments along these lines are in progress.
Acknowledgment. This work was supported in part by a Grant-in-
Aid (10650873) for Scientific Research from the Ministry of Education,
Science, and Culture, Japan.
Supporting Information Available: Synthesis of monomer 1b and
polymerization procedure (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA001871B
(15) In the monomer-addition experiment, it was crucial to use 0.95 equiv
of 6 in the first-stage polymerization. An excess amount of 6 did not react
with the polymer end group during polymerization but did after the
consumption of monomer to give the amide end group, which could not initiate
the polymerization in the second-stage.
(13) The Mns of polyamides were estimated by gel permeation chroma-
tography (GPC) based on polystyrene standards. However, the Mns determined
by GPC were in good agreement with those determined by 1H NMR spectra.
(14) For a recent review of living polymerizations, see: Kobayashi, S.
Catalysis in Precision Polymerization; John Wiley & Sons: New York, 1997.
(16) For reviews, see: (a) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed.
Engl. 1999, 38, 885. (b) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem.
ReV. 1999, 99, 1665.
(17) For examples, see references in ref 12.