Cobalt-catalyzed carbonylative copolymerization of N-alkylazetidines and tetrahydrofuran

Article abstract of DOI:10.1002/anie.200502116

(Chemical Equation Presented) A living polymerization: Poly(amide-coester)s with either gradient or segmental ester distribution are formed by the cobalt-catalyzed polymerization of azetidine in THF in the absence or presence of Lil, respectively. Periodic addition of azetidine results in copolymers with multiple amide blocks separated by ester segments (see picture). The polymers undergo a two-stage chemical degradation.

Full text of DOI:10.1002/anie.200502116

Angewandte
Chemie
Polymerization
carried out in THF under CO (1000 psi) at 60–808C (Table 1).
For 2, in addition to the major carbonylative enchainment
that resulted in the amide units, a simple ring-opening
enchainment without carbonylation was also observed,
which resulted in amine units in the polymer chain (entries 1
and 2).[7c] For the sterically bulkier and less nucleophilic 3,
selective carbonylative enchainment occurred, as assessed by
NMR spectroscopy (entries 3–8).[11] Somewhat surprisingly,
carbonylative enchainment of THF also took place and
resulted in d-oxyvaleroyl ester units in the polymer prod-
ucts.[12] Thus, the reactions of 2 and 3 in THF produced
poly(amide-co-amine-co-ester)s (PAAEs) and poly(amide-
co-ester)s (PAEs), respectively [Table 1, Eqs. (1) and (2)].
DOI: 10.1002/anie.200502116
Cobalt-Catalyzed Carbonylative
Copolymerization of N-Alkylazetidines and
Tetrahydrofuran**
Guosheng Liu and Li Jia*
The metal-catalyzed carbonylative polymerization of hetero-
cycles (COPH) has received much attention in the last few
years as a novel method for the synthesis of
Table 1: Carbonylative polymerization of azetidine and THF (CO pressure 1000 psi).[a]
polyamides and polyesters.[1–7] Motivation
for these research activities is the practical
demand for polymers with heteroatom-con-
taining backbones that are susceptible to
hydrolytic and/or enzymatic attack in bio-
logical or ecological environments.[8,9] We
are interested in the development of COPH
using an approach based on single-site
catalysts, under the idea that well-defined
molecular catalysts would allow rational
design and control of the chemical compo-
sition, molecular weight, and microstructure
of the polymer product.
Entry THF Azetidine Azetidine/1 Cocatalyst Selectivity[b]
Ester abun-
dance [%]
Mn ”10À3
[mL]
(molar
ratio)
(PDI)[c]
1[d,e] 20
2
2
3
3
3
3
3
3
3
3
3
3
3
3
8:1
16:1
8:1
16:1
32:1
48:1
80:1
100:1
18:1
18:1
–
–
–
–
–
–
–
–
(81:9)[f]:10 8.3
2.16 (1.47)
3.78 (1.55)
1.43 (1.18)
2.68 (1.20)
5.43 (1.20)
7.42 (1.23)
13.3 (1.22)
15.5(1.25)
4.38 (1.12)
2.63 (1.35)
–
2[d,e] 20
3[d,e] 20
4[d,e] 20
5[d,e] 20
6[d,e] 30
7[d,g] 35
8[d,g] 40
9[d,e] 30
10[d,e] 30
11[d,e] 30
(84:8)[f]:8
75:25
4.7
16
Herein, we report the carbonylative
polymerization of azetidines catalyzed by
[Co(CH3CO)(CO)3P(o-tol)3] (1; tol = tolyl),
and the participation of the tetrahydrofuran
(THF) solvent in the polymerization to give
ester units in the polymer products. The use
of LiI as a cocatalyst eliminates the g-lactam
by-product and influences both the amount
and distribution of the ester units in the
polymer backbone. The well-controlled dis-
tribution of the ester units and the living
character of the polymerization have
allowed the synthesis of polymers with
alternating amide blocks and ester segments,
which undergo two-stage degradation.[10]
Herein, N-n-butylazetidine (2) and N-
isobutylazetidine (3) are used as represen-
tative monomers. The polymerization was
82:18
86:14
83:17
85:15
9.4
6.7
4.3
3.3
2.4
16.7
8.5
0
82:18
LiI
>99:1
84:16
LiBPh4
nBu4NI
LiI
LiI
LiI
18:1
>99[h] :1
12[i]
13[i]
14[i]
65
70
75
(18”2):1
(18”3):1
(18”4):1
>99:1
>99:1
>99:1
15
9.1
8.7
7.51 (1.18)
11.65 (1.21)
14.92 (1.23)
[a] Catalyst 1 (0.12 mmol) used in all runs; cocatalyst (0.24 mmol) used for entries 9–14. Quantitative
total conversion was achieved, except for entry 11. [b] Molar ratio of azetidine converted to polymer and
g-LA. [c] Determined by GPC with a refractive index detector against polystyrene standards with CHCl3 as
the eluent. [d] Oil-bath temperature 708C. [e] Reaction time 48 h. [f] Molar ratio of amide and amine
units estimated by 1H NMR spectroscopy. [g] Reaction time 72 h. [h] Monomer conversion 58%.
[i] Autoclave interior temperature 628C; 18 equiv of 3 added 10 h after the previously added 3 was
exhausted.
[*] Dr. G. Liu, Prof. Dr. L. Jia[+]
Department of Chemistry
The extent of THF carbonylative enchainment differed when
2 and 3 were used as the monomer: twofold as many ester
units were incorporated with 3 than with 2 (entries 1vs. 3, and
2 vs. 4).
Lehigh University
6 East Packer Avenue, Bethlehem, PA 18015 (USA)
Fax: (+1)610-758-6536
E-mail: ljia@rohmhaas.com
The ester abundance, which is defined as the number of
[+] Current address: Rohm and Haas Electronic Materials
ester units divided by the total number of all monomer units,
was also a function of the initial monomer-to-catalyst ratio.
Higher monomer loadings resulted in a lower degree of ester
incorporation (entries 3–8), which suggests that the reaction
of THF was favored when the azetidine concentration was
low. Directly monitoring the amide and ester growth by in situ
IR spectroscopy confirmed that the ester abundance in the
455 Forest Street, Marlborough, MA 01752 (USA)
[**] This research was supported by the National Science Foundation
(CHE 01-34285) and the Petroleum Research Fund (40241-AC1).
Supporting information for this article (polymerization procedures,
plots of Mn versus conversion, NMR spectra of polymer products,
and mechanistic hypotheses on g-lactam formation) is available on
the WWW under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 129 –131
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
129

Communications
polymer increased as the reaction proceeded (Figure 1). The
polymerization has a living character, as evidenced by the
narrow polydispersity (Table 1, entries 3–8) and the linear
acylazetidinium intermediate rather than that of the acyl-
Co(CO)4 intermediate.[13] Based on this hypothesis, one could
formulate that the addition of nucleophiles, such as IÀ ions,
capable of opening the azetidium ring would bias the reaction
toward polymerization. Indeed, the g-LA by-product was
eliminated with the addition of two equivalents of LiI relative
to 1 (entry 9). The role of IÀ in suppressing the formation of g-
LA was corroborated by the fact that the amount of 3
converted to g-LA was hardly affected by the addition of
LiBPh4 (entry 10). Furthermore, nBu4NI was also effective in
suppressing the formation of g-LA, although the polymeri-
zation was slower, and converted only 58% of 3 in 48 h under
otherwise identical conditions (entries 9 vs. 11). The latter
observation suggests that the Li+ cation must also play a role
in the polymerization.
In addition, LiI exerted an unexpected effect on the
distribution and abundance of ester units in PAE. In situ IR
spectroscopic monitoring of the polymerization revealed that
in the presence of LiI, ester units did not form until complete
consumption of 3 (Figure 1). The formation of the ester units
began once the growth of the amide units stopped, and the
final ester abundance was appreciably higher with LiI than
without LiI for the same reaction time (entries 4 vs. 9). Thus,
the PAEs produced in the presence of LiI are best charac-
terized as consisting of an amide block followed by a very
short ester segment. Note that again, Li+ cations must play a
role in the reaction of THF, as no ester units were formed in
the presence of nBu4NI (entry 11).
The combination of the segmental ester distribution and
the living character of the polymerization provides an
opportunity for the synthesis of PAEs with multiple, alter-
nating amide blocks and ester segments. This was achieved by
periodically feeding 3 into the reaction 10 hours after the
previously added 3 was exhausted. The multiple additions
under high pressure preserved the living polymerization, as
demonstrated by the relatively narrow polydispersity indices
(PDI; entries 12–14 in Table 1) and the linear increase of Mn
with increasing number of monomer additions (Figure 2).
Methanolysis of the PAEs at room temperature under acidic
conditions completely eroded the ester linkages in the
Figure 1. Change of ester abundance in the polymer chain as a
function of reaction time. Line a: reaction in the absence of LiI; line b:
reaction in the presence of LiI.
increase of molecular weight (Mn) with increasing monomer
conversion,[11] and hence the ester units must populate the
polymer chain in a gradient fashion that increases from the
head to the tail. The gradient ester distribution and the
variation in the ester abundance can likely be attributed to
competition for the acyl site by THF, azetidine, and
[Co(CO)4]À ions and the relative ring-opening rates of
azetidine and THF (Scheme 1).[12] Note that although the
incorporation of THF was observed by Osakada and co-
workers in the carbonylative polymerization of oxiranes,[5] we
have never observed such reactivity in the carbonylative
polymerization of aziridines.[7]
Scheme 1. Plausible mechanistic scenario affecting THF enchainment.
The selectivity of the polymerization of both 2 and 3
suffered from the ring-expanding side reaction that produced
the g-lactam (g-LA). The selectivity did not deteriorate over a
12-fold increase in monomer loading (entries 3–8). The total
conversions to the polymer and g-LA were quantitative in all
runs (entries 1–8). The formation of g-LA did not annihilate
the living character of the polymerization. Therefore, we
tentatively attribute the formation of g-LA to a process
intrinsically coupled to the polymerization, most likely “back-
biting”[6] as opposed to catalyst decomposition.[7a] We further
hypothesize that back-biting happens at the stage of the
Figure 2. The linear increase of Mn values of PAEs with increasing
number of additions of 3 (c; PAEs from entries 9, 12–14, Table 1).
The dashed line shows the nearly constant Mn values of the methanol-
ysis products independent of the Mn values of the original PAEs.
130
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Angewandte
Chemie
[2] T. L. Lee, H. Alper, Macromolecules 2004, 37, 2417 – 2421.
backbone in 12 hours. The Mn values of the resulting poly-
amides were significantly reduced to the narrow range of
3570–3880 gmolÀ1, and their PDI values were rather small in
the range of 1.11–1.30. The uniform Mn and the narrow PDI of
the methanolysis products are consistent with the anticipated
microstructure of the PAEs, that is, amide blocks with
uniform block length separated by segments of ester units.
In comparison, the polyamides from the degradation of PAEs
synthesized in the absence of LiI under otherwise identical
conditions by multiple additions of 3 displayed a much
broader molecular weight distribution.[11] Finally, the poly-
amides that resulted from methanolysis at room temperature
can be completely degraded in refluxing water in one day
under acidic conditions. Thus, the chemical compositions and
their distributions in the above PAEs dictate that they would
undergo two-stage degradation.
The Co-catalyzed living carbonylative polymerization of
N-alkylazetidines provides a method that circumvents the
thermodynamic problem for the synthesis of poly(g-lactam)s
via the ring-opening polymerization of g-LAs.[13] The partici-
pation of THF in the polymerization is an interesting feature
of this reaction. The ability of LiI to control the selectivity of
the reaction and the microstructure of the polymer product is
likely not limited to azetidines, but may also be applicable to
other heterocyclic monomers. Combination of the above
attributes has allowed the synthesis of PAEs with controlled
molecular weight and microstructure that impart unique
chemical degradation properties.
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Molnar, B. Rieger, J. Organomet. Chem. 2004, 689, 971– 979;
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dinger, R. Eberhardt, G. Luinstra, B. Rieger, J. Am. Chem. Soc.
2002, 124, 5646 – 5647.
[4] K. Nakano, K. Fumitaka, K. Nozaki, J. Polym. Sci. A 2004, 42,
4666 – 4670.
[5] D. Takeuchi, Y. Sakaguchi, K. Osakada, J. Polym. Sci. A 2002, 40,
4530 – 4537.
[6] D. J. Darensbourg, A. L. Phelps, N. Le Gall, L. Jia, J. Am. Chem.
Soc. 2004, 126, 13808 – 13815.
[7] a) H. Xu, N. Le Gall, J. Zhao, L. Jia, W. W. Brennessel, B. E.
Kucera, J. Organmet. Chem. 2005, 690, 5150 – 5158; b) G. Liu, L.
Jia, J. Am. Chem. Soc. 2004, 126, 14716 – 14717; c) J. Zhao, E.
Ding, L. Jia, J. Polym. Sci. A 2003, 41, 376 – 385; d) L. Jia, H. Sun,
J. T. Shay, A. M. Allgerier, S. D. Hanton, J. Am. Chem. Soc. 2002,
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[8] E. S. Stevens, Green Plastics, Princeton University Press, Prince-
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[9] Biomedical Polymers and Polymer Therapeutics (Eds.: E.
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moto), Kluwer, New York, 2001.
[10] H. Abe, Y. Doi, Macromol. Rapid Commun. 2004, 25, 1303 –
1308.
[11] See the Supporting Information.
[12] THF can be carbonylatively oligomerized under identical
conditions in the absence of azetidines but at a slower rate.
Details of this finding will be reported in due course. No ether
linkages were observed in any experiments with or without
azetidine.
[13] a) V. P. Kolesov, I. E. Paukov, S. M. Skuratov, Zh. Fiz. Khim.
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Experimental Section
Azetidine monomers were synthesized with modified methods from
the literature,[14] and were dried with Bu2Mg at room temperature.
The synthesis of catalyst 1 was previously reported.[6] THF was
refluxed over Na/benzophenone and freshly distilled.
The polymerizations were carried out in 300-mL stainless-steel
reactors equipped with a stainless-steel tube as the reservoir for
addition of azetidines. The tube was connected to the reactor by a
stainless-steel ball-valve fitting. The reactor was located in a well-
ventilated hood, around which CO detectors were placed. Catalyst 1
was dissolved in a small amount of THF and added to the stainless-
steel tube with a syringe under a gentle flow of CO. After being
charged with the solvent, the reactor was immediately pressurized
with CO (1000 psi) and was heated with either an oil bath or a heating
jacket. The stainless-steel tube holding the monomer was slightly
overpressurized to allow the addition of azetidine. Multiple additions
of monomer under high pressure could be performed in the same way.
After the reaction was complete, the reactor was cooled to ambient
temperature and the pressure was released into the fume hood. The
polymer product was isolated by removal of solvent under vacuum,
washing with ether/hexane (1:3 v/v), and drying under vacuum at
room temperature.
Received: June 17, 2005
Published online: November 22, 2005
Keywords: azetidines · block copolymers · carbonylation ·
.
cobalt · polymerization
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Komiya, Chem. Lett. 2004, 33, 858 – 859.
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