Regiocontrolled copolymerization of methyl acrylate with carbon monoxide

Article abstract of DOI:10.1021/ja802161n

An alternating copolymer of methyl acrylate with carbon monoxide has been synthesized for the first time via coordination polymerization using palladium complexes of phosphine-sulfonic acid as catalysts. The highly controlled head-to-tail structure of the

Full text of DOI:10.1021/ja802161n

Published on Web 06/07/2008
Regiocontrolled Copolymerization of Methyl Acrylate with Carbon Monoxide
Akifumi Nakamura, Kagehiro Munakata, Takuya Kochi,† and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo, 7-3-1
Hongo, Bunkyo-ku, 113-8656, Japan
Received March 24, 2008; E-mail: nozaki@chembio.t.u-tokyo.ac.jp
Scheme 1
The use of polar vinyl monomers for coordination polymerization
has been a major challenge for decades1 because it potentially provides
a wide variety of unique functionalized polymer architectures with
precise structural control.2 Late transition metal-catalyzed alternating
copolymerization of olefins with CO is one of the most efficient
processes that provides polar functionalized polymers by the
coordination-insertion mechanism.3,4 Several olefins bearing polar
functional groups have been applied to the copolymerization.5 How-
ever, no example had ever been reported for copolymerization of CO
with polar vinyl monomers, the C-C double bond moiety of which
is directly attached to polar groups, until our recent report on the
copolymerization of vinyl acetate with CO.6
Table 1. Alternating Copolymerization of Methyl Acrylate with COa
Methyl acrylate is one of the most important and readily available
polar vinyl monomers. However, application of methyl acrylate for
coordination polymerization has only been achieved for copolymeri-
zation with ethylene and R-olefins.7 Despite considerable efforts
devoted toward copolymerization with CO, all of the reports described
observation of a five-membered chelate substituted by a methoxycar-
bonyl group at the R-position (C), formed by migratory insertion of
CO into the Pd-Me bond of A and subsequent 2,1-insertion of methyl
acrylate (Scheme 1). Previous studies also identified several serious
obstacles to the copolymerization, including (i) suppressed CO
coordination by strong intramolecular ketone coordination in C,8b (ii)
low nucleophilicity of the R-carbon impaired by the electron-
withdrawing group in D, even if existed,8b,9 and (iii) low coordination
ability of the C-C double bond of methyl acrylate.7a,10,11
entry Pd source ligand PCO (MPa) time (h) TOF (h-1
)
Mn (×103) Mw/Mn
1
2
3
4
5
6
7
Pd(dba)2 1a
Pd(dba)2 1b
Pd(dba)2 1a
Pd(dba)2 1a
Pd(dba)2 1a
6.0
6.0
6.0
3.0
8.0
6.0
6.0
20
20
0.5
20
20
20
21
6.0
30
26
1.6
1.2
1.1
1.6
1.7
1.6
1.1
39
16
24
25
57
3.1
21
17
24
2a
2a
-
-
0.5
3.5
In this report, we describe the first example of alternating
copolymerization of methyl acrylate with CO. The resulting
copolymer has a high level of head-to-tail regularity, which has
not been achieved for polar vinyl monomer/CO copolymerization.
a Conditions: 0.010 mmol of Pd source, 0.012 mmol of ligand, 2.5
mL of methyl acrylate, 70 °C. The product contained poly(methyl
acrylate) with less than 1 wt %.
The alternating copolymers of methyl acrylate with CO were
obtained using catalysts generated in situ by combination of
Pd(dba)2 and phosphine-sulfonic acid ligand (1).12,13 Treatment
of methyl acrylate with 6.0 MPa of CO at 70 °C for 20 h in the
presence of the Pd(dba)2/1a catalyst afforded the copolymer with
Mn ) 30 000 (Table 1, entry 1). Methoxy-substituted ligand 1a
offers both higher activity and higher molecular weights of the
copolymers than does unsubstituted 1b (entries 1, 2). The turnover
frequency of the copolymerization was dependent on the CO
pressure (entries 1, 4, 5). An increase in CO pressure led to an
enhancement in catalytic activity, which may suggest the high
barrier for CO coordination-insertion into the chelate C.
Figure 1. 13C NMR spectrum of poly(methyl acrylate-alt-CO) (in CDCl3).
The alternating structure of the copolymer was confirmed by
mass spectrometry and NMR spectroscopy. MALDI-TOF mass
spectrometry of the low-molecular-weight product (Table 1, entry
3) shows signal intervals with 114 indicating the formation of
copolymers were in good agreement with the alternating structure
but were distinct from those of poly(methyl acrylate).14 The highly
controlled head-to-tail structure of the copolymer was also suggested
by the NMR spectra. In fact, the 13C NMR spectrum (Figure 1)
exhibits only one relatively sharp signal in the carbonyl region (δ
) 201.0), which can be interpreted as a regiocontrolled copolymer
on the basis of comparisons with model compounds and other
olefin/CO copolymers.15
1
alternating copolymer. H and 13C NMR spectra of the resulting
† Present address: Department of Chemistry, Faculty of Science and Technology,
Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522,
Japan.
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8128 J. AM. CHEM. SOC. 2008, 130, 8128–8129
10.1021/ja802161n CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
The strong tendency for 2,1-insertion of methyl acrylate into the
Pd-C bond, leading to the head-to-tail structure, was consistent
with density functional theory (DFT) calculations11,13c and experi-
mental observations7a using an R-diimine ligand or 1a. However,
the origin of the difference in regioregularity between methyl
acrylate/CO and vinyl acetate/CO copolymerization is still unclear.
According to the DFT calculation on the R-diimine Pd system by
Goddard and co-workers,11 the preference for 2,1-insertion into a
Pd-C bond over 1,2-insertion is greater for methyl acrylate by 1.2
kcal/mol than for vinyl acetate. Although the energy difference may
suggest a disparity in regioregularity, further studies are necessary
to clarify the difference.
In summary, the first highly regiocontrolled copolymerization
of a polar vinyl monomer with CO is reported, using methyl acrylate
as a comonomer. Further mechanistic investigations by experimental
and computational methods are currently in progress.
Figure 2. Synthesis and molecular structure of 2a. Hydrogen atoms are
omitted. Selected bond distances (Å): Pd(1)-C(21) 2.042(6), Pd(1)-O(8)
2.137(4), Pd(1)-O(3) 2.106(4), Pd(1)-P(1) 2.2163(17).
Acknowledgment. The authors are grateful to Assoc. Prof. Y.
Nishibayashi and Dr. Y. Miyake at the University of Tokyo for
high-resolution FAB-MS analysis of 4a.We also thank Rigaku for
the X-ray crystal analysis of 2a
Supporting Information Available: Experimental procedures and
characterization (PDF, CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Boffa, L. S.; Novak, B. M. Chem. ReV. 2000, 100, 1479.
(2) Patil, A. O., Schulz, D. N., Novak, B. M., Eds.; Functional Polymers:
Modern Synthetic Methods and NoVel Structures; ACS Symposium Series
704; American Chemical Society: Washington, DC, 1998.
(3) (a) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663. (b) Bianchini,
C.; Meli, A. Coord. Chem. ReV. 2002, 225, 35.
(4) Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and Cooli-
gomers; Sen, A., Ed.; Kluwer Academic Publishers: Dordrecht, The
Netherlands, 2003.
Figure 3. Plausible mechanism of copolymerization of methyl acrylate
with CO catalyzed by (a) 1 and Pd(dba)2 or (b) 2a.
(5) (a) Kacker, S.; Jiang, Z. Z.; Sen, A. Macromolecules 1996, 29, 5852. (b)
Lee, J. T.; Alper, H. Chem. Commun. 2000, 2189.
(6) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc. 2007,
129, 7770.
(7) (a) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem.
Soc. 1998, 120, 888. (b) Sen, A.; Borkar, S. J. Organomet. Chem. 2007,
692, 3291. See also refs 12a, 13a–c.
To investigate the mechanism of the copolymerization, a five-
membered chelate species (2a) corresponding to C in Scheme 1
was isolated using 1a (Figure 2). Exposure of 3a13b to ambient
CO pressure quantitatively provided acetylpalladium complex 4a.13d
2,1-Inseriton of methyl acrylate into the Pd-C bond of 4a
proceeded in the presence of AgOTf to yield γ-ketoalkylpalladium
complex 2a in 60% yield. No formation of the 1,2-insertion product
was observed. The structure of 2a was determined by NMR and
X-ray crystallography. The Pd-C (2.042(6) Å) and Pd-O (2.137(4)
Å) bond lengths in the five-membered chelate are similar to those
of reported examples of C bearing neutral bidentate ligands (Pd-C,
2.046-2.059 Å; Pd-O, 2.112-2.161 Å).8f,g Copolymerization of
methyl acrylate with CO was also successfully initiated and
catalyzed by 2a (Table 1, entries 6, 7).
(8) (a) Ozawa, F.; Hayashi, T.; Koide, H.; Yamamoto, A. J. Chem. Soc., Chem.
Commun. 1991, 1469. (b) Dekker, G. P. C. M.; Elsevier, C. J.; Vrieze, K.;
Vanleeuwen, P. W. N. M.; Roobeek, C. F. J. Organomet. Chem. 1992,
430, 357. (c) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc.
1996, 118, 4746. (d) Braunstein, P.; Frison, C.; Morise, X. Angew. Chem.,
Int. Ed. 2000, 39, 2867. (e) Braunstein, P.; Durand, J.; Knorr, M.;
Strohmann, C. Chem. Commun. 2001, 211. (f) Reddy, K. R.; Surekha, K.;
Lee, G. H.; Peng, S. M.; Chen, J. T.; Liu, S. T. Organometallics 2001, 20,
1292. (g) Agostinho, M.; Braunstein, P. Chem. Commun. 2007, 58.
(9) Shen, H.; Jordan, R. F. Organometallics 2003, 22, 1878.
(10) Kang, M. S.; Sen, A.; Zakharov, L.; Rheingold, A. L. J. Am. Chem. Soc.
2002, 124, 12080.
(11) Philipp, D. M.; Muller, R. P.; Goddard, W. A.; Storer, J.; McAdon, M.;
Mullins, M. J. Am. Chem. Soc. 2002, 124, 10198.
A plausible mechanism for the regiocontrolled alternating
copolymerization is shown in Figure 3.16 When a mixture of 1 and
Pd(dba)2 was used, the reaction was initiated by formation of a
Pd-H bond via protonation of the Pd(0) species, followed by
insertion of methyl acrylate. Further insertion of CO and 2,1-
insertion of methyl acrylate in an alternating fashion leads to the
formation of the alternating copolymer. In fact, a 1-methoxycar-
(12) Phosphine-sulfonic acid ligand 1a was originally reported by the group
of Shell for the copolymerization of ethylene with methyl acrylate to afford
linear copolymer12a12a and the non-perfectly alternating copolymerization
of ethylene with CO.12b (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van
Oort, B.; Pugh, R. I. Chem. Commun. 2002, 744. (b) Drent, E.; van Dijk,
R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964.
(13) Several groups have investigated the chemistry of 1a: (a) Hearley, A. K.;
Nowack, R. A. J.; Rieger, B. Organometallics 2005, 24, 2755. (b) Kochi,
T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25. (c) Haras, A.;
Anderson, G. D. W.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics
2006, 25, 4491. (d) Newsham, D. K.; Borkar, S.; Sen, A.; Conner, D. M.;
Goodall, B. L. Organometallics 2007, 26, 3636. (e) Luo, S.; Vela, J.; Lief,
G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 8946. (f) Kochi, T.;
Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 8948.
(g) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 15450.
See also ref 6.
1
bonylethyl group was detected at the initiating end by H NMR
for relatively low molecular weight copolymers (Table 1, entry 3),
as a result of the 2,1-insertion of methyl acrylate into the Pd-H
bond.15 In contrast, copolymers formed with 2a (entry 7) showed
the signal for an acetyl initiating end group, instead of the
1-methoxycarbonylethyl group. It is noteworthy that this system
conquered the difficulties in coordination and insertion of CO to
the five-membered chelate of C, which had never been ac-
complished using other ligands.8
(14) Less than 1% of poly(methyl acrylate) units compared to the methyl acrylate
units in the alternating copolymers were also observed.
(15) See Supporting Information.
(16) Copolymerization via a radical process seems to be denied. See Supporting
Information.
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