C O M M U N I C A T I O N S
compared to the in situ generated catalysts (compare entry 3 with
ethylene by Pd/phosphine-sulfonate catalysts proceeds via the
coordination-insertion mechanism.
entries 5 and 6), providing the copolymers with a VAc ratio of up
11
to ca. 2%. The activity of alkylphosphine complexes 3 was higher
than that of arylphosphine complex 4 (compare entries 5 and 6
with entry 7).
In summary, we have developed the copolymerization of vinyl
acetate with ethylene by Pd/alkylphosphine-sulfonate catalysts,
leading to highly linear copolymers possessing in-chain and chain-
end VAc units. The present study provides the first example of the
coordination copolymerization of VAc with ethylene. Further
investigations to enhance the catalytic activity and VAc incorpora-
tion ratio as well as mechanistic studies are currently underway in
our group.
The characterization of the products revealed that the polymers
obtained are undoubtedly copolymers rather than a mixture of
homopolymers. According to the 13C NMR spectrum (Figure 2),
the copolymers possess highly linear polyethylene backbones
3
without a detectable branching unit (<1 branch/10 C). The acetoxy
groups were linked to the main chain as well as to initiating and
terminating chain ends. The comparison of the 13C NMR data with
those of some model compounds also supports the NMR assign-
Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research on Priority Areas “Advanced Molecular
Transformations of Carbon Resources” and the Global COE
Program “Chemistry Innovation through Cooperation of Science
and Engineering” from MEXT, Japan. A.N. is grateful for a
Research Fellowship for Young Scientists from JSPS.
9
Supporting Information Available: Experimental procedures and
product characterization (PDF, CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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1
3
Figure 2. C NMR spectrum (1,2,4-trichlorobenzene, 120 °C) and the
assignments for the vinyl acetate/ethylene copolymers obtained in entry 5,
Table 1. Further assignments for other minor signals are shown in the
Supporting Information.
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Chain-end analysis revealed the presence of two major initiating
chain ends, n-alkyl (C) and 2-acetoxypropyl (D) groups (C:D )
ca. 2:3). The copolymerization initiated by 3a would begin with
the insertion of either ethylene or VAc into the Pd-Me bond. In
fact, however, only group C was detected when the copolymeri-
8
949. (f) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129,
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9
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1
29, 7770–7771. See also: (b) Nakamura, A.; Munakata, K.; Kochi, T.;
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only pathway to initiate the copolymerization. Initiating chain ends
D gradually appeared with a longer reaction time, which can be
attributed to the initiation by the 2,1-insertion of VAc into a Pd-H
bond. There were two types of terminating chain ends, vinyl (E)
and (E)-2-acetoxyethenyl (F) groups (E:F ) ca. 3:1). Group E is
formed via ꢀ-H elimination after ethylene insertion or via ꢀ-OAc
elimination after 1,2-insertion of VAc, and group F is formed via
Nozaki, K. J. Am. Chem. Soc. 2008, 130, 8128–8129.
(
6) In a patent literature, Drent reported that Pd/arylphosphine-sulfonate
catalysts copolymerized ethylene and VAc with a trace amount of VAc
incorporation, but no detailed product characterization was described. Drent,
E.; Pello, D. H. L.; Jager, W. W. Eur. Pat. Appl. 1994, 589527.
7) Compared with arylphosphine-sulfonate ligands, alkylphosphine-sulfonate
ligands have been less investigated. For a report on the synthesis of an
alkylphosphine-sulfonate ligand and its nickel complex, see: Zhou, X.;
Bontemps, S.; Jordan, R. F. Organometallics 2008, 27, 4821–4824.
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K. J. Am. Chem. Soc. 2009, in press (DOI: 10.121/ja9047398).
9) See the Supporting Information for experimetal details.
(
(
(
ꢀ
-H elimination after 2,1-insertion of VAc. The resulting Pd-H
species can initiate the copolymerization to form initiating groups
(
10) For a similar purification method, see: (a) Gibson, V. C.; Tomov, A. Chem.
Commun. 2001, 1964–1965. (b) Li, X. F.; Li, Y. G.; Li, Y. S.; Chen, Y. X.;
Hu, N. H. Organometallics 2005, 24, 2502–2510.
11) The incorporation of a small amount of polar functional groups can change
the surface properties of polyolefins. For example, isotactic polypropylenes
modified with less than 1% of maleic anhydride can make the polymers
exhibit better surface properties sufficient for the application to compati-
bilizing agents; see: Nov a´ k, I.; Borsig, E.; Hr c´ kov a´ , L.; Fiedlerov a´ , A.;
Kleinov a´ , A.; Poll a´ k, V. Polym. Eng. Sci. 2007, 47, 1207–1212.
C and D.
(
Control experiments clearly exclude the possibility of radical
polymerization: (i) A radical inhibitor, galvinoxyl, did not affect
the copolymerization of VAc with ethylene by 3a. (ii) The AIBN-
initiated radical polymerization afforded only VAc homopolymers.
These results suggest that the copolymerization of VAc with
9
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