Initiation steps for the polymerization of vinyl ethers promoted by cationic palladium aqua complexes

Article abstract of DOI:10.1021/om020662s

Both deuterium-labeled experimental and NMR spectroscopic analyses of poly(vinyl ether) offer the mechanistic insight into the polymerization, indicating that the cationic palladium aqua imine - phosphine complexes [(P～N)PdMe(H₂O)]BF₄

Full text of DOI:10.1021/om020662s

5382
Organometallics 2002, 21, 5382-5385
In itia tion Step s for th e P olym er iza tion of Vin yl Eth er s
P r om oted by Ca tion ic P a lla d iu m Aqu a Com p lexes
Chi-Li Chen, Yi-Chun Chen, Yi-Hong Liu, Shie-Ming Peng, and
Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China
Received August 14, 2002
Both deuterium-labeled experimental and NMR spectroscopic analyses of poly(vinyl ether)
offer the mechanistic insight into the polymerization, indicating that the cationic palladium
aqua imine-phosphine complexes [(P ∼N)PdMe(H2O)]BF4 (P ∼N ) o-C6H4(PPh2)(NdCHAr))
promote the reaction via proton-transfer initiation. Insertion of vinyl ether into the Pd-Me
bond in [(P ∼N)PdMe(H2O)]BF4 does not proceed, but the single insertion of the same
substrate into the Pd-acetal bond of [(P ∼N)PdCOCH3(L)]BF4 provides the stable inserted
product [(P ∼N)PdCH(OEt)CH2COCH3]BF4, which has been characterized by an X-ray
structural determination. However, these palladium complexes do not catalyze the poly-
merization or copolymerization of vinyl ether and carbon monoxide.
In tr od u ction
Utilization of a Lewis acid, particularly with early-
transition-metal complexes, to promote the carbocationic
polymerization of vinyl ethers is well-documented,1-8
but the use of late-transition-metal complexes as cata-
lysts on such polymerization is rarely reported.1a,b,6
Recently, Brookhart and Eisenberg’s groups reported
that the polymerization of vinyl ethers could be cata-
lyzed by palladium1a and platinum6 diimine complexes,
respectively. In our earlier investigation, cationic com-
plexes 1 and 2 were also excellent catalysts for such
polymerization.7a In these studies the mechanism for
polymerization was attributed to the cationic pathway
based on the end-group analysis and solvent effect.4-8
However, the initiation of polymerization still remains
an interesting aspect. It is suggested that the cationic
metal center might initiate the carbon-carbon bond
formation by heterolytic cleavage of the π-bond of the
olefin followed by an addition process.8 Here, we report
our effort to determine the mechanistic pathway for the
polymerization and hydrolysis of vinyl ether catalyzed
by palladium phosphine-imine complexes.
* To whom correspondence should be addressed. E-mail: stliu@
ccms.ntu.edu.tw.
(1) (a) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169. (b) Baird, M. C. Chem. Rev. 2000, 100, 1471. (c) Coates, G.
W. Chem. Rev. 2000, 100, 1223. (d) Chen, E. Y.-X.; Marks, T. J . Chem.
Rev. 2000, 100, 1391. (e) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000,
100, 1479. (f) Siemeling, U. Chem. Rev. 2000, 100, 1495 and references
therein.
(2) (a) Sawamoto, M.; Higashimura, T. Makromol. Chem., Macromol.
Symp. 1992, 60, 47. (b) Kennedy, J . P. Macromol. Chem. 1990, 1, 260.
(c) Kazanskii, K. Makromol. Chem., Macromol. Symp. 1992, 60, 259.
(d) Aoshima, S.; Kobayashi, E. Makromol. Chem., Macromol. Symp.
1995, 95, 91. (e) Patrickios, C. S.; Forder, C.; Armes, S. P.; Billingham,
N. C. J . Polym. Sci., Part A: Polym. Chem. 1997, 35, 1181. (f) Pugh,
C.; Kiste, A. L. Prog. Polym. Sci. 1997, 22, 601. (g) Spange, S.; Eismann,
U.; Hoehne, S.; Langhammer, E. Macromol. Symp. 1998, 126, 223. (h)
Yamada, K.; Miyazaki, M.; Ohno, K.; Fukuda, T.; Minoda, M. Macro-
molecules 1999, 32, 290. (i) Matsuzaki, K.; Ito, H.; Kawamura, T.; Uryu,
T. J . Polym. Sci., Polym. Chem. Ed. 1979, 11, 971.
(3) Higashimura, T.; Sawamoto, M. In Comprehensive Polymer
Science; Eastmond, G. C., Ledwith, A., Russo, S., Sigwait, P., Eds.;
Pergamon Press: New York, 1989; Vol. 3, p 673.
(4) (a) J ohnson, L. F.; Heastley, F.; Bovey, F. A. Macromolecules
1970, 3, 175. (b) Nuyken, O.; Aechtner, S. Polym. Bull. 1992, 28, 117.
(c) Fumio, S.; Mitsu, M. Macromolecules 1995, 28, 6911.
(5) Wang, Q.; Baird, M. C. Macromolecules 1995, 28, 8021.
(6) Albietz, P. J ., J r.; Yang, K.; Eisenberg, R. Organometallics 1999,
18, 2747.
Resu lts a n d Discu ssion
Polymerization of ethyl vinyl ether is carried out in a
dichloromethane solution of monomer with the pal-
ladium complex as the catalyst at 30 °C under a
nitrogen atmosphere but not absolutely dry conditions
(see below). Results of the polymerization of ethyl vinyl
ethers catalyzed by cationic palladium complexes are
summarized in Table 1. In all instances, the palladium
complexes catalyze the polymerization with excellent
conversions. The molecular weights of poly(vinyl ether)
are around 8000, whereas poly(dihydrofuran) weights
are in the range of 16 000-20 000. It is interesting to
find that all of the polymerization reactions require an
induction period, but less time is needed for the hydrate
complex 4. In fact, the aqua-coordinated complex 4
initiates the polymerization within several minutes,
whereas the corresponding acetonitrile-coordinated com-
(7) (a) Chen, Y.-C.; Reddy, K. R.; Liu, S.-T. J . Organomet. Chem.
2002, 656, 198. (b) Reddy, K. R.; Surekha, K.; Lee, G.-H.; Peng, S.-M.;
Chen, J .-T.; Liu, S.-T. Organometallics 2001, 20, 1292. (c) Chen, Y.-
C.; Chen, C.-L.; Chen, J .-T.; Liu, S.-T. Organometallics 2001, 20, 1285.
(d) Reddy, K. R.; Tsai, W.-W.; Surekha, K.; Lee, G.-H.; Peng, S.-M.;
Chen, J .-T.; Liu, S.-T. J . Chem. Soc., Dalton Trans. 2002, 1776. (e)
Reddy, K. R.; Chen, C.-L.; Liu, Y.-H.; Peng, S.-M.; Chen, J .-T.; Liu,
S.-T. Organometallics 1999, 18, 2574.
(8) Sen, A. Acc. Chem. Res. 1988, 21, 421.
10.1021/om020662s CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/02/2002

Polymerization of Vinyl Ethers
Organometallics, Vol. 21, No. 24, 2002 5383
Ta ble 1. P olym er iza tion of Vin yl Eth er sa ,b
time conversn
entry
monomer
catalyst (h)
(%)
MN
PDIc
1
2
3
4
5
6
7
ethyl vinyl ether
2,3-dihydrofuran
ethyl vinyl ether
ethyl vinyl ether
ethyl vinyl ether
2,3-dihydrofuran
2,3-dihydrofuran
2
2
3
4
4
4
4
2
100
98
94
92
100
99
8476 3.32
20359 2.40
7139 3.20
7983 3.19
7598 3.39
16721 2.89
17909 2.77
1.5
1.8
1.8
2
2
1.5
98
a
Conditions: vinyl ether (2 mL), palladium complex (10 mg)
b
in CH2Cl2 (10 mL) under a nitrogen atmosphere, at 30 °C. MN/
MW by GPC; using THF as eluant at room temperature and
polystyrene calibration curve for analyses. c PDI ) polydispersity
index.
plexes 1-3 require 10 min or more. However, the
addition of trace water into the reaction medium with
3 as the catalyst shortens the induction period. This
induction period is related to the hydrolysis of vinyl
ether into acetaldehyde and acetal. In a NMR tube
loaded with 4 and ethyl vinyl ether (15 equiv relative
to Pd) in CDCl3, hydrolysis of vinyl ether takes place at
room temperature to form acetaldehyde, 1,1-diethoxy-
ethane (acetal), and ethanol initially. The rationale of
this hydrolysis step is due to the Lewis acid nature of
the metal complex and the dissociation of water from
4. When the water decreases to ∼20% of the original
amount of 4 by the NMR integration relative to Pd-
CH3, the polymerization of vinyl ether starts, as evi-
denced by the appearance of the resonances correspond-
ing to the polymer. In the presence of an excess of water,
the reaction yields the hydrolyzed products exclusively.
This result clearly illustrates that the rate of hydrolysis
is much faster than that of polymerization. It is also
noticed that the NMR signals corresponding to the
palladium complex stay the same as those of the original
complex, particularly without any decrease in the
intensity of the Pd-CH3 resonance. The intact func-
tionality of Pd-Me clearly reveals that the polymeri-
zation process is not via migratory insertion. Also, only
a small fraction of 4 is the initiator for the reaction
followed by rapid chain growth.
F igu r e 1. ORTEP plot of the cationic portion of 6.
bond at room temperature was isolated,7e but not for
ethyl vinyl ether. Concerning the insertion pathway, a
product with a vinyl ether unit inserted into the Pd-
acyl bond was isolated and characterized by single-
crystal analysis. Thus, a mixture of complex 1 (0.18
mmol), vinyl ether (0.9 mmol), and carbon monoxide (50
psi) in dichloromethane (5 mL) was stirred at room
temperature for 12 h. Complex 6 was isolated as a light
yellow crystalline solid (61%) and characterized by both
spectral and elemental analysis. X-ray-quality crystals
of 6‚H2O were obtained by slow evaporation of a
dichloromethane/ether solution. An ORTEP diagram of
the cationic portion of 6 is shown in Figure 1. The
structural study shows that complex 6 exists as a
slightly distorted square-planar complex with a carbon
ligand derived from the insertion of a vinyl ether
molecule into Pd-COCH3. As expected, the carbonyl
moiety is coordinated toward the metal center, allowing
the complex to be quite stable through the chelation
effect.7e This coordination mode is also consistent with
the infrared absorption of carbonyl stretching at 1621
cm-1. All bond distances and bond angles lie within
normal ranges, except P(1)-Pd-N(1) and O(1)-Pd-
C(4), illustrated in Table 2. The smaller angle of P(1)-
Pd-N(1) is due to the rigidity of the o-phenylene system
and the substituent of the donor atoms, which are sim-
ilar to those of related complexes previously prepared,7b-e
whereas the smaller angle of O(1)-Pd-C(4) is presum-
ably caused by the chelation effect of the carbon ligand.
However, no further insertion of vinyl ethers takes
place with 6, and the complex 6 does not catalyze the
polymerization of vinyl ether. Results of this experiment
clearly rule out the possibility of polymerization or
Characterization of polymers was performed by both
1
GPC and NMR. The H NMR spectrum of the polymer
shows broad signals at δ 3.6 (R-O-CH), 3.48 (O-CH2),
1.7 (â-CH2), and 1.2 (-Me), typical for the polymer. As
expected, the acetal functionality is established as the
1
end groups by the H NMR spectrum, which is consis-
tent with the cationic polymerization process.3-6 How-
ever, it is still not possible to identify the other end
group (the initiated side), due to the overlapping of
peaks. However, a deuterium-labeling experiment al-
lows us to answer this query. With 3 as the catalyst in
the presence of trace D2O, the mass spectrum of the
products clearly shows the incorporation of deuterium
into the oligomers by ∼70% on the basis of the intensity
of m/z at M + 1 for the formula H[CH2CH(OEt)]nOEt.
In addition, the hydrolysis of ethyl vinyl ether catalyzed
by 3 also provides DCH2CHO and DCH2C(OCH2CH3)2,
1
as evidenced by H NMR. These results suggest that
the proton transfer to vinyl ether is the initiation step
for polymerization.
In our previous investigation, the stable chelating
product 5 of insertion of vinyl acetate into the Pd-CH3

5384 Organometallics, Vol. 21, No. 24, 2002
Chen et al.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) of 6
than chain growth. This is why the hydrolyzed product
forms first, before the polymerization takes place.
Apparently, the water molecules in the medium play
an important role in the reaction. Even though the
solvent and monomer were purified according to the
standard procedure under dry nitrogen conditions, a
trace of water molecules (ca. 50-70 ppm) is still present
in the reaction medium, as analyzed by the Karl Fischer
method,12 which explains why complexes 1-3 can
initiate the polymerization. Under extremely dry condi-
tions, we found that ethyl vinyl ether does not undergo
polymerization with 3 as a catalyst within hours.
However, this mixture still undergoes polymerization
upon prolonged standing, presumably due to the trace
amount of alcohol present in the monomers. Of course,
these polymerizations are readily quenched by the
addition of a base such as pyridine or triethylamine, as
expected.
Pd-C(4)
2.049(4)
2.195(4)
2.206(1)
2.156(3)
1.290(6)
1.532(7)
1.481(7)
1.232(5)
P(1)-Pd-N(1)
P(1)-Pd-O(1)
N(1)-Pd-C(4)
O(1)-Pd-C(4)
O(1)-C(2)-C(3)
Pd-N(1)-C(13)
P(1)-Pd-C(4)
81.7(1)
170.12(9)
178.0(2)
81.8(2)
120.0(4)
127.7(3)
96.6(2)
Pd-N(1)
Pd-P(1)
Pd-O(1)
N(1)-C(13)
C(4)-C(3)
C(3)-C(2)
C(2)-O(1)
Sch em e 1
In summary, the proton transfer process is believed
to be the initiation step for polymerization of vinyl ether
catalyzed by the cationic palladium aqua complexes. The
acidity of these palladium aqua complexes is currently
under investigation.
Exp er im en ta l Section
Gen er a l In for m a tion . All reactions, manipulations, and
purification steps were performed under
a dry nitrogen
atmosphere. Tetrahydrofuran was distilled under nitrogen
from sodium benzophenone ketyl. Dichloromethane and ac-
etonitrile were dried over CaH2 and distilled under nitrogen.
Other chemicals and solvents were of analytical grade and
were used as received unless otherwise stated. Complexes 1-3
were prepared as described previously.7
Nuclear magnetic resonance spectra were recorded in CDCl3
or acetone-d6 on either
a Bruker AC-E 200 or AM-300
spectrometer. Chemical shifts are given in parts per million
relative to Me4S for 1H and 13C NMR and relative to 85% H3-
PO4 for 31P NMR. Due to the complicated aromatic region,
chemical shifts of nonaromatic carbons were reported. Infrared
spectra were measured on a Nicolet Magna-IR 550 spectrom-
eter (Series-II) as KBr pellets, unless otherwise noted. Gel
permeation chromatography (GPC) data were obtained from
a Waters Model 590 liquid chromatograph equipped with a
Lab Allience RI 2000 detector using THF as eluant at room
temperature and a polystyrene calibration curve for analyses.
Differential scanning calorimetric (DSC) measurements were
carried out on a TA 2920 system.
copolymerization of vinyl ether via migratory insertion.
It also appears that the palladium center of the coor-
dinated saturated complex 6 is not accessible as a site
for the coordination of water molecules; i.e., water is
not able to compete with the chelating CdO. Therefore,
complex 6 cannot be used as an initiator for the
polymerization of vinyl ethers.
A possible polymerization pathway for vinyl ether
initiated by palladium complexes is revealed in Scheme
1, which involves the key step of the proton transfer
from the palladium aqua species to vinyl ether. It has
been reported that the pKa value for [(SCS)Pd(H2O)]+
is 10.7.11 Thus, the pKa value of the palladium with an
imine donor in 4 should be in a similar range, which
supports the possibility of the protonation of vinyl ether
to generate the oxonium intermediate I, which then
undergoes the chain growth with the monomers. As for
the hydrolysis of vinyl ether, Kobayashi and co-workers
reported that the oxonium intermediate I could be easily
hydrolyzed by the catalysis of the palladium complexes,
due to its small pKh value (hydrolysis constant).10 In
other words, the hydrolysis would proceed much faster
Com p lex 4. To a solution of [(P ∼N)PdMeCl] (159 mg, 0.29
mmol) in CH
2Cl2 (10 mL) was added an equimolar amount of
AgOTf in CH2Cl2 (5 mL) and H2O (21 µL) under a nitrogen
atmosphere. After it was stirred for 2 h at room temperature,
the reaction mixture was filtered and the filtrate was concen-
trated to yield the crude product, which was washed with ether
several times to provide the desired complex 4 (141 mg, 83%):
1H NMR δ 8.78 (s, 1 H, -NdCH), 8.35 (m, 2 H), 7.65-7.01
(m, 17 H, Ar H), 1.74 (s, 2 H, OH2), 0.86 (s, 3 H, Pd-Me); 31P
NMR δ 39.4; 13C NMR δ 168.6 (-NdCH), 155.5-118.3
(aromatic), 2.0 (Pd-CH3). Anal. Calcd for C27H25F3NO4PPd:
C, 49.59; H, 3.85; N, 2.14. Found C, 49.24; H, 3.68; N, 2.05.
Com p lex 6. A solution of 1 (0.1 g, 0.18 mmol) in dichlo-
romethane (10 mL) was pressurized with carbon monoxide (50
psi). After it was stirred for 2 h, the reaction mixture was
checked by 31P NMR to make sure that all of the complex had
been converted into the palladium-acetyl complex. Ethyl vinyl
ether (0.065 g, 0.9 mmol) was then added to the above reaction
mixture, and the mixture was kept stirring for 12 h at room
(9) Matsuzaki, K.; Ito, H.; Kawamura, T.; Uryu, T. J . Polym. Sci.,
Polym. Chem. Ed. 1979, 11, 971.
(10) (a) Kobayashi, S.; Nagayama, S.; Busujima, T. J . Am. Chem.
Soc. 1998, 120, 8288. (b) Coordination Chemistry; Martell, A. E., Ed.;
ACS Monograph 168; American Chemical Society: Washington, DC,
1978; Vol. 2.
(11) Nakai, H.; Ogo, S.; Watanabe, Y. Organometallics 2002, 21,
1674.
(12) Yunxiang, C.; Xin, J . Talanta 1984, 31, 556.

Polymerization of Vinyl Ethers
Organometallics, Vol. 21, No. 24, 2002 5385
temperature. The reaction mixture was concentrated, and the
residue was recrystallized to give the desired complex as light
yellow crystalline solids (76 mg, 61%): IR (KBr): 1621 cm-1
(νCO); 1H NMR δ 9.3 (s, 1 H), 8.59 (m, 2 H), 8.1-7.3 (m, 16 H),
5.6 (m, 1 H), 3.2 (br, 2 H), 2.29 (s, 3 H), 2.20 (t, J ) 7 Hz, 3 H).
13C NMR δ 229.1, 166.2, 153.9, 133.9-132.4, 118. 9, 116.7,
116.5, 85.2 (d, J ) 9 Hz), 67.7, 57.4, 28.4, 15.0; 31P NMR δ 35.
Anal. Calcd for C31H30BF5NO2PPd: C, 53.82; H, 4.37; N, 2.02.
Found: C, 53.76; H, 4.09; N, 1.97.
a nitrogen atmosphere. The resulting mixture was stirred at
room temperature for a certain period. After the reaction was
completed, the reaction mixture was concentrated. The residue
was filtered through a silica gel column with elution of
chloroform. The desired polymer was obtained upon concentra-
tion. All results are summarized in Table 1.
Poly(ethyl vinyl ether): 1H NMR δ 3.6 (br, 1 H, OCH-),
3.48 (br, 2 H, OCH2-), 1.70 (2 H, -CH2-), 1.2 (t, 3 H, -CH3);
13C NMR δ 74 (OCH2), 64 (OCH-), 40 (-CH2-), 16 (-CH3).
Cr ysta llogr a p h y. A crystal suitable for X-ray determina-
tion was obtained for 6‚H2O by slow evaporation of a dichlo-
romethane/ether solution at room temperature. Cell param-
eters were determined by a Siemens SMART CCD diffract-
ometer. Crystal data are as follows: C31H32BF5NO3PPd, fw )
709.76, monoclinic, P21/c, a ) 10.2058(1) Å, b ) 19.8003(3) Å,
c ) 15.7152(2) Å, R ) 90°, â ) 108.025(1)°, γ ) 90°, V )
3019.84(7) Å3, Z ) 4, Dcalcd ) 1.561 Mg/m3, F(000) ) 1440, 0.20
× 0.15 × 0.15 mm, θ ) 1.71-27.50°, 18 065 reflections
collected, 6793 independent reflections (Rint ) 0.0472), good-
ness of fit on F2 ) 1.092, R1 ) 0.0592, wR2 ) 0.1166 for I >
Poly(2,3-dihydrofuran): 1H NMR δ 3.52-3.72 (br, 3 H,
OCH- and OCH2-), 1.86-2.32 (br, 3 H, -CH- and -CH2-);
13C NMR δ 79-84 (OCH-), 65-67 (OCH2-), 43-47 (-CH-),
25-30 (-CH2).
Ack n ow led gm en t. We thank the National Science
Council (Grant No. NSC90-2113-M002-38) for financial
support.
Su p p or tin g In for m a tion Ava ila ble: Complete descrip-
tion of the X-ray crystallographic structure determination of
6‚H2O, including atomic coordinates, isotropic and anisotropic
thermal parameters, bond distances and angles, and hydrogen
coordinates. This material is available free of charge via the
Internet at http://pubs.acs.org.
2σ(I), largest difference peak and hole 1.004 and -0.635 e Å-3
.
Other crystallographic data are deposited as Supporting
Information.
P olym er iza tion of Vin yl Eth er . The palladium complex
(10 mg) was placed in a flask, which was evacuated and
flushed with nitrogen three times. Vinyl ether (2 mL) and
dichloromethane (10 mL) were syringed into the flask under
OM020662S