Copolymerization of ethylene and propylene with functionalized vinyl monomers by palladium(II) catalysts

Full text of DOI:10.1021/ja953247i

J. Am. Chem. Soc. 1996, 118, 267-268
267
effectively retard chain transfer. We report here the first
examples of metal-catalyzed copolymerizations of nonpolar
olefins (ethylene and propylene) with alkyl acrylates to give
high molar mass polymers.
Copolymerization of Ethylene and Propylene with
Functionalized Vinyl Monomers by Palladium(II)
Catalysts
The copolymerization reactions were initiated by the diethyl
ether adducts 1 or the more stable chelate complexes 6, which
were easily prepared from [(N∧N)PdMeCl] (2), acrylate, and
NaBAr′4 (eq 1) and isolated as air- and temperature-stable
solids.8 Exposure of 1 or 6 to ethylene or propylene in the
Lynda K. Johnson, Stefan Mecking, and Maurice Brookhart*
Department of Chemistry
UniVersity of North Carolina at Chapel Hill
Chapel Hill, North Carolina 27599-3290
ReceiVed September 22, 1995
Ziegler-Natta and related catalysts based on early transition
metal d0 complexes are extensively used for the coordination
polymerization of nonpolar olefins such as ethylene and
propylene.1 However, due to their highly oxophilic nature, these
catalysts are incompatible with functionalized vinyl monomers.2
Late transition metal catalysts are less oxophilic, but they most
often dimerize or oligomerize olefins3 rather than form high
molar mass polymers.4 Thus, ethylene-acrylate or ethylene-
vinyl acetate copolymers are still exclusively produced by radical
routes, which often require high pressure.5,6 Recently we
reported the development of highly active Ni(II)- and Pd(II)-
based catalysts of the general type [(ArNdC(R)sC(R)dNAr)M-
(CH3)(OEt2)]BAr′4 (cf. Scheme 1) that polymerize ethylene and
presence of alkyl acrylates results in the formation of high molar
mass random copolymers (Table 1). Using simultaneous
refractive index and UV detection, GPC analysis indicates that
the fraction of acrylate comonomer is equally distributed over
all molecular weights of the monomodal distribution.8 Similar
to the corresponding ethylene homopolymers,7 the ethylene
copolymers are amorphous, highly branched materials with
∼100 branches/1000 carbon atoms.9 Typical Tg values fall in
the range of -67 to -77 °C. The ester groups are predomi-
nantly located at the ends of branches in the manner shown in
Scheme 1 (x g 0). Evidence of this structural feature, which
requires a 2,1-insertion of acrylate into the Pd-C bond, comes
from observation of a triplet for HR at 2.2-2.4 ppm and a
multiplet for Hâ at 1.6 ppm.10 Similar properties were also
determined for an ethylene-methyl vinyl ketone copolymer
(entry 12) and for the amorphous propylene-acrylate copoly-
mers (entries 14 and 15).11
Scheme 1
Productivities of the copolymerizations are greatly reduced
relative to those of the homopolymerizations (entry 13). As
expected, the fraction of acrylate incorporation is directly
proportional to its concentration in the reaction solution (entries
1-3), and productivity falls correspondingly. Gas solubility
experiments12 show that the ethylene/methyl acrylate (MA)
molar ratio is ∼1:6 under the conditions of entry 4, which
implies relative rates of incorporation of ethylene and MA of
∼150:1 at equal molar concentrations. Variation of the diimine
backbone substituents R does not significantly affect the
percentage of acrylate incorporation in the copolymer (entries
4-6). Productivities, however, are dependent upon the nature
of R (Me > An ≈ H) and follow the same trend as observed
for the ethylene homopolymerizations.7
Low-temperature NMR studies provide insight into the
mechanism of the copolymerization. The reaction of the ether
adduct 1a with MA at -80 °C produces the π-acrylate complex
3a, which undergoes 2,1-migratory insertion with ∼95% regi-
oselectivity to yield the four-membered chelate 4a (eq 2). At
-80 to -60 °C, complex 4a isomerizes to the five-membered
chelate complex 5a, which rearranges to the six-membered
chelate complex 6a at -20 °C. Insertion of fluorinated octyl
acrylate (FOA) also yields predominantly the six-membered
chelate 6 as the final product. In contrast, with tert-butyl
acrylate, a significant percentage of 1,2-insertion to give a
R-olefins.7 The bulky substituents on the aryl groups of the
diimine ligand block associative olefin exchange and thus
(1) As leading references, see: (a) Coates, G. W.; Waymouth, R. M.
Science 1995, 267, 217-219. (b) Yang, X.; Stern, C. L.; Marks, T. J. J.
Am. Chem. Soc. 1994, 116, 10015-10031. (c) Coughlin, E. B.; Bercaw, J.
E. J. Am. Chem. Soc. 1992, 114, 7606-7607. (d) Crowther, D. J.; Baenziger,
N. C.; Jordan, R. F. J. Am. Chem. Soc. 1991, 113, 1455-1457. (e)
Kaminsky, W.; Ku¨lper, K.; Brintzinger, H. H.; Wild, F. R. W. P. Angew.
Chem., Int. Ed. Engl. 1985, 24, 507-508. (f) Ewen, J. A. J. Am. Chem.
Soc. 1984, 106, 6355-6364.
(2) Polymerization of olefins containing functional groups in a position
remote from the vinyl group by early transition metal catalysts has been
reported: (a) Chung, T. C. Macromolecules 1988, 21, 865-869. (b) Chung,
T. C.; Rhubright, D. Macromolecules 1993, 26, 3019-3025. (c) Kesti, M.
R.; Coates, G. W.; Waymouth, R. M. J. Am. Chem. Soc. 1992, 114, 9679-
9680.
(3) (a) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-
1138. (b) Peuckert, M.; Keim, W. Organometallics 1983, 2, 594-597. (c)
Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 185-206.
(4) Polymerization of ethylene has been described: (a) Schmidt, G. F.;
Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443-1444. (b) Brookhart,
M.; Volpe, A. F., Jr.; Lincoln, D. M.; Horvath, I. T.; Millar, J. M. J. Am.
Chem. Soc. 1990, 112, 5634-5636. (c) Keim, W.; Kowaldt, F. H.; Goddard,
R.; Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1978, 17, 466-467. (d)
Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134. (e) Wang, L.;
Lu, R. S.; Bau, R.; Flood, T. C. J. Am. Chem. Soc. 1993, 115, 6999-7000.
Certain Ni(II) catalysts convert R-olefins to oligomers with degrees of
polymerization of ∼4-20: (f) Mo¨hring, V. M.; Fink, G. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 1001-1003.
second five-membered chelate [(N∧N)PdCH2CHMeC(O)O-t-
Bu]+ (5′) also occurs.8
(7) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415.
(5) Doak, K. W. In Encyclopedia of Polymer Science and Engineering;
Mark, H. F., Ed.; John Wiley and Sons: New York, 1986; Vol. 6, pp 386-
429.
(6) For polymerization of olefins containing functional groups in a
position remote from the olefinic function, cf. refs 2 and 4d. A palladium-
catalyzed reaction of ethylene with methyl acrylate has been claimed to
give low molecular weight materials (Mn e 4100), but no detailed product
characterizations were given: Drent, E.; Pello, D. H. L.; Jager, W. W. Eur.
Pat. Appl. 589527, 1994.
(8) For complete data, cf. supporting information.
(9) C(O)OR carbon atoms are excluded.
(10) Detailed characterization of these polymers by 13C NMR spectros-
copy supports the proposed copolymer structure: McCord, E.; McLain, S.,
unpublished results.
(11) The propylene-acrylate copolymers possess ∼210 methyl groups/
1000 carbon atoms.9 This implies 2,1-insertion of propylene and subsequent
“chain straightening” by chain migration to the terminal carbon atom.
(12) Mecking, S.; Brookhart, M., unpublished results.
0002-7863/96/1518-0267$12.00/0 © 1996 American Chemical Society

268 J. Am. Chem. Soc., Vol. 118, No. 1, 1996
Table 1. Olefin Polymerization Dataa
Communications to the Editor
results
reaction conditions
polymer properties
TONe
f
concn
comon (M)
polymer
mass (g)
comon
Mh n
entry
cat.b
monomersc
P (atm)
incorpd (%)
E re. P
comon
(×10-3
)
Mh w/Mh n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
6b
6b
6b
6b
6a
6c
6b
6b
1a
1b
6b
6b
6b
6b
6b
E/MA
E/MA
E/MA
E/MA
E/MA
E/MA
E/tBuA
E/tBuA
E/FOA
E/FOA
E/FOA
E/MVK
E
0.6
2.9
5.8
5.8
5.8
5.8
3.4
0.4
0.6
0.6
1.8
3.0
2
2
2
6
6
6
6
1
1
1
1
6
6
6
2
22.2
4.3
1.8
11.2
1.2
1.2
2.8
1.9
1.5
27.5
9.5
1.0
6.1
12.1
4.0
5.0
4.7
0.7
0.4
0.3
0.6
0.9
1.3
7710
1296
455
3560
355
364
956
78
84
63
148
19
18
7
3
2
54
27
8
88
26
11
42
0.3g
10
25
6
1.8
1.6
1.6
1.8
1.8
1.6
1.8
1.6
3.1
2.7
1.5
3.1
1.8
1.8
665
506
3
8928
2962
626
37127
1179
145
106
95
7
384
37
18
1.8
10.3
5.0
P/MA
P/FOA
0.6
1.8
1.1
5.6
13
9
1.0
a Conditions: 0.1 mmol of catalyst (entry 13, 0.01 mmol); solvent, CH2Cl2 (total volume CH2Cl2 and comonomer, 100 mL; entries 9 and 10, 60
mL); temperature, 35 °C (entries 8-11 and 13, 25 °C); reaction time, 18.5 h (entries 8-10, 24 h; entry 11, 37 h). b Complexes 6: R′ ≡ Me (entry
8, R′ ≡ tBu). c Ethylene (E), propylene (P), methyl acrylate (MA), tert-butyl acrylate (tBuA), H2CdCHC(O)OCH2(CF2)6CF3 (FOA), methyl vinyl
ketone (MVK). d In mol %. e Turnover number ≡ moles of substrate converted per mole of catalyst. f Determined by GPC Vs polystyrene standards,
1
uncorrected. g Determined by H NMR spectroscopy of the nonvolatile product fraction; ∼0.5 g of volatile products formed additionally.
Scheme 2
Exposure of the ether adduct 1a to 5 equiv each of ethylene
and MA at -80 °C results in selective formation of the ethylene
complex [(N∧N)PdMe(C2H4)]+.7 After complete consumption
The NMR studies and the structure of the copolymers support
of ethylene occurs at -30 °C, 1 equiv of MA is incorporated
to form a chelate complex analogous to 6a by insertion into
the Pd-(CH2)2nMe bond. The much more rapid insertion of
acrylates into the Pd-R bond (-80 °C) than ethylene insertion
(-30 °C)7 clearly shows that the predominant incorporation of
ethylene into the copolymers is due to the low relative binding
constant of MA to the electrophilic metal center.
the mechanistic pathway depicted in Scheme 2. During the
copolymerization, a six-membered chelate is formed after
insertion of MA. Further chain growth requires coordination
and insertion of ethylene,13 and under the conditions of entries
1-4 in Table 1, this is the turnover-limiting step. Therefore,
the chelate complex is the catalyst resting state. In accord with
this proposed mechanism, raising the ethylene pressure results
in an increase in ethylene and MA turnovers (entry 4 Vs 3).
In conclusion, these are the first transition metal catalysts
capable of copolymerizing ethylene and propylene with polar-
functionalized vinyl monomers to high molar mass polymers
by a coordination-type polymerization.
Reversible substitution of the chelating carbonyl group of 6
by ethylene was observed at low temperatures for both the MA
and FOA chelates (eq 3). In contrast to the former, the FOA
Acknowledgment is made to the National Science Foundation
(CHE-9412095) and DuPont for financial support. L.K.J. thanks the
NSF for a postdoctoral fellowship, and S.M. thanks the Alexander von
Humboldt Foundation for a Feodor Lynen Fellowship. GPC analyses
were provided by DuPont and J. M. DeSimone’s group.
chelates open readily at -80 °C in the presence of 1 equiv of
ethylene.8,14 For the MA chelates, study of the temperature
dependency of the equilibria 6 + C2H4 h 7 at -100 to -58
°C gave, for 6a, ∆H° ) -8.1 ( 0.2 kcal/mol, ∆S° ) -34 (
1 eu; for 6b, ∆H° ) -5.3 ( 0.3 kcal/mol, ∆S° ) -31 ( 2 eu;
and for 6c, ∆H° ) -5.7 ( 0.1 kcal/mol, ∆S° ) -27 ( 1 eu.
Extrapolation to the conditions of entry 4 in Table 1 gives a
ratio of the chelate to the ethylene complex, 6b/7b, of ∼1000:
1. Warming 6 (R′ ≡ Me) to temperatures above -30 °C results
in ethylene polymerization. The only organometallic species
observed during this process is 6, indicating that initiation to
form the active species for polymerization is much slower than
chain propagation.
Supporting Information Available: Details of catalyst and polymer
synthesis and characterization and descriptions of low-temperature NMR
experiments (16 pages). This material is contained in many libraries
on microfiche, immediately follows this article in the microfilm version
of the journal, can be ordered from the ACS, and can be downloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
JA953247I
(13) Insertion of acrylate at this point is unlikely, as complexes 6 react
only slowly (days) with excess acrylate.8
(14) The more facile substitution of the carbonyl group of the fluorinated
chelate by ethylene may be partially responsible for the higher productivities
in the FOA copolymerizations.