![Table 4. Equilibrium Constants for [(N∧N)PdMe(L)]+ + L′ a [(N∧N)PdMe(L′)]+ + L (N∧N t ArNdC(H)-C(H)dNAr, Ar t 2,6--iPr2C6H3 (a))b](/figures/table-4-equilibrium-constants-for-n-n-pdme-l-l-a-n-n-pdme-l-lv6e2y54.webp)
Mechanistic Studies of the Palladium-Catalyzed Copolymerization of
Ethylene and R-Olefins with Methyl Acrylate
Stefan Mecking,
†
Lynda K. Johnson,
†,‡
Lin Wang,
‡
and Maurice Brookhart*
,†
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290, and DuPont Central Research and DeVelopment,
Wilmington, Delaware 19880-0328
Abstract: Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and R-olefins
with methyl acrylate to give high molar mass polymers are described. Complexes [(N
∧
N)Pd(CH
2
)
3
C(O)-
OMe]BAr′
4
(2)or[(N
∧
N)Pd(CH
3
)(L)]BAr′
4
(1:L) OEt
2
; 3:Lt NCMe; 4:Lt NCAr′)(N
∧
N ≡
ArNdC(R)-C(R)dNAr, e.g., Ar t 2,6-C
6
H
3
(i-Pr)
2
,Rt H(a), Me (b); Ar′ t 3,5-C
6
H
3
(CF
3
)
2
) with bulky
substituted R-diimine ligands were used as catalyst precursors. The copolymers are highly branched, the acrylate
comonomer being incorporated predominantly at the ends of branches as -CH
2
CH
2
C(O)OMe groups. The
effects of reaction conditions and catalyst structure on the copolymerization reaction are rationalized. Low-
temperature NMR studies show that migratory insertion in the η
2
-methyl acrylate (MA) complex [(N
∧
N)-
PdMe{H
2
CdCHC(O)OMe}]
+
(5) occurs to give initially the 2,1-insertion product [(N
∧
N)PdCH(CH
2
CH
3
)C-
(O)OMe]
+
(6), which rearranges stepwise to yield 2 as the final product upon warming to -20 °C. Activation
parameters (∆H
q
) 12.1 ( 1.4 kcal/mol and ∆S
q
)-14.1 ( 7.0 eu) were determined for the conversion of
5a to 6a. Rates of ethylene homopolymerization observed in preparative-scale polymerizations (1.2 s
-1
at 25
°C, ∆G
q
) 17.4 kcal/mol for 2b) correspond well with low-temperature NMR kinetic data for migratory
insertion of ethylene in [(N
∧
N)Pd{(CH
2
)
2n
Me}(H
2
CdCH
2
)]
+
. Relative binding affinities of olefins to the
metal center were also studied. For [(N
∧
N)PdMe(H
2
CdCH
2
)]
+
+ MA h 5a + H
2
CdCH
2
, K
eq
(-95 °C) )
(1.0 ( 0.3) × 10
-6
was determined. Combination of the above studies provides a mechanistic model that
agrees well with acrylate incorporations observed in copolymerization experiments. Data obtained for equilibria
2 + H
2
CdCHR′′ h [(N
∧
N)Pd{(CH
2
)
3
C(O)OMe}(H
2
CdCHR′′)]
+
(R′′ t H, Me,
n
C
4
H
9
) shows that chelating
coordination of the carbonyl group is favored over olefin coordination at room temperature. Formation of
chelates analogous to 2 during the copolymerization is assumed to render the subsequent monomer insertion
a turnover-limiting step.
Introduction
The development of an insertion-type olefin polymerization
catalyst compatible with readily available polar-functionalized
monomers H
2
CdCHX (X t e.g., C(O)OR, OC(O)R) would
potentially offer a low-pressure, low-temperature route to a wide
range of functionalized copolymers. This development might
enable the synthesis of new polymer structures or offer an
alternate route to commercially available copolymers such as
ethylene-acrylate and ethylene-vinyl acetate copolymers, which
are presently produced by radical polymerizations in high-
pressure reactors.
1
Although Ziegler-Natta and related catalysts
based on early transition metal d
0
-complexes are extensively
used for the coordination polymerization of nonpolar olefins
such as ethylene and propylene, the use of these catalysts in
the copolymerization of functionalized olefins has been limited
due to their highly oxophilic nature.
2
For example, the
polymerizations by early metals of olefins containing a func-
tional group in a remote position to the vinyl group have been
reported, but these reactions are currently limited to special
substrates.
3,4
The development of polymerization catalysts incorporating
late transition metals is a promising area of research, since late
metals are typically less oxophilic, and thus more functional-
group tolerant, than early metals. For example, Ru-based
metathesis catalysts have been developed that are compatible
with a wide range of functional group-containing olefins and
are active in aqueous media.
5
Examples of the functional-group
†
University of North Carolina at Chapel Hill.
‡
DuPont Central Research and Development.
(1) 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.
(2) 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. For recent reviews, see: (g) Brintzinger, H.
H.; Fischer, D. Mulhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1143-1170. (h) Coates, G. W.; Waymouth, R. M.
In ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.; Hegedus, L., Vol. Ed.; Pergamon Press: 1995;
Vol. 12, pp 1193-1208.
(3) (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. (d) Aaltonen, P.; Lo¨fgren, B. Macromolecules 1995, 28, 5353-5357.
(e) Galimberti, M.; Giannini, U.; Albizatti, E.; Caldari, S.; Abis, L. J. Mol.
Catal. 1995, 101,1-10.
(4) The preparation of ethylene-acrylate A-B block-copolymers has
recently been reported. However, the ethylene block must be grown first,
presumably due to an irreversible change in polymerization mechanism:
Yasuda, H.; Ihara, E. Macromol. Chem. Phys. 1995, 196, 2417-2441.
888
Konstanzer Online-Publikations-System (KOPS)
URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/6186/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-61868
First publ. in: Journal of the American Chemical Society 120 (1998), 5, pp. 888-899
tolerance of late metals in insertion-type reactions include reports
on Ru, Rh, Ni, and Pd catalysts for the dimerization of acrylates
and the codimerization of ethylene and acrylates.
6,7
In these
acrylate dimerizations and in numerous other insertion reactions
of late metals with olefins, β-hydride elimination competes
effectively with chain growth, resulting in the formation of
dimers or oligomers.
6,8
A prominent example is the synthesis
of linear R-olefins from ethylene in the Ni-catalyzed Shell
Higher Olefin Process.
8a,b
Recently, however, a number of
ethylene polymerization catalysts based on Co, Rh, and Ni have
been described.
9
Although the polymerization of R-olefins by
these catalysts has not been achieved,
10,11
a Ni catalyst has been
reported to copolymerize ethylene with olefins containing a
functional group in a remote position to the olefin.
12,13
We recently reported the development of highly active,
cationic Ni(II)- and Pd(II)-based catalysts of the general type
[(N
∧
N)M(Me)(L)]
+
(Scheme 1) that polymerize ethylene and
R-olefins.
14
The palladium complexes also catalyze the co-
polymerization of ethylene and R-olefins with functionalized
olefins such as acrylates and methyl vinyl ketone.
15
A key
feature of these Ni and Pd catalysts is the incorporation of bulky
substituents R′ on the aryl groups of the R-diimine ligand, which
block associative olefin exchange and thus effectively retard
chain transfer (Scheme 2).
14,16
We present here a full account
of the mechanistic aspects of the copolymerization of ethylene
and R-olefins with methyl acrylate. The effect of catalyst
structure and reaction conditions is discussed based on mecha-
nistic insight obtained from low-temperature NMR experiments.
Full details of the characterization of the unique, highly branched
copolymers
17
and the functional-group tolerance of these
catalysts
18,19
will be reported separately.
Results and Discussion
A. Copolymerization Reactions. Employing cationic pal-
ladium catalysts with bulky substituted R-diimine ligands to
copolymerize ethylene or an R-olefin with methyl acrylate yields
a high molecular weight random copolymer (eq 1). Similar to
the corresponding ethylene or R-olefin homopolymers synthe-
sized with these catalysts, the copolymers are highly branched,
the ester groups being located predominantly at the ends of
branches in the manner shown (x g 0).
17
The chelate complexes 2 or the nitrile adducts 3 and 4 were
generally preferred to the ether-adducts 1 for preparative-scale
(5) As leading references, see: (a) Moore, J. S. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Hegedus, L., Vol. Ed.; Pergamon Press: 1995; Vol. 12, pp 1209-
1232, and references therein. (b) Novak, B. M.; Grubbs, R. H. J. Am. Chem.
Soc. 1988, 110, 7542-7543. (c) Nguyen, S. T.; Johnson, L. K.; Grubbs, R.
H. J. Am. Chem. Soc. 1992, 114, 3974-3975. (d) Lynn, D. M.; Kanaoka,
S.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 784-790. (e) Stumpf, A.
W.; Saive, E.; Demonceau, A.; Noels, F. J. Chem. Soc., Chem. Commun.
1995, 1127-1128.
(6) (a) Alderson, T.; Jenner, E. L.; Lindsey, R. V., Jr. J. Am. Chem.
Soc. 1965, 87, 5638-5645. (b) Barlow, M. G.; Bryant, M. J.; Haszeldine,
R. N.; Mackie, A. G. J. Organomet. Chem. 1970, 21, 215-226. (c) Oehme,
G.; Pracejus, H. Tetrahedron Lett. 1979, 343-344. (d) Nugent, W. A.;
Hobbs, F. W., Jr. J. Org. Chem. 1983, 48, 5364-5366. (e) Grenouillet, P.;
Neibecker, D.; Tkatchenko, I. Organometallics 1984, 3, 1130-1132. (f)
Nugent, W. A.; McKinney, R. J. J. Mol. Catal. 1985, 29,65-76. (g)
McKinney, R. J.; Colton, M. C. Organometallics 1986, 5, 1080-1085. (h)
Wilke, G. Angew. Chem. 1988, 100, 189-211. (i) Brookhart, M.; Sabo-
Etienne, S. J. Am. Chem. Soc. 1991, 113, 2777-2779. (j) Brookhart, M.;
Hauptman, E. J. Am. Chem. Soc. 1992, 114, 4437-4439. (k) Hauptman,
E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.; Garner, J. M.; Fagan, P.
J ; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 8038-8060, and references
cited therein. (l) DiRenzo, G. M.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 1996, 118, 6225-6234.
(7) Behr, A. In Industrial Applications of Homogeneous Catalysis;
Mortreux, A., Petit, F., Eds.; D. Reidel Publishing Company: Dordrecht,
1988; pp 156-167.
(8) (a) Peuckert, M.; Keim, W. Organometallics 1983, 2, 594-597. (b)
Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235-244. (c) Rix, F. C.;
Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-1138.
(9) (a) Keim, W.; Kowaldt, F. H.; Goddard, R.; Kru¨ger, C. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 466-467. (b) Schmidt, G. F.; Brookhart, M. J.
Am. Chem. Soc. 1985, 107, 1443-1444. (c) Brookhart, M.; Volpe, Jr., A.
F.; Lincoln, D. M.; Horvath, I. T.; Millar, J. M. J. Am. Chem. Soc. 1990,
112, 5634-5636. (d) Ostaja Starzewski, K. A.; Witte, J.; Reichert, K. H.;
Vasiliou, G. In Transition Metals and Organometallics as Catalysts for
Olefin Polymerization; Kaminsky, W., Sinn, H., Eds.; Springer-Verlag:
Berlin Heidelberg, 1988; pp 349-360. (e) Wang, L.; Lu, R. S.; Bau, R.;
Flood, T. C. J. Am. Chem. Soc. 1993, 115, 6999-7000.
(10) Certain Ni(II) catalysts convert R-olefins to oligomers with degrees
of polymerization of ca. 4-20: (a) Mo¨hring, V. M.; Fink, G. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 1001-1003. (b) Schubbe, R.; Angermund, K ; Fink,
G.; Goddard, R. Macromol. Chem. Phys. 1995, 196, 467-468.
(11) For an early example of the observation of both 2,1- and 1,2-
insertions in R-olefin dimerizations by Pd(II)-based catalysts modified with
bipyridyl-type ligands, see: (a) Drent, E. Eur. Pat. Spec. 1986, 170311.
(b) Drent, E. Pure Appl. Chem. 1990, 62, 661-669.
(12) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134.
(13) A palladium-catalyzed reaction of ethylene with methyl acrylate
has been reported to give low molecular weight materials (M
n
e 4100),
but no detailed product characterizations were given: Drent, E.; Pello, D.
H. L.; Jager, W. W. Eur. Pat. Appl. 1994, 589527.
(14) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415 (experimental details and NMR data are given in
the Supporting Information).
(15) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267-268 (experimental details and NMR data are given in the
Supporting Information).
(16) The use of R-diimine ligands in transition-metal-catalyzed reactions
has been extensively researched. As leading references, see: (a) van Koten,
G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151-239. (b) van Asselt,
R.; Gielens, E. E. C. G.; Rulke, R. E.; Vrieze, K.; Elsevier: C. J. J. Am.
Chem. Soc. 1994, 116, 977-985. For the development and synthesis of the
N-aryl-substituted R-diimine ligands used in this research, see: (c) R ) H,
Me: tom Dieck, H.; Svoboda, M.; Grieser, T. Z. Naturforsch 1981, 36b,
823-832. (d) R ) An: van Asselt, R.; Elsevier: C. J.; Smeets, W. J. J.;
Spek, A. L.; Benedix, R. Recl. TraV. Chim. Pays-Bas 1994, 113,88-98.
(17) The detailed nature of the branching in the novel ethylene- and
R-olefin polymers and the corresponding copolymers with functionalized
olefins has been extensively studied by 2D-
13
C NMR spectroscopy: McLain,
S.; McCord, E., manuscript in preparation.
(18) For example, methyl vinyl ketone, numerous acrylates besides MA
(e.g., acrylic acid), and other polar monomers have been copolymerized
with ethylene by the palladium catalysts.
15,19
In addition, polymerizations
using the palladium catalysts have been carried out in water.
19
The synthesis
and reactivity of nickel chelate complexes and the functional-group tolerance
of the nickel catalysts has also been explored.
19
(19) (a) Brookhart, M. S.; Johnson, L. K.; Killian, C. M.; Arthur, S. D.;
Feldman, J.; McCord, E. F.; McLain, S. J ; Kreutzer, K. A.; Bennett, A.
M.; Coughlin, B. E.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J. Pat. Appl.
WO 96/23010, 1996. (b) Brookhart, M. S.; Arthur, S. D.; Feldman, J.;
Johnson, L. K.; McLain, S. J.; McCord, E. F.; Mecking, S.; Wang, L.; Wang,
Y., unpublished results.
Scheme 1
889
polymerizations, as the former are air- and temperature-stable
and can be prepared readily from [(N
∧
N)PdMeCl], NaBAr′
4
and
the acrylate or nitrile.
20
Under the conditions reported here,
the results of polymerization experiments were largely inde-
pendent of the precursor employed.
21
1. Copolymerization of Ethylene with Methyl Acrylate.
The effects of catalyst structure and reaction parameters were
studied extensively using ethylene and methyl acrylate (MA)
as comonomers. The effect of monomer concentrations is
shown in Table 1. Gas solubility experiments showed that the
solubility of ethylene in neat MA and in methylene chloride
(used as a solvent in copolymerization experiments) does not
differ significantly. Thus, varying the MA concentration at a
given ethylene pressure does not change the ethylene concentra-
tion. The solubility of ethylene in methylene chloride was
determined to be 0.18 mol/L atm in the range from 1 to 11 atm
at 25 °C. The fraction of MA incorporation is directly
proportional to the MA concentration in the reaction solution
(Table 1, entries 1-3 and Figure 1). The polymer yield falls
as the MA incorporation increases, due to a decrease in ethylene
turnovers, while the turnover numbers for MA remain roughly
the same. In comparison to ethylene homopolymerization
experiments
22
(Table 1, entries 5-8), productivities of the
copolymerizations are greatly reduced. These observations are
in accord with the mechanistic model depicted in Scheme 3,
which is supported by low-temperature NMR experiments (vide
infra). After an insertion of acrylate, rearrangement can yield
a stable chelate complex. Coordination of the carbonyl group
to palladium inhibits the next olefin insertion and renders it a
turnover-limiting step.
(20) The high lability of Et
2
O results in the thermal instability of 1;
however, the lability of Et
2
O also makes 1 a very useful precursor for low-
temperature NMR studies. Decomposition of 1 (R′ ) i-Pr) occurs upon
activation of one of the CH
3
bonds of the isopropyl substituent by Pd,
resulting in the evolution of methane and formation of a chelate complex:
Johnson, L. K.; Tempel, D.; Brookhart, M., unpublished results.
(21) Concerning activation of ethylene homopolymerizations using
chelates 2 as catalyst precursors, the relative amount of unactivated chelate
2b was determined by
1
H NMR analysis (integrals of OMe-groups) of the
ester end group of the polymer vs the unreacted chelate in 1 h runs. After
1 h, at 2 atm 71% of the chelate had been activated, whereas at 29 atm
virtually no unactivated chelate remained. These data correspond well to
the productivities observed in these experiments and to productivity observed
using the ether complex 1, which is activated instantaneously, as a catalyst
precursor. Thus, the rate of activation of 2 is only significant at low ethylene
pressure in combination with short reaction times; in 18 h experiments,
even at only 1 atm, the rate of activation does not affect the outcome of
polymerizations significantly.
(22) Additional data for Pd-catalyzed homopolymerizations of ethylene
(1 atm) and R-olefins by the Et
2
O adducts 1 is published in Table 1 of ref
14. Please note, in this table M
n
and M
w
reported in entry 1 for the ethylene
homopolymer produced by 1a are off by a factor of 10; correct values are
M
n
) 6 000 and M
w
) 18 000. The low values for the molecular weights
of the Pd-produced homopolymers in ref 14 as compared to the values
reported in Table 1 of this report are mainly due to mass-transfer-limited
conditions in ref 14.
Scheme 2
Table 1. Ethylene-MA Copolymerization and Ethylene Homopolymerization: Effect of Reaction Conditions
f
results polymer properties
react. conditions TON
d
entry cat. [MA] (M) p(atm)
e
polymer
yield (g)
comon.
incorp.
b
(%) E MA
M
n
a
(× 10
-3
) M
w
/M
n
branches/
1000C
1 2b 0.6 2 22.2 1.0 7710 78 88 1.8 103
2 2b 2.9 2 4.3 6.1 1290 84 26 1.6 103
3 2b 5.8 2 1.8 12.1 455 63 11 1.6 105
4 2b 5.8 6 11.2 4.0 3560 148 42 1.8 97
5 2b 2 8.8 30160 297
c
3.5
c
102
6 2b 11 13.7 48700 490
c
2.7
c
100
7 2b 29 8.1 28480 496
c
3.0
c
98
8 2d 11 3.7 13300 445/28
c
106
a
Determined by GPC vs polystyrene standards, uncorrected.
b
Mol %.
c
Bimodal distribution.
d
Turnover number t mole substrate converted per
mole catalyst.
e
E.g. 2 atm t 1 atmg.
f
0.1 mmol catalyst (entries 5-8: 0.01 mmol); solvent: CH
2
Cl
2
(total volume CH
2
Cl
2
and comonomer: 100
mL); temperature: 35 °C (entries 5-8: 25 °C); reaction time: 18.5 h.
890
Correspondingly, increasing the ethylene pressure results in
an increase in ethylene and MA turnovers (Table 1, entry 4 vs
3). The monomer concentrations have no significant effect on
the total number of methyl- and ester-ended branches for a given
catalyst. The number of branches observed is similar to that
for the corresponding ethylene homopolymers (entries 5-7).
This suggests that chain-running (Scheme 2) is fast compared
to chain growth in the case of the palladium catalysts. In all
experiments given in Tables 1 and 2, the ester groups are
predominantly located at the ends of branches with x g 2 (see
eq 1). The average length of the ester-ended branch decreases
with increasing ethylene pressure; that is, after an MA-insertion
the resulting Pd-alkyl complex may be trapped prior to
isomerization to a six-membered chelate or further chain
running.
17,23
Figures 2 and 3 show the progress of the copolymerization
over time. Catalyst deactivation occurs within 50 h with the
ArNdC(Me)-C(Me)dNAr (Ar t 2,6-C
6
H
3
(i-Pr)
2
) ligand (b)
(Figure 2), whereas for the catalyst with the corresponding 2,6-
dimethyl substituted ligand d (Figure 3), no loss in activity was
observed for over 3 days.
24,25
In both cases, varying the reaction
time in the range of 24-72 h had little effect on polymer
molecular weight. A run under identical conditions as entry 2
in Table 2 (18 h) terminated after 3 h yielded a polymer of
2-fold lower molecular weight (M
n
) 23 kg/mol; 870 TO). That
is, in the 3 h experiment the rate of chain transfer is on the
order of the reaction time, whereas in the longer experiments,
the molecular weight is controlled by chain transfer and thus
reflects the properties of the specific catalyst under steady state
conditions of chain initiation and chain transfer. Assuming that
the copolymerization is retarded by formation of a chelate as a
stable catalyst resting state as discussed above, but with
otherwise identical mechanisms of chain propagation and chain
transfer operating as in the case of ethylene homopolymerization,
one would expect the molecular weights of the copolymers to
approach those of the homopolymers at sufficiently long reaction
times. However, the molecular weights of the copolymers are
much lower than those of ethylene homopolymers. Additional
modes of chain transfer must be operative. Possibly, intermedi-
ates of the type [(N
∧
N)PdH{RCHdCHC(O)OMe}]
+
are in-
volved. It is interesting in this respect that for 2c, a slow
reaction with excess MA resulted in elimination of the chelate
moiety and formation of a new chelate [(N
∧
N)Pd{(CH
2
)
2
C(O)-
OMe}]
+
. In addition, exposure of 2a to 6000 equiv of MA for
18 h (0.02 mmol; 10 mL MA; 10 mL methylene chloride)
resulted in decomposition of the complex and formation of a
small amount of MA-dimers, predominantly dimethyl hex-3-
enedioate. In the absence of excess MA, 2a is stable in
methylene chloride solution for days.
The mechanistic experiments depicted in eq 2 (vide infra)
revealed that MA binds to the electrophilic metal center via its
olefinic functionality (as opposed to coordination via its ester
group).
26
To further probe for solvent effects or other effects
of the ester functionality of the MA monomer present in the
copolymerization reactions, an ethylene homopolymerization
was run in a 3:1 mixture (v:v) of methylene chloride and methyl
propionate under conditions otherwise identical with entry 6 of
Table 1. Productivity and polymer molecular weight are
reduced (32 000 TO; M
n
) 306 kg/mol), but compared to the
copolymerization experiments the effect is small, and it can be
concluded that the presence of ester groups in the reaction
mixture does not significantly effect the outcome of polymer-
ization experiments with these catalysts.
(23) The observation that chain running is fast relative to chain growth
in general, whereas the intermediate formed after an MA-insertion may be
trapped by ethylene, is in accord with the result that rearrangement of the
chelate complexes (eq 1) is much slower than isomerization of an
unsaturated or weakly ligated alkyl complex [(N
∧
N)PdR(L)]
+
(L t solvent;
agostic interaction): Tempel, D. J.; Johnson, L. K.; Brookhart, M.,
unpublished results.
(24) The instability of complexes containing the 2,6-diisopropyl-
substituted ligand as compared to the 2,6-dimethyl-substituted ligand may
be related to the greater accessibility of the C-H bond of the i-Pr substituent
to the metal center.
20
(25) In homopolymerizations using 2d, turnover frequency ina1h
experiment was the same as that in the 18.5 h experiment (entry 8, Table
1) under identical conditions. This implies little or not catalyst deactivation
over 18 h.
Scheme 3
Figure 1. Linear dependence of MA incorporation on the MA
concentration in the reaction solution.
891
Table 2 shows the effects of variations of ligand structure
on the copolymerizations. Variation of the diimine backbone
substituents R does not significantly affect the percentage of
acrylate incorporation in the copolymer (Table 2, entries 1-3).
However, the nature of R affects productivities (Me > An ≈
H) and molecular weights (Me > An > H), and the trends
follow those observed for the ethylene homopolymerizations.
14
Reduction of the steric bulk of the substituents on the aryl
moieties R′ results in an increase of the relative acrylate
incorporation, presumably due to the greater accessibility of the
metal center for the binding of the sterically larger olefin (Table
2, entry 2 vs 4 and 5-7). However, at the same time, less
effective retardation of associative olefin exchange enhances
chain transfer and thus lowers the molar mass of the copolymer.
Interpretation of the relative catalyst activities is complex:
mechanistic experiments (vide infra) suggest that opening of a
chelate complex by ethylene coordination, believed in part to
control the TO frequency of the copolymerization reaction, is
favored by smaller diimine substituents R′. On the other hand,
the smaller diimine substituents also result in lower catalyst
activity in ethylene homopolymerization experiments due to
slower chain growth (entry 6 vs 8 in Table 1). For example,
for the production of a copolymer of given MA incorporation
(26) (a) To convert 1a completely to 5a in a reasonable time at
temperatures low enough to prevent migratory insertion, an excess of MA
was utilized. This results in formation of an additional species, identi-
fied as the MA adduct of 6a, [(N
∧
N)Pd{CH(Et)C(O)OMe}-
{H
2
CdCHC(O)OMe}]
+
(t at δ 0.53, J ) 7Hz(CD
2
Cl
2
, -62 °C)). This
does not effect the rate of migratory insertion, which was found to be
independent of [MA] in the range of 1.3-18 equiv MA in the limit of
error. Some subsequent rearrangement of 6a to 7a also occurred. (b) Starting
from 1b, displacement of ether by MA occurs more slowly than with 1a
and thus interferes with observation of the migratory insertion. Therefore,
complexes of ligand a were preferred for mechanistic studies. In addition
to the η
2
-olefin complex 5b (
1
H NMR (CD
2
Cl
2
, -71 °C) δ 5.01 (d, 1, J )
15 Hz, dCHH′), 4.82 (dd, 1, J ) 15 Hz, J ) 8 Hz, dCHC(O)), 4.48 (d,
1, J ) 8 Hz, dCHH′), 3.60 (s, 3; OMe), 2.38 and 2.27 (s, 3 each; NdC-
(Me)-C′(Me)dN), 0.32 (s, 3; PdMe)), a second species 5b′ with δ 2.96 (s,
3, OMe), 2.18 (s, 6, NdC(Me)-C′(Me)dN), 0.26 (s, 3H.; PdMe) (CD
2
Cl
2
,
-71 °C) is observed. The latter is assigned as the κ-O complex analogous
to 5b, in which the MA-ligand is bound via its carbonyl group. At -71 °C,
conversion of 5b′ to 5b was observed (in addition to migratory insertion).
This rearrangement presumably occurs very rapidly at room temperature,
so that coordination of the acrylate via the carbonyl-group is relatively
insignificant in typical copolymerization experiments.
Table 2. Ethylene-MA Copolymerization: Effect of the R-Diimine Ligand
a
ligand results polymer properties
TON
d
entry -NdC(R)-
subst. of aryl
ring Ar-Nd
rctn.
cond.
polymer
yield (g)
MA-incorp.
b
(%) E MA
M
n
a
(×10
-3
) M
w
/M
n
branches/
1000C
1 a H 2,6-
i
Pr
2
A 1.2 5.0 355 19 0.3
c
n.d.
2 b Me 2,6-
i
Pr
2
A 11.2 4.0 3560 148 42 1.8 97
3 c An 2,6-
i
Pr
2
A 1.2 4.7 364 18 10 1.8 109
4 d Me 2,6-Me
2
A 2.3 14.2 542 90 7 2.1 116
5 b Me 2,6-
i
Pr
2
B 8.5 7 827 58 17 1.4 128
6Me2-
i
Pr 6-Me B 4.0 15 283 50 12 1.3 170
7 d Me 2,6-Me
2
B 2.5 20 154 41 8 1.4 203
8 Me 2,4,6-Me
3
B 3.0 25 162 54 8 1.4 255
9 Me 2,6-Me
2
4-Br B 3.6 15 266 47 7 1.4 178
a
Determined by GPC vs polystyrene standards, uncorrected.
b
Mol %.
c
Determined by
1
H NMR spectroscopy of the nonvolatile product fraction;
ca. 0.5 g of volatile products were formed additionally.
d
Turnover number t mole substrate converted per mole catalyst.
e
Reaction conditions A:
0.1 mmol catalyst precursor 2; solvent: 50 mL of CH
2
Cl
2
;50mLofMA(t 6 M); 6 atm (i.e., 5 atmg); temperature: 35 °C; reaction time: 18.5
h. B: 0.3 mmol catalyst precursor 3; solvent: 40 mL CH
2
Cl
2
;5mLMA(t 1.2 M); 1 atm (ambient pressure); temperature: 25 °C; reaction time:
72 h.
Figure 2. Reaction profile of ethylene-MA copolymerization employing ligand ArNdC(Me)-C(Me)dNAr (Ar t 2,6-C
6
H
3
(i-Pr)
2
)(b). Reaction
conditions of entry 5, Table 2.
892