Efficient incorporation of polar comonomers in copolymerizations with ethylene using a cyclophane-based Pd(II) α-diimine catalyst

Article abstract of DOI:10.1021/ja072502j

Polar olefins were copolymerized with ethylene using a cyclophane-based palladium(II) α-diimine catalyst. The incorporation levels of both methyl acrylate and tert-butyl acrylate are unusually high compared to what is obtained with the corresponding acyclic catalyst. Mechanistic studies by low-temperature NMR spectroscopy revealed that the differences between the insertion barriers for ethylene and MA (ΔΔG?) are similar for the acyclic and cyclophane catalysts. The equilibration of comonomers, fast on experimental time scales for the acyclic catalyst, was too slow to be measured for the cyclophane catalyst. These results suggest that ligand substitution is significantly retarded by the bulky cyclophane ligand. The reduction of the rate of monomer exchange with respect to monomer insertion reduces the catalyst's ability to discriminate between comonomers, hence resulting in the high incorporation levels of acrylates in copolymerizations with ethylene. Copyright

Full text of DOI:10.1021/ja072502j

Published on Web 07/26/2007
Efficient Incorporation of Polar Comonomers in Copolymerizations with
Ethylene Using a Cyclophane-Based Pd(II) r-Diimine Catalyst
Christopher S. Popeney, Drexel H. Camacho, and Zhibin Guan*
Department of Chemistry, UniVersity of California, 1102 Natural Sciences 2, IrVine, California 92697
Received April 10, 2007; E-mail: zguan@uci.edu
This communication reports our polymerization and mechanistic
studies of a cyclophane-based Pd(II) R-diimine catalyst for efficient
copolymerization of ethylene with polar olefins. Incorporation of
polar monomers into polyolefins can significantly enhance important
physical properties such as toughness and adhesion, thereby
increasing the utility of olefin-based polymers. Currently, polar
olefin copolymers are commercially manufactured through free
radical polymerization but under very harsh conditions. As a result,
there remains a tremendous driving force for developing milder
transition-metal-catalyzed polymerizations that can produce polar
olefin copolymers in a controllable manner. While early transition
metal polymerization catalysts are in general not very effective for
direct copolymerization with polar olefins because of their oxo-
philicity,1 the polar functional group tolerance of recently developed
late transition metal catalysts is highly promising.2 Indeed, a few
late metal catalysts have been reported for olefin copolymerization
with polar monomers, prominent examples being the palladium-
(II) complexes3-6 and the neutral nickel(II) systems.7-10 Although
these catalyst systems can incorporate polar olefins, efficiencies
of functional olefin incorporation are usually low, which warrants
further search of new catalysts.
Figure 1. (A) Incorporation ratio of MA into copolymer versus MA
concentration for catalysts 1a and 2a. (B) Incorporation ratio of TBA into
copolymer versus TBA concentration for catalysts 1a and 2a. Red circle is
for catalyst 2a, and blue diamond is for catalyst 1a.
Chart 1. Complexes Employed in the Copolymerization Study
Our laboratory has recently developed a cyclophane R-diimine
ligand 2 for late transition metal polymerization catalysis.11 The
Ni(II) complex of 2 exhibited significantly increased thermal
stability and reduced chain transfer,12 even exhibiting living
polymerization behavior at relatively high temperatures.13 Encour-
aged by these positive results, we recently investigated the
copolymerization of ethylene with polar olefins using cyclophane-
based Pd(II) catalyst 2a. The new system shows an unusually high
efficiency in incorporating polar comonomers such as methyl
acrylate (MA) and tert-butyl acrylate (TBA) as compared to the
acyclic analogue 1a. Furthermore, our low-temperature NMR
studies revealed unique mechanistic origins for their increased
efficiency for polar olefin incorporation.
In this study, a series of copolymerizations of ethylene with MA
or TBA were carried out at 88 psi ethylene, 35 °C, for 18 h, but
with different comonomer concentrations (Chart 1). The copolymers
were analyzed by gel permeation chromatography (GPC) for
molecular weight and NMR for composition and branching density
(see the Supporting Information). The monomodal GPC traces and
the characteristic chemical shifts in NMR spectra prove that these
are statistical copolymers with the polar comonomers incorporated
at the ends of branches. The incorporation level of polar monomer
(mol % of polar comonomer incorporated) is plotted against the
polar monomer feed concentration for both catalysts 1a and 2a
(Figure 1). The comparison shows clearly that the cyclophane-
Pd(II) catalyst 2a is much more efficient in incorporating polar
comonomers than the acyclic analogue 1a. For example, 2a afforded
copolymer of over 20% MA at [MA] ) 4.0 M, whereas catalyst
1a achieved only 4% MA at 5.8 M. The contrast in incorporation
efficiency is even bigger for copolymerization of ethylene with
TBA, with 2a again giving a polymer of high acrylate incorporation.
The surprisingly high efficiency in incorporating acrylates for 2a
is most likely due to the unique microenvironment of the cyclophane
ligand.
To reveal the origin of the high efficiency for acrylate incorpora-
tion for the cyclophane-Pd(II) catalyst 2a, a detailed comparative
mechanistic study was undertaken. According to the established
mechanistic model,14 fast pre-equilibrium of monomers occurs
before monomer insertion; therefore, incorporation will be related
to the relative binding abilities of the olefins and their insertion
rates. The labile adducts 1b and 2b served as suitable precursors
to the corresponding ethylene or MA adducts (1,2c and 1,2d,
respectively) for our low-temperature NMR investigations.15 The
reactions to prepare the precursors proceeded far more sluggishly
for the cyclophane-based systems than for their acyclic analogues,
suggesting that ligand substitution is hindered by the cyclophane
ligand. In particular, the formation of 2b by silver(I) activation of
the Pd(II) methyl iodide was complete after 2 days as opposed to
only 2 h for the preparation of 1b.
The rates of migratory insertion of ethylene and MA for both
catalysts were measured by low-temperature 1H NMR techniques.
The first-order decay of the Pd-Me signal of the methyl olefin
complexes was monitored at various temperatures, and the activation
parameters, calculated from the Eyring plots, are shown in Table 1
(details in Supporting Information). The insertion barriers (∆Gq)
agree with the polymerization data, with catalyst 2a being 3 times
9
10062
J. AM. CHEM. SOC. 2007, 129, 10062-10063
10.1021/ja072502j CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Kinetic Data for Olefin Insertion into the Pd-Me Bond
the likelihood of one or the other comonomers binding should be
less dependent on the relative energies of complexation, which
would normally strongly favor ethylene, allowing a weakly binding
comonomer higher incorporation.
In summary, we report on the unusually high efficient incorpora-
tion of polar olefins in copolymerizations with ethylene using a
cyclic cyclophane-based Pd(II) catalyst. The unique structure of
the cyclophane ligand, especially its ability to shield the axial
binding sites, significantly reduces the rate of comonomer exchange.
Presumably, this suppresses the catalyst’s ability to discriminate
between monomers for binding, thus enhances the incorporation
for the polar olefins. Further mechanistic studies and structural
modifications of the cyclic ligand are currently underway.
cat
olefin
∆
Hq (kcal/mol)
∆
Sq (cal/mol
‚K)
∆
Gq (298 K, kcal/mol)
1c
1d
2c
2d
C2H4
MA
C2H4
MA
16.72 ( 0.35
14.68 ( 0.60
18.58 ( 0.49
16.54 ( 0.35
-4.60 ( 1.44
-4.44 ( 2.81
-0.89 ( 1.92
+0.82 ( 1.62
18.14 ( 0.56
16.05 ( 1.05
18.58 ( 0.77
16.29 ( 0.61
slower than 1a for ethylene homopolymerization, for example (see
Supporting Information, Table S1). The differences between the
insertion barriers for ethylene and MA (∆∆Gq), however, are
surprisingly similar for the two catalysts, suggesting that there must
be other factors contributing to the unusually high efficiency in
incorporating MA by catalyst 2a.
Acknowledgment. We thank the National Science Foundation
(DMR-0135233 & Chem-0456719) for financial support. Z.G.
acknowledges a Camille Dreyfus Teacher-Scholar Award and a
Humboldt Bessel Research Award. C.P. thanks an Allergan
Graduate Fellowship. We thank Professors Keith Woerpel and
James Nowick for helpful discussions, Drs. Phil Dennison and
Dennis Leung for NMR assistance, and Christopher Levins for aid
in ligand synthesis.
Because the relative binding affinities of the olefins will also
influence their incorporation ratio, we then carried out olefin
exchange studies. Unlike the acyclic complexes,14 olefin equilibria
were not measurable for complexes of cyclic ligand 2 since
displacement of one olefin by an added olefin did not occur up to
the temperatures of olefin insertion (220-260 K, depending on
monomer, see Supporting Information). As a result, we sought to
obtain estimates of the rates of olefin exchange through one-
dimensional inverse recovery NMR.16 The method was successful
in observing exchange between free and bound ethylene for the
acyclic complex 1c at temperatures as low as 200 K. No exchange,
however, was discernible between free and bound ethylene for 2c
at temperatures up to 260 K. The lower limit for detectable exchange
processes by this method is 10-2 s-1, which is only 15-fold higher
than the calculated insertion rate of ethylene into 2c at 260 K, 6.9
× 10-4 s-1. The exchange of MA was not observed, being too slow
at the low temperatures in which complexes 1d and 2d are stable.
The evidence indicates a slow monomer equilibration for catalysts
bearing ligand 2, a conclusion also corroborated by the observations
of reduced reactivity for ligand substitution in the preparation of
2b. The rates of monomer exchange may approach or be less than
the insertion rates, suggesting that fast pre-equilibrium of olefins
does not occur for the cyclophane catalyst. As proposed previously
by Brookhart and co-workers,3,14 olefin exchange proceeds through
association of a new olefin at the axial coordination sites followed
by replacement of the olefin bound to the metal. The dramatic
reduction of associative ligand substitution processes, critical steps
in chain transfer, halide exchange, and olefin monomer exchange,
is attributed to the effective steric blocking of the axial coordination
sites of the metal by the bulky cyclophane ligand. Without the
establishment of equilibrium prior to insertion, the catalyst’s ability
to discriminate between monomers of different steric and electronic
properties would be suppressed. With this decrease in selectivity,
Supporting Information Available: Experimental details for the
synthesis and characterization of complexes, polymerization data, low-
temperature NMR experiments, and complete ref 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Aaltonen, P.; Fink, G.; Loefgren, B.; Seppaelae, J. Macromolecules 1996,
29, 5255-5260.
(2) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 1169-
1203.
(3) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267-268.
(4) Luo, S.; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 12072-12073.
(5) Chen, G.; Ma, S. X.; Guan, Z. J. Am. Chem. Soc. 2003, 125, 6697-
6704.
(6) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem.
Commun. 2002, 964-965.
(7) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134.
(8) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs,
R. H.; Bansleben, D. A. Science 2000, 287, 460-462.
(9) Johnson, L.; et al. ACS Symp. Ser. 2003, 857, 131-142.
(10) Li, X. F.; Li, Y. G.; Li, Y. S.; Chen, Y. X.; Hu, N. H. Organometallics
2005, 24, 2502-2510.
(11) Camacho, D. H.; Salo, E. V.; Guan, Z. Org. Lett. 2004, 6, 865-868.
(12) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan, Z. Angew. Chem., Int.
Ed. 2004, 43, 1821-1825.
(13) Camacho, D. H.; Guan, Z. Macromolecules 2005, 38, 2544-2546.
(14) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc.
1998, 120, 888-899.
(15) Williams, B. S.; Leatherman, M. D.; White, P. S.; Brookhart, M. J. Am.
Chem. Soc. 2005, 127, 5132-5146.
(16) Led, J. J.; Gesmar, H. J. Magn. Reson. 1982, 49, 444-463.
JA072502J
9
J. AM. CHEM. SOC. VOL. 129, NO. 33, 2007 10063