A mechanistic investigation on copolymerization of ethylene with polar monomers using a cyclophane-based Pd(II) α-diimine catalyst

Article abstract of DOI:10.1021/ja904471v

A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate (MA) by a Pd(II) cyclophane-based R-diimine catalyst is reported. Our previous observations of unusually high incorporations of acrylates in copolymerization usin

Full text of DOI:10.1021/ja904471v

Published on Web 08/10/2009
A Mechanistic Investigation on Copolymerization of Ethylene
with Polar Monomers Using a Cyclophane-Based Pd(II)
r-Diimine Catalyst1
Chris S. Popeney and Zhibin Guan*
Department of Chemistry, UniVersity of California, 1102 Natural Sciences 2,
IrVine, California 92697
Received June 2, 2009; E-mail: zguan@uci.edu
Abstract: A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate
(MA) by a Pd(II) cyclophane-based R-diimine catalyst is reported. Our previous observations of unusually
high incorporations of acrylates in copolymerization using this catalyst (J. Am. Chem. Soc. 2007, 129,
10062) prompted us to conduct a full mechanistic study on ethylene/MA copolymerization, which indicates
a dramatic departure from normal Curtin-Hammett kinetic behavior as observed in copolymerization using
the normal Brookhart type of Pd(II) R-diimine catalysts. Further investigation reveals that this contrasting
behavior originates from the axial blocking effect of the cyclophane ligand hindering olefin substitution and
equilibration. In equilibrium studies of ethylene with nitriles, the cyclophane catalyst was found to more
strongly favor the linearly binding nitrile ligands as compared to the standard acyclic Pd(II) R-diimine catalysts.
Ethylene exchange rates in the complexes [(N∧N)PdMe(C2H4)]+ (N∧N ) diimine) were measured by 2D
EXSY NMR spectroscopy and found to be over 100 times slower in the cyclophane case. Measurement of
the slow equilibration of ethylene, methyl acrylate, and 4-methoxystyrene in cyclophane-based Pd(II) olefin
complexes by 1H NMR and fitting of the obtained kinetic plots allowed for the estimation of exchange rates
and equilibrium constants of the olefins. After extrapolation to typical polymerization temperature, ∆Gq )
20.6 and 16.4 kcal/mol for ethylene-methyl acrylate exchange in the forward (ethylene displacement by
methyl acrylate) and reverse directions, respectively. These values are of similar magnitude to the previously
determined migratory insertion barriers of ethylene (∆Gq ) 18.9 kcal/mol) and methyl acrylate (∆Gq ) 16.3
kcal/mol) under equivalent conditions, but contrast strongly to the rapid olefin exchange seen in the Brookhart
acyclic catalyst. The large barrier to olefin exchange hinders olefin pre-equilibrium, decreasing the
cyclophane catalyst’s ability to preferentially incorporate one monomer (in this case ethylene) over the
other, thus giving rise to high comonomer incorporations.
promise because of their superior functional group tolerance,3,9-11
permitting copolymerizations in water,12 the incorporation of
Introduction
Transition metal catalyzed copolymerization of ethylene with
polar-functionalized olefins is a highly attractive but challenging
endeavor. In the past decade or so, interest has intensified
dramatically as a large body of new work involving olefin
polymerization catalyst systems utilizing late transition metals
has been published.2-6 A series of cationic Pd(II) R-diimine-
based catalysts, reported by Brookhart and co-workers, were
the first successful systems able to copolymerize important polar
feedstock monomers, like acrylates, with ethylene.7,8 Since then,
neutral Ni(II) and Pd(II) catalyst systems have shown particular
notoriously difficult comonomers like acrylonitrile,13 and in-
corporations of acrylates in excess of 50%.14 Copolymerizations
with the cationic Pd(II) R-diimine catalysts like 1 (Chart 1),
however, have been more thoroughly investigated by mecha-
nistic15 and computational16,17 studies. Brookhart and co-
workers discovered that the incorporation ratio of methyl
(8) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267–268.
(9) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217–3219.
(10) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744–745.
(1) This paper has been adapted in part from the following thesis: Ph.D.
Dissertation. University of California at Irvine, CA. Popeney, C. S.
2008.
(11) Luo, S.; Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007,
129, 8946–8947.
(12) Gottker-Schnetmann, I.; Korthals, B.; Mecking, S. J. Am. Chem. Soc.
2006, 128, 7708–7709.
(2) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169–1203.
(13) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc.
2007, 129, 8948–8949.
(3) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–462.
(4) Boffa, L. S.; Novak, B. M. Chem. ReV. 2000, 100, 1479–1493.
(5) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283–315.
(6) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143–
7144.
(14) Guironnet, D.; Roesle, P.; Ruenzi, T.; Goettker-Schnettmann, I.;
Mecking, S. J. Am. Chem. Soc. 2009, 131, 422–423.
(15) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem.
Soc. 1998, 120, 888–899.
(16) Michalak, A.; Ziegler, T. J. Am. Chem. Soc. 2001, 123, 12266–12278.
(17) Szabo, M. J.; Jordan, R. F.; Michalak, A.; Piers, W. E.; Weiss, T.;
Yang, S.-Y.; Ziegler, T. Organometallics 2004, 23, 5565–5572.
(7) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414–6415.
9
12384 J. AM. CHEM. SOC. 2009, 131, 12384–12393
10.1021/ja904471v CCC: $40.75  2009 American Chemical Society

Copolymerizations with Pd(II) Cyclophane Catalysts
A R T I C L E S
Chart 1. Acyclic (1) and Cyclophane-Based (2) R-Diimine Pd(II)
Olefin Polymerization Catalysts
Figure 1. Incorporation ratio of MA and TBA into copolymer from
catalysts 1 and 2 (ref 22).
Scheme 1. Curtin-Hammett-Based Model of Ethylene-Acrylate
Copolymerization with Catalyst 1 (ref 15)
Table 1. Ethylene Homopolymerizations and Copolymerizations
with Methyl Acrylate Using Catalysts 1 and 2 (ref 22)a
TON
[MA] yield
(mol/L) (mg)
mol %
Mn
entry cat.
C2H4
MA
Mw/Mnc
Bd
MAb (kg/mol)b
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
0e
6600 23500
340 1200 10
-
-
33.7
1.54 101
1.48 104(105)
1.51 100(108)
1.0
2.5
5.8
0e
1.0
2.5
4.0
0.8 26.8
2.4 19.3
120
80
400 10
270 11
4.1
-
5.1 10.1
7.3
12.9
1.61
97(112)
2280 8100
-
1.68 106
100
30
30
310 16
70 12 14.4
60 16 21.8
1.35
1.35
1.18
96(114)
72(130)
71(153)
8.03
6.67
acrylate follows the standard Curtin-Hammett relationship,
given in eq 1.15 By this mechanism, both metal-ethylene and
metal-acrylate complexes are in rapid equilibrium while the
migratory insertion of monomer into the growing polymer chain
takes place comparatively slowly (Scheme 1). The ratio of
monomer incorporation is dependent upon the relative rates of
ethylene and acrylate insertion, given by rate constants kE and
kA, and the equilibrium constant Keq of complexation of the two
olefins. The incorporation of acrylates by these catalysts was
low, a result of the far higher coordination strength of ethylene
compared to MA.
a Polymerization conditions: 10.0 µmol of catalyst, 88 psi ethylene
pressure, 35 °C, 18 h. b Determined by 1H NMR. c Determined by
SEC-MALS. d Branching density per 1000 carbons determined by 1H
NMR. Values in parentheses include ester groups as chain ends.
e Ethylene homopolymerizations shown for comparison.
Quite unexpectedly, we discovered that the cyclophane-based
Pd(II) R-diimine catalyst 2 could incorporate both methyl
acrylate (MA) and tert-butyl acrylate (TBA) monomers in
copolymerizations with ethylene far more effectively than the
corresponding acyclic catalyst 1 (Figure 1).22 Copolymers with
molar ratios of acrylate above 20%, nearly 5 times the maximum
incorporation level observed in polymerizations with acyclic
Pd(II) R-diimine catalyst 1,15 were produced (Table 1). The rate
constant ratios of ethylene and MA insertion (kE/kA) were
measured for each catalyst, but the values were similar within
experimental error. This implies that the enhanced efficiency
for acrylate incorporation for the cyclophane catalyst is not due
to the change of relative insertion barrier caused by the new
ligand framework. From eq 1, perturbation of the monomer
binding equilibrium could also have explained the excess of
MA incorporation. The direct measurement of ethylene-MA
exchange equilibrium constants was not possible with catalyst
2, however, because of the apparently low reactivity of the
cyclophane Pd(II) complex to ligand substitution.
kA[A]
eq kE[E]
[Acrylate]
[Ethylene] Polymer
) K
(1)
(
)
In order to address general shortcomings of the cationic
R-diimine-based catalysts, namely their sensitivity at high
temperature,18,19 our laboratory prepared a new class of Ni(II)
and Pd(II) catalysts bearing an R-diimine ligand with a cyclic
cyclophane-type structure.20 Presumably due to the conforma-
tional inflexibility of the closed rigid cyclophane ligand present-
ing groups that severely hinder access to the axial metal
coordination sites, these catalysts have exhibited thermal stabili-
ties significantly higher than the original acyclic R-diimine
catalysts. Polymerizations at elevated temperature with the
cyclophane catalysts afforded polyolefins of higher molecular
weight, believed to result from a reduction in the rate of
associative chain transfer processes.21
In accordance with reduced rates of associative chain transfer,
suppression by the cyclophane ligand of associative substitution
was hypothesized to account for the unexpectedly high acrylate
incorporation ability. Retardation of fast olefin equilibrium, we
reasoned, would reduce the selectivity of the metal to the
monomers based on their binding strengths, rendering the
Curtin-Hammett assumption invalid and permitting an over-
(18) Gates, D. P.; Svejda, S. K.; Onate, E.; Killian, C. M.; Johnson, L. K.;
White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320–2334.
(19) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,
M. J. Am. Chem. Soc. 2000, 122, 6686–6700.
(20) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan, Z. Angew. Chem.,
Int. Ed. 2004, 43, 1821–1825.
(22) Popeney, C. S.; Camacho, D. H.; Guan, Z. J. Am. Chem. Soc. 2007,
129, 10062–10063.
(21) Camacho, D. H.; Guan, Z. Macromolecules 2005, 38, 2544–2546.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12385

A R T I C L E S
Popeney and Guan
Chart 2. Pd(II) R-Diimine Methyl Complexes Used in Mechanistic Studies
Scheme 2. Ligand Exchange in Pd(II) Complexes
following the general second-order rate law given in eq 2. If the
concentrations of each free ligand is in a large enough excess
compared to the Pd complex with which it is paired in the terms
of eq 2, the pseudo-first-order approximation may be applied to
allow simplification to eq 3, where k1 and k-1 are the pseudo-first-
order rate constants.25 Integration then affords eqs 4 and 5, where
xeq is the change in concentration of all species during equilibration,
or the departure from equilibrium (eq 6). Equations 4 and 5,
respectively, describe the time-dependent concentration of the
complexes PdLA and PdLB as functions of the equilibrium
incorporation of acrylate. Since our initial communication,22 we
have carried out a detailed mechanistic and kinetic investigation
using one-dimensional and two-dimensional NMR methods on
a series of cyclophane-based Pd(II) olefin complexes (Chart 2).
The exchange rates of olefins were measured and the energetic
barriers to exchange processes were compared to the barriers
of migratory insertion. The results reveal that olefin exchange
by associative substitution is dramatically suppressed by the
presence of the cyclic cyclophane ligand of 2, resulting in a
decrease of the cyclophane catalyst’s ability to preferentially
incorporate one monomer (in this case ethylene) over the other,
thus giving rise to high comonomer incorporations.
concentrations, xeq, and the sum of the rate constants k1 and k-1
.
The pseuso-first-order approximation is valid in the regime where
xeq is small, meaning at small departures from equilibrium,
compared to the actual concentrations of the complexes and free
ligands.25
d[PdLA]
-
) k1[PdLA] - k-1[PdLB]
(3)
dt
-1)t
[PdLA](t) ) [PdLA]eq + xeq e-(k +k
1
(4)
(5)
(6)
Experimental Section
-1)t
[PdLB](t) ) [PdLB]eq - xeqe-(k +k
1
General Considerations. NMR spectra were recorded on Bruker
DRX400 and DRX500 FT-NMR instruments. Proton and carbon
NMR spectra were recorded in ppm and were referenced to
indicated solvents. Data was reported as follows: chemical shift,
multiplicity (s ) singlet, d ) doublet, t ) triplet, q ) quartet),
integration, and coupling constant(s) in Hertz (Hz). Multiplets (m)
were reported over the range (ppm) at which they appear at the
indicated field strength. All synthetic procedures, including the
preparation and characterization of new complexes 4e and 4f and
additional details regarding substance preparation and handling, is
described in the Supporting Information.
Kinetic Treatment. Ligand substitution (Scheme 2) at square
planar d8 complexes, such as Pd(II), is widely held to proceed by an
associative mechanism.23,24 Associative chain transfer has been
proposed by Brookhart, for example, to proceed through a 5-coordinate
transition state,19 as should the exchange of olefins prior to monomer
insertion (Scheme 2). The rate of such a process is dependent on the
concentrations of the free olefins as well as the Pd(II) complexes. The
second-order rate law is shown in eq 2, where k2 and k-2 are the
forward and reverse rate constants, respectively.
xeq ) [PdLA]0 - [PdLA]eq
)[PdLB]eq - [PdLB]0
Following the introduction of the second olefin, the concentra-
tions of all species were monitored over a period of up to eight
1
hours at low temperature by H NMR. The concentrations of the
complexes were plotted as functions of time and the curves fitted
to a first-order exponential function26,27 in the form of eqs 4 or 5
by the Origin software program. In this manner, the exchange rate
constants and the equilibrium concentrations could be calculated.
In order to ensure the validity of the pseudo-first-order approxima-
tion, only measurements made at later time, in which the concentra-
tion of free ligand was at least 5 times higher than the concentration
of the complex existing in the same term as eq 2, were included in
the fit. In practice, these conditions were realized by using a large
excess (at least 10-fold) of the weakly binding ligand LA and then
adding approximately a 3-fold excess of the strongly binding ligand
LB. Only data points following ∼70% equilibration were used in
calculations. The concentrations of species in these experiments
are tabulated in the Supporting Information, Tables S2, S3, and
S4. It can be seen that the concentrations of the free ligands are at
least 5 times the concentration of the Pd complex and reach far
higher ratios at later equilibration times, indicating that the pseudo-
first-order approximation is valid.
d[PdLA]
-
) k2[PdLA][LB] - k-2[PdLB][LA]
(2)
dt
Addition of a second olefin LB to a solution of a Pd(II) complex
of olefin LA (or PdLA) results in the equilibration of the system
(23) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals,
3rd ed.; John Wiley and Sons, Inc: New York, 2001.
(24) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry, 3rd ed.; W. H.
Freeman and Company: New York, 1999.
(25) Houston, P. L. Chemical Kinetics and Reaction Dynamics; McGraw-
Hill: New York, 2001.
(26) Eigen, M. Discuss. Faraday Soc. 1954, 17, 194–205.
(27) Bracken, D. E.; Baldwin, H. W. Inorg. Chem. 1974, 13, 1325–1329.
9
12386 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Copolymerizations with Pd(II) Cyclophane Catalysts
A R T I C L E S
In the simple case of the exchange of bound ethylene with free
ethylene, eq 2 can be rewritten as eq 7, where k2 is the second-
order rate constant. Equation 7 describes the total amount of
exchange “events”, which can be detected in dynamic NMR
methods such as EXSY by the transfer of magnetization that occurs.
Since this system is in dynamic equilibrium and the concentrations
of all species remain constant, the pseudo-first-order approximation
may be applied, affording eqs 8 and 9. The pseudo-first-order rate
constants k1 and k-1 could then be related to k2 by eqs 10 and 11,
respectively.
second-order rate constant k2 was determined from the plot of
ethylene concentration dependence on k1, where the slope of the
linear fit is equal to k2. Under the assumption the temperature
dependence of k2 was equal to the temperature dependence of k1,
the activation parameters of exchange ∆Hq and ∆Sq were then
calculated from the Eyring plot of k2, allowing for extrapolation of
exchange rates to polymerization temperature.
(4) Mixing Time Consideration. As tm increases, the cross-
peak intensity increases in a linear manner. At some point, however,
T2 relaxation mechanisms begin to affect magnetization transfer
and the linear correlation is affected.28 Thus tm should be chosen
to be small enough so that the cross-peak intensities are still
increasing linearly but large enough that cross-peaks are large
enough to be accurately measured.29 In the case of the acyclic
catalyst, exchange was fast enough that short values of tm of 10 to
20 ms, well within the linear growth region, could suffice to give
intense cross-peaks. With the cyclophane catalyst, longer mixing
times, from 100 to 800 ms, were necessary because of the slower
exchange of the system. This corresponds well to the optimal mixing
time of about 420 ms according to the formula below,28 calculated
using T1 of the Pd
rate ) k2[Pd(C2H4)][C2H4]
(7)
) k1[Pd(C2H4)]
(8)
) k-1[C2H4]
(9)
k1 ) k2[C2H4]
(10)
k-1 ) k2[Pd(C2H4)]
(11)
1
tm,opt
≈
Low-Temperature One-Dimensional 1H NMR Experiments.
A 10 ppm spectral window was used, and irradiation was centered
at 4.5 ppm. A single 90° pulse was performed, preceded by at least
a 60 s delay, with an 8.0 s acquisition time. If possible, the probe
was tuned to optimize sensitivity at the measurement temperature.
The duration between acquisitions in kinetic studies was adjusted
by changing the delay.
T-1 + k1 + k-1
complex (0.45 s). Values of k1, calculated from different mixing
times, were plotted versus ethylene concentration (as in the
determination of k2), and the mixing times corresponding to the
best linear fits were selected (see the Supporting Information, Table
S1). Longer mixing times were found to give best results.
Measurement of Ethylene Exchange Rates by EXSY
NMR. (1) Acquisition Parameters. A standard gradient NOESY
pulse sequence was employed in the EXSY experiments, with a
spectral width of 2.0 ppm, an offset of 4.8 ppm, 128 experiments
in the F1 domain with two scans each, and delays of 2 s between
each scan (approximate experiment time was 15 min). Since the
two exchanging sites in this studied system lie on different
molecular species (and are therefore far apart), it can be assumed
only exchange processes contributed to the transfer of magnetization.
(2) Experimental Setup. Sets of experiments, conducted at
specific temperature and concentration conditions, consisted of a
reference EXSY experiment with mixing time set to zero and two
or three experiments of varying mixing times tm, all under specific
temperature and concentration conditions. To account for sample
or spectrometer drift during the sets, one-dimensional spectra (using
standard low temperature parameters) were acquired before all
EXSY runs to permit the measurement of the exact concentration
of species by comparison to a known amount of added diphenyl-
methane standard. These experiment sets were acquired at four
different concentrations of free ethylene at constant temperature to
measure the concentration dependence of k1. Additionally, sets were
acquired at two other temperatures, at highest or lowest ethylene
concentrations in the case of the cyclophane or acyclic system,
respectively, to measure the temperature dependence of k1. The
intensity of diagonal and cross-peaks in the EXSY spectra were
obtained by volume integration following appropriate phase and
baseline correction. Regions of both positive and negative intensity
were included in the integration to minimize the effect of baseline
noise.
(5) Error Analysis. Errors in k2, δk2, were calculated from the
linear fit of the k1 versus [C2H4] plot by the least squares method.
To propagate this error to the temperature dependence measure-
ments, two new values of k2 equal to k2 ( δk2 were determined. In
addition to k2, values of k2 ( δk2 proportional to the temperature
dependence of k1 were calculated at the other two temperatures.
An Eyring plot consisting of k2 + δk2, k2, and k2 - δk2 for the
three temperatures was constructed, and errors in ∆Hq and ∆Sq were
calculated from the linear fit by the least squares method.
Ligand Equilibration Studies. An NMR tube was charged with
10 µmol of complex 3a or 4a under nitrogen atmosphere and closed
with a septum cap. The tube was precooled in the instrument to
-100 °C, and 0.6 mL of CD2Cl2 was added along with a known
amount of an internal standard: diphenylmethane (s, δ 3.95 ppm)
in studies with ethylene or 1,2,2,3-tetrachloropropane (s, δ 4.15
ppm) for experiments with MA. The more weakly binding ligand
was added in large excess followed by a 30 min period of
equilibration during which its complexation took place. A spectrum
was acquired by the standard low temperature acquisition param-
eters. The stronger binding ligand was added later, the temperature
was adjusted as desired, and a loop routine was used to carry out
the time-lapsed kinetic study, with delay increased to the desired
time interval between experiments and the standard acquisition
parameters used. The more weakly binding olefin was usually added
in far larger amounts to allow for a measurable equilibration.
Displacement of the first olefin by the second was rapid for the
acyclic catalyst system but required many hours for the cyclophane
system for all cases except experiments involving nitriles, which
equilibrated completely within one hour. Following the experiment,
an external anhydrous methanol standard was used to accurately
determine the experiment temperature. Concentrations of the Pd-
bound species were determined by peak height measurements of
the corresponding Pd-Me resonances normalized to the intensity
of the internal standard. Equilibrium constants were measured from
the concentration of Pd adducts and free olefins at equilibration. A
five percent error was assumed in all peak height measurements.
(3) Determination of Rate Constants. The EXSYCalc software
package, available online at www.mestrec.com, was used to
determine the pseudo-first-order rate constants k1 and k-1. The
intensities of the diagonal peaks and cross-peaks are divided by
the peak intensities of the reference spectrum. The program then
evaluates the rate matrix R,28 the diagonal terms of which are
related to the T1 relaxation of each site and the exchange rates while
the off-diagonal elements are the rate constants k1 and k-1. The
(29) Dimitrov, V. S.; Vassilev, N. G. Magn. Reson. Chem. 1995, 33, 739–
(28) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935–967.
744.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12387

A R T I C L E S
Popeney and Guan
Table 2. Equilibrium Constants and Energetic Parameters of Ethylene-Nitrile Exchange for Acyclic (3) and Cyclophane-Based (4) Pd(II)
Complexes Based on the Mechanism in Scheme 2
entry
diimine series
LA-LB
T (K)
Keq
∆G (kcal/mol)
∆H (kcal/mol)b
0.38 ( 0.12
∆S (kcal/mol)b
-4.7 ( 0.6
1
2
3
4
5
6
7
8
9
3
3
3
4
4
4
4
4
4
4
4
MeCN-C2H4
190
208
224
190
208
224
190
224
298
190
224
0.27 ( 0.02
0.49 ( 0.02
0.58 ( 0.03
0.66 ( 0.03
2.1 ( 0.1
2.2 ( 0.1
2.4 ( 0.1
1.2 ( 0.1
1.4 ( 0.1
1.7 ( 0.1
3.3 ( 0.2c
3.8 ( 0.2c
MeCN-C2H4
0.24 ( 0.02
MeCN-C2H4
0.23 ( 0.02
CH2ClCN-C2H4
CH2ClCN-C2H4
CH2ClCN-C2H4
MeCN-CH2ClCN
MeCN-CH2ClCN
MeCN-CH2ClCN
MeCN-C2H4
(3.9 ( 0.3) × 10-3
(4.1 ( 0.3) × 10-3
(4.6 ( 0.4) × 10-3
0.040 ( 0.003
0.40 ( 0.08
-8.9 ( 0.4
0.048 ( 0.004
0.32 ( 0.06
0.72 ( 0.14d
-4.7 ( 0.2
0.055 ( 0.004
c
c
10
11
(1.5 ( 0.3) × 10-4
-13.6 ( 0.6d
MeCN-C2H4
(2.2 ( 0.4) × 10-4
a Determined by 1H NMR. Solvent ) CD2Cl2. b Calculated from plots of ln Keq versus 1/T. c Calculated from the products of Keq from CH2ClCN/
C2H4 and MeCN/CH2ClCN experiments at given temperature. d Caluclated from ∆H and ∆S values for entries 4-6 and 7-9.
For slowly equilibrating systems, the equilibrium constant and the
pseudo-first-order exchange rate constants were derived by fitting
eqs 4 or 5 to plots of the concentrations of the Pd-bound species
with time. Errors in exchange rates were calculated from the
exponential fits by the least-squares method.
Results
The preparation of cyclophane-based Pd(II) methyl cationic
complexes with a labile ligand for use in NMR investigations
has been previously described. The complexes involved in
mechanistic studies are shown in Chart 2, which include the
acyclic ether complex 3a8 and cyclophane-based water complex
4a.22 The complexation of olefins was then performed in situ
as necessary for NMR experiments by addition of the gaseous
or liquid olefin into a cooled solution of 3a or 4a in CD2Cl2
Figure 2. PM3 model of cation 4b showing the ethylene ligand in green.
Palladium is magenta, carbon is black, nitrogen is blue, and hydrogen is
white.
affording the ethylene, MA, and 4-methoxystyrene (MS) adducts
3b/4b, 3c/4c, and 3d/4d, respectively.
Equilibration of Nitriles and Olefins. Late transition metal
complexes containing a nitrile ligand can be conveniently
prepared and are relatively stable; therefore, we began our
investigation with nitrile complexes. Acetonitrile complexes of
Pd(II) bearing the acyclic or cyclophane-based R-diimine
ligands, 3e and 4e, respectively, were studied in pairwise NMR
experiments with ethylene. The calculated equilibrium constants
from various ligand pairs are shown in Table 2 for the acyclic
(3)andcyclophane-based(4)catalysts.Theresultsofnitrile-ethylene
equilibration show that the cyclophane catalyst exhibited a 3
orders of magnitude greater preference toward the binding of
nitrile instead of ethylene as compared to the acyclic catalyst
(Table 2, entries 3 and 10). Because ethylene displacement of
dicular to the metal coordination plane. Compared to a linear,
η1 σ-donating ligand like acetonitrile, the accommodation of
ethylene in the sterically encumbered cyclophane complex
should be significantly more demanding. To illustrate this point,
a simple model of ethylene complex 4b derived from semiem-
pirical calculations was constructed (Figure 2).30 The olefin can
be seen coordinating to the metal from within a binding pocket
formed by the specific orientation of the cyclophane phenyl
rings.
Rates of Ethylene Exchange. The simplest olefin exchange
scenario, the exchange of free and bound ethylene, was first
examined for both the acyclic and cyclophane Pd(II) catalysts.
In our previous study, attempts to measure the rate of ethylene
exchange for the cyclophane system, important in testing the
applicability of the Curtin-Hammett principle, were not suc-
cessful.22 In the current investigation, exchange was observed
between free ethylene and cyclophane-based ethylene complex
4b by two-dimensional exchange spectroscopy, or EXSY
NMR.28
1
acetonitrile was too weak to measure directly by H NMR in
the cyclophane system, the displacement of more weakly binding
chloroacetonitrile from the adduct 4f (entries 4-6 and 7-9,
respectively), was used to compute the C2H4-MeCN equilib-
rium constant for comparison to that of the acyclic catalyst.
The temperature dependence on Keq was then used to
determine ∆H and ∆S of the equilibrium process. Concerning
∆H, displacement of acetonitrile by ethylene is almost thermo-
neutral for both the acyclic and cyclophane catalysts. However,
the entropic change is much more pronounced for the cyclo-
phane catalyst as compared to the acyclic one. The strongly
negative ∆S value of -13.6 cal/mol*K for the cyclophane
catalyst suggests that the ethylene complex 4b is highly ordered
compared to acetonitrile adduct 4e. This can be attributed to
steric interactions that exist between the preferred conformation
of the bound olefin and the bulky cyclophane ligand. Olefins
are well-known to bind to metals through an η2-π-type
interaction, in which the olefin C-C bond is oriented perpen-
The EXSY technique provided the pseudo-first-order rate
constants k1 and k-1 from experiments with the acyclic complex
3b as well as cyclophane complex 4b. Details of data acquisition
(30) The equilibrium geometry of cyclophane ethylene adduct 4b was
generated by the Spartan software package (Wavefunction, Inc., of
Irvine, CA). The structure of the methyl chloride complex obtained
from X-ray crystallography (ref 20) was imported, the chloride ligand
removed, and an ethylene molecule substituted. Semi-empirical
calculations (PM3) were performed, which converged to the shown
structure regardless of the starting configuration of the olefin (whether
in or perpendicular to the metal coordination plane).
9
12388 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Copolymerizations with Pd(II) Cyclophane Catalysts
A R T I C L E S
Figure 3. EXSY with 600 ms mixing time for the cyclophane adduct of ethylene 4b at 220 K. The signals at 5.30 and 3.95 ppm are dichloromethane and
diphenylmethane, respectively.
Figure 4. Plot of ethylene exchange rate constant k1 versus free ethylene concentration for (a) the acyclic complex 3b at 190 K and (b) the cyclophane-
based complex 4b at 220 K. The slope of the linear fit is the second-order rate constant k2.
and data analysis can be found above in the experimental
section. The results of the EXSY experiments for cyclophane
ethylene complex 4b under various reaction conditions and
mixing times, tm, are tabulated in the Supporting Information,
Table S1. Rate constants were calculated by the program
EXSYCalc from comparison of diagonal and cross-peak vol-
umes for a spectrum at selected tm to those from a reference
spectrum with tm set to zero under the same conditions. The
rate constant k1 was directly related to the concentration of free
ethylene, as expected for a second-order process and eq 10.
Constant k-1, however, remained nearly unchanged (aside from
its temperature dependence) owing to its dependence from eq
11 on the concentration of Pd(II) complex, which was not varied
in these experiments.
An accurate value of the second-order rate constant k2 was
calculated from the slope of a plot of measured k1 at different
concentrations of free ethylene. The calculated values of k2 were
1.4 ( 0.1 s-1M-1 and 320 ( 10 s-1M-1 for the cyclophane
system 4b at 220 K and the acyclic system 3b at 190 K,
respectively. Plots of k1 versus ethylene concentration and the
linear fits are shown in Figure 4. The strong dependence of k1
on ethylene concentration illustrates the associative nature of
the exchange.
By measurement of k1 at different temperatures, its temper-
ature dependence was determined. Because k1 and k2 have the
same temperature dependence, the respective values of k2 at
each temperature could be determined from k1 at those tem-
peratures. The Eyring plots of ethylene exchange for both the
cyclophane and acyclic catalyst are shown in Figure 5. The
following activation parameters for ethylene exchange were
calculated from the Eyring plot of the acyclic system: ∆Hq )
2.5 ( 0.1 kcal/mol, ∆Sq ) -33 ( 1 cal/mol*K. Extrapolation
A representative EXSY spectrum obtained for the cyclophane
complex 4b, along with the regions of integration, is shown in
Figure 3. As can be seen, the crosspeaks were weak as a result
of the slow exchange kinetics but, nevertheless, still visible.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12389

A R T I C L E S
Popeney and Guan
Figure 5. Eyring plot of ethylene exchange for (a) the acyclic complex 3b and (b) the cyclophane-based complex 4b.
to the polymerization temperature (35 °C) gives ∆Gq ) 12.6 (
0.3 kcal/mol. For the cyclophane system, the same treatment
led to the following activation parameters: ∆Hq ) 5.3 ( 0.2
kcal/mol, ∆Sq ) -33 ( 1 cal/mol*K, and ∆Gq ) 15.5 ( 0.4
kcal/mol at 35 °C. The enthalpies of activation are markedly
different in both catalyst systems. The significantly higher ∆Hq
for ethylene exchange with the cyclophane system leads to the
far slower exchange rates of ethylene, over 2 orders of
magnitude slower than the acyclic catalyst. This phenomenon
is most likely due to the increased steric blocking effect of the
cyclophane ligand. The large negative and nearly identical values
of ∆Sq for both systems confirm that the exchange of ethylene,
and most likely the other olefins as well, proceeds by the
associative ligand substitution mechanism. Consequentially, the
cyclophane ligand, with its axial steric bulk, greatly decreases
the rate of substitution processes.
Exchange Rates of Ethylene with Other Olefins. Olefin
equilibration was monitored by 1H NMR analysis in CD2Cl2 at
temperatures below olefin insertion rates. Ethylene binds to
Pd(II) so much more strongly than does MA that the
ethylene-MA equilibrium constant was unable to be determined
from a simple ethylene-MA pairwise experiment. As in
Brookhart’s study,15 4-methoxystyrene (MS) was employed as
an olefin of complexation strength between that of ethylene and
MA to aid in the measurement of the ethylene-MA equilibrium
constant. Attempted measurements of the equilibrium constants
for olefins at low temperature with the cyclophane catalyst,
however, were complicated by the extremely slow exchange
rates that were apparent. Several hours were often not sufficient
to reach full equilibrium, placing unreasonable demands on
instrument time. As an alternative strategy using the pseudo-
first-order approximation, the time-dependent plots of the
concentration of Pd-olefin species were fitted to first-order
exponential functions and the equilibrium concentrations and
exchange rate constants thereby derived.26,27 Details of the data
analysis are provided above in the Experimental Section, along
with our rationale in applying the pseudo-first-order approxima-
tion. The concentrations of species present in the NMR
experiments are also tabulated in the Supporting Information.
The equilibrium constants for the olefin pairs ethylene-MS
and MS-MA with the cyclophane system were calculated by
fitting the plots of Pd(II) complex concentration to eqs 4 or 5
(Figure 6), allowing for the determination of the concentrations
of the species at equilibrium. Fit correlations were of high
quality, with values of R2 at least 0.996 for all equilibrating
complexes. Evaluation of the exchange rate constants, in the
form of the sum k1 + k-1, was also performed by the fit. All
olefin equilibration data is compiled in Table 3. In contrast to
the cyclophane system, the acyclic system equilibrated too
rapidly for its progress to be effectively monitored and exchange
rate constants to be determined. Although an equilibrium
constant was obtained for the ethylene-MS pair, the Pd-Me
signals were obscured by ligand resonances in the MS-MA
experiment. Instead, the previously reported values using a
similar acyclic ligand are given for the MS-MA and the overall
ethylene-MA equilibria (see Table 3, footnote d).
While the calculated ethylene-MS equilibrium constant for
the cyclophane system was similar to that of the acyclic catalyst,
the MS-MA equilibrium constant was significantly higher.
From the product of Keq from both pairs (Table 3, entries 4 and
5), the ethylene-MA equilibrium constant was determined to
be 2.7 × 10-5 at 202 K. If ∆S of equilibrium is assumed to be
negligible, the equilibrium constant is then estimated to be 1.0
× 10-3 when extrapolated to 35 °C, or about 3-4 times higher
(favoring MA complexation) in the cyclophane catalyst than in
the acyclic catalyst. Although this favorable complexation of
MA could partially contribute to the larger incorporation of MA
by the cyclophane catalyst, the slow ligand exchange rates
deserve further consideration. The exchange rates for both olefin
pairs calculated from the fits were found to be extremely slow
for the cyclophane complex. Lastly, the rate of ethylene-MA
exchange, especially meaningful for the mechanistic analysis,
was also measured and found to be the fastest of the three
exchange processes (Table 3, entry 6, and Figure 6c).
Discussion
The applicability of the Curtin-Hammet treatment for the
cyclophane Pd(II) catalyst system given by eq 1 is in question
on the basis of the results of rate measurements for ethylene
exchange and for the ethylene-MA pair. Ethylene exchange
for the cyclophane catalyst is approximately 100 times slower
than the acyclic catalyst at 35 °C, but the discrepancy of the
rates of exchange with other olefins appears to be even larger.
To arrive at more meaningful energetic parameters and to permit
the estimation of exchange rates under realistic polymerization
conditions, approximate values for the second-order rate con-
stants of exchange processes were calculated. From eqs 2 and
3, both k2 and k-2 can be approximated as eqs 12 and 13. The
above-determined values of the equilibrium constant Keq,
rewritten in the form of eq 14, and use of the predetermined
sum k1 + k-1 as a boundary condition then permit the calculation
of k2 and k-2
.
k1
k2 ≈
(12)
[LB]
9
12390 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Copolymerizations with Pd(II) Cyclophane Catalysts
A R T I C L E S
k-1
insertion rates likely become faster at polymerization temper-
ature. Assuming an entropy of activation of -30 cal/mol*K
based on the results of the ethylene exchange experiments, the
free energy barrier of exchange at the polymerization temper-
ature of 35 °C were estimated. From these results, exchange
between ethylene and the other olefins appears significantly
slower than the exchange of ethylene, even in the favorable
reverse direction. For example, values of k-2 for ethylene-MS
and ethylene-MA, 4.6 s-1 M-1 and 12 s-1 M-1, respectively,
are less than the ethylene exchange rate constant of 63 s-1 M-1.
Steric hindrance as a result of the larger olefins is a likely factor
leading to reduced associative ligand exchange. Comparisons
between the estimated rate constants of exchange processes and
the olefin insertion rate constants previously determined are also
tabulated in Table 4. In addition, the actual rates were estimated
under typical copolymerization conditions using the given rate
expressions. These calculations suggest that the rate of
ethylene-MA exchange is of the same order of magnitude as
the rates of olefin insertion for the cyclophane system. These
rates are calculated under the assumption that the complexes
reach equilibrium as measured above. Since there is likely a
larger quantity of MA complex due to slow equilibration with
the ethylene complex, as elaborated below, the rates of MA
insertion with respect to ethylene insertion shown in Table 4
can be taken as a minimum value.
k-2
≈
(13)
(14)
[LA]
k2
k1[LA]
Keq
)
≈
k-2 k-1[LB]
Values of the second-order rate constants for the exchange
of ethylene-MS, MS-MA, and ethylene-MA pairs are shown
in Table 4. At low temperatures, exchange rates are fast enough
relative to migratory insertion that they can be measured without
complication. Because of the strongly negative entropy of
activation of associative ligand exchange, in contrast to insertion,
On the basis of the energetic values calculated in Table 4, a
free energy diagram of the copolymerization of ethylene and
MA is constructed (Figure 7). Whereas the overall energetic
landscape of the cyclophane system has some similarity to the
acyclic case, the cyclophane system has significantly larger
barrier to equilibration of the olefin comonomers. The activation
barrier for displacement of ethylene by MA is 20.6 kcal/mol.
On the basis of the presumptive 5-coordinate trigonal bipyra-
midal model of the transition state in associative ligand
exchange,31 the transition state structure 5 is proposed. Due to
the steric bulkiness of the cyclophane ligand, formation of the
5-coordinate transition state is very unfavorable, leading to much
higher activation energy.
By inspection of Figure 7, it is apparent that the fast-
equilibrium prerequisite for the Curtin-Hammett scenario does
not exist in the cyclophane system. In fact, our data suggest
that the activation energy of equilibration in either direction is
larger than both MA and ethylene insertion at polymerization
temperature, and that the transition state of ligand exchange is
likely the energetic maxima of the system. The barrier for
displacement by MA of ethylene adduct 4b is disfavored by
about 2 kcal/mol over ethylene insertion. This means that
complex 4b present will be directed toward insertion versus
MA displacement by a factor of kE/(k2[MA]) ≈ 7.2, under the
polymerization conditions of Table 1, entry 7. Regarding MA
complex 4c, the barriers to MA insertion and displacement by
ethylene are comparable (kA/(k-2[C2H4]) ≈ 1.4 under the same
conditions) meaning at least a substantial amount of 4c will
undergo MA insertion before displacement by ethylene.
The olefin complexes are being continuously created by the
complexation of free olefins to the alkyl agostic polymerization
intermediate 6 that results after each monomer insertion. As a
result, the olefin complexes form in amounts dependent on the
rates of ethylene versus MA trapping of the alkyl agostic
intermediate 6. To the best of our knowledge, rate constants
for the trapping of MA and ethylene have never been accurately
Figure 6. Plots and first-order exponential fits of Pd-olefin adduct
concentrations in equilibrium measurements of (a) MA and MS cyclophane
adducts 4c and 4d at 202 K, (b) ethylene and MS cyclophane adducts 4b
and 4d at 202 K, and (c) MA adduct 4c in equilibrium measurements with
ethylene at 196 K.
(31) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001,
123, 11539–11555.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12391

A R T I C L E S
Popeney and Guan
a
Table 3. Equilibration of Olefin Complexation in [(N∧N)PdMe(L)]SbF6
b c
,
kex (s-1
)
entry
ligand series
LA-LB
T (K)
Keq
∆G (kcal/mol)
d
1
2
3
4
5
6
3
3
3
4
4
4
MS-MA
178
202
178
202
202
196f
(1.8 ( 0.4) × 10-4
(7.1 ( 1.4) × 10-3
(1.3 ( 0.4) × 10-6
(3.0 ( 1.1) × 10-3
(9.0 ( 1.5) × 10-3
(2.7 ( 1.7) × 10-5
3.1 ( 0.1
2.0 ( 0.1
5.1 ( 0.2
2.3 ( 0.1
1.9 ( 0.1
4.2 ( 0.2
faste
fast
fast
C2H4-MS
C2H4 -MA
MS-MA
d
c
c
g
(1.0 ( 0.1) × 10-4
(6.8 ( 0.2) × 10-5
(2.6 ( 2.0) × 10-4
C2H4-MS
C2H4-MA
a
b
c Determined by first-order exponential fitting of the equilibration plots. d From ref 15
1
Determined by H NMR. Solvent ) CD2Cl2. kex ) k1 + k-1
.
for the catalyst bearing ligand ArNdC(H)-C(H)dNAr, Ar ) 2,6-i-Pr2C6H3. e Too fast to measure. f Temperature at which exchange rate was
determined. g Calculated from the product of entries 5 and 6.
Table 4. Comparison of the Rates of Insertion and Exchange Processes Related to the Copolymerization of MA and Ethylene by the
Cyclophane Catalysta
∆Gq (kcal/mol)
at 35 °Cb
typical rate at
polymerization conditionsc
process
rate constants at 35 °Cb
rate law expression
d
migratory insertion of ethylene
migratory insertion of MA
ethylene exchange
k1 ) 0.27 s-1
18.9 ( 0.8d
16.3 ( 0.6d
15.5 ( 0.4e
19.0 ( 1.0j
kE[Pd(C2H4)]
kA[Pd(MA)]
2k2[Pd(C2H4)][C2H4]
5.4 × 10-5 M/s
8.5 × 10-6 M/s
0.013 M/s
d
k1 ) 17 s-1
e
k2 ) 63 s-1 M-1
g h
ethylene-MS exchangef
k2 ) 0.21 s-1 M-1
k-2 ) 4.6 s-1 M-1
k2 ) 0.015 s-1 M-1
k2[Pd(C2H4)][MS] ) k-2[Pd(MS)][C2H4]
N/A
,
g i
,
ethylene-MA exchangef
20.6 ( 1.4j
k2[Pd(C2H4)][MA] ) k-2[Pd(MA)][C2H4]
7.0 × 10-6 M/s
g h
,
k-2 ) 12 s-1 M-1g i
,
a Conditions of Table 1, entry 7: [Pd] ) 0.20 mM, [C2H4] ) 1 M at 88 psi (ref 15), [MA] ) 2.5 M. b Calculated by extrapolation to 35 °C from low
temperature 1H NMR experimental measurements. k1 and k2 designate rate constants of first- and second-order processes, respectively. c Under
equilibrium conditions. Rate expressions written for the disappearance of equilibrating Pd complex, e.g. in eq 2, or inserting Pd complex. d Extrapolated
from activation parameters presented in Table 1 of ref 22. e Extrapolated from activation parameters calculated from the results of EXSY NMR
experiments. f Displacement of ethylene denotes the forward direction. g Estimated from equilibrium constants and pseudo-first-order rate constants found
in Table 3. h Forward rate constant. i Reverse rate constant. j Barrier in forward direction.
Figure 7. Free energy diagram of ethylene/MA copolymerizations at 35 °C by the cyclophane catalyst. Energetic values are given in kcal/mol.
measured but must be very fast. Spectroscopic studies routinely
detect the alkyl olefin complexes such as 4b and c, but the
agostic intermediates 6 are never observed in the presence of
free olefins. Their energies are normally shown as being far
higher than that of the olefin complexes, which are considered
to be the polymerization resting states.15,19,31 In computational
studies by Ziegler and co-workers, for example, the complex-
ation of ethylene to the agostic intermediate 6 was calculated
to be favored by 16.4 kcal/mol.16 From Hammond’s postulate,
the transition states of ethylene and MA olefin trapping will
more closely resemble the structure and energy of the common
intermediate 6 than the respective olefin complexes; thus, the
energy barriers and rate constants to trapping should be
approximately the same for both olefins (Figure 8). Because
the concentrations of free olefins in solution will affect the
trapping rates, comonomer loading will have a large effect on
the concentrations of the olefin complexes.
Because of the smaller energy difference in the trapping barriers
and the slow exchange of olefins for the cyclophane catalyst, the
MA complex will be generated in amounts larger than would be
9
12392 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Copolymerizations with Pd(II) Cyclophane Catalysts
A R T I C L E S
(2) Ethylene exchange, monitored by two-dimensional EXSY
NMR methods, was found to proceed over 100 times more slowly
than with the acyclic catalyst. From determination of the temper-
ature dependence on ethylene exchange, the entropy of activation
was found to be almost the same for the two catalysts. The value
of -33 cal/mol*K was consistent with an associative mechanism
of olefin exchange. The enthalpy of activation of the cyclophane
catalyst, however, was found to be 2.8 kcal/mol higher than the
acyclic catalyst. This increase in the barrier height was attributed
to the increased steric bulk of the cyclophane ligand, which
obstructs access of the incoming ligand to the axial coordination
sites on the metal.
(3) Dramatically reduced equilibration rates of cyclophane-
bound olefin adducts were observed. The rate constants of
exchange of ethylene with other olefins, most importantly MA,
were determined by exponential curve fitting to the time-
dependence of equilibrating olefin concentrations. The pseudo-
first-order rate constants for the displacement of ethylene by
MA were calculated to be 0.015 and 12 s-1 M-1 at 35 °C in the
forward and reverse directions, respectively. Rates of ligand
exchange are diminished to be on the order of the monomer
insertion rates. This observation is also attributed to the steric
bulk of the cyclophane ligand blocking the metal axial sites,
inhibiting associative ligand substitution.
(4) Equilibrium constants of ethylene and MA complexation
were also determined by exponential fitting to the olefin concentra-
tion curves. The complexation of MA was favored by 3 to 4 times
in the cyclophane catalyst as compared to the acyclic catalyst.
Although this would normally be expected to contribute to the
relatively high incorporation of MA in the cyclophane catalyst,
the slow equilibration rates of the cyclophane catalyst make the
Curtin-Hammet scenario inapplicable to this system.
(5) The second-order rate constants and energy barriers for
the exchange of ethylene with MA were calculated following
the pseudo-first-order approximation. Exchange barrier heights
were found to be similar to or to even exceed the insertion
barrier heights.
The loss of the prerequisite of fast monomer equilibration relative
to monomer insertion thus renders the Curtin-Hammett ap-
proximation invalid for the cyclophane system. The net result is
significant loss of selectivity of the catalyst for the preferential
incorporation of strongly binding olefins. Instead, the relative
trapping rates of MA and ethylene to the alkyl agostic polymeri-
zation intermediate are believed to factor significantly in the
resulting comonomer incorporation ratios. These results have
successfully demonstrated the control of the composition of polar
olefin copolymers by kinetic regulation of polymerization processes
using a ligand with a unique cyclophane structure.
Figure 8. Free energy diagram of the trapping of ethylene and MA by
alkyl agostic intermediate 6.
permitted by olefin equilibrium (∆∆Gq < ∆G, Figure 8). This
situation contrasts dramatically with that of the acyclic catalyst,
where the concentrations of the olefin complexes should always
exist at their equilibrium levels because of rapid exchange. In the
so-called kinetic quenching scenario,32 the opposite condition from
the Curtin-Hammet scenario in which insertion is much faster
than exchange, the incorporation of acrylate is dependent solely
on the ratio kA/kE. In the cyclophane system, however, the exchange
rates are likely within an order of magnitude of the insertion rates,
constituting an intermediate situation in between the Curtin-
Hammett and kinetic quenching scenarios. This intermediate case
cannot be treated by a simple analytical solution.
The acyclic catalyst exhibits high selectivity toward ethylene
insertion because of the greatly favored complexation of ethylene
over acrylate comonomers. In the cyclophane-based catalyst,
however, this selectivity is dramatically reduced, allowing for the
insertion of monomers that bind weakly to the metal. Because of
the small difference in olefin trapping barriers and the large barrier
for olefin exchange, enrichment in acrylate complex concentration
above what would be permitted by relative olefin binding strengths
occurs. Since olefin exchange rates are less than insertion rates,
the concentrations of olefin complexes are unable to return to their
equilibrium concentrations. Therefore, a disproportionately large
amount of acrylate complex undergoes insertion without the
establishment of equilibrium. In this manner, the structure of the
cyclophane ligand is able to kinetically control comonomer
incorporation ratios.
Conclusions
The copolymerization of ethylene with methyl and tert-butyl
acrylate using the cyclophane-based Pd(II) R-diimine catalyst led
to dramatically higher polar monomer incorporation compared to
what was observed with the acyclic catalyst. A detailed mechanistic
investigation was carried out to determine the cause of this unusual
behavior. Efforts were devised to analyze the applicability of the
existing Curtin-Hammett fast olefin exchange scenario for acrylate
incorporation with the cyclophane catalyst. Key findings from
mechanistic studies of the cyclophane-based catalyst are as follows:
(1) In ligand binding studies, a large preference for the
complexation of nitriles compared to that of olefins was observed.
This effect was attributed to the steric bulk of the cyclophane ligand
disrupting the binding orientation of olefins and thus destabilizing
η2-π-type olefin complexes as compared to η1 σ-donating complexes.
Acknowledgment. We thank the National Science Foundation
(Chem-0456719, Chem-0723497, DMR-0703988) for funding. C.P.
acknowledges the Allergan Graduate Fellowship, the UCI Depart-
ment of Chemistry, and Joan Rowland for financial support. We
also thank Dr. Phil Dennison for valuable NMR assistance and Prof.
Keith Woerpel and Dr. Fiona Lin for helpful discussions.
Supporting Information Available: Details of the synthesis
and characterization of complexes, data on EXSY experiments
on 4b at various mixing times, and concentration data for olefin
equilibration experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
(32) Seeman, J. I. Chem. ReV. 1983, 83, 83–134.
JA904471V
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12393