Effect of ligand electronics on the stability and chain transfer rates of substituted Pd(II) α-diimine catalysts (1)

Article abstract of DOI:10.1021/ma100220n

The effect of electron-donating and -withdrawing groups on the ligands of Pd(II) α-diimine olefin polymerization catalysts on catalyst stability, activity, and polymer molecular weight is investigated. The polyethylene molecular weight and the productivit

Full text of DOI:10.1021/ma100220n

Macromolecules 2010, 43, 4091–4097 4091
DOI: 10.1021/ma100220n
Effect of Ligand Electronics on the Stability and Chain Transfer Rates of
Substituted Pd(II) R-Diimine Catalysts1
Chris S. Popeney and Zhibin Guan*
Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697
Received January 28, 2010; Revised Manuscript Received March 30, 2010
ABSTRACT: The effect of electron-donating and -withdrawing groups on the ligands of Pd(II) R-diimine
olefin polymerization catalysts on catalyst stability, activity, and polymer molecular weight is investigated.
The polyethylene molecular weight and the productivity of catalysts bearing substituted bis-
(aryl)dimethyldiazabutadiene (Me2DAB) and bis(aryl)acenaphthenequinonediimine (BIAN) ligands were
analyzed over time at room temperature and 40 °C to monitor catalyst stability and chain transfer processes.
The introduction of electron-donating groups led to a dramatic increase in polymer molecular weight, with
polymer chains still growing after 24 h of polymerization. The amino-substituted Me2DAB analogue
afforded polymer of more than twice the molecular weight compared to the polymer made with the
unsubstituted analogue after 24 h of polymerization. The unsubstituted catalysts and those bearing
electron-withdrawing groups, however, reached a maximal molecular weight, generally lower, after a
comparatively short time, which was presumably due to higher chain transfer rates. Electron-donating
groups also provided increased stability to the catalysts leading to longer catalyst lifetimes. Both of these
effects are likely due to stabilization of the reactive, electron-deficient, and coordinatively unsaturated alkyl
agostic intermediate, the reactivity of which is key to both chain transfer and decomposition processes.
Introduction
electronics, which are widely known to play a substantial role in
various organometallic reactions.23-25 Recent reports have de-
Since the discovery of cationic Ni(II) and Pd(II) catalysts
15 years ago,2 the development of late-transition-metal cata-
lysts for the polymerization of olefins has been spurred by their
high functional group tolerance and their ability to incorporate
useful comonomers.2-9 These systems, the cationic Ni(II) and
Pd(II) R-diimine catalysts in particular, have also become especi-
ally attractive due to their ability to afford novel branched
polymers in a controllable manner. While research in neutral
Ni(II) and Pd(II) catalysts has recently taken off due to further
improvements on functional group tolerance and thermal stabi-
lity,10-15 the original cationic Ni(II) and Pd(II) R-diimine cata-
lysts are the subjects of a large body of work. The Pd(II) catalysts
are particularly well-studied because of their unique ability to
introduce chain branching, giving rise to varied polymer archi-
tectures that have made them subjects of numerous mechanistic
investigations.6,16-20 Because of this ability to control polymer
topology and the aforementioned tolerance of polar olefins,
the Pd(II) R-diimine catalysts have been employed in the con-
struction of complex polymeric structures in a simple, one-pot
syntheses.9,21,22
Despite these special attributes, these catalysts suffer from
relatively low thermal stability, as evidenced by their rapid
decomposition and drastically reduced polymer molecular weight
at temperatures above 50 °C.16 Furthermore, polymerization
under low olefin concentrations or in the presence of polar
comonomers leads to greatly reduced catalytic activities. The
combination of low thermal stability and activity at desired
conditions hinders further development of this family of catalysts
and advocates the need to develop improved catalyst systems.
Previous research on Pd(II) R-diimine catalyst performance
has focused primarily on ligand structure as opposed to ligand
scribed the placement of substituents onto R-diimine ligands and
their effect on the σ-donating ability to Pd(II) and thus the Lewis
acidity of the metal.26,27 In response, we carried out an investiga-
tion of the polymerization behavior of a series of Pd(II) R-diimine
catalysts bearing substituents in the para-aryl position on the
ligand aromatic rings.28 We discovered that this substitution was
able to perturb the metal electronics in a regular manner, as
evidenced by the remarkably linear correlation observed in the
stretching frequencies of the corresponding carbonyl complexes.
Furthermore, several trends were observed in the physical prop-
erties of the resulting polymers. The introduction of substituents
of increasing electron-donating ability led to catalysts that
afforded polymers of higher molecular weight, slightly reduced
branching density, and a corresponding shift toward less den-
dritic polymer topologies. In the case of copolymerizations with
methyl acrylate, a clear and pronounced trend of increasing
acrylate incorporation ratio and overall tolerance to polar func-
tionality was also observed over the range of substitution explored.
In contrast to the significant trends previously described, the
effect of substitution on other catalytic properties such as activity
and thermal stability were difficult to discern with the data then
available.28 Since then, more detailed experiments have been
performed on the substituted R-diimine complexes of types 1 and
2 (Chart 1) which we now describe herein. Particular emphasis of
the new study was placed on the elucidation of trends in catalyst
thermal stability through careful observation of polymerization
over time. Further analysis on the dependence of polymer
molecular weight (MW) on substitution will also be presented.
Results
The Pd(II) complexes 1a-g bearing R-diimine ligands with
backbone methyl substituents, the bis(aryl)dimethyldiazabutadiene
*Corresponding author. E-mail: zguan@uci.edu.
r 2010 American Chemical Society
Published on Web 04/14/2010
pubs.acs.org/Macromolecules

4092 Macromolecules, Vol. 43, No. 9, 2010
Chart 1. Aryl-Substituted Pd(II) r-Diimine Complexes
Popeney and Guan
Scheme 1. Synthesis of Diimines 3a and 3e and Pd(II) Complexes 2a and 2e
Table 1. Properties of Polyethylene Produced at 22 °Ca
Mn (kg/mol)b
Table 2. Properties of Polyethylene Produced at 40 °Ca
Mn (kg/mol)b
catalyst
24 h Mw/Mn Rg (nm)d Be lifetime (h)f
catalyst
24 h Mw/Mn Rg (nm)d Be lifetime (h) f
c
c
entry catalyst 3 h
entry catalyst 3 h
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
2a
2d
2e
220
192
193
225
241
332
170
39.4
32.8
30.4
627
433
425
253
261
400
178
70.4
43.5
34.0
1.25 27.2 ( 1.1 95.5
1.31 23.2 ( 2.1 95.9
1.34 23.0 ( 2.5 95.3
1.15 15.1 ( 4.1 95.2
1.17 14.2 ( 4.1 96.7
1.18 20.3 ( 4.7 96.7
1.50 13.9 ( 3.5 100.3
1.28 10.0 ( 6.8 103.9
>24
24
24
>24
12
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
2a
2d
2e
231
246
255
230
249
195
24.6
19.1
17.0
234
275
271
252
256
195
24.2
19.3
19.6
1.39 18.9 ( 3.8 94.4
1.33 19.5 ( 2.7 96.8
1.36 19.1 ( 2.3 95.8
1.37 19.0 ( 2.5 94.6
1.32 18.8 ( 1.9 97.3
1.28 13.9 ( 3.8 97.1
6
12
12
6
3
3
6
24
12
6
3
1.27 N/Ag
1.24 N/Ag
1.21 N/Ag
104.1
107.1
106.7
>24
>24
>24
1.31 N/Ag
1.37 N/Ag
103.8
106.3
a Initial polymerization conditions: 5.0 μmol of catalyst and 1.5 equiv
of NaBAF in 25 mL of 3:1 toluene/chlorobenzene, 1 atm of ethylene
pressure. Aliquots of 5 mL of the polymerization mixture were taken at
3, 6, and 12 h, and the remaining mixture was collected at 24 h.
b Number-averaged molecular weight at 3 and 24 h aliquots obtained
from SEC-MALLS. c Polydispersity obtained from SEC-MALLS for
24 h aliquots. d Radius of gyration obtained from MALLS for 24 h
aliquots. e Polymer branching density per 1000 carbons obtained from
1H NMR for 24 h aliquots. f Catalyst lifetimes estimated from Figure 2.
g Rg too small for accurate measurement by MALLS.
10
a Initial polymerization conditions: 5.0 μmol of catalyst and 1.5 equiv
of NaBAF in 25 mL of 3:1 toluene/chlorobenzene, 1 atm of ethylene
pressure. Aliquots of 5 mL of the polymerization mixture were taken at
3, 6, and 12 h, and the remaining mixture was collected at 24 h.
b Number-averaged molecular weight at 3 and 24 h aliquots obtained
from SEC-MALLS. c Polydispersity obtained from SEC-MALLS for
24 h aliquots. d Radius of gyration obtained from MALLS for 24 h
aliquots. e Polymer branching density per 1000 carbons obtained from
1H NMR for 24 h aliquots. f Catalyst lifetimes estimated from Figure 1.
g Rg too small for accurate measurement by MALLS.
(Me2DAB) ligands, and a series of aryl substituents ranging from
strongly donating amino to withdrawing nitro groups have been
described previously.28 Since we frequently work with catalysts
derived from R-diimine ligands bearing the aromatic bis-
(aryl)acenaphthenequinonediimine (BIAN) backbone, we also
prepared in this study two new substituted BIAN analogues 2a
and 2e, bearing amino and chloro substitution, respectively, to
accompany the existing unsubsituted complex 2d. The BIAN
ligands 3a and 3e were prepared by condensation of acenaphthe-
nequinone with the appropriate substituted aniline in metha-
nol and catalytic formic acid (Scheme 1).29 The corresponding
Pd(II) complexes were then prepared by displacement of 1,5-
cyclooctadiene (COD) from the precursor Pd(COD)MeCl.30
Polymerizations were carried out at 1 atm of ethylene pressure
in toluene-chlorobenzene mixtures by direct in situ activation of
the methyl chlorides 1a-g and 2a,d,e by the addition of an excess
of sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF)
to solutions of the complexes.31 To accurately monitor catalyst
activity, thermal stability, and polymer chain growth, aliquots of
known volume were periodically taken from the polymerization
mixtures. Catalyst productivities were determined by measure-
ment of polymer yield, and polymer properties were then deter-
mined as a function of time to examine the time-dependent
behavior of the catalysts. Details of the polymerizations can be
found in Tables 1 and 2.
Catalyst Thermal Stability. The results of the catalyst
activity and stability analysis at room temperature (22 °C)
for catalysts 1a-g bearing the Me2DAB series ligands are
shown in Figure 1a as a collection of catalyst productivity, in
terms of polymer mass per mole of catalyst, versus time
profiles. The trends in catalyst activity and stability over
the ligand substitution series were complicated by the exist-
ence of substituent-specific effects. As previously reported,28
the nitro-substituted catalyst 1g was deactivated almost
immediately after the initiation of polymerization. This
instability might be attributed to a side reaction facilitated
by the nitro group involving prompt reduction of Pd(II) to
palladium black, which could be seen in the reaction mixture
after a short time. The dimethylamino-substituted catalyst
1a and unsubstituted catalyst 1d were the most active and

Article
Macromolecules, Vol. 43, No. 9, 2010 4093
Figure 1. Plots of catalyst productivity vs time at 22 °C for (a) Me2DAB series catalysts 1a-g and (b) BIAN series catalysts 2a,d,e.
Figure 2. Plots of catalyst productivity vs time at 40 °C for (a) Me2DAB series catalysts 1a-g and (b) BIAN series catalysts 2a,d,e.
stable at room temperature, exhibiting productivities that
were still increasing after 24 h. Among the other catalysts, Cl-
and CF3-substituted catalysts 1e,f showed high activity
initially but appeared to undergo complete deactivation
within 6-10 h while catalysts 1b,c were still active after 24 h.
Thus, the following stability trend can be summarized for
substituted Me2DAB catalysts at room temperature: NO2 ,
CF3 ∼ Cl < Me ∼ OMe < H ∼ NMe2. The BIAN-based cata-
lysts (Figure 1b) were found to be roughly half as active at room
temperature as the Me2DAB-based catalysts, in agreement
with published reports of ethylene homopolymerizations.4 All
three BIAN catalysts 2a,d,e gave results comparable to one an-
other and appeared to have higher stability in general than the
Me2DAB-based systems. For clarity, tables of the actual pro-
ductivities are given in Table S1 of the Supporting Information.
The sensitivity of the catalysts to temperature was inves-
tigated by conducting analogous experiments at 40 °C. The
low thermal stability of NMe2-substituted catalyst 1a at
40 °C was particularly striking. The reasons for the rapid
deactivation of 1a at elevated temperature are unclear,
although it will be seen that this is a general phenomenon
for amino-substituted catalysts. Otherwise, the same trend in
stability was observed. On the basis of the data shown in
Figure 2a and with the amino catalyst excluded, the follow-
ing trend of stability at 40 °C can be generalized for the
Me2DAB series catalysts: NO2 , CF3 ∼ Cl < H ∼ Me ∼
OMe. Most of the Me2DAB series of catalysts appear to be
deactivated after ∼10 h of polymerization, and none of them
remained active after 24 h of polymerization at 40 °C. The
high productivity seen after short polymerization times is
evidence of initial activities that are higher than at room
temperature, although only the most stable catalysts 1b and
1c were stable for long enough to have higher overall
productivities after a 24 h polymerization period. Among
the BIAN series catalysts (Figure 2b), catalyst 2a, like 1a,
deactivated quickly at 40 °C, presumably by the same
process. On the other hand, 2d and 2e exhibited higher
thermal stability compared to their Me2DAB analogues.
For example, catalyst 2e was significantly more stable than
1e, an effect that likely resulted from the rigidity of the planar
acenaphthyl backbone (see below). The productivities are
tabulated in Table S2 of the Supporting Information.
The measurement of productivity after short reaction
times permitted the analysis, although with limited accuracy,
of intrinsic catalyst activity. At 22 °C, the Me2DAB series
catalysts exhibited turnover numbers after 3 h that varied by
as much as a factor of 2 (Figure 1a) if NO2-substituted 1g is
excluded. The electron-deficient catalysts 1e and 1f seemed
more active than the electron-rich analogues 1b and 1c. The
CF3-substituted catalyst 1f appeared to have the highest
initial activity at 22 °C, which was further supported by the
high polymer MW afforded by the catalyst after short
reaction times (see below). Curiously, substitution had little
effect on the apparent catalyst activity of the three BIAN
catalysts until the temperature was increased to 40 °C, upon
which a similar trend could be seen. For the Me2DAB series
at higher temperatures the opposite trend was observed: the
electron-rich catalysts (aside from amino-substituted 1a)
exhibited higher activities. This effect, contrary to what
was seen at room temperature, probably stemmed from the
low thermal stabilities of the electron-poor catalysts and
their appreciable decomposition at 40 °C after only 3 h, lead-
ing to a detrimental effect on catalyst productivity. Because
the observed productivity is dependent on both the inherent

4094 Macromolecules, Vol. 43, No. 9, 2010
Popeney and Guan
Figure 3. Plots of number-averaged MW vs time at 22 °C for (a) Me2DAB series catalysts 1a-g and (b) BIAN series catalysts 2a,d,e.
activity and stability of the catalysts, it is difficult to decon-
volute and separately analyze the two individual properties
from the current experimental data.
(Scheme 2). This highly electrophilic agostic complex is presumed
to undergo C-H activation of the diimine isopropyl groups in the
2,6-aryl positions, affording palladacycle 6. Electron-donating
R-diimine ligands could stabilize intermediate 5 by making the
metal center less reactive toward C-H activation. Intuitively, this
process should become more favorable as metal electrophilicity
increases.
Polymer Molecular Weight. The MW of polyethylene obta-
ined by the catalysts in the polymerizations above was mea-
sured by multiangle laser light scattering (MALLS) analysis
after separation by gel permeation chromatography (GPC).
A significant dependence on substituent was observed in the
data with an overall trend of higher MW polymer produced
from catalysts bearing more electron-donating ligands. As shown
in Table 1, polymers of high number-averaged molecular
weight (Mn) were produced with the electron-rich Me2DAB
catalysts 1a-c (entries 1-3). Catalyst 1a afforded polymer
of more than double the Mn of the unsubstituted catalyst 1d
(entry 4) after a polymerization period of 24 h at room tem-
perature. The BIAN catalysts afforded lower MW polymer
than the Me2DAB catalysts because of an increased rate of
chain transfer (entries 8-10), which is in agreement with pre-
vious studies,6,16 although the same trend was evident. The
Mn of polymer produced by 2a was nearly twice that of catalyst
2d after 24 h of polymerization. Polymer chain growth behavior
at 22 °C can be seen in the MW vs time profiles in Figure 3.
Further discussion of polymer MW can be found in the follow-
ing section.
At 40 °C, Mn peaked early during polymerizations with all
the catalysts as a result of increased chain transfer and
catalyst decomposition rates (Table 2). Overall, there was
less influence from the para-aryl substituents at this tem-
perature, and the R-diimine backbone substituent instead
became the most dominant structural feature controlling
MW. Although both chain transfer and deactivation pro-
cesses were expected to become more prevalent at higher
temperature, catalyst deactivation was likely more signifi-
cant among Me2DAB catalysts than BIAN catalysts due to
the lower stability of the former.
In the literature, the reported electronic effects on C-H acti-
vation reactivity are complex and seem case dependent.23,32,33 It
is thus difficult to rationalize the trends regarding electronic
effects on the stability of Pd(II) R-diimine polymerization cata-
lysts based on existing examples. It is known that ligand steric
effects play a significant role in complex stability. For example,
catalysts with smaller 2,6-dimethyl groups on the aryl rings are
known to show increased stability compared to the isopropyl-
substituted analogues described herein because access of the
smaller 2,6-dimethyl groups to the metal is restricted.16 The
rigidity of the diimine backbone also imparts a positive effect
on stability for this reason,34 at least partly explaining the higher
stabilities of the BIAN series complexes when compared to the
Me2DAB complexes. From these observations, it is possible that
the substitution of electron-donating groups could decrease the
favorability of C-H activation by perturbing the ligand geome-
try to limit the close approach of the metal to the ligand axial
alkyl moieties. It has been observed in R-diimine complexes,
however, that the Pd-N coordination strength has little influence
on metal-ligand bond lengths,35 likely resulting in little change in
complex geometry as substituents are introduced. Finally, elec-
tron-deficient R-diimine ligands should be more prone to decom-
plexation, which could provide an additional route to catalyst
decomposition. To the best of our knowledge, this route of
deactivation has not been explored.
The ultimately reachable polymer MW is dependent on three
distinct processes, in which polymer chain growth (catalyst
activity) is offset by the negative effects of chain transfer and
catalyst decomposition. The relative importance of each of these
rates onMW control canbedetermined for each catalyst fromthe
above thermal stability and MW data. The stabilities of all the
catalysts can be estimated from the approximate time of complete
catalyst deactivation and is shown in Tables 1 and 2 as “catalyst
lifetime”. In the first case, catalysts 1d, 2d, and 2e (Table 1, entries
4, 9, and 10), although quite stable at 22 °C, nevertheless
exhibited little increase in Mn after an initial period of polymer
growth. Chain transfer was thus concluded to be an important
chain termination process leading to maximal MW after short
reaction times. On the other hand, catalyst deactivation can be
concluded as being significant among the less robust electron-
deficient catalysts 1e-g (Table 1, entries 5-7) in addition to
chain transfer. Nitro-substituted catalyst 1g represents an ex-
treme example in which decomposition occurred completely
Discussion
Results from these polymerization experiments lend further
support to the previously identified role of ligand electronics on
catalyst stability. The presence of electron-donating groups on
the R-diimine ligands has an overall stabilizing effect on the
catalysts and slows the rate of deactivation processes (with
exception of the dimethylamino-substituted catalyst at high
temperatures). Although the decomposition pathways are still
not well understood, detailed mechanistic studies by Brookhart
have detected the products of C-H activation of the ligand with
coordinatively unsaturated Pd(II) intermediates.16 This inter-
mediate is most likely alkyl agostic complex 5, which forms after
migratory insertion of bound olefin in alkyl olefin complex 4

Article
Macromolecules, Vol. 43, No. 9, 2010 4095
Scheme 2. Catalyst Decomposition via C-H Activation of Agostic Complex 5
Scheme 3. Possible Chain Transfer Pathways in Pd(II) r-Diimine-Catalyzed Polymerization
during the initial rapid chain growth phase, leading to small
amounts of low-MW polymer of high polydispersity. Although
chain growth with CF3-substituted catalyst 1f was also of short
duration, it produced polymers of the highest MW for 3 h
polymerization time. This suggests that the intrinsic activity of
the catalyst was higher than that of the others and able to offset
the higher chain transfer or decomposition rates. Similar activity
enhancement appeared to take place with chloro-substituted 1e
as well, albeit to a lesser extent. Lastly, catalysts 1a-c and 2a were
not only stable over the course of the polymerization runs but
also afforded polymers of steadily increasing Mn during that time
(Table 1, entries 1-3 and 8, and Figure 3). These electron-rich
catalysts exhibited both low chain transfer and low catalyst
decomposition rates, thereby giving polymers of high MW after
sufficient polymerization time.
The positive effect on polymer molecular weight from the
presence of an electron-rich R-diimine ligand can be attributed to
two effects. First, the stability of the catalyst itself is increased,
which allows for a longer active polymerization time. In addition,
the chain transfer rate is appreciably reduced, resulting in a longer
residence time of a single polymer chain on the metal. The means
by which substitution affects the chain transfer rate remains
unclear, although some insight can be gained by examination of
the polymerization mechanism, represented in Scheme 3. The two
most important species in the catalytic cycle are the alkyl olefin
complex 4, the catalyst resting state, and the coordinatively
unsaturated alkyl agostic insertion product 5. The alkyl agostic
complex can undergo a wide range of transformations, including
β-hydride elimination to olefin hydride complex 7. As proposed
by Brookhart and co-workers,4,5,16 chain transfer in the Pd(II)
R-diimine system proceeds by an associative mechanism by which
the axial approach of ethylene displaces the growing polymer
chain.16-18 A more strongly electron-donating ligand could
stabilize the electron-deficient agostic complex 5 and reduce the
rate of β-hydride elimination and formation of 7 and subsequent
chain transfer. In fact, a small decrease in the polymer branching
density B (branches per 1000 carbons, Tables 1 and 2) of about
5% from electron-deficient catalyst 1g to 1a supports this
hypothesis, since the chain walking mechanism responsible for
branching requires the formation of olefin hydride complexes like
7. In addition, a more strongly electron-donating ligand could
stabilize the cationic intermediates such as 4 and 7, making the
coordination of the incoming ethylene more difficult. In other
words, the dz2 orbital of the Pd(II) is pushed to higher energy by a
more strongly donating ligand, decreasing its tendency to co-
ordinate incoming ethylene.
Computational studies by Morokuma and co-workers have
suggested that the barrier to hydride reinsertion in complex 7 is
extremely low (less than 1 kcal/mol), and thus this intermediate
exists only briefly.36,37 Further theoretical calculations by Ziegler
and co-workers have proposed that the alkyl ethylene complexes
4 themselves may be prone to chain transfer by a concerted
hydride shift from polymer chain to bound ethylene (alkyl olefin
complexes 4, 40, and 400 are all capable of undergoing chain

4096 Macromolecules, Vol. 43, No. 9, 2010
Popeney and Guan
transfer as long as the alkyl group contains a β-hydrogen).38,39
Since these computational models also predict a transition state
geometry with ligands in the axial sites, electronic changes to
the ligands should affect this mechanism in a manner similar to
the more traditional associative chain transfer mechanism dis-
cussed above. Because neither proposed mechanism has been
disproved, both remain potential routes for chain transfer in the
Pd(II) R-diimine polymerization system and deserve consideration.
complexes,4,5 2,6-diisopropyl-4-(dimethylamino)aniline,41 4-chloro-
2,6-diisopropylaniline and all substituted Me2DAB series li-
gands and Pd complexes,28 Pd(COD)MeCl,42 and sodium
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate43 (NaBAF) were
prepared by known literature procedures. Chlorobenzene
(Aldrich, anhydrous) was used as received.
ArNdC(An)-C(An)dNAr, Ar ꢀ 2,6-iPr2-4-NMe2C6H2
(
NMe2NNAn
, 3a). To a suspension of 2,6-diisopropyl-4-
(dimethylamino)aniline (1.497 g, 6.79 mmol) and acenaphthe-
nequinone (0.557 g, 3.056 mmol) in 20 mL of methanol was
added 0.5 mL of formic acid. The mixture was heated to reflux
with stirring for 4 h, cooled, and filtered. The solid was washed
with cold methanol and dried in vacuo to afford 3a as a purple
solid (1.566 g, 79%). 1H NMR (500 MHz, CDCl3): δ 0.95
(d, 12H, J = 6.8 Hz), 1.22 (d, 12H, J = 6.7 Hz), 2.90-3.15
(m, 16H), 6.72 (s, 4H), 6.78 (d, 2H, J=7.2 Hz), 7.36 (t, 2H, J=
7.6 Hz), 7.84 (d, 2H, J=8.2 Hz). 13C NMR (125 MHz, CDCl3):
δ 23.41, 23.57, 29.0, 41.6, 109.0, 123.4, 128.02, 128.63, 129.98,
131.26, 136.4, 139.40, 140.76, 148.3, 162.1. HR-MS calcd for
[C40H50N4 þ H]þ: 587.4114. Found: 587.4127.
Conclusions
A series of Pd(II) R-diimine polymerization catalysts posses-
sing a range of electron-donating and -withdrawing groups
were examined to probe the effects of substitution on catalyst
activity, thermal stability, and polymer chain growth. The poly-
mer yield and polymer molecular weight were analyzed at fixed
time increments during polymerization to monitor the catalytic
behavior and obtain insight on the relative importance of chain
transfer and catalyst decomposition. Several catalysts bearing
substituted Me2DAB ligands bearing dimethyl backbones and a
few BIAN-type catalysts with rigid aromatic backbones were
studied.
Limited information was obtained regarding intrinsic catalyst
activity that indicated an increase in activity with more electron-
withdrawing substituents. A far clearer trend was observed in
catalyst stability, in which catalysts bearing electron-donating
groups underwent deactivation far more slowly than their elec-
tron-deficient analogues. The amino-substituted catalysts did not
follow these trends at higher temperatures, becoming dramati-
cally less stable under such conditions for unknown reasons.
Overall, the increased stability of the electron-rich catalysts likely
results from stabilization of the reactive electrophilic and co-
ordinatively unsaturated alkyl agostic intermediates.
A pronounced dependence of polymer MW on the ligand
electronic properties was also observed. Catalysts bearing stron-
ger electron-donating groups were able to produce polymer of
increasing Mn at prolonged reaction time. Therefore, it was con-
cluded that these catalysts are in fact more resistant to chain
transfer than the unsubstituted catalysts or analogues bearing
electron-withdrawing groups. Presumably, the stronger electron-
donating character of the ligands of these catalysts stabilizes the
alkyl agostic catalytic intermediate, inhibiting β-hydride elimina-
tion and subsequent chain transfer.
ArNdC(An)-C(An)dNAr, Ar ꢀ 2,6-iPr2-4-ClC6H2 (ClNNAn
,
3e). To a suspension of 4-chloro-2,6-diisopropylaniline (2.34 g,
11.1 mmol) and acenaphthenequinone (0.906 g, 4.97 mmol) in
30 mL of methanol was added 1.0 mL of formic acid. The
mixture was allowed to stir at room temperature overnight and
was filtered, and the solid was washed with cold methanol. The
solid was recrystallized in 10:1 ethanol/chloroform and dried in
vacuo to afford 3e as an orange solid (1.736 g, 61%). 1H NMR
(500 MHz, CDCl3): δ 0.96 (d, 12H, J=6.8 Hz), 1.22 (d, 12H, J=
6.8 Hz), 2.98 (septet, 4H), 6.78 (d, 2H, J=7.2 Hz), 7.25 (s, 4H),
7.44 (t, 2H, J=7.6 Hz), 7.93 (d, 2H, J=8.2 Hz). 13C NMR
(125 MHz, CDCl3): δ 23.07, 23.40, 29.0, 123.64, 124.12, 128.24,
129.33, 129.58, 129.97, 131.4, 137.7, 141.1, 146.0, 161.6. HR-MS
calcd. for [C36H38N2Cl2 þ H]þ: 569.2490. Found: 569.2494.
(
NMe2NNAn)PdMeCl (2a).To a solution of 3a (0.880 g, 1.50 mmol)
in 40 mL of CH2Cl2 was added 398 mg of Pd(COD)MeCl
(1.50 mmol), and the solution was stirred for 3 h at room tempera-
ture. The mixture was concentrated, and the residue was dried
in vacuo overnight to afford 2a as a dark green solid (1.080 g,
1
97%). H NMR (400 MHz, CDCl3): δ 0.85 (s, 3H), 0.90 (d, 6H,
J=6.9 Hz), 0.95 (d, 6H, J=6.9 Hz), 1.37 (d, 6H, J=6.8 Hz), 1.47 (d,
6H, J=6.7 Hz), 3.05 (s, 6H), 3.07 (s, 6H), 3.39 (septet, 4H, J=6.8
Hz), 6.66 (d, 1H, J=7.2 Hz), 6.70 (s, 2H), 6.71 (s, 2H), 6.87 (d, 1H,
J=7.2 Hz), 7.43 (t, 1H, J=7.7 Hz), 7.45 (t, 1H, J=7.7 Hz), 7.97 (d,
1H, J=8.0 Hz), 8.02 (d, 1H, J=8.1 Hz). 13C (125 MHz, CDCl3): δ
3.6, 23.38, 23.48, 23.82, 24.10, 28.85, 29.35, 40.79, 40.88, 107.99,
108.15, 124.41, 124.75, 127.00, 127.75, 128.63, 128.79, 130.20, 130.69,
131.23, 132.36, 133.61, 139.15, 140.07, 143.4, 149.74, 150.20, 168.3,
172.7. Anal. Calcd for C41H53N4ClPd: C, 66.21; H, 7.18; N, 7.53.
Found: C, 65.94; H, 7.09; N, 7.30.
Experimental Section
General Considerations. All catalyst handling was carried out
in a Vacuum Atmospheres glovebox filled with nitrogen. All
other moisture and air-sensitive reactions were carried out in
flame-dried glassware using magnetic stirring under a positive
pressure of argon or nitrogen. Removal of organic solvents was
accomplished by rotary evaporation and is referred to as con-
centrated in vacuo. Flash column chromatography was per-
formed using forced flow on EM Science 230-400 mesh silica
gel. 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 at indicated temperature, if different than ambient.
Data were 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. Elemental analysis (for new compounds) was per-
formed by Atlantic Microlab, Norcross, GA.
(ClNNAn)PdMeCl (2e). To a suspension of 3e (250 mg,
0.439 mmol) in 20 mL of diethyl ether was added 116 mg of
Pd(COD)MeCl (0.439 mmol), and the solution was stirred
overnight at room temperature. The mixture was filtered, and
the solid was washed with cold diethyl ether then dried in vacuo
overnight to afford 2e as a red solid (282 mg, 88%) found to
contain ∼12% of the monochlorinated side product. 1H NMR
(500 MHz, CDCl3): δ 0.86 (s, 3H), 0.92 (d, 6H, J=6.9 Hz), 0.97
(d, 6H, J=6.9 Hz), 1.39 (d, 6H, J=6.7 Hz), 1.48 (d, 6H, J=
6.8 Hz), 3.29-3.41 (m, 4H), 6.62 (d, 1H, J=7.3 Hz), 6.82 (d, 1H,
J=7.2 Hz), 7.32 (s, 2H), 7.38 (s, 2H), 7.52 (t, 1H, J=7.8 Hz), 7.54
(t, 1H, J=7.8 Hz), 8.09 (d, 1H, J=8.1 Hz), 8.13 (d, 1H, J=
8.3 Hz). 13C (125 MHz, CDCl3): δ 4.0, 23.29, 23.57, 23.74, 24.14,
29.13, 29.58, 124.53, 124.99, 125.13, 125.21, 126.41, 127.17,
129.13, 129.27, 131.45, 131.63, 131.92, 133.24, 134.22, 139.96,
140.67, 140.76, 141.77, 144.1, 168.1, 172.4. Anal. Calcd for
C37H41N2Cl3Pd þ 0.15 C37H42N2Cl2Pd: C, 61.55; H, 5.74; N,
3.88. Found: C, 61.74; H, 5.69; N, 3.87.
Materials. Toluene, tetrahydrofuran (THF), diethyl ether,
and dichloromethane were purified by passing through solvent
purification columns following the method introduced by
Grubbs and are referred to as dry.40 Unless otherwise stated,
all solvents and reagents were purchased from commercial sup-
pliers and used as received. Unsubstituted ligands and Pd
In-Situ-Activated Polymerization Procedure. An oven-dried
50 mL two- or three-neck flask fitted with septa and a water-cooled

Article
Macromolecules, Vol. 43, No. 9, 2010 4097
condenser (for elevated temperature polymerizations) was
charged with 10 μmol of Pd(II) complex and 15 μmol of NaBAF
under an inert atmosphere. A 3:1 ratio of toluene to chloroben-
zene was added to bring the reaction volume to 25 mL. Ethylene
was added by slowly bubbling through the reaction mixture to
maintain ambient pressure during the course of the polymeri-
zation. Temperature was controlled by placement in an external
oil bath as needed. After 3, 6, and 12 h, 5.0 mL aliquots of the
polymerization mixtures were taken and placed into preweighed
vials and dried overnight under soft vacuum at 70 °C. After the
withdrawal of each aliquot, the volume of remaining mixture
was measured by syringe. After 24 h the leftover mixture (∼10 mL)
was dried by the same means. The yield of polymer and catalyst
turnover number (TON) were calculated from the mass of poly-
mer and the amount of catalyst in each aliquot, as determined
from the remaining mixture volume to account for solvent
evaporation and/or polymer generation.
(8) Kang, M.; Sen, A.; Zakharov, L.; Rheingold, A. L. J. Am. Chem.
Soc. 2002, 124, 12080–12081.
(9) Chen, G.; Ma, X. S.; Guan, Z. J. Am. Chem. Soc. 2003, 125, 6697–
6704.
(10) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–462.
(11) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217–3219.
(12) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744–745.
(13) Gottker-Schnetmann, I.; Korthals, B.; Mecking, S. J. Am. Chem.
Soc. 2006, 128, 7708–7709.
(14) Luo, S.; Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007,
129, 8946–8947.
(15) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc.
2007, 129, 8948–8949.
(16) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,
M. J. Am. Chem. Soc. 2000, 122, 6686–6700.
(17) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 11539–11555.
SEC-MALLS Characterization of Polymers.19,20 All the poly-
mers were characterized by size-exclusion chromatography
(SEC) coupled to a multiangle laser light scattering detector
(MALLS) for obtaining both polymer MW (Mn and Mw) and
Rg. Measurements were made on highly dilute fractions eluting
from a SEC consisting of a HP Aglient 1100 solvent delivery
system/auto injector with an online solvent degasser, tempera-
ture-controlled column compartment, and an Aglient 1100
differential refractometer. A 30 cm column was used (Polymer
Laboratories PLgel Mixed C, 5 μm particle size) to separate
polymer samples. The mobile phase was THF, and the flow rate
was 1.0 mL/min. Both the column and the differential refract-
ometer were held at 35 °C. A 30 μL sample of a 2 mg/mL
solution was injected into the column. A Dawn DSP 18-angle
light scattering detector (Wyatt Technology, Santa Barbara,
CA) was coupled to the SEC to measure both the MW and sizes
for each fraction of the polymer eluted from the SEC column.
ASTRA 4.7 software from Wyatt Technology was used to
acquire data from the 18 scattering angles (detectors) and the
differential refractometer and compute Mn and Mw from classi-
cal light scattering treatments.
(18) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975–
3982.
(19) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999,
283, 2059–2062.
(20) Cotts, P. M.; Guan, Z.; McCord, E.; McLain, S. Macromolecules
2000, 33, 6945–6952.
(21) Chen, G.; Guan, Z. J. Am. Chem. Soc. 2004, 126, 2662–2663.
(22) Chen, G.; Huynh, D.; Felgner, P. L.; Guan, Z. J. Am. Chem. Soc.
2006, 128, 4298–4302.
(23) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2002, 124, 1378–1399.
(24) Lanza, G.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122,
12764–12777.
(25) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599–2602.
(26) Gasperini, M.; Ragaini, F.; Cenini, S. Organometallics 2002, 21,
2950–2957.
(27) Gasperini, M.; Ragaini, F. Organometallics 2004, 23, 995–1001.
(28) Popeney, C.; Guan, Z. Organometallics 2005, 24, 1145–1155.
(29) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L.; Benedix,
R. Rec. Trav. Chim. Pays-Bas 1994, 113, 88–98.
(30) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L. Inorg.
Chem. 1994, 33, 1521–1531.
(31) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
Organometallics 2001, 20, 2321–2330.
Acknowledgment. We thank the National Science Founda-
tion (Chem-0456719, Chem-0723497, DMR-0703988) for funding.
C.P. acknowledges the Allergan Foundation, the UCI Department
of Chemistry, and Joan Rowland for financial support.
(32) DeYonker, N. J.; Foley, N. A.; Cundari, T. R.; Gunnoe, T. B.;
Petersen, J. L. Organometallics 2007, 26, 6604–6611.
(33) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.;
Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 1400–1410.
(34) Liu, F-.S.; Hu, H-.B.; Xu, Y.; Guo, L-.H.; Zai, S-.B.; Song, K-.M.;
Gao, H-.Y.; Zhang, L.; Zhu, F-.M.; Wu, Q. Macromolecules 2009,
42, 7789–7796.
Supporting Information Available: Tables of catalyst pro-
ductivities with time. This material is available free of charge via
the Internet at http://pubs.acs.org.
(35) Gasperini, M.; Ragaini, F.; Gazzola, E.; Caselli, A.; Macchi, P.
Dalton Trans. 2004, 3376–3382.
(36) Musaev, D. G.; Svensson, M.; Morokuma, K.; Stromberg,
S.; Zetterberg, K.; Siegbahn, P. E. M. Organometallics 1997, 16,
1933–1945.
(37) Froese, R. D. J.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc.
1998, 120, 1581–1587.
References and Notes
(1) This paper has been adapted in part from the following thesis:
Popeney, C. S. Ph.D. Dissertation, University of California at
Irvine, 2008.
(2) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414–6415.
(3) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169–1203.
(38) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 1094–
1100.
(39) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am.
Chem. Soc. 1997, 119, 6177–6186.
(4) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283–315.
(5) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 267–268.
(6) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am.
Chem. Soc. 1998, 120, 888–899.
(7) Gates, D. P.; Svejda, S. K.; Onate, E.; Killian, C. M.; Johnson,
L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33,
2320–2334.
(40) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(41) Carver, F. J.; Hunter, C. A.; Livingstone, D. J.; McCabe, J. F.;
Seward, E. M. Chem.;Eur. J. 2002, 8, 2848–2859.
(42) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; Vanleeuwen,
P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769–5778.
(43) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24,
3579–3581.