Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes

Article abstract of DOI:10.1021/jacs.8b04712

Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)-P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes with M_w up to ca. 2 × 10⁵ g mol^-1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.

Full text of DOI:10.1021/jacs.8b04712

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Article
Electron-Rich Metal Cations Enable Synthesis of High
Molecular Weight, Linear Functional Polyethylenes
Wei Zhang, Peter M. Waddell, Margaret A. Tiedemann,
Christian E. Padilla, Jiajun Mei, Liye Chen, and Brad P. Carrow
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04712 • Publication Date (Web): 26 Jun 2018
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Journal of the American Chemical Society
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Electron-Rich Metal Cations Enable Synthesis of High Molecular
Weight, Linear Functional Polyethylenes
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Wei Zhang, Peter M. Waddell, Margaret A. Tiedemann, Christian E. Padilla, Jiajun Mei, Liye Chen, and Brad P.
Carrow*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
ABSTRACT: Group 10 metal catalysts have shown much promise for the copolymerization of non-polar with polar alkenes to directly generate
functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this
field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)−P(III) chelating
ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization cata-
lysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP)
Pd catalysts are competent to generate linear, functional polyethylenes with Mw up to ca. 2 × 105 g mol−1, significantly higher than prototypical
catalysts in this field, and with polar content up to ca. 9 mol%. Functional groups are positioned by these catalysts almost exclusively along the
main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectro-
scopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species,
which may correlate to their complimentary catalytic properties versus privileged catalysts such as electrophilic a-diimine (Broohart-type) or
neutral phosphine-sulfonato (Drent-type) complexes. While steric blocking within the catalyst coordination sphere has long been a reliable
strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complimentary concept
less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.
Scheme 1. Common Functional Polyolefin Synthesis Methods.
INTRODUCTION
Tailored applications of polymeric materials demand access to a
characteristic: long chain branching
disadvantage: harsh, poor control
a) free radical
continuum of material properties, which in turn requires flexible
polymerization methods with precise control over macromolecular
structure. Polyolefin materials, polyethylene (PE) and polypropyl-
ene (PP) being the most common examples, are thus attractive tar-
gets for new methods development because they comprise the large
majority of all polymer production by weight.1 While the semi-crys-
talline nature of PE and isotactic PP engender strength and tough-
ness, the associated hydrophobicity of these polymers also restricts
their potential applications by limiting surface properties, compati-
bility, and rheology. The incorporation of polar functional groups
into the parent polyolefin structure, even in relatively minor
amounts, can significantly alter the material properties of the result-
ing ‘functional’ polyolefins and has long been the focus of synthetic
efforts.
FG
+
FG
free radical
FG
FG
FG
b) insertion, Brookhart-type
characteristic: short chain branching
disadvantage: catalyst stability
FG
i-Pr
i-Pr
N
Br
FG
Pd
Br
i-Pr
+ MAO
N
FG
FG
i-Pr
High molecular weight, linear functional polyethylenes (f-PE)
represent a particularly attractive target for insertion polymerization
because of their structural analogy to linear low-density polyeth-
ylene (LLDPE). Even though the $40 billion global market volume
(2013) for LLDPE is forecast to continue expanding,2 established
polyolefin markets such as this are still insufficient to fully capture
the massive volume of light olefins that have resulted from shale gas
production.3 The potential applications of polar analogues of
LLDPE are manifold because of enhanced surface properties, com-
patibility in polymer blends, printability, dye retention, and degrada-
bility. New polyolefin classes therefore have added value both in
their fundamental material properties but also for their potential to
c) insertion, Drent-type
characteristic: linear copolymers
disadvantage: generally modest MW
O
O
CH3
P
FG
FG
FG
Pd
FG
S
Me
Me
S
O
O
O
O
capture carbon that would otherwise be burned as fuel. A funda-
mental challenge to this end, however, is that few methods for func-
tional polyolefin synthesis selectively generate linear polymer micro-
structures with high molecular weight.
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Common synthetic approaches to incorporate polar functional
Scheme 2. General Considerations of Catalyst Steric Effects on
Insertion Copolymerizations.
1
2
3
4
5
6
7
8
groups into the hydrocarbon backbone of polyolefins are summa-
rized in Scheme 1. High energy intensity methods, such as free radi-
cal copolymerizations of ethylene with polar alkenes, have been
commercialized (Scheme 1a).4 Copolymers formed by such meth-
ods are mechanistically biased toward microstructures with long
chain branching and low crystallinity. On the other hand, the direct
metal-catalyzed insertion copolymerization of olefins and industrial
polar monomers represents a potentially ideal and tunable route to
functional polyolefin materials. Significant progress has been made
in this area over the last two decades using group 10 metal catalysts.5
Brookhart's cationic Pd and Ni complexes coordinated by an a-
diimine ligand have been very successful in this regard, which toler-
ate acrylates or several other polar vinyl monomers to generate
branched copolymers (Scheme 1b).6 Mechanistic studies on these
systems have established that branch formation occurs through a
a) chain transfer
b) competing insertions c) this work
R
R
P
O
R
R
M
RL
O
RS
RS
R
S
X
O
RL
P
P
O
O
vs.
M
M
R
H
H
R
R
R
R
P
O
9
M
i.e., Drent catalyst
(RS = small; RL = large)
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FG
O size: Mw control,
less impact on ΔΔG‡
P size ΔΔG‡
P size
Mw
Computational studies on Drent-type phosphine-sulfonato Pd
catalysts have also suggested that isomerization of the reacting lig-
ands occurs prior to each monomer enchainment and that the most
reactive intermediate has mutually cis p-alkene and tertiary phos-
phine ligands.9 Based on this characteristic feature of (P^O)-coordi-
nated polymerization catalysts, we hypothesized that steric bulk em-
anating from the O ligand rather than from the P ligand should still
enhance polymer molecular weight but also lessen the impact of lig-
characteristic “chain walking” mechanism due to facile b-hydrogen
elimination and reinsertion allowing monomer enchainment to oc-
cur at internal positions along the polymer chain. As a consequence
of this mechanistic behavior, polar functional groups are positioned
at the ends of branches.
Strictly linear functional polyolefins can be generated by ring
opening metathesis polymerization (ROMP) or acyclic diene me-
tathesis polymerization (ADMET).7 Both of these methods provide
highly controlled and tunable routes to well-defined copolymers, but
the requisite post-polymerization hydrogenation and specialty mon-
omers nevertheless impose major barriers to large-scale applications.
Alternatively, Drent and Pugh discovered that phosphine-sulfonato
Pd catalysts generate linear (co)polymers by insertion polymeriza-
tion (Scheme 1c).8 The contrasting catalyst behavior of Brookhart-
type versus Drent-type polymerization catalysts has been estab-
lished through theoretical studies to originate from the electronic
(a)symmetry of the respective chelating ligands.9 A variety of group
10 metal catalysts with other (P^O)-type ligands have since been
shown to conserve the linear polymer selectivity of Drent-type cata-
lysts and can make high molecular weight PE in some cases,10 but
very few can access high f-PE molecular weights (i.e., ≥ 105 g mol−1)
needed for many applications.11 Even Drent-type complexes, argua-
bly the most successful catalyst class,12 rarely achieve (co)polymer
Mw exceeding 104 g mol−1.13 The direct synthesis of f-PE with high
molecular weight therefore remains a significant ongoing challenge
that could benefit from new catalysts and rational design strategies.
and size on DDG‡ for competing monomer enchainment because co-
ordinated monomer is positioned trans to the O ligand (Scheme 2c).
The substituents of P(V) oxides seem well-suited as a platform to
test this hypothesis.10,14 In particular, a phosphonic diamide group
was attractive to us because it is derived from abundant secondary
amines that could facilitate a high degree of structural tunability. An
initial challenge to this end was the lack of any precedent for metal
catalysis involving a phosphonic diamide in a chelating (P^O)-type
ligand scaffold. We report here the first examples of single compo-
nent Pd catalysts featuring a chelating P(V)−P(III) ligand based on
a phosphonic diamide-phosphine (PDAP) motif. We have found
these types of ligands do indeed provide a tunable platform for gen-
erating highly active Pd catalysts for synthesis of linear f-PE from in-
dustrial polar monomers, including high molecular weight examples
that few other catalysts can match.
RESULTS AND DISCUSSION
PDAP-Pd Catalyst Synthesis and Steric Benchmarking.
The highly polarized P−O bond in phenylphosphonic diamides
functions as a good directing group for ortho-metalation, which pro-
vided a straightforward synthesis of PDAP ligands 1a−1i by a one-
pot sequence of directed ortho-lithiation followed by nucleophilic
substitution at a chlorophosphine reagent (Table 1).15 The PDAP
ligands possessing a diarylphosphino moiety are air-stable and were
purified by simple flash chromatography.16 Air sensitive al-
kylphosphines were carried forward without purification. Ligand ex-
change of the PDAP for cyclooctadiene in (COD)Pd(CH3)Cl oc-
curred within minutes at rt and gave the neutral complexes 2a−2i.
Finally, exchange of the chloride ligand for 2,6-lutidine generated
the final air and moisture-stable single component precatalysts
(3a−3i) in 15%−76% isolated yield over three steps. We next pur-
sued a quantitative assessment of the space filling properties of
PDAP ligands to test our initial hypothesis that the phosphonic dia-
mide provides suitable steric properties.
A typical design strategy to increase polymer molecular weight us-
ing Drent-type and related catalysts is based on a steric blocking
model (Scheme 2a) to inhibit the rate of chain transfer at these
square planar metals that occurs through associative ligand substitu-
tion by monomer.9a Increased steric bulk at the ancillary ligand,
which must emanate from phosphorus(III) substituents in Drent-
type catalysts, therefore tends to increase polymer molecular weight
by blocking monomer coordination at the apical sites. The liability
of this common approach during insertion copolymerizations is that
bulkier P(III) substituents exacerbate the relative energy barrier of
enchaining any comonomer relative to ethylene, the smallest alkene
(Scheme 2b). Drent-type catalysts therefore tend to exhibit an an-
tagonistic relationship between polymer molecular weight and
comonomer enchainment, and it is not clear a priori how catalysts
could be designed to avoid this mechanistic paradox by variation of
the P(III) substituents alone.
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Solid-state data for complexes 2g and 4−8 were analyzed by
Table 1. Synthesis of Phosphine-Phosphonic Diamide (PDAP)
Ligands and Single Component Precatalysts.a
1
2
Cavallo's SambVca 2.0 program to generate topographical steric
maps (Figure 1).17 Complexes possessing small ester (4), sulfonate
(7), or carboxylate (8) groups expectedly display significant open
space in the western hemisphere (O side) of their coordination
sphere. On the other hand, the complex with a diisopropyl phos-
phonic diamide group (2g) fills significantly more space. Percent
buried volume data for these complexes also indicate the diispropyl
phosphonic diamide group in 2g fills more space than do other O
ligands based on P(V) moieties, such as diethyl phosphonate in 5 or
di-tert-butyl phosphine oxide in 6. Variation of the amine substitu-
ents in the PDAP ligand subsequently provided a straightforward
means for catalyst tuning (vide infra).
R2
R2
3
R3
R3
R3
R3
X
O
P
4
P
P
Me
Cl
Me
Me
N
R1
c
b
Pd
Pd
N
5
6
R1
R1
R1
N
P
O
P
O
N
N
R2
R1
Me
R1
R1
R1
N
N
R1
R1
R1
R1
7
8
BArF
X = H
X = P(R3)2 (1a−1i)
4
a
2a−2i
3a−3i
9
complex
3a
R1
R2
R3
yieldb (%)
10
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60
Me
i-Pr
i-Pr
i-Pr
Me
Et
H
H
H
H
H
H
H
H
i-Pr
i-Pr
51
43
55
76
54
48
43
28
15
3b
Ligand-Dependent Catalyst Activity and Stability. The sin-
gle component PDAP-Pd complexes 3a−3i were next benchmarked
as catalysts for ethylene polymerization (Table 2) to identify prom-
ising candidates for functional polyolefin synthesis. Catalysts 3a−3c
possessing only alkyl substituents on both the P(V) and P(III)
groups formed PE with decreasing activity as the size of the alkyl sub-
stituents increased (entries 1−3). In contrast, PDAP-Pd complexes
were much more reactive with more hindered arene P(III) substitu-
ents. Complex 3d that contains a simple diphenylphosphino group
formed only trace PE after 30 min (entry 4), but complexes pos-
sessing ortho-arene substituents were highly reactive (entries 5−11).
The highest average turnover frequency (TOF) of 3.6 × 105 h−1 (100
s−1) was observed using 3h (entry 12) with electron-poor 2,6-
difluorophenyl P(III) substituents, which ranks among the highest
activities reported for any Pd polymerization catalyst13a,14c and even
3c
i-C5H11
Ph
3d
3e
2-(MeO)-C6H4
2-(MeO)-C6H4
2-(MeO)-C6H4
2,6-F2C6H3
3f
3g
i-Pr
i-Pr
Me
3h
3i
NMe2
2,4-(MeO)2C6H3
aReagents and conditions: (a) tert-butyl lithium (1.2 equiv), THF at
0 °C for 1 h the ClP(R3)2 (1.2 equiv), THF at 0 °C then warming to rt
for 0.5 h; (b) (COD)PdCl(Me) (1.0 equiv), CH2Cl2 at rt for 0.5 h; (c)
NaBArF4 (1.0 equiv), 2,6-lutidine (1.1 equiv), CH2Cl2 at −78 °C warm-
ing to rt for 0.5 h. bYield over three steps. ArF = 3,5-(CF3)2C6H3.
compares favorably to values first reported for cationic a-diimine
nickel complexes (TOF up to 3.9 × 105 h−1) or classic homogenous
metallocene complexes such as [Cp2ZrMe]+[MeB(C6F5)3]– (TOF
up to 1.6 × 105 h−1).6a,18
Importantly, PE molecular weight also increased significantly
upon variation of the amino substituents in 3e−3g from Me or Et to
i-Pr (e.g., entries 5, 9 and 10). Complex 3g formed PE with the high-
est molecular weight (Mw 2.4 × 105 g mol−1), and this molecular
To quantitate how the coordination chemistry of the aforemen-
tioned phosphonic diamide group differs from other common types
of neutral and anionic oxygen ligands, we conducted a systematic
steric analysis of a PDAP versus other common (P^O)-type ligands.
To do so, we prepared and crystallographically characterized six
(P^O)Pd(CH3)Cl coordination compounds relevant to olefin
polymerization and oligomerization. The structures of these com-
plexes differ only by the identity of the O group and should thus pro-
vide a clear indication of how the new PDAP ancillary ligand fills the
primary coordination sphere relative to existing catalysts and, by cor-
ollary, could affect chain transfer during alkene polymerization.
y
y
3.0
y
y
y
y
2.0
1.0
x
x
x
x
x
x
x
0.0 Å
O
P
O
P
O
P
O
P
O
P
O
P
−1.0
−2.0
−3.0
O
O
O
O
O
O
O
O
O
O
O
P
P
P
P
P
P
CH3
CH3
CH3
CH3
CH3
Pd
Pd
Pd
Pd
Pd
i-Pr
Cl
O
Cl
Cl
Cl
P
O
Cl
S
O
P
O
O
P
O
O
EtO
N
MeO
O
O
i-Pr
OEt
N
i-Pr
2g
49.6
i-Pr
4
5
6
7
8
%Vbur
:
41.5
45.2
48.4
45.6
43.6
Figure 1. Topographical steric maps of (P^O)Pd(CH3)Cl complexes 2g and 4−8 in which the Pd atom defines the center of the xyz coordinate system,
the metal square plane defines the xz-plane and the z-axis bisects the O−Pd−P angle. Pd(CH3)Cl fragment omitted from each plot. Colors indicate
occupied space in the +z direction toward the covalent ligands (red) or −z direction toward the dative ligands (blue).
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Table 2. Ligand-Dependent Catalyst Activity and Polymer Structure Effects During Ethylene Polymerization.a
1
2
3
4
5
6
7
8
b
d
entry
catalyst
[C2H4]
(bar)
30
30
23
time
(min)
60
15
30
yield
(g)
activity
(kg molPd−1 h−1
2900
Mw
Đb
Me br.c
Tm
)
(× 10−3 g/mol)
(°C)
130
131
124
1
2
3e
4
3a
3b
3c
3d
3e
7.14
1.08
2.60
trace
3.60
3.56
2.48
1.19
1.56
1.35
1.35
6.30
2.12
5.2
39
81
1.8
1.6
1.4
0.5
2.5
16f
1700
1000
30
30
30
15
5
5800
2800
2000
950
2500
2200
270
120
130
100
21
120
240
16
1.6
1.6
1.5
2.5
1.4
1.4
1.6
1.8
1.4
1.4
2.9f
2.7f
7.0f
3.4f
13
137
134
131
126
136
127
105
130
133
9
6
7
8
9
10
11
12
13g
15
7
3.5
30
30
30
30
30
15
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3f
3g
15
3.5
30
30
120
15
15
21
3.0f
3.0f
3h
3i
10000
6800
80
120
aConditions: catalyst (2.5 µmol) and toluene (0.1 L) were added to a 450 mL stainless steel autoclave at rt under N2, equilibrated to 90 °C, then
b
c
d
charged with ethylene. Determined by GPC with multi-detection. Methyl branches per 1000 C as determined by quantitative 13C NMR. Peak Tm
determined by DSC, second heating cycle. e5 µmol catalyst. fDetermined by 1H NMR. g[Pd] = 12.5 µM.
weight is substantially higher than the PE formed by any other
(P^O)Pd catalyst reported to date that possesses the same P(III)
moiety. For instance, a phosphine-sulfonato palladium catalyst with
identical ligand backbone and bis-o-anisylphosphino group gener-
ated PE with 6× lower Mw under similar reaction conditions.19 Addi-
tionally, the dependence of PE molecular weight on pC2H4 saturates
above ca. 7 bar using catalyst 3e (entries 5−8), but the catalyst activ-
ity increased proportionally with pC2H4 from 3.5−30 bar.
Thermal stability was assessed from the relationship between pro-
duction of PE versus time during batch polymerizations. It was nec-
essary to decrease the catalyst concentration (4.2 µM) and ethylene
pressure (3.5 bar) to avoid fast saturation of the autoclave with PE
during polymerizations at 90 °C. Under these conditions, steady
production of PE was observed using catalyst 3i over a period of 24
h before a deviation in linearity was apparent (Figure 2). In contrast,
a representative Brookhart (9) catalyst exhibited an initial burst of
PE formation followed by rapid deactivation, consistent with litera-
ture observations. The prototypical Drent-type catalyst (e.g., 11, eq
1) displayed a slightly lower activity than did the PDAP-Pd catalyst
3i. The PDAP catalyst 3g was also tested, which gave a more meas-
ured accumulation of PE over time. Using this catalyst, linear PE
Methyl branching increases with increasing amino substituent
size in 3e−3g, from 1 to 13 per 1000 methylene carbons, leading to
a corresponding depression of peak PE melting temperature (Tm) by
10 °C, respectively. Monomer pressure also affects branch density,
which is most pronounced for catalyst 3g for which depression of Tm
by 22 °C was observed upon decreasing pC2H4 from 30 to 3.5 bar
(entries 10 and 11). This modulation of short chain branching con-
trasts the uniformly linear PE produced by Drent-type catalysts and
highlights an advantage of the modular oxygen ligand in the PDAP
scaffold.20
i-Pr
i-Pr
O
i-Pr
Br
i-Pr
Ph
O
Me
N
N
N
O
P
Ni
Pd
Me
(1)
Ni
N
PPh3
S
O
Br
i-Pr
O
Me
O
Another important catalyst property for practical applications in
olefin polymerization is high thermal stability to allow for high turn-
over numbers and frequencies at reaction temperatures required for
i-Pr
9
10
11
industrial reactors (ca. 80−140 °C).21 While group 10 a-diimine
complexes developed by Brookhart and coworkers (e.g., 9, eq 1) are
very reactive at mild temperatures, they can suffer from facile de-
composition at reaction temperatures above ca. 60 °C.6a,6b,21-22 Many
subsequent modifications to the classic a-diimine ligands have since
been made with the goal of improved thermal stability,23,13a but the
absolute activities reported for these are diminished compared to
Brookhart's original room temperature reactions. Salicylaldiminato
nickel complexes developed by Grubbs (e.g., 10, eq 1) are similarly
thermally sensitive, displaying excellent low temperature reactivity
but deactivation occurs upon heating.24 It is therefore instructive to
benchmark the lifetime of PDAP-Pd catalysts during polymeriza-
tions at high temperature.
Figure 2. Batch ethylene (3.5 bar) polymerization catalyzed by 3g, 3i,
or 9−11 (1.25 µmol) in toluene (300 mL) at 90 °C. Methylaluminoxane
(1.25 mmol) added to the reaction with 9; Ni(cod)2 (1.25 mmol) added
to reaction with 10.
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Table 3. Copolymerization of Ethylene and Polar Alkenes Catalyzed by Cationic PDAP-Pd Complexes.a
1
2
3
4
3e or 3i
(50 µM)
FG
+
FG
Me
n
m
toluene
90 °C, 48 h
5
6
7
8
b
e
DHe
entry catalyst comonomer
(M)
yield
(g)
productivity
Mw
Đb Me brc inc. ratioc
FG distr.
Tm
(× 10−3 g/mol)
(mol %) (main/chain ends)c,d (°C)
−1
(J g−1
)
(kg molPd
)
1f
2
3e
3i
MA (2.2)
BA (1.1)
MA (1.0)
MA (2.0)
MA (3.0)
BA (1.0)
BA (1.0)
BA (1.0)
BA (0.5)
BA (2.0)
BA (3.0)
iBorA (1.0)
iBorA (2.0)
iBorA (3.0)
AA (1.0)
AA (2.0)
AA (3.0)
BVE (1.0)
BVE (2.0)
BVE (3.0)
0.26
2.8
26
570
740
300
150
470
1000
1000
1800
260
190
390
200
180
110
61
21
47
1.4
1.4
1.3
1.1
1.2
1.2
1.6
1.8
2.0
1.5
1.5
1.2
1.2
1.5
1.6
1.4
1.3
1.7
1.6
2.4
10
3.4
1.7
2.9
2.7
1.3
0.5
1.6
2.0
1.3
0.8
n.d.
n.d.
n.d.
3.1
6.1
6.6
1.3
1.3
1.5
2.3
4.6
1.7
4.1
5.4
2.7
0.7
0.3
0.3
8.2
9.6
1.5
2.4
4.3
1.5
3.1
4.0
0.5j
0.2j
0.4j
94:6
>98:2
n.d.i
107
116
113
106
100
113
125
135
135
104
97
−
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
116
107
95
3
11.1
4.5
85
4
49
n.d.i
5
2.3
28
>97:3
n.d.i
n.d.i
100
108
140
160
169
82
6
7.0
84
7g
8h
9g
10
11
12
13
14
15
16
17
18
19
20
15.4
15.2
26.5
3.9
220
190
190
53
n.d.i
n.d.i
n.d.i
2.9
62
n.d.i
72
5.9
92
n.d.i
116
107
100
121
116
111
128
130
128
108
88
3.0
37
n.d.i
2.7
18
n.d.i
65
1.7
31
n.d.i
125
120
83
0.91
0.63
0.74
0.70
0.50
12
n.d.i
42
3.0
21
85:15
90:10
n.d.i
49
186
165
165
47
39
33
19
87:13
aConditions: catalyst (15 µmol), comonomer, and toluene (300 mL) were added to a 450 mL autoclave at rt under N2, charged with ethylene (30
bar), then heated to 90 °C. bDetermined by GPC with multi-detection. cMethyl branches per 1000 C, comonomer incorporation, and functional group
distributions determined by quantitative 13C NMR or H NMR. Ratio of functional groups in the polymer main chain versus the chain ends. ePeak
melting temperature determined by DSC, second heating cycle. f10 µmol [Pd], 15 mL total volume, 95 °C for 12 h. gpC2H4 = 40 bar, 70 °C. hpC2H4 =
40 bar, 60 °C. iChain-ends were not detectible by 1H NMR; see SI for limit of detection estimation for each sample. n.d. = not detected. jDetermination
of BVE incorporation was complicated by overlapping 1H NMR resonances.
1
d
productivity (R2 > 0.99) was observed over an even longer period
(48 h) at 90 °C, which is indicative of excellent thermal stability at
this temperature. The yield of PE (13.7 g) formed by 3g corresponds
to 1.2 × 106 catalyst turnovers. A deviation from linearity was evident
between 48 and 72 h, but the similar mass yields at 48 h using 3g, 3i,
or 11 suggest to us that the autoclave may be saturated with polymer
at this point. In any case, these data are consistent with the PDAP
ligand engendering excellent catalyst thermal stability.
more electron-rich yet isosteric to 3e, also generated promising pol-
ymer molecular weight (Table S6, entry 4). Catalysts 3e and 3i were
selected for subsequent reactions.
A series of copolymerizations (300 mL total volume) was next
conducted using 3e or 3i (Table 3). The typical conditions for these
reactions involved 50 µM [Pd], 30 bar of ethylene, and 1−3 M con-
centrations of comonomer in toluene at 90 °C.25 Under these condi-
tions, complex 3i generated poly(ethylene-co-butyl acrylate) with
nearly double the molecular weight (Mw 8.9 vs. 4.7 × 104 g mol−1, re-
spectively) than did 3e (entries 2 and 6), which contrasts the weak
poly(ethylene-co-methyl acrylate) molecular weight dependence on
remote P(III) substituent electronic effects reported for Drent-type
catalysts.26 The magnitude of this difference was surprising because
it is unlikely to originate from a classic steric blocking effect given
that 3e and 3i are approximately isosteric. A rationalization for these
polymer molecular weight differences based on ligand electronic ef-
fects is therefore favored (vide infra). A considerable difference in f-
PE Mw is also observed between a prototypical Drent catalyst (11)
and PDAP-Pd catalyst 3i with P(III) substituents of comparable
size, the latter 10× larger under identical reaction conditions (see
Copolymerizations with Polar Alkenes. A preliminary series
of small-scale reactions (i.e., 15 mL total volume) using PDAP-Pd
complexes for copolymerizations of ethylene with a polar alkene
(Table S6) was conducted to identify promising candidates for fur-
ther study. During this initial screening, the most active catalyst for
ethylene homopolymerization (3h) was found to generate poly(eth-
ylene-co-methyl acrylate) with quite low molecular weight (Mw 9 ×
103 g mol−1, Table S6, entry 2), similar to the copolymers formed us-
ing prototypical Drent catalysts (e.g., 11).8c On the other hand, co-
polymerization of ethylene with methyl acrylate catalyzed by the
more electron-rich 3e generated the highest Mw (2.1 x 104 g mol-1)
in the initial survey (Table 3, entry 1). The analogue 3i, which is
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Table S8 versus Table 3, entry 3). Increasing pC2H4 to 40 bar and/or
lowering the reaction temperature from 90 °C to 60−70 °C (Table 3,
entries 6−8) generated poly(ethylene-co-butyl acrylate) with even
higher molecular weights (Mw ~ 2 × 105 g mol−1), which is highly de-
sirable yet rare among linear functional polyolefins formed by inser-
tion polymerization.11b,13 The incorporation ratio (e.g., 2.7−9.6
mol%; Table 3, entries 6, 10 and 11) increased concomitantly with
butyl acrylate (BA) concentration (1−3 M). This trend also oc-
curred using methyl acrylate (MA) or isobornyl acrylate (iBorA)
that possess smaller and larger ester substituents, respectively. All
poly(ethylene-co-acrylate) copolymers generated by 3i are highly
linear with few methyl branches (≤3 CH3 per 1000 C).
Catalyst 3i is also compatible with other more challenging polar
monomers. The copolymerization ofethylene with acrylic acid (AA)
or butyl vinyl ether (BVE) to generate statistical copolymers demon-
strates the tolerance of PDAP-Pd catalysts to both acidic and nucle-
ophilic reagents, albeit with reduction in catalyst activity and copol-
ymer molecular weights (Table 3, entries 15−20). Contact angle (q)
measurements for the poly(ethylene-co-acrylic acid) with 1.5 mol%
AA incorporation (78(3)°) or 3.1 mol% AA incorporation (65(3)°)
reflect a significantly more hydrophilic polymer surface than com-
mercial low density PE (102°).27 This reinforces the substantial ef-
fects polar functional groups can exert on polyolefin material prop-
erties even at relatively low incorporation ratios (i.e., <10 mol%).
state, a Pd-ester enolate complex, formed by enchainment of methyl
1
acrylate.12o Analyses of 13C and H NMR data for other poly(eth-
1
2
3
4
5
6
7
8
ylene-co-acrylate) samples in entries 3−14 indicate the sequence dis-
tribution of functional groups, heavily concentrated in the main
chain versus chain ends in the copolymer formed by 3e, is conserved
when using catalyst 3i. This microstructure formed by a PDAP-Pd
catalyst compliments the functional polyolefins typically formed by
Drent-type catalysts, which generally have a significant fraction of
unsaturated chain ends with functional groups. A potential connec-
tion between Pd-ester enolate stability and polymer molecular
weight control is therefore conceivable but has received little atten-
tion to date. The frequent correlation between decreasing copoly-
mer molecular weight and increasing polar monomer content in
functional polyolefin synthesis may suggest this effect deserves fur-
ther consideration.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
E-co-AA
E-co-iBorA
co iBorA
co
E-co-MA
KE-imco/P-ohciltip(Ksim)
E-ccoo-BA
140
120
100
80
60
main chain
chain ends
40
O
OCH3
a b
O
OCH3
0
2
4
6
8
10
CH
3
n
c
e
CH3
q
m
Comonomer/ mol %
p
u
d
l
o
t
f
Figure 4. Relationship between melt temperature and comonomer con-
tent in f-PE generated by 3i (Table 3, entries 3−8 and 10−17). Literature
values for hydrophobic analogues shown in open circles.28
CH3
k
s
O
n
g
h
i
r
v
j
OCH3
(not detected)
The range of accessible molecular weights and comonomer con-
tent in the f-PE formed by 3i, which in the best cases are comparable
to single site LLDPE samples, could provide a useful platform for
comparative material property studies. An initial indication that un-
anticipated properties could be manifested in these functional ana-
logues is apparent from their melting behavior, as determined by dif-
ferential scanning calorimetry (DSC). The relative depression in
peak melting temperature (Tm) of poly(ethylene-co-acrylate) sam-
ples with increasing comonomer content (Figure 4) track closely
with literature values for LLDPE reported by Kim at lower comono-
mer content (ca. 0−5 mol%).28a At higher comonomer content (ca.
5−10 mol%), however, Tm decreases to a larger extent for the non-
polar poly(ethylene-co-octene) samples than for the functional ana-
logues. The inclusion of methyl branching in this analysis maintains
the same general trend (see Figure S1). The origin of these differ-
ences requires further study, but one potential cause could be statis-
tical differences in the spacing between functional groups (i.e., ester
versus hexyl) in the nominally random copolymer structures that
arise from the catalytic behaviors of the respective metallocene or
PDAP-Pd catalysts.
Figure 3. Assignment of characteristic 13C NMR resonances in
poly(ethylene-co-methyl acrylate) in Table 3, entry 1. See SI for full
spectrum. (* = galvinoxyl)
Structure analysis of the poly(ethylene-co-methyl acrylate)
formed in Table 3, entry 1 by quantitative 13C NMR spectroscopy
revealed a statistical distribution of methyl ester groups almost ex-
clusively along the main chain (94 mol%). Only 6 mol% of incorpo-
rated acrylate was positioned at the saturated chain-end, which cor-
responds to initiation of a new polymer chain via acrylate insertion
into a Pd−H species. More importantly, no alkenyl ester groups were
detectable at the unsaturated chain ends. This observation suggests
chain transfer does not occur from the presumed catalyst resting
Differentiating O Ligand Electronic Properties. In addition
to the beneficial space filling characteristics of this new ligand class,
our copolymerization data (e.g., Table 3, entries 1 vs. 4 or 2 vs. 6)
suggested there could be an electronic component to the polymer
molecular weight control because 3e and 3i are nearly isosteric and
a steric blocking model (Scheme 2a) does not adequately account
for the near doubling of Mw using the latter catalyst with either MA
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Journal of the American Chemical Society
or BA. On the other hand, assessment of any significant ligand elec- L-type oxygen ligands. The sum of the C−N−C and C−N−P bond
angles was 355.5(3)° for N1 and 359.5(3)° for N2.30 This trigonal
planar geometry at each nitrogen atom is consistent with strong N−P
multiple bond character through negative hyperconjugation. A more
polarized P−O bond is then expected for the phosphonic diamide
versus other P(V) oxides, such as phosphonates or phosphine ox-
ides. A more polarized P−O bond has previously been correlated to
stronger coordination of monodentate P(V) donors to lanthanide
metals with the trend: phosphonate < phosphine oxide < phospho-
ramide.31
1
2
3
4
5
6
7
8
tronic differences is hindered by the general lack of systematic and
quantitative data benchmarking the electronic properties of oxygen-
based ligands to late transition metals,29 particularly moieties com-
monly used in (P^O)-type ligands for alkene polymerization cata-
lysts. We thus conducted additional experiments to build some un-
derstanding of how the coordination chemistry of the aforemen-
tioned phosphonic diamide group differs from other common neu-
tral and anionic oxygen ligands. To do so, we returned to the struc-
tures of (P^O)Pd(CH3)Cl compounds 2g, 4−8, which differ only by
the identity of the O group.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
P
CH3
Pd
Cl
X
2g, 4–8
O
O
P
CO
O
P
O
S
O
O
O
Rh
OEt
O
O
P
O
P
Cl
OMe
X
O
OEt
tBu
NiPr2
tBu
NiPr2
12–17
Figure 5. Systematic variation in
O
ligand trans influence in
(P^O)Pd(CH3)Cl complexes 2g and 4−8 determined from solid state
data, and atomic charge in analogous (P^O)Rh(CO)Cl complexes
12−17, determined by NBO analysis. Error bars indicate 3σ.
Figure 6. ORTEP diagram of 2g. Thermal ellipsoids are shown at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond dis-
tances (Å) and angles (°): Pd−P2 2.2181(4), Pd−O1 2.163(1), Pd−C1
2.034(2), Pd−Cl1 2.3824(4), P1−O1 1.490(1), P2−Pd−O1 89.02(3),
C23−N2−C26 114.8(1), C23−N2−P1 120.8(1), C26−N2−P1
123.9(1), C29−N1−C32 113.2(1), C29−N1−P1 113.3(1),
C32−N1−P1 129.0(1).
A systematic change in trans influence is evident in complexes 2g
and 4−8 as measured by the change in Pd−C bond length (Figure 5)
for the methyl ligand situated trans to the O ligand. Elongation of the
Pd−C bond reflects a more strongly s-donating oxygen atom in
these complexes. The L-type oxygen ligands in 4 (ester) and 5
(phosphonate) are poor donors and exhibit expectedly weak trans
influence. On the other hand, the X-type anionic donors in 7 (sul-
fonate) and 8 (carboxylate) displayed higher trans influence. A sur-
prising observation was that the formally L-type phosphonic dia-
mide group in 2g actually falls within the strongly donating X-type
A departure in the bonding trends between hard lanthanide and
soft late transition metals is suggested from a natural bond order
(NBO) analysis conducted on a related series of (P^O)Rh(CO)Cl
complexes (12−17) that possess the same ligands as 2g and 4−8.
These complexes, which are geometrically similar to Pd(II), were in-
itially targeted for their CO reporter ligand that reflects the net metal
electron density (vide infra). The atomic charge (q) on the metal is
plotted in Figure 5, which indicates that (i) higher trans influence
generally correlates to decreasing metal charge in this series of com-
plexes, and (ii) the PDAP ligand is a significant outlier in this trend.
region. This suggests the phosphonic diamide s-donicity is unusu-
ally strong for a neutral ligand and beyond what other P(V) ligands
(i.e., phosphonate and phosphine oxide) exhibit.
The solid-state structure of 2g (Figure 6) provides some indica-
tion as to why the phosphonic diamide is such a strong donor among
Table 4. Systematic Effects of O Ligand Identity on the Electronic Properties of 2g, 4−8, and 12−17.
atomic chargec
q(Rh)
dC Pd−CH3 (ppm)
n COexptl (cm-1)a
n COtheor (cm-1)b
entry
X
q(O)
1
2
3
4
−CO2Me
−P(O)(OEt)2
−P(O)tBu2
−1.2
−1.3d
1990
1982
1975
1980f
1971g
1973
1971f
1960g
2083
2079
2075
2077
−0.3502
−0.3475
−0.3704
−0.3719
−0.5950
−1.029
−1.048
−0.9528
−2.3
−
−SO3
−2.1e,f
−3.5g
−2.5
5
6
−P(O)(NiPr2)2
2076
2060
−0.3251
−0.3764
−1.055
−
−CO2
−4.2f
−5.3g
−0.7041
aDetermined by13C NMR in CD2Cl2 for 2g and 4−8,. bCalculated for 12−17 at wB97XD/6-31G(d)/LANL2DZ (Rh) level; CPCM solvent model
(CH2Cl2). cDetermined by NBO analysis for for 12−17. From Ref 14b. eFrom Ref 12c. f(Et3NH)+ cation. g[Ph3P=N=PPh3]+ cation.
d
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The atomic charge (q) on oxygen in (1g)Rh(CO)Cl (16) was also hydride in a P^O-coordinated Pd complex is the one with hydride
1
2
3
4
5
6
7
8
the most negative versus any other complex in the series (Table 4),
which suggests the PDAP exhibits the weakest ligand-to-metal
charge transfer (LMCT) even though the phosphonic diamide is the
strongest s-donating L-type ligand. This contrasts the bonding
trend of P(V) oxides to lanthanides for which phosphoramides show
the highest LMCT.31
and oxygen ligands mutually trans. Higher trans influence in the ox-
ygen ligand, such as for the phosphonic diamide group, should then
destabilize this intermediate and thereby disfavor chain transfer.
This effect could contribute to the higher polymer molecular weight
of 3i versus 3e because the dimethylamino group in the former
should make the O ligand a stronger donor. Such an electronic effect
may work synergistically with the higher steric shielding by the phos-
phonic diamide as compared to small sulfonate or carboxylate
groups, thereby slowing chain transfer during ethylene propagation
and also potentially from the catalyst resting state (Scheme 3, top)
generated immediately after polar alkene enchainment. Future stud-
ies will attempt to scrutinize this hypothesis.
Additional spectroscopic data for complexes 2g, 4−8, and 12−17
further support the general trend in O ligand s-donicity and also the
disconnection between donicity and metal electron density for
PDAP-coordinated complexes (Table 4). A consistent increase in
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
shielding of the Pd-CH3 resonances in 2g and 4−8 is observed by 13
C
NMR spectroscopy as the oxygen ligand fluctuates from the weakest
L-type ester group (dC −1.2 ppm) to the strongest X-type carbox-
ylate anion (dC −4.2 to −5.3 ppm),32 corroborating the general trend
in s-donicity suggested by the trans influence data. Note that com-
plex 2g exhibited a Pd-CH3 chemical shift of dC −2.5 ppm that is sim-
Scheme 3. Mechanistic Pathways Controlling Polar Group Dis-
tribution in Functional Polyolefins.
R
CO2R'
P
Pd
OR'
ilar to the complex with a sulfonate (dC −2.1 to −3.5), which again
suggests the donicity of this neutral phosphonic diamide group rivals
oxygen anions.
O
P
O
R2N
propagation
major
NR2
(postulated resting state)
The (P^O)Rh(CO)Cl complexes (12−17) used for NBO analy-
sis were also synthesized to determine the experimental carbonyl
stretching frequency as a reporter of net metal electron density (Ta-
ble 4). A general red shift in these stretching frequencies, measured
in CH2Cl2 solution, was observed for (P^O) ligands that exhibit
higher trans influence. The complex (1g)Rh(CO)Cl (16) with a
phosphonic diamide group exhibited a CO stretch (1973 cm–1) that
was blue shifted compared to either the phosphine-sulfonato (1971
cm–1) or phosphine-carboxylate rhodium complexes (1960 cm–1)
that possess a non-coordinating cation (PPN+).27 The theoretical
stretching frequencies determined for 12−17 using DFT calcula-
tions generally agree with the experimental trend. The PDAP ligand
thus appears to again be an outlier because the electron density at Rh
is less than would be expected based on the high s-donicity of the
phosphonic diamide donor alone, which suggests that localization of
positive charge at the metal by the PDAP exerts a significant com-
peting electrostatic effect.
slow
H
P
– [Pd]-H
R
Pd
CO2R'
not observed
P
O
R2N
chain transfer
NR2
CO2R'
slow
R
R'O2C
P
1-2
1-2
Pd
P
O
O
OR'
R2N
chain walking
not observed
NR2
CONCLUSION
A series of cationic palladium complexes ligated by a chelating
phosphine-phosphonic diamide (PDAP) have been developed. Sev-
eral of these PDAP-Pd complexes (e.g., 3e and 3i) are potent cata-
lysts for insertion copolymerization with polar vinyl monomers. Im-
portantly, poly(ethylene-co-acrylate) materials with Mw up to 2 ×
105 g mol−1 were formed, which is a molecular weight threshold far
higher than functional polyethylenes (f-PE) formed by the im-
portant Drent-type catalysts that possess analogous P(III) substitu-
ents (e.g., o-anisyl). This increase in molecular weight control high-
lights the utility of tunability in the O hemisphere of the coordina-
tion sphere of P^O-coordinated late transition metal catalysts,
which is enabled by the modular amino groups the PDAP structure.
Analysis of the f-PE structures formed by PDAP-Pd catalysts
showed that the polar comonomer was incorporated almost exclu-
sively into the polymer main chain, which complements copolymers
formed by existing catalysts that tend to have significant functional
group content at chain ends or at the ends ofbranches. Furthermore,
the linear production of polymer over 24−48 h at 90 °C by PDAP-
Pd catalysts 3g and 3i established the excellent thermal stability en-
gendered by PDAP coordination, which is an important property
needed for practical applications.
In total, these experimental and computational data establish a
spectrum of electronic properties for common oxygen ligands used
for the design of alkene polymerization and oligomerization cata-
lysts. These data are also consistent with the conclusion that com-
plexes with a phosphonic diamide group are less electron-rich than
would be expected based purely on this O ligand’s high s-donicity.
Considering also that PDAP ligand 1g is still substantially more elec-
tron-releasing than is a prototypical Brookhart-type a-diimine lig-
and, such as Dipp-BIAN (DnCO = 25 cm–1),6a,33,34 we propose that the
PDAP-Pd polymerization catalysts 3a−3i are best described as elec-
tron-rich metal cations. Consequently, these catalysts are electroni-
cally differentiated from both the very electrophilic, cationic
Brookhart-type catalysts and the electron-rich, neutral SHOP-type35
or Drent-type catalysts.
Finally, a potential electronic contribution to the polymer molec-
ular weight control by PDAP-Pd catalysts can be inferred from the
above data. Mechanisms of chain transfer (Scheme 3, middle) and
chain-walking (Scheme 3, bottom) during group 10 metal-catalyzed
insertion polymerization both generally proceed through initial for-
Quantitative analysis of the steric properties of a prototypical
PDAP ligand (1g) versus other common chelating (P^O)-type lig-
ands confirmed that the former fills more space than typical ester,
mation of a metal hydride by b−H elimination from the propagating
organometallic intermediate. The most stable isomer of this metal
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Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger,
B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K.; Ortho-
Phosphinobenzenesulfonate: Superb Ligand for Palladium-Catalyzed
Coordination–Insertion Copolymerization of Polar Vinyl Monomers. Acc.
Chem. Res. 2013, 46, 1438-1449.
sulfonate, phosphonate, phosphine oxide, or carboxylate groups,
1
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which helps to inhibit coordinative chain transfer. Spectroscopic,
solid-state structural, and computational data are consistent with the
phosphonic diamide being a more potent s-donor to late metals,
such as Pd or Rh, than any other neutral oxygen donor that was
tested. We therefore conclude that PDAP-Pd catalysts can be de-
scribed as electron-rich metal cations, which is distinct from existing
catalyst types such as electrophilic, cationic Brookhart-type a-
diimine Pd catalysts and electron-rich, neutral Drent-type catalysts.
The potential of these ligands to access new and complementary cat-
alytic behaviors in alkene polymerization when coordinated to Pd,
and potentially other transition metals, should provide opportuni-
ties to explore material properties of high molecular weight polar-
functionalized polyethylenes.
A
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Macromolecules 2000, 33, 8963-8970; (d) Schwendeman, J. E.; Church, A. C.;
Wagener, K. B.; Synthesis and Catalyst Issues Associated with ADMET
Polymerization. Adv. Synth. Catal. 2002, 344, 597-613; (e) Hillmyer, M. A.;
Laredo, W. R.; Grubbs, R. H.; Ring-Opening Metathesis Polymerization of
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures, crystallographic data, characterization data
for ligands, metal compounds, and polymers.
The Supporting Information is available free of charge on the ACS Pub-
lications website.
Functionalized Cyclooctenes by
a Ruthenium-Based Metathesis Catalyst.
Macromolecules 1995, 28, 6311-6316; (f) Lehman, S. E.; Wagener, K. B.; Baugh,
L. S.; Rucker, S. P.; Schulz, D. N.; Varma-Nair, M.; Berluche, E.; Linear
Copolymers of Ethylene and Polar Vinyl Monomers via Olefin
Metathesis−Hydrogenation:ꢀ Synthesis, Characterization, and Comparison to
Branched Analogues. Macromolecules 2007, 40, 2643-2656; (g) Scherman, O.
A.; Walker, R.; Grubbs, R. H.; Synthesis and Characterization of Stereoregular
Ethylene-Vinyl Alcohol Copolymers Made by Ring-Opening Metathesis
Polymerization. Macromolecules 2005, 38, 9009-9014.
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H.; Jager, W. W. Appl. 0,589,527 Mar 30, 1994; (c) Drent, E.; van Dijk, R.; van
Ginkel, R.; van Oort, B.; Pugh, R. I.; Palladium catalysed copolymerisation of
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Chem. Commun. 2002, 744-745.
(9) (a) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki,
K.; Mechanistic Studies on the Formation of Linear Polyethylene Chain
Catalyzed by Palladium Phosphine−Sulfonate Complexes: Experiment and
Theoretical Studies. J. Am. Chem. Soc. 2009, 131, 14088-14100; (b) Hasanayn,
F.; Achord, P.; Braunstein, P.; Magnier, H. J.; Krogh-Jespersen, K.; Goldman, A.
S.; Theoretical Structure–Reactivity Study of Ethylene Insertion into Nickel–
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in Determining Agostic Bond Strengths. Organometallics 2012, 31, 4680-4692;
(c) Nakano, R.; Chung, L. W.; Watanabe, Y.; Okuno, Y.; Okumura, Y.; Ito, S.;
Morokuma, K.; Nozaki, K.; Elucidating the Key Role of Phosphine−Sulfonate
Ligands in Palladium-Catalyzed Ethylene Polymerization: Effect of Ligand
Structure on the Molecular Weight and Linearity of Polyethylene. ACS Catal.
2016, 6, 6101-6113; (d) Szabo, M. J.; Jordan, R. F.; Michalak, A.; Piers, W. E.;
AUTHOR INFORMATION
Corresponding Author
*bcarrow@princeton.edu
Funding Sources
We thank the NSF (CHE-1654664), the Princeton Center for Com-
plex Materials, a MRSEC supported by NSF Grant DMR 1420541, and
Princeton University for financial support.
Notes
The authors declare the following competing financial interest: a pa-
tent was filed by Princeton University: Carrow, B.P.; Zhang, W. WO
2015/200849 A2, December 30, 2015.
ACKNOWLEDGMENT
We thank Richard Register for helpful suggestions and access to analyt-
ical instrumentation, Adam Burns for assistance with DSC measure-
ments, and Jason Brandt for assistance with manuscript revisions. CEP
thanks the NSF for a graduate fellowship. Bruker is thanked for sample
analysis on a 10 mm cryoprobe 500 MHz NMR spectrometer.
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(32) The spectroscopic data of the anionic Pd complexes were sensitive to the
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ligands.
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P(V)–P(III) ligand: modularꢀlocalizes chargeꢀhigh σ-donicity
e.g.,
P
CH3
L
O
O
O
O
O O
FG
Pd
P
O
N
Mw & microstructures
comparable to LLDPE
N
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high FG tolerance (organic acid, enol ether)
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