Nickel-Catalyzed Copolymerization of Ethylene and Vinyltrialkoxysilanes: Catalytic Production of Cross-Linkable Polyethylene and Elucidation of the Chain-Growth Mechanism

Article abstract of DOI:10.1021/jacs.7b10281

Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CH₃CN)⁺ complexes 4a,b/B(C₆F₅)₃ yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of -Si(OR)₃ groups whose ratios depend on temperature and ethylene pressure. Detailed low-temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt₂)⁺ reacts rapidly at -60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(C₂H₄)⁺ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for formation of chain-end -Si(OR)₃ groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the Ni center to the ether moiety, R-O-Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene homopolymerization, but still remarkably high TOFs of up to 4.5 × 10⁵ h^-1 and overall productivities can be achieved. Activation of readily available (α-diimine)NiBr₂ complexes 2 with a combination of AlMe₃/B(C₆F₅)₃/[Ph₃C][B(C₆F₅)₄] yields a highly active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example, copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are precursors to the commercial cross-linked polyethylene, PEX-b.

Full text of DOI:10.1021/jacs.7b10281

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
Nickel-Catalyzed Copolymerization of Ethylene and
Vinyltrialkoxysilanes: Catalytic Production of Cross-Linkable
Polyethylene and Elucidation of the Chain-Growth Mechanism
Zhou Chen,† Mark D. Leatherman,‡ Olafs Daugulis,*,† and Maurice Brookhart*,†,‡
†Center for Polymer Chemistry, Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
ABSTRACT: Copolymerizations of ethylene with vinyl-
trialkoxysilanes using cationic (α-diimine)Ni(Me)(CH3CN)+
complexes 4a,b/B(C6F5)3 yield high molecular weight
copolymers exhibiting highly branched to nearly linear
backbones depending on reaction conditions and catalyst
choice. Polymerizations are first-order in ethylene pressure and
inverse-order in silane concentration. Microstructural analysis
of the copolymers reveals both in-chain and chain-end
incorporation of −Si(OR)3 groups whose ratios depend on
temperature and ethylene pressure. Detailed low-temperature
NMR spectroscopic investigations show that well-defined
complex 3b (α-diimine)Ni(Me)(OEt2)+ reacts rapidly at −60
°C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such
chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)-
(C2H4)+ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for
formation of chain-end −Si(OR)3 groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the
Ni center to the ether moiety, R−O−Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene
homopolymerization, but still remarkably high TOFs of up to 4.5 × 105 h−1 and overall productivities can be achieved. Activation
of readily available (α-diimine)NiBr2 complexes 2 with a combination of AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4] yields a highly
active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example,
copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg
copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are
precursors to the commercial cross-linked polyethylene, PEX-b.
cationic Pd(II) catalysts derived from bisphosphine monoxide
type ligands have been developed for ethylene homo- and
copolymerizations.6 Many of these systems successfully copoly-
merize ethylene with a variety of vinyl polar monomers including
methyl acrylate,7 vinyl acetate,8 vinyl fluoride,9 acrylonitrile,10
vinyl ethers,11 and other important polar monomers.12 In
contrast to the palladium α-diimine systems, linear function-
alized polyethylenes are obtained. Activities of these systems in
copolymerizations are invariably substantially less than for
ethylene homopolymerizations, and low molecular weight
copolymers are generally produced.
INTRODUCTION
■
The study of late metal-catalyzed olefin polymerizations has
advanced substantially in recent years.1 A primary motivating
factor for the development of late-metal catalysts is that their less
oxophilic nature, in comparison with early metal catalysts,
renders them more compatible with polar monomers. Of
particular interest is the copolymerization of ethylene and readily
available, inexpensive polar vinyl monomers.2 An early report
from our group showed that cationic α-diimine Pd(II) complexes
copolymerize ethylene and methyl acrylate (MA) to form highly
branched copolymers in which acrylate is preferentially
incorporated at chain ends.3 Mechanistic studies revealed that
a six-membered carbonyl chelate intermediate was formed
following 2,1 insertion of MA and rapid chain-walking (Scheme
1a).4 This chelate, the catalyst resting state, is highly favored over
the “chelate-opened” ethylene complex required for further chain
growth.
Nickel(II) analogues supported by α-diimine,13 phosphine−
sulfonate,14 and bisphosphine monoxide ligands6e noted above
have received limited attention for ethylene/polar vinyl
monomer copolymerizations. While these systems generally
exhibit much higher activities for homopolymerization of
An extensive family of neutral Pd(II) catalysts based on
Received: September 26, 2017
phosphine−sulfonate ligands (“Drent-type” catalysts)5 and
© XXXX American Chemical Society
A
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Scheme 1. Proposed Mechanism of α-Diimine Pd(II)-Catalyzed Copolymerization of Ethylene with (a) MA and (b)
Vinyltrialkoxysilane
ethylene relative to Pd analogues, copolymerizations that have
been examined generally show strong rate retardation or
complete suppression by the polar functional group.15−17 The
development of practical catalysts for ethylene/polar vinyl
monomer copolymerizations which exhibit commercially viable
activities, productivities, and molecular weights remains a
continuing challenge for this field.
Due to the high cost of palladium and the relatively low
activities and lifetimes of these Pd(II) catalysts, Pd-catalyzed
copolymerizations hold little commercial interest. In the same
patent disclosure, the DuPont group reported extensive
screening of nickel α-diimine catalysts for copolymerization of
ethylene and vinyltrialkoxysilanes.20a The most active catalysts as
shown in eq 1 were generated via activation of the bis-
trimethylsilyl-functionalized Ni complex 1 at 60 °C with a
combination of B(C6F5)3 and LiB(C6F5)4.
In this paper, we report a comprehensive synthetic and
mechanistic investigation of this copolymerization reaction
employing well-defined cationic nickel α-diimine catalysts. A
combination of bulk copolymerization data, low-temperature
NMR characterizations of intermediates and their reactivities,
and the analysis of copolymer molecular weights and micro-
structures provide a complete profile of the chain growth process,
including identification of the catalyst resting states, the mode of
insertion of the vinylsilanes, the dependence of the turnover
frequency on ethylene and vinylsilane concentrations, and the
role of chelate intermediates and their isomerization reactions in
determining the polymer enchainment modes of the vinylsilanes.
Furthermore, we report a simple method for activating readily
available α-diimine nickel dibromide complexes 2 (Figure 1),
Polyethylene that is functionalized with trialkoxysilane
(−Si(OR)3) groups can be cross linked to form PEX-b, a
tough material that is widely used for power cable insulation and
hot water piping systems.18 This cross-linkable polymer is
produced commercially either by radical-induced grafting of
vinyl silanes to polyethylene (PE) or radical-initiated copoly-
merization of ethylene and the vinyl silane, a process requiring
exceptionally high pressures and temperatures. Since only low
levels of −Si(OR)3 are required for cross-linking (ca. 0.1−2.0
mol %),19 metal-catalyzed copolymerization of ethylene and
vinyltrialkoxysilanes provides a potentially viable alternative
method for the synthesis of this modified PE.16b,20 Recently, we
reported mechanistic studies following up a DuPont patent
disclosure20a that α-diimine Pd(II) complexes catalyze copoly-
merization of ethylene and vinyltriethoxysilane to yield low
molecular weight polymers/oligomers.20b,21 We showed that
copolymerizations proceed at rates comparable to ethylene
homopolymerization since the catalyst resting state is an alkyl
ethylene complex. The turnover frequency is independent of
ethylene pressure and is controlled by the rate of migratory
insertion. Low-temperature NMR studies revealed that a silyl
ether chelate intermediate is formed after 1,2 insertion of
vinyltrialkoxysilane, but this chelate is rapidly and completely
opened with ethylene. Thus, unlike the methyl acrylate system,
formation of the chelate does not retard copolymerization rates
(Scheme 1b). The chelate undergoes facile β-silyl elimination
and displacement of the unsaturated chain by ethylene, resulting
in chain transfer and formation of low molecular weight
copolymers exhibiting chain-end incorporation of vinyltrialkox-
ysilane.21
Figure 1. α-Diimine nickel dibromide complexes 2.
which allows convenient synthesis of high molecular weight
copolymers with productivities of up to 560 kg copolymer/g Ni
at 60 °C over 4 h with 0.23 mol % incorporation of silane.
RESULTS
■
Copolymerization Reactions. The well-defined nickel
catalysts employed here for the mechanistic studies of the
copolymerization reactions are the cationic α-diimine nickel
methyl ether complexes 3a,b and the nickel methyl acetonitrile
complexes 4a,b, which were readily synthesized from nickel ether
complexes 3a,b (Figure 2; see the SI for details).22 Catalysts
4a,b23 are activated with B(C6F5)3, which abstracts the nitrile
ligand. They show activities in ethylene homopolymerization
B
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Copolymerizations were then carried out at 60 °C for 30 min
under various ethylene pressures and concentrations of VTEoS
(entries 2 and 6−11). As expected, at a fixed ethylene pressure,
the incorporation of VTEoS into the copolymer is nearly
proportional to the VTEoS concentration. At the same time, the
TOF is inversely proportional to the concentration of VTEoS
(compare entries 2, 6, 7 and entries 9−11). Likewise, at a given
VTEoS concentration, the TOF increases proportionally to the
ethylene pressure (compare entries 6, 8, 10). Plots illustrating
these dependencies are shown in the SI. The fact that the TOFs
in these copolymerizations are essentially first-order in ethylene
shows that the catalyst resting state is not a nickel alkyl ethylene
complex in contrast to the analogous palladium systems.20b As
expected from homopolymerization studies, increasing ethylene
pressure results in decreasing branching densities due to more
rapid trapping and insertion relative to chain-walking.24b
Notably, complex 4a exhibits a TOF of 2.0 × 104 h−1, reduced
by a only factor of 7 in absence of activator B(C6F5)3 (entries 9 vs
12).
We wished to examine the potential role of coordination of the
Si−O−R ether linkage to the Ni center with respect to
competition with ethylene and retardation of the turnover
frequency. Si(OEt)4, which lacks a reactive vinyl group, was used
as a model for vinyltriethoxysilane. Table 2 summarizes several
control experiments. The homopolymerization of ethylene (200
psig, 60 °C, 30 min) by 4a/B(C6F5)3 is retarded by a factor of ca.
2 when carried out in the presence of 0.5 M Si(OEt)4 (entries 1
vs 2), which indicates that the silyl ether groups of Si(OEt)4
effectively compete with ethylene for binding to the Ni center,
and a major resting state under these conditions is an (α-
diimine)Ni(R)[Si(κ-OEt)(OEt)3]+ species. When the copoly-
merization using 0.5 M VTEoS is compared to the
homopolymerization, the suppression factor, 4X, is greater
than for Si(OEt)4 (entries 1 vs 3), suggesting a Ni ether complex
(α-diimine)Ni(R)[Si(κ-OEt)(OEt)2(CHCH2)]+ may play a role
in retarding the rate of copolymerization relative to homo-
polymerization, but this is likely not the only factor involved.
This is further confirmed by comparing entry 3 with a
copolymerization run using 0.5 M VTEoS and an additional
0.5 M Si(OEt)4 (entry 4). The added 0.5 M Si(OEt)4 further
suppresses the rate by a factor of 2.5, implying that the Ni
complex in the copolymerization in entry 3 cannot be completely
tied up as an ether complex; otherwise, no additional rate
suppression would have been observed on addition of Si(OEt)4
Figure 2. Well-defined α-diimine Ni(II) complexes 3 and 4.
similar to those of typical (α-diimine)NiBr2 catalysts 2a,b
activated with modified methylaluminoxane (MMAO) (see SI).
Vinyltriethoxysilane (VTEoS) was chosen as a model CH2
CHSi(OR)3 monomer.
Investigation of Tetraisopropyl-Substituted Complex
4a. Results of copolymerizations of ethylene and VTEoS using
4a/B(C6F5)3 are summarized in Table 1. Highly branched
polymers are formed from 4a/B(C6F5)3 via the well-established
chain-running mechanism24 with branching densities ranging
from ca. 25−95 branches per 1000 carbons depending on
reaction conditions. Copolymerization temperatures were
screened from 40 to 100 °C at 200 psig pressure of ethylene
(entries 1−4). Branching densities and the mole fraction
incorporation of VTEoS into the polymer increase as temper-
ature increases with a corresponding decrease in Mn. (The
increase in silane incorporation with increasing temperature is
likely primarily due to deceasing solubility of ethylene at higher
temperatures.)25 For example, at 40 °C, 0.37 mol %
incorporation of silane is obtained with 22 br/1000 C and a
Mn of 173 kg/mol, while at 100 °C, 0.93 mol % incorporation of
silane is achieved in the polymer, with a high branching density of
94 br/1000 C and a lower Mn of 27 kg/mol. It is well-known that
α-diimine nickel catalysts decompose rapidly in ethylene
homopolymerizations at temperatures above ca. 60 °C.26,27
Interestingly, the turnover frequency (TOF) of copolymeriza-
tion (0.5 h runs) decreases only slightly from 5.3 × 104 h−1 at 60
°C to 4.1 × 104 h−1 at 80 °C. Moreover, 4a/B(C6F5)3 exhibits
fairly constant copolymerization activity over 1 h at 60 °C
(compare TOF entry 2, 30 min to TOF entry 5, 60 min). These
results suggest that this catalyst exhibits better thermal stability
under copolymerization conditions relative to homopolymeriza-
tion conditions.
a
Table 1. Copolymerizations Using (α-Diimine)Ni(Me)(MeCN)+ Catalyst 4a/B(C6F5)3
b
c
c
b
d
entry T (°C) C2H4 (psig) [VTEoS] (M) yield (g) TOF (× 10−4 h−1
)
branches/1000 C
Mn (kg/mol) Mw/Mn
incorp (mol %) Tm (°C)
1
2
3
4
5
6
7
8
9
40
60
80
100
60
60
60
60
60
60
60
60
200
200
200
200
200
200
200
400
600
600
600
600
0.5
0.5
0.5
0.5
0.5
1.0
1.5
1.0
0.5
1.0
1.5
0.5
1.0
4.8
5.3
4.1
0.6
4.5
2.7
1.4
4.8
22
50
73
94
51
51
49
40
34
32
34
36
173
104
62
1.4
1.6
1.8
1.6
1.9
1.7
1.7
1.5
1.4
1.4
1.5
1.8
0.37
0.69
0.79
0.93
0.68
1.30
2.20
0.72
0.23
0.46
0.72
0.24
99
64
1.1
0.85
0.13
1.9
22
27
−29
66
e
106
85
0.56
0.29
1.0
57
61
57
112
138
128
110
84
78
2.9
14
89
10
11
12
1.4
6.8
4.4
2.0
89
0.92
0.41
85
f
87
a
b
c
V(total) = 50 mL, 1.5 μmol of 4a, 10 equiv of B(C6F5)3, 30 min, toluene solvent. Determined by using 1H NMR spectroscopy. Mn and Mw/Mn
d
e
f
were determined by SEC in trichlorobenzene at 150 °C. Determined by differential scanning calorimetry, second heating. 60 min. No B(C6F5)3.
C
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a
Table 2. Control Polymerizations Using (α-Diimine)Ni(Me)(MeCN)+ Catalyst 4a/B(C6F5)3
b
c
c
b
d
entry monomer [0.5 M] additive [0.5 M] yield (g) TOF (× 10−4 h−1
)
branches/1000 C
Mn (kg/mol) Mw/Mn
incorp. (mol %) Tm (°C)
1
2
3
4
5
6
4.5
2.6
1.1
0.42
3.0
1.7
22
13
54
57
50
53
60
53
125
126
104
90
1.6
1.6
1.6
1.6
1.7
48
59
Si(OEt)4
Si(OEt)4
Si(OEt)4
VTEoS
VTEoS
5.3
2.0
43
25
0.69
0.71
64
63
55
60
e
e
135
112
1.7
a
b
1
V(total) = 50 mL, 1.5 μmol of 4a, 10 equiv of B(C6F5)3, 30 min, 60 °C, 200 psig C2H4, toluene solvent. Determined by using H NMR
c
d
spectroscopy. Mn and Mw/Mn were determined by SEC in trichlorobenzene at 150 °C. Determined by differential scanning calorimetry, second
heating. 10 min.
e
a
Table 3. Copolymerizations Using (α-Diimine)Ni(Me)(MeCN)+ Catalyst 4b/B(C6F5)3
b
c
c
b
b
d
entry C2H4 (psig) [VTEoS] (M) yield (g) TOF (× 10−4 h−1
)
branches/1000 C Mn (kg/mol) Mw/Mn incorp (mol %) ratio (x/z) Tm (°C)
1
2
3
200
200
200
200
600
600
600
600
0.5
0.5
1.5
1.5
1.5
0.5
0.1
0
3.4
6.3
2.4
1.6
3.8
6.1
4.1
3.8
1.2
0.8
4.6
14.5
45
16
16
14
29
9
14
14
2.2
2.1
1.6
1.5
2.0
2.0
2.1
2.2
3.0
3.1
8.5
1.0/1.9
1.0/1.9
1.0/1.9
1.0/2.8
1.0/1.1
1.0/1.1
95
95
e
f
8.8
6.9
14
48
f g
4 ,
10
17
5
3.2
97
h
6
h
9
23
29
85
1.0
115
115
135
,
i
j
7
19
10
0.23
n.d.
ik
8 ,
6.2
177
5
a
b
c
V(total) = 50 mL, 6.0 μmol of 4b, 10 equiv of B(C6F5)3, 30 min, 60 °C, toluene solvent. Determined by using 1H NMR spectroscopy. Mn and
d
e
f
Mw/Mn were determined by SEC in trichlorobenzene at 150 °C. Determined by differential scanning calorimetry, second heating. 60 min. 15
μmol of 4b. 80 °C. 3 μmol of 4b, 20 equiv of B(C6F5)3. V(total) = 200 mL. Not determined due to low VTEoS incorporation. 1.5 μmol of 4b,
20 equiv of B(C6F5)3, 35 °C, 5 min.
g
h
i
j
k
(mechanistic studies below will clarify the nature of the catalyst
resting states). In contrast to the enhanced lifetime of the
copolymerization, the decrease in activity of homopolymeriza-
tion with time in the presence of Si(OEt)4 at 60 °C is similar to
that of the homopolymerization with no additive (compare
entries 2 vs 6 with entries 1 vs 5, Table 2).
function of the ratio of VTEoS: ethylene (compare entry 1 to
entry 3, and entry 5 to entry 6, Table 3) and the TOF is
approximately proportional to ethylene pressure (entries 1 vs 6)
and inversely proportional to VTEoS concentration (entries 1 vs
3 and 5 vs 6). An increase in temperature results in an increase in
comonomer incorporation, and at 80 °C, 10 mol % VTEoS
incorporation is achieved at 200 psig ethylene, 1.5 M silane
(entry 4). As expected, the branching density decreases from 16
br/1000 C at 200 psig ethylene pressure to 9 br/1000 C at 600
psig ethylene (entries 1 vs 6). Notably, the TOF reaches 4.5 ×
105 h−1, under 600 psig ethylene, 0.1 M silane, 30 min, 60 °C,
which is only ca. 4-fold lower than that of ethylene
homopolymerization at 35 °C, with an incorporation ratio of
0.23 mol % (entries 7 vs 8, Table 3).
The microstructures of the copolymers obtained using catalyst
4b/B(C6F5)3 were analyzed by high temperature NMR spec-
troscopy to determine if the −Si(OEt)3 groups were
incorporated “in-chain” (on secondary carbons) or at chain
ends (on primary carbons) (Scheme 2, see the SI for details).
The mode of polar monomer incorporation is of particular
interest as the reported ethylene-MA copolymers produced by α-
diimine Ni catalysts contain predominantly in-chain acrylate
groups,15a,b which differs from copolymers of ethylene-MA
generated with Pd α-diimine catalysts where the functional group
occurs at the ends of branches.4 As noted previously, ethylene/
Very low molecular weight copolymers are produced by the
analogous Pd complexes,20b,21 while much higher molecular
weight copolymers are generated by 4a/B(C6F5)3. The
molecular weights of copolymers are increased at higher ethylene
pressures and lower concentrations of VTEoS; however, high
molecular weights are seen under all conditions. Mn values reach
138 kg/mol under 600 psig ethylene and 0.5 M of VTEoS (entry
9, Table 1). In the Pd-catalyzed copolymerizations, only one
−Si(OR)3 unit is incorporated per chain due to silane-induced
chain transfer.20b,21 In contrast, polymers reported here contain
numerous −Si(OEt)3 groups per chain. For example, there are
ca. 26 −Si(OR)3 units on average per chain for the copolymer in
entry 2, Table 1. Mechanistic studies described below will show
that β-silyl elimination does occur in these systems but seldom
leads to chain transfer.
Investigation of Tetramethyl-Substituted Complex 4b.
While highly branched copolymers are produced by tetraiso-
propyl-substituted catalyst 4a, sparsely branched, nearly linear
copolymers are generated using the less bulky tetramethyl-
substituted complex 4b.26 Results are summarized in Table 3.
Compared with 4a/B(C6F5)3, 4b/B(C6F5)3 produces copoly-
mers with higher VTEoS incorporation under similar reaction
conditions and with similar TOFs. For example, compare entry 1,
Table 3 (3.0 mol % incorporation, 4.1 × 104 TOF) with entry 2,
Table 1 (0.69 mol % incorporation, 5.3 × 104 TOF). 4b/
B(C6F5)3 also shows good thermal stability at 60 °C; the TOF for
4b/B(C6F5)3 is nearly constant over 60 min (entries 1 vs 2, Table
3). Like catalyst 4a/B(C6F5)3, the incorporation of VTEoS is a
Scheme 2. In-Chain vs End-of-Chain Microstructures of
Ethylene/VTEoS Copolymers
D
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VTEoS copolymers produced with palladium α-diimine catalysts
are end-capped with −Si(OR)3 due to silane-induced chain
transfer. Table 3 summarizes the ratio of in-chain and chain-end
silane incorporation (i.e., x/z) for polymers generated under
various conditions (Scheme 2). Varying the concentration of
VTEoS does not affect the ratio of in-chain to chain-end
incorporation (entries 1 vs 3 and 5 vs 6, Table 3). However, the
ratio is sensitive to the ethylene pressure and temperature. For
example, the ratio decreases from 1.0:1.9 at 60 °C to 1.0:2.8 at 80
°C (entries 3 vs 4, Table 3) and increases from 1.0:1.9 at 200 psig
ethylene to 1.0:1.1 at 600 psig ethylene (entries 3 vs 5, Table 3).
A mechanistic rationalization of these results will be presented
later in the paper.
insertion product 6a and 1,2-insertion product 6b in a ratio of
1:1. Dynamic behavior identical to the previous VTMoS case is
observed, with the chelated and free ethoxy groups of both
complexes distinguishable in the NMR spectrum at −80 °C (δ
2.91 ppm for κ-OCHH′CH3 and 2.78 ppm for κ-OCHH′CH3 in
complex 6a; δ 3.32 ppm for κ-OCHH′CH3 and 3.22 ppm for κ-
OCHH′CH3 in complex 6b).
Chelate Opening and Chain Propagation: Reactions of
5a,b with Ethylene. The addition of excess ethylene to a
mixture of 5a and 5b at −80 °C in CD2Cl2 results in much more
rapid site exchange of the bound −OMe group with the two free
−OMe groups as shown by NMR line-broadening experiments.
Furthermore, the rate of site exchange increases with ethylene
concentration.28 This implies that, in the presence of ethylene,
chelates rapidly open to form η2-ethylene complexes, 7a and 7b,
but that the equilibrium lies to the side of the chelates (Scheme
4). Upon warming solutions of 5a and 5b under 20 equiv of
Mechanistic Studies. Extensive low-temperature NMR
spectroscopic investigations have been carried out in order to
determine the structures and behavior of intermediates during
chain growth and the nature of the catalyst resting state(s) and to
elucidate the full details of the chain growth process.
Chelate Formation: Reactions of (α-Diimine)Ni(Me)-
(OEt2)+ 3b with Vinyltrialkoxysilanes. Diethyl ether complex
3b was used for low-temperature studies since it has been
previously established that the weakly bound diethyl ether ligand
can be readily displaced at exceptionally low temperatures.22,24b
In order to generate simplified NMR spectra, vinyltrimethox-
ysilane (VTMoS) was initially used in place of VTEoS. Addition
of 2 equiv of VTMoS to a CD2Cl2 solution of 3b at −80 °C leads
to slow insertion of the monomer without detectable formation
of either the κ-O complex, (α-diimine)Ni(Me)[(κ-OMe)Si-
(OMe)2(C2H3)]+, or the η2-olefin complex (Scheme 3). The
Scheme 4. Chelate Opening by Ethylene and Chain
Propagation
Scheme 3. Reactions of (α-Diimine)Ni(Me)(OEt2)+ 3b with
Vinyltrialkoxysilanes
ethylene to −40 °C, rapid ethylene consumption is observed
with formation of polymer, but with no concomitant significant
decrease in concentration (less than 5%) of 5a and 5b. This behavior
is due to slow initiation. Once 7a and 7b undergo insertion,
chelate formation is disfavored and fast multiple insertions of
ethylene ensue, so only a small fraction of 5a and 5b is
consumed.29
In order to obtain the insertion rate from a “chelate-opened”
ethylene complex, bulky vinyltri(tert-butoxy)silane (VTBoS) was
evaluated (Scheme 5). Treatment of 3b with 2 equiv of VTBoS at
−60 °C leads to rapid insertion without detection of a κ-O
complex (α-diimine)Ni(Me)[(κ-OBut)Si(OBut)2(C2H3)]+ or
η2-olefin-bound complex intermediates. The 1,2-insertion
product is exclusively formed, which, surprisingly, is a stable
agostic species 8a presumably due to the extreme steric bulk of
insertion is fast and complete at −60 °C, yielding chelate
complexes from both 2,1- and 1,2-insertions. The 2,1-insertion
reaction generates four-membered chelate complex 5a and is
favored over 1,2-insertion that forms five-membered chelate
complex 5b by a ratio of 3:2. Both complexes exhibit fluxional
behavior due to C−Si bond rotation and interchange of the
coordinated −OMe groups (δ = 3.81 ppm for 5a, 2.86 ppm for
5b) with the two free −OMe groups, respectively. The rotational
barriers for this process were measured via line-broadening
techniques (ΔG⧧ = ca. 10.5 kcal/mol at −55 °C for 5a, ΔG⧧ = ca.
11.5 kcal/mol at −45 °C for 5b). Key 1H NMR resonances for
the static structure of complex 5a include signals at δ 0.79 ppm
for the Ni-CH methine proton, δ 1.19 ppm and −0.22 ppm for
the methylene protons of NiCHCH2CH3, and δ 0.70 ppm for the
methyl group of NiCHCH2CH3. Characteristic resonances of
insertion product 5b include signals at δ 1.43 and 0.87 ppm for
the diastereotopic methylene protons of Ni-CH2, δ −0.31 ppm
for the Si-CH methine proton and δ 1.17 ppm for the methyl
group at C-β of the chelate.
Scheme 5. Chain Propagation of VTBoS via Complexes 9a,b
In addition to VTMoS, VTEoS was also investigated.
Treatment of 3b with 2 equiv of VTEoS in CD2Cl2 at −80 °C
results in slow diethyl ether displacement by olefin again with no
observation of the intermediate η2-olefin complex. Insertion at
more convenient rates is observed at −60 °C, yielding both 2,1-
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the tert-butoxy group which hinders chelate formation.
Characteristic resonances of complex 8a were observed at δ
1.54 and 1.44 ppm for the diastereotopic methylene protons of
Ni-CH2, δ −10.55 ppm for the agostic proton of C(μ-H)CH3
and 1.28 ppm for the methyl protons of C(μ-H)CH3. Upon
warming to 0 °C, complex 8a slowly isomerized to the complex
8b. Characteristic resonances of complex 8b were observed at δ
1.75−1.77 ppm (m, NiCH(CH3)CH(μ-H)), −0.13 ppm (d,
Scheme 7. Proposed Mechanism of Chelate Isomerization
3JHH = 5.4 Hz, NiCH(CH3)CH(μ-H)), −0.86 ppm (dd, 2JHH
=
8.4 Hz and 3JHH = 12.0 Hz, NiCH(CH3)CH(μ-H)), and −8.96
ppm (d, 2JHH = 8.4 Hz, NiCH(CH3)CH(μ-H)). Addition of 20
equiv of ethylene to the CD2Cl2 solution of complex 8a at −80
°C leads to quantitative formation of a complex assigned as the
ethylene-trapped complex 9a, based on the disappearance of the
agostic resonance and identification of bound ethylene
resonances (δ 5.04, 4.54, 3.92, and 3.70 ppm, 1H each). For
the complex 9a, the turnover frequency (1.4 × 10−2 s−1)
corresponding to the average rate of migratory insertion during
chain growth was measured by the rate of decrease of signal for
free ethylene in solution at -60 °C; the corresponding activation
barrier, ΔG⧧, was determined to be 14.1 kcal/mol, which is
nearly the same as that for the n-propyl ethylene nickel complex
in ethylene homopolymerization.24b Similarly for complex 9b,
the turnover frequency was found to be 2.9 × 10−3 s−1 (ΔG⧧ =
14.8 kcal/mol, −60 °C). This barrier is slightly higher than that
of the complex 9a and similar to that of the isopropyl ethylene
nickel complex in ethylene homopolymerization.24b These
activation barriers are nearly identical to ethylene homopolyme-
rization barriers and suggest that the insertion barriers of chelate-
opened ethylene complexes are not a factor in retarding the
activity of copolymerizations.30
system).20b 2,1-Migratory insertion of 10 forms complex 5c.
Subsequent “chain-walking” via intermediate 11 yields the
complex 5a.
If all steps are reversible, low concentrations of intermediate
10 could potentially be intercepted by an incoming ligand to
generate free propylene (a major pathway in the Pd systems20b).
Addition of 4 equiv of MeCN to the CD2Cl2 solution of
complexes 5a and 5c (7:2) at −80 °C results in rapid formation
of chelate-opened MeCN complexes 12a and 12c in a 7:2 kinetic
ratio (consistent with the starting 7:2 ratio of 5a:5c) (Scheme 8).
Scheme 8. Interception of Intermediate 10 with MeCN
Chelate Isomerization via β-Silyl Elimination. As noted
above, the copolymers produced using these nickel catalysts
exhibit both in-chain and chain-end incorporation of −Si(OEt)3,
and the ratio of these two modes of enchainment is affected by
ethylene pressure and reaction temperature. Isomerizations of
initially formed chelates likely play a role in determining in-chain
vs chain-end incorporation of −Si(OEt)3. Thus, the isomer-
ization behavior of the chelates was studied in the absence of
ethylene.
Warming the mixture of 12a and 12c at 20 °C over 12 h indeed
results in observation of some free propylene (ca. 25%) which
seems likely to result from interception of 10 formed via nitrile
loss from 12a,12c and isomerization as shown in Scheme 7.
Warming a mixture of chelates 5a and 5b to −20 °C results in a
decrease and eventual disappearance of 5b as complex 5c grows
in (Scheme 6). The initial ratio of 5a:5b is ca. 3:2, while the final
DISCUSSION
Scheme 6. Isomerizations of Chelate Complexes 5a,b
■
The mechanistic studies described above for the α-diimine nickel
catalysts yield a detailed picture of the chain-growth process and
a rationale for the modes of enchainment of the VTEoS
monomer as summarized below. Furthermore, these studies
provide instructive comparisons with the quite different behavior
of palladium α-diimine systems for the analogous copolymeriza-
tions.
Chain Growth. The proposed general mechanism of chain
growth in the nickel system is shown in Scheme 9. Refinement of
this mechanism to account for chelate isomerization is discussed
in the following section in conjunction with Scheme 10. Chelate
complexes 14a,b are the major catalyst resting states since
mechanistic studies establish that the equilibria between 14a,b
and the ethylene complexes 15a,b, favor 14a,b, and in addition,
the equilibria with ether-bound complexes 13a,b also strongly
favor 14a,b.31 Analyzing chain growth starting from resting states
14a,b, the productive cycle involves reaction of these chelates
with ethylene to form alkyl ethylene complexes 15a,b, followed
product ratio of 5a:5c is ca. 7:2, which suggests that 5a can be
formed from both 5b and 5c and that the final ratio represents an
equilibrium distribution of these three species. Characteristic
resonances of chelate 5c include signals at δ 1.48 ppm and −1.64
ppm for the diastereotopic methylene protons of Si-CH2, δ 1.67
ppm for the Ni-CH methine proton and δ −0.10 ppm for the
methyl group at C-α of the chelate.
The mechanism proposed for the isomerization of complex 5b
to complexes 5a and 5c is shown in Scheme 7. Opening of chelate
complex 5b followed by β-silyl elimination results in the
formation of trimethoxysilyl η2-propylene intermediate 10
(precedence for such an intermediate has been seen in the Pd
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and that TOFs are constant over many minutes so 14a is not an
unreactive sink; otherwise, activity would soon cease. Fur-
thermore, rearrangements of these chelates can occur so 14a
could rearrange to a more reactive chelate (see below).) Multiple
rapid ethylene insertions from 15a,b occur through the primary
species responsible for chain growth, 15+n.32 We established
previously that homopolymerization of ethylene as well as these
copolymerizations are retarded by Si(OEt)4 implying that
VTEoS can compete with ethylene to remove quite significant
concentrations of 15+n from the cycle by forming unreactive
ether complexes 16, thus retarding the rate of chain growth. The
binding affinity of VTEoS via a π-interaction is weaker than that
of ethylene so the concentration of 17 will be low relative to
14a,b, 15+n, and 16.
Scheme 9. Proposed Mechanism of Chain Growth
This mechanistic scenario is consistent with an effective TOF
that is first-order in ethylene and inverse order in VTEoS and
contrasts with the palladium system in which the ethylene
complex is the resting state and the TOF is zero-order in
ethylene. Despite the fact that the alkyl ethylene complexes
responsible for chain growth are only a fraction of the species in
solution, the Ni systems, due to lower ethylene insertion barriers,
are much more active than the Pd systems20b (ca. 400-fold under
similar conditions) in which the dominant resting state(s) are the
alkyl ethylene complexes. The nickel catalysts show enhanced
lifetimes in copolymerization relative to homopolymerization,
and thus, higher productivities than expected are observed based
on comparisons of initial TOFs. The enhanced catalyst lifetimes
are likely a result of higher thermal stabilities of the chelate
complexes 14a,b relative to 15+n and 16.
Scheme 10. Proposed Mechanism of Modes of Silane
Enchainment
Chelate Isomerizations: In-Chain vs Chain-End −Si-
(OR)3 Incorporation/Chain Transfer. In Scheme 9, resting
states 14a,b were proposed on the basis of initial products of
reaction of methyl ether complex 3b with vinyltrialkoxysilanes to
form 5a,b and 6a,b. From analysis of the copolymers formed
from 4b (Table 3), both in-chain and chain-end −Si(OEt)3
groups are observed. Since insertion of ethylene into 14a,b yields
only in-chain −Si(OEt)3 structures, the chain growth process
must be more complex than shown in Scheme 9. As shown in
Scheme 7, initially formed chelates can isomerize via β-Si(OMe)3
elimination to form 5c in which the −Si(OMe)3 group is
attached to a primary carbon. Thus, chain-end −Si(OEt)3 groups
can arise via the process shown in Scheme 10. β-Silyl elimination
from 14b produces the silyl complex 18. Displacement of the
unsaturated chain (chain transfer) in 18 by ethylene yields 19,
which would proceed on to give a −CH2CH2Si(OEt)3 end-
functionalized polymer.33 This is the dominant pathway in the
Pd systems20b but can only be a minor pathway here since there
are many −CH2Si(OEt)3 groups per chain.34 From 18, 2,1-
insertion yields chelate 14c (analogous to 5c, Scheme 7).
by multiple insertions of ethylene to yield 15+n, then
coordination of VTEoS to yield 17 followed by insertion to
return to resting states 14a,b. We have established that the rate of
the first ethylene insertion starting from 14a,b will be
proportional to kinsKeq[C2H4] and will be slower than subsequent
insertions since the equilibria strongly favor 14a,b (although we
do not know the precise value of Keq, see Scheme 4 and
associated text). Judging from previous results for the Pd
analogue, kins involving the initially opened chelate 15b is likely
very similar to known kins values for 15+n. This contention is
supported by results obtained here for the barriers to insertion of
the tri-tert-butoxy systems 9a,b. (While we have no models for
insertion of 15a, we know 14a is in rapid equilibrium with 15a
a
Table 4. Copolymerization of Ethylene and VTEoS Using (α-Diimine)NiBr2 2
c
b
d
C2H4
(psig)
time
(h)
TOF
productivity
(kg copolymer/g Ni)
branches/
b
Mn
M /
wc
incorp
Tm
entry precatalyst
(× 10−4 h−1
)
1000 C
(kg/mol)
Mn
(mol %)
(°C)
1
2
3
4
5
2a
2a
2b
2b
2b
2b
200
200
600
600
600
600
0.5
1.0
0.5
0.5
1.5
4.0
12.9
11.0
23.6
44.3
38.1
29.0
29
53
54
54
9
112
114
39
1.8
1.8
1.4
1.6
1.6
1.9
0.60
0.60
0.84
0.41
0.43
0.23
66
65
58
113
115
116
115
e
e
106
275
560
10
10
11
35
,
f
35
fg
6 ,
33
a
V(total) = 50 mL, 1.5 μmol of 2, [0.5 M] VTEoS, 60 °C, 15 μmol B(C6F5)3, 0.3 mmol of AlMe3, 15 μmol of [Ph3C][B(C6F5)4], toluene solvent.
b
c
d
Determined by using 1H NMR spectroscopy. Mn and Mw/Mn were determined by SEC in trichlorobenzene at 150 °C. Determined by differential
e
f
g
scanning calorimetry, second heating. V(total) = 200 mL. [0.2 M] VTEoS. 0.6 mmol of AlMe3, V(total) = 400 mL. [0.1 M] VTEoS.
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Trapping by ethylene and insertion yields 22 resulting in chain-
end incorporation of silane.35 This pathway is likely the
dominant pathway for chain-end −Si(OEt)3 incorporation.
This scheme is consistent with the dependence of the in-chain
to chain-end ratios on temperature and ethylene pressure as
shown in Table 3. As ethylene pressure is increased, one would
expect faster trapping of chelates 14a,b and insertion to yield in-
chain silane relative to chelate isomerization to 18/14c and
formation of chain-end silanes. This trend is verified in
comparing entry 1 (200 psig ethylene) with entry 6 (600 psig
ethylene) where chain-end silanes decrease substantially (1.9:1.0
vs 1.1:1.0). Similarly, the rate of chelate isomerization (first-order
process) relative to trapping (second-order process) should
increase with temperature. Indeed comparing entry 3 (60 °C)
with entry 4 (80 °C) shows such a trend (1.9:1.0 vs 2.8:1.0).
Since copolymers exhibit 50% or greater chain-end incorporated
silane, we must conclude that species 14c plays a significant role
in chain growth. From this data, we cannot say whether 14c is a
significant resting state or whether conversion of 14c to 22 is
simply faster than conversion of 14a,b to 15+n and thus never
builds up in significant concentrations.
Convenient Synthesis of Vinyltriethoxysilane/Ethyl-
ene Copolymers. Finally, a highly active and productive
catalyst system has been devised via activation of easily
synthesized and air-stable (α-diimine)NiBr2 complexes 2a,b.36
Selected copolymerization results are summarized in Table 4
using 2a,b activated by a combination of commercially available
activators AlMe3, B(C6F5)3 and [Ph3C][B(C6F5)4] (see Table S2
for details). Cross-linking of isolated copolymers does not occur
under these conditions as is common using other activation
procedures.20a Compared with (α-diimine)Ni(Me)(CH3CN)+
complexes 4, this catalyst system produces similar copolymers
but with a higher (ca. 2-fold) activity (compare entry 2, Table 1
vs entry 1, Table 4 and entry 6, Table 3 vs entry 3, Table 4). The
productivity of the system is found to be ca. 2 times higher than
the DuPont system with a similar VTEoS incorporation (ca. 0.4
mol %) under similar conditions (eq 1 vs entry 5). Furthermore,
the productivity reaches 275 kg copolymer/g Ni in 1.5 h with
0.43 mol % silane incorporation (entry 5) and 560 kg
copolymer/g Ni in 4 h with 0.23 mol % silane incorporation
(entry 6). Since only low levels of −Si(OEt)3 are needed for
cross-linking these results suggest possible commercial potential
for production of cross-linkable PE using this method.
Experiments demonstrating that these copolymers can be
cross-linked have been added to the SI.37
reduced catalyst lifetimes and a single −Si(OEt)3 group
incorporated at the chain end.20b
Low temperature NMR studies show that the vinyltrialkox-
ysilanes insert into Ni−R bonds in both 2,1- and 1,2-modes to
yield 4- and 5-membered chelates involving coordination of the
Si−O−R moiety to the Ni center. These chelates are the catalyst
resting states and account for a TOF that is first-order in ethylene
and inverse-order in silane. The chelates react rapidly with
ethylene to yield “opened” ethylene alkyl complexes but the
chelates are energetically favored. In contrast, the resting state(s)
of the Pd systems are alkyl olefin complexes and the TOF is
independent of silane concentration and ethylene pressure.
The ratio of in-chain/chain-end incorporation of −Si(OR)3
groups increases with ethylene pressure and decreases with an
increase in temperature. Low-temperature NMR studies show
that chain-end incorporation of −Si(OR)3 groups arises from
chelate rearrangement via a β-silyl elimination mechanism. In the
Pd analogues, β-silyl elimination accounts for the primary chain
transfer mechanism resulting in low Mn polymers with
incorporation of only one −Si(OR)3 group per chain. Such
chain transfer events are rare in the Ni systems.
Polyethylene functionalized with −Si(OR)3 groups is a
commercial polymer used to prepare PEX-b via cross-linking
of the −Si(OR)3 groups by exposure to moisture. Si(OR)3-
functionalized PE is prepared commercially by grafting vinyl
silanes onto PE or via radical-initiated copolymerization of
ethylene and a vinyl silane under high pressures and temper-
atures. We have developed here a convenient Ni-catalyzed
copolymerization route using as precatalysts easily accessible α-
diimine nickel dibromides in combination with commercial
activators, AlMe3/B(C6F5)3/[Ph3C][B(C6F5)4]. Excellent activ-
ities and productivities using this catalyst system can be obtained.
For example, copolymers containing 0.23 mol % triethoxysilane
groups can be generated at 60 °C, 600 psig ethylene over 4 h with
a productivity of 560 kg of copolymer/g Ni.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b10281.
Detailed experimental procedures, characterization data
for complexes 4a,b, 5a−c, 6a,b, 8a,b, 9a,b, and 12a,c,
NMR spectra of copolymers, kinetic plots, and cross-
linking experiments (PDF)
AUTHOR INFORMATION
SUMMARY
■
■
Corresponding Authors
*E-mail: mbrookhart@unc.edu.
*E-mail: olafs@uh.edu.
Comprehensive synthetic and mechanistic studies of the α-
diimine nickel-catalyzed copolymerizations of ethylene and
vinyltrialkoxysilanes, including low-temperature NMR studies
of intermediates, provide a complete description of the chain
growth process that differs in numerous crucial details with
respect to the analogous palladium chemistry. Key findings are
summarized below:
The cationic α-diimine Ni complexes 4a,b/B(C6F5)3 catalyze
copolymerization of ethylene and VTEoS at 60 °C to give nearly
linear to highly branched high Mn copolymer depending on
catalyst choice and reaction conditions with high turnover
frequencies (up to 4.5 × 105 h−1), good lifetimes, and high overall
productivity. Incorporation levels of VTEoS range from 0.23 to
10.0 mol % with multiple −Si(OEt)3 groups incorporated per
chain. In contrast, palladium analogs produce low Mn polymers/
oligomers with modest overall productivities due to low TOFs,
ORCID
Zhou Chen: 0000-0003-4345-7070
Olafs Daugulis: 0000-0003-2642-2992
Notes
The authors declare the following competing financial
interest(s): The authors have submitted a provisional patent
on portions of this work.
ACKNOWLEDGMENTS
■
This research was supported by the National Science Foundation
(CHE-1010170 to M.B.), DuPont, and the Welch Foundation
(Chair E-0044 to O.D. and Grant E-1893 to M.B.).
H
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(28) A quantitative dynamic NMR line broadening study of chelate 5a
shows line-broadening of the bound −OCH3 resonance to be directly
proportional to ethylene concentration at −80 °C. Exchange rates and
concentrations were observed: 84 s−1, 0.0050 M C2H4; 181 s−1, 0.0096
M C2H4; 287 s−1, 0.014 M C2H4. These results show clearly that chelate
5a is in rapid equilibrium with ethylene complex 7a prior to insertion.
(29) A polyethylene would be produced with a terminal −Si(OMe)3
group, but its concentration is too low to be detected.
(30) As noted, the rate constants measured are an average over
multiple insertions. Due to overlap of signals 9a,9b with subsequent
ethylene complexes it was not possible to precisely measure “first
insertion” barriers of 9a, 9b. However, we describe in the SI experiments
I
DOI: 10.1021/jacs.7b10281
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
that suggest these barriers are slightly but not significantly higher than
subsequent insertion barriers.
(31) This was established by treating 5a,b with 0.05 M Si(OMe)4 and
observing that no adduct was formed in measurable concentrations.
(32) As established previously (ref 24), in homopolymerizations of
ethylene using these nickel α-diimine catalysts, β-agostic alkyl species
will form as intermediates in chain propagation via successive ethylene
insertions of 15+n. The concentrations of these agostic species relative
to ethylene complexes 15+n will be low, since under these conditions,
homopolymerizations are zero-order in ethylene and the resting state(s)
are the ethylene alkyl complexes. However, even if the β-agostic species
were present in significant amounts relative to the alkyl ethylene species,
this would have no effect on the general mechanistic/kinetic analysis
presented here.
(33) A reviewer has suggested that −CH2CH2Si(OEt)3 polymer end
groups could arise not only via β-silyl elimination, chain transfer, and
ethylene insertion into Ni−Si(OEt)3 but also via vinyl silane insertion
into a Ni−H bond generated from β-H elimination. While vinyl silane
competes poorly with ethylene for insertion into a Ni−R bond, it may
compete well for insertion into the less bulky Ni−H group as
demonstrated in other systems (refs 7−12). NMR analysis of the
copolymers does not allow us to distinguish between these possibilities.
(34) This chain-transfer process is dominant in the Pd system (ref 20b)
resulting in low Mn polymer/oligomers. Since high polymers are
generated in the Ni systems, the rate of chain-transfer via this process
relative to chain propagation must be small. Homopolymers generated
have somewhat higher Mn values relative to copolymers generated under
similar conditions. It seems plausible, since traces of propylene are
observed on decay of 12a,b, that Mn values are reduced via a minor β-
silyl elimination chain-transfer pathway.
(35) Chain-walking from species 14c and trapping could yield a
+
complex of the type (α-diimine)(ethylene)NiCHPCH2CH2Si(OEt)3 .
Insertion would then yield
a copolymer exhibiting a
PCH(CH2CH2Si(OEt)3)CH2CH2P’ microstructural feature, with
silane separated from the main chain by two methylene units. This
possibility draws support from a preliminary observation that trapping
5a/5c with Me2S yields the expected two products at −60 °C which
upon warming to −20 °C yields an additional product, (α-diimine)
+
(SMe2)NiCH2CH2CH2Si(OMe)3 .
(36) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65−74.
(37) Copolymers with the Mn of 30−40 kg/mol and Mw of 60−80 kg/
mol should surely be sufficient to provide a tough cross-linked polymer.
For a reported example, see: Zhang, G.; Wang, G.; Zhang, J.; Wei, P.;
Jiang, P. J. Appl. Polym. Sci. 2006, 102, 5057−5061. Experiments
showing generation of highly crossed-linked insoluble material with high
gel content are described in the SI.
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DOI: 10.1021/jacs.7b10281
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX