Secondary alkene insertion and precision chain-walking: A new route to semicrystalline polyethylene from α-olefins by combining two rare catalytic events

Article abstract of DOI:10.1021/ja502130w

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low T_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the sandwich α-diimines 3-6 yielded semicrystalline polyethylene comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events.

Full text of DOI:10.1021/ja502130w

Communication
pubs.acs.org/JACS
Secondary Alkene Insertion and Precision Chain-Walking: A New
Route to Semicrystalline “Polyethylene” from α‑Olefins by
Combining Two Rare Catalytic Events
Tulaza Vaidya,† Kristine Klimovica,‡ Anne M. LaPointe,† Ivan Keresztes,† Emil B. Lobkovsky,†
Olafs Daugulis,*,‡ and Geoffrey W. Coates*,†
†Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
‡Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
S
* Supporting Information
with the α-diimine Ni(II) complexes reported by Brookhart and
coworkers (e.g., 1, Scheme 1) is regiorandom,5a,b while related
ABSTRACT: While traditional polymerization of linear
α-olefins (LAOs) typically provides amorphous, low Tg
polymers, chain-straightening polymerization represents a
route to semicrystalline materials. A series of α-diimine
nickel catalysts were tested for the polymerization of
various LAOs. Although known systems yielded amor-
phous or low-melting polymers, the “sandwich” α-diimines
3−6 yielded semicrystalline “polyethylene” comprised
primarily of unbranched repeat units via a combination
of uncommon regioselective 2,1-insertion and precision
chain-walking events.
catalysts (e.g., 2) exhibit predominantly 1,2-insertion and precise
chain-walking at low temperatures.7 Osakada and coworkers
have also reported controlled chain-walking polymerization of 4-
alkyl cyclopentenes8 and alkyl substituted 1,6-dienes9 using α-
diimine Pd(II) catalysts to afford regioregular polymers.
Scheme 1 outlines the chain-walking polymerization of α-
olefins (where ω = the number of carbon atoms in the α-olefin).
Provided that the active metal undergoes predominant chain-
walking to the ω-carbon following insertion, 1,2-insertion leads
to an ω,2-unit while 2,1-insertion installs an ω,1-unit in the
polymer backbone.10 If the metal fails to chain-walk after 1,2-
insertion, the final polymer will contain n-alkyl branches or 1,2-
units found in common poly(α-olefin)s. The amount and type of
branching will dictate the properties of chain-straightened
polymers, especially the crystallinity. For example, 1,2-units
will disrupt the crystallinity more than ω,2-units since methyl
branches can pack into the polyethylene crystals, but n-alkyl
branches cannot.11 As such, polymers with highly branched
microstructures will be amorphous or semicrystalline with low-
melting points.
We hypothesized that the presence of substantial axial bulk
and appropriate functionality on the α-diimine framework might
favor the desired 2,1-insertion and permit precise chain-walking
to give predominantly ω,1-units in the polymer. Molecular
modeling suggests that sterically hindered aryl substituted
naphthyl α-diimine catalysts (3−6) may influence the
regiochemistry of insertion and inhibit associative displacement
of the growing polymer chain. Brookhart, Daugulis, and
coworkers recently reported a “sandwich” (naphthyl α-diimine)-
nickel(II) catalyst (3) for ethylene polymerization to generate
highly branched polymers.12 Despite the nonlinear topology of
polyethylene generated from 3, we decided to investigate this
class of catalysts for polymerization of LAOs. Herein, we describe
the unique polymerization behavior of aryl naphthyl nickel(II)
polymerization catalysts (3−6) for the conversion of commodity
α-olefins to substantially linear, semicrystalline polymers,
structurally equivalent to linear low-density polyethylene. At
low monomer concentration, these catalysts show predominant
2,1-insertion and precise chain-walking. To the best of our
inear α-olefins (LAOs) are versatile monomers that can be
Lreadily sourced from both petrochemical and bioderived
feedstocks. Synthetic routes to LAOs include decarbonylative
dehydration of fatty acids,1 ethenolysis of their unsaturated
derivatives,2 and oligomerization of ethylene.3 The latter
generates a distribution of α-olefins of varying chain-length.
These olefins have diverse applications: 1-hexene and 1-octene
are used as comonomers for the synthesis of linear low density
polyethylene (LLDPE), low molecular weight poly(1-decene) is
primarily used in synthetic lubricants, and C12−C18 olefins are
converted into detergents.3b,4 Homopolymerization of these α-
olefins with conventional Group IV heterogeneous and metal-
locene catalysts results in amorphous polyolefins with n-alkyl
side-chains (see Scheme 1, 1,2-enchained units) that have
important uses as industrial lubricants, but limited use as
thermoplastics. We are interested in developing new catalysts
that can polymerize α-olefins via unique reaction mechanisms. In
particular, we have focused on generating semicrystalline
polymers similar to polyethylene from LAOs such as 1-decene.
The desired process involves two relatively uncommon
mechanistic steps: (1) a regioselective 2,1-olefin insertion and
(2) controlled “chain-walking”a series of sequential β-hydride
eliminations, alkene rotations, and reinsertions that relocates the
active metal center along the growing polymer chain.5 Only a
handful of olefin polymerization catalysts are known to enchain
terminal olefins via a secondary insertion mechanism.6 Despite
the partial chain-straightening ability of known α-diimine
complexes of Ni and Pd,7 competing olefin insertion pathways
and nonselective enchainment lead to branched polymers with a
low degree of crystallinity. For example, the mode of insertion
Received: March 1, 2014
Published: April 28, 2014
© 2014 American Chemical Society
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Scheme 1. Modes of α-Olefin Enchainment and α-Diimine Nickel Catalysts
a
Table 1. Polymerization of 1-Decene by α-Diimine Nickel(II) Complexes
d
mole fraction of C10 units (χ)
c
e
[1-decene]
(M)
toluene
(mL)
trxn
(h)
yield
(g)
Mn
linear
(χL)
methyl-branched
alkyl-branched
Tm
b
c
entry cat.
TON
(kg/mol)
Mw/Mn
(χM)
(χA)
(°C)
f
1
2
1
1
2
2
3
3
3
4
4
4
4
4
5
6
3.53
2.07
3.53
2.07
3.53
2.07
0.10
3.53
2.07
1.13
0.10
0.05
0.10
0.10
0.0
5.0
0.2
0.2
1.31
0.51
1.06
0.63
0.29
0.33
0.09
0.30
0.25
0.43
0.06
0.06
0.02
0.05
1868
727
1517
903
415
472
128
431
361
612
86
148
86
1.7
1.8
1.5
1.8
1.1
1.1
1.4
1.1
1.1
1.1
1.2
1.3
1.2
1.3
0.32
0.43
0.30
0.34
0.54
0.59
0.68
0.52
0.65
0.68
0.77
0.77
0.74
0.76
0.15
0.25
0.20
0.29
0.24
0.22
0.21
0.18
0.16
0.17
0.16
0.17
0.53
0.32
0.50
0.37
0.22
0.19
0.11
0.30
0.19
0.15
0.07
0.06
−
f
−
f
3
0.0
1.0
131
100
58
−
f
4
5.0
1.0
−
g
5
0.0
2.0
60
g
6
5.0
3.0
69
69
g
7
37.0
0.0
24.0
2.0
21
96
g
8
70
63
g
9
5.0
3.0
76
75
g
10
11
12
13
14
15.0
37.0
38.0
37.0
37.0
24.0
24.0
48.0
24.0
24.0
151
32
84
106
106
108
109
81
25
h
h
i
i
8
8
−
0.15
−
0.09
22
10
a
b
Polymerization conditions: Ni = 5 μmol in chlorobenzene (2 mL), [MAO]/[Ni] = 200, Trxn = 22 °C, toluene. TON (turnover number) = (mol 1-
c
decene consumed)/(mol Ni). Determined using gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C vs polyethylene
d
e
f
g
h
standards. Determined using 1H NMR spectroscopy. Peak melting point (second heat) determined using DSC. No Tm. Broad endotherm. Ni =
i
17 μmol. χM + χA = 0.26, overlapping NMR signals.
knowledge, this is the first example of polymerization of LAOs
such as 1-decene to semicrystalline polyethylene-type materials
with melting temperatures (Tm) >100 °C.5a,b,7b,13
decene concentration leads to a significant increase in the Tm and
χL of the resulting polymer with a considerable decline in χA
(entries 5−7). With [1-decene] = 0.10 M, χL rises to 0.68,
generating semicrystalline polymer with Tm = 96 °C (entry 7).
The observed trend suggests that lowering the concentration of
1-decene results in fewer long-chain branches in the final
polymer.
To probe the substituent effects of the naphthyl sandwich
framework on polymer properties, catalyst 4 (R1 = R3 = H, R2 =
CF3; see Figure 1 for X-ray structure) was screened under a
parallel set of reaction conditions (entries 8−12). DSC analysis
of polymers produced by 3 and 4 revealed broad endotherms for
both polymers at high monomer concentration (3.53 M).
However, a closer analysis of the DSC traces indicated that 4
generates polymer with higher melting points in comparison to 3
(see SI). In addition, there is a remarkable correlation between
the concentration of 1-decene and the Tm of the resulting
polymer from 4 (Figure 2). As monomer concentration is
decreased, the melting endotherms begin to resemble that of
semicrystalline polyethylene. In-depth analysis of the polymer
architecture using quantitative 13C NMR spectroscopy revealed
We initially screened six different methylaluminoxane (MAO)
activated α-diimine Ni(II) catalysts at varying 1-decene
concentration (Table 1). Polymers were analyzed by quantitative
NMR spectroscopy to obtain mole fractions of the three major
C10 units: linear (χL), methyl-branched (χM), and alkyl-branched
(χA) (see SI for details).14 Polymerization using the prototypical
Brookhart catalyst (1) at two different concentrations (3.53 and
2.07 M) leads to amorphous polymers (no Tm observed) with
relatively broad molecular weight distributions (entries 1−2).
While the mole fraction of linear enchainment (χL) increases
from 0.32 to 0.43, the polymer remains amorphous. A more
sterically encumbered Ni(II) α-diimine, 2, which is known for
preferential 1,2-insertion, is also ineffective at producing
crystalline material (entries 3−4). To probe the behavior of
sandwich α-diimines, complex 3 (R1 = R3 = H, R2 = Me) was
tested. The use of the 3 results in a considerable decline in χA and
χM, while χL increases to 0.54 (entry 5). In addition to the effect
observed by the use of the sandwich ligand, a reduction in 1-
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Journal of the American Chemical Society
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in the amount of branching with decreasing 1-decene
concentration. In sharp contrast to the results observed for 1−
3, at 0.10 M of 1-decene, the observed Tm of the polymer
produced by 4 is 106 °C with χL = 0.77 (entry 11). Lowering the
1-decene concentration below 0.10 M does not further enhance
the polymer crystallinity (entry 12). Surprisingly, decreasing the
concentration of 1-decene has a profound effect on χL, while χM
remains relatively constant. It is possible that monomer
concentration determines the microstructure either by influenc-
ing the regiochemistry of olefin insertion or by diminishing
partial chain-walking.14
To further investigate the effect of naphthyl substituents on
the chain straightening polymerization, complexes 5 and 6 were
tested. It is worth noting that 5 (R1 = R3 = CF3, R2 = H) creates
polymer of higher Tm (108 °C) but shows lower activity than 4
(entry 13). Altering the backbone diimine from acenaphthene to
dimethyl (6, R1 = R3 = H, R2 = CF3) results in a polymer with Tm
of 109 °C (entry 14). While both 4 and 6 produce semicrystalline
material, a closer analysis of the DSC traces reveals that the
polymer from 4 has a relatively sharp melting transition
compared to 6. In light of overall performance and polymer
properties, 4 was selected for further investigation.
Figure 1. X-ray crystal structure for 4. Hydrogen atoms are omitted for
clarity. Atoms drawn at 40% probability level.
Optimization of 1-decene polymerization with respect to
catalyst concentration, various types and amounts of activators,
1-decene concentration, and solvents led to the following
observations: (a) the catalyst concentration has no significant
effect on the Tm, (b) dried MAO is the optimal cocatalyst for 4,
and (c) chlorobenzene is an effective solvent compared to less
polar solvents. Reaction temperatures above 22 °C reduce the
efficacy of 4 in terms of Mn, Mw/Mn, and Tm (see SI). Catalyst 4
provides the best combination of activity and polymer
crystallinity, although it should be noted that these materials
are more similar to linear low-density PE rather than high-density
PE on the basis of their melting points. In addition, polymers
produced by 4 at 22 °C have a very narrow molecular weight
distribution (<1.3), consistent with living polymerization
behavior. At 0 °C, 4 generates semicrystalline polyethylene
Figure 2. DSC traces of polymers obtained using 4 at different
concentrations of 1-decene at 22 °C (Table 1, entries 8−12 and SI).
f
the presence of a relatively intense polyethylene (CH2) signal at
29.2 ppm and other low intensity signals from methyl and alkyl
branch defects (Figure 3). Similar to the trend observed with
DSC measurements, 13C NMR spectra show a drastic reduction
with a Tm of 107 °C, χL of 0.80 and enthalpy of fusion (−ΔHm )
of 112 J/g. A further reduction in temperature does not result in
an increase in Tm, and catalyst activity decreases substantially.
These results prompted us to evaluate the scope of this
polymerization with other LAOs at 0.1 M (Table 2). Each LAO
leads to semicrystalline polyethylene with primarily ω,1
enchainment units with χL ranging from 0.63 to 0.77 (entries
1−4). An increase in the monomer chain length shows a
corresponding increase in Tm. With 1-octadecene as the
monomer, the Tm of the polymer rises to 113 °C (entry 4).
Complex 4 is successful at generating semicrystalline polymers
from different monomers; qualitatively, the polymers exhibit very
similar 13C NMR spectra, i.e., a distinct polyethylene CH2 signal
at 29.2 ppm with minor signals from branching (Figure 3 and SI).
To assess the practicality of the process, an equimolar mixture
of multiple LAOs (C6, C8, C10, and C18) was polymerized by 4/
MAO. For comparison purposes, the total olefin concentration
was maintained at 0.1 M (0.025 M each). Remarkably, the
polymerization led to semicrystalline polyethylene with a melting
point of 103 °C (Figure 4). This result suggests that
semicrystalline polyethylene can be prepared from mixed α-
olefin feeds, reducing the need for tedious separation processes.
In conclusion, we report that bulky “sandwich” α-diimine
Ni(II) catalysts convert α-olefins into semicrystalline polymers
with Tm >100 °C. Quantitative NMR analysis of the polymers
suggests that the catalysts are highly selective for 2,1-insertion
Figure 3. 13C NMR spectra and assignments of polymers obtained using
4 (Table 1, entries 8 and 12).
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Journal of the American Chemical Society
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a
Table 2. Polymerization of Linear α-Olefins by 4
d
mole fraction of units (χ)
b
c
c
e
e
entry
α-olefin
yield (g) TON
Mn (kg/mol) Mw/Mn
linear (χL) methyl-branched (χM) alkyl-branched (χA) −ΔHm (J/g) Tm (°C)
1
2
3
4
1-hexene
0.08
0.04
0.06
0.06
195
78
24
18
32
25
1.3
1.3
1.2
1.3
0.76
0.73
0.77
0.63
0.17
0.17
0.16
0.14
0.07
0.10
0.07
0.23
77
79
76
97
95
99
1-octene
1-decene
86
106
113
1-octadecene
45
a
Polymerization conditions: [α-olefin] = 0.1 M, catalyst 4 = 5 μmol in 2 mL chlorobenzene, [MAO]/[4] = 200, Trxn = 22 °C, trxn = 24 h, toluene (37
b
c
mL). TON = (mol α-olefin consumed)/(mol Ni). Determined using GPC in 1,2,4-trichlorobenzene at 150 °C vs polyethylene standards.
d
e
Determined using 1H NMR spectroscopy, see SI for detailed calculations. Enthalpy and peak melting point (second heat) determined using DSC.
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Figure 4. Single-pot polymerization of a mixture of linear α-olefins using
catalyst 4 (see Figure 3 for assignment of defect peaks).
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and data. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
coates@cornell.edu; olafs@uh.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.W.C. thanks the NSF Center for Chemical Innovation (CHE-
1136607) for financial support. O.D. is grateful to the Dreyfus
and Welch Foundations (Grant E-5171).
(14) Although methyl- and alkyl-branched units likely result from 1,2-
insertion with and without chain-walking, respectively, we cannot rule
out other mechanisms, including incomplete chain-walking.
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