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Synthesis of Semicrystalline Polyolefin Materials: Precision Methyl Branching via Stereoretentive Chain Walking David N. Vaccarello, Kyle S. O'Connor, Pasquale Iacono, Jeffrey M Rose, Anna E Cherian, and Geoffrey W. Coates J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02963 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018
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Journal of the American Chemical Society
Synthesis of Semicrystalline Polyolefin Materials: Precision Methyl Branching via Stereoretentive Chain Walking David N. Vaccarello,‡ Kyle S. O’Connor,‡ Pasquale Iacono, Jeffrey M. Rose, Anna E. Cherian, and Geoffrey W. Coates* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
Supporting Information Placeholder ABSTRACT: We report the discovery of C2-symmetric nickel catalysts capable of the regio- and isoselective polymerization of 1-butene to produce isotactic 4,2-poly(1-butene), a new semi-crystalline polyolefin. The catalyst exhibits enantioface selectivities as high as 84% and the resulting polymers display melting temperatures up to 86 °C. This system marks a rare example of preserving stereochemistry through a chain walking polymerization process.
Given the massive global production of polyethylene (PE) and isotactic polypropylene (iPP) and the importance of these materials to modern society, new semicrystalline polyolefins produced from readily available, inexpensive feedstocks are of significant interest. The olefin 1-butene meets these criteria and can easily be accessed from both petroleum and biorenewable sources.1 Polymerization of 1-butene using Ziegler-Natta catalysis results in isotactic 1,2-poly(1-butene) which exhibits superior mechanical properties relative to iPP and PE.2 However, its complex crystallization hinders its use in many practical applications.3 Crystallization from the melt gives a kinetically favored polymorph, which slowly transforms over several days at room temperature to yield a more thermodynamically stable structure resulting in dimensional changes of the material. Recent advances have attempted to address this issue, potentially allowing iso-1,2-poly(1-butene) to be more widely utilized.3 Since their discovery for olefin polymerization, α-diimine nickel(II) and palladium(II) catalysts have received considerable attention.4 A unique mechanistic feature of these catalysts is their ability to undergo an isomerization event involving successive β-hydrogen eliminations followed by metal hydride reinsertions with opposite regiochemistry. Commonly known as “chain walking,” this isomerization event places the active species at various positions along the polymer backbone, allowing for the preparation of numerous polymer topologies from simple olefin feedstocks. Exhibiting control over polymer tacticity using late metal catalysts, due to chain walking, is a challenge. By nature of the chain walking mechanism, previously installed stereocenters can easily be planarized and potentially racemized during polymerization. Notable advancements have been achieved, with Brookhart and DuPont reporting the polymerization of cyclopentene using α-diimine Ni(II) and Pd(II) complexes to form partially isotactic cis-1,3-polycyclopentene.5 Takeuchi has accessed a variety of stereoregular structures through the polymerization of non-conjugated dienes and cycloolefins.6 Additionally, Sigman and coworkers identified a rare example of enantioselective chain walking in a redox-relay oxidative Heck-arylation using chiral
palladium catalysts. After arylation, the catalyst walks through a branch point and maintains a high enantiomeric ratio of the original stereocenter.7 Despite this progress, there are very few reports for the chain walking polymerization of simple linear α-olefins into precision branched, stereoregular structures. Wagener and coworkers have pursued acyclic diene metathesis as an elegant strategy for producing precision branched polyolefins of the type desired from precise chain-walking polymerization, but this method has not yet provided stereoregularity.8 We reported the polymerization of trans-2-butene into iso-1,3-poly(2butene);9 however, the catalyst system was only modestly isoselective ([mm] = 0.41–0.64) with the resulting polymers exhibiting low melting temperatures (Tm < 67 °C). We believed a highly isoselective catalyst capable of preserving the stereochemistry from the initial insertion event could produce isotactic materials with an improved melting temperature from more readily available 1-butene (Scheme 1). Herein, we report on the development of α-diimine nickel complexes for the polymerization of 1-butene (Figure 1). To the best of our knowledge, there are no reports of catalysts that can perform both isoselective 1alkene insertion and stereoretentive chain walking polymerization in tandem.
Scheme 1. 4,2-Polymerization of 1-Butene via Stereoretentive Chain Walking. Ar R'
1-butene
4,2-poly(1-butene) 4
R
N Ni N Br 2 R' Ar + [Al(Me)O] x cocat.
+
LNi P
R
+
P
β-H
β-H
P +
NiL H
+
LNi
ins.
ins.
LNi
1
LNi
P + NiL
H
P LNi
+
H
2
n
+
stereoretentive chain walking
P
3
P
ins. β-H
P +
L Ni H
Application of complex 1 activated with methylaluminoxane (MAO) for the polymerization of 1-butene resulted in amorphous polymer (Table 1, entry 1).10 Analysis of the material by 13C NMR spectroscopy revealed a microstructure composed primarily of 4,2poly(1-butene) (Scheme 1). However, a lack of stereocontrol as evidenced by a low [mm] triad value resulted in a lack of crystallinity (Table 1, entry 1). We hypothesized that using a cumyl-derived
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Table 1. Catalyst Screen for the Polymerization of 1-Butene. a
entry complex
yield (mg)
TOFb (h−1)
Mn(theo) (kDa)
Mnc (kDa)
Đc (M w /M n )
44
59.8
70.0
enchainment typed (mole fraction)
[mm]d
αe
Tm f (°C)
4, 2
4, 1
1, 2
1.26
0.88
0.06
0.02h
0.43
0.74
–g
1
1
598
2
2
122
9
12.2
12.8
1.60
0.94
0.04
0.02
0.75
0.91
77
3
3
233
17
23.3
23.5
1.48
0.93
0.03
0.04
0.79
0.92
80
4
4
220
16
22.0
21.0
1.51
0.91
0.04
0.05
0.73
0.90
73
0.80
0.93
85
h
5
5
234
17
23.4
14.7
1.57
0.93
0.03
0.03
6
6
200
15
20.0
18.5
1.60
0.91
0.03
0.06
0.80
0.93
86
7
7
298
22
29.8
20.5
1.69
0.93
0.03
0.04
0.84
0.94
85
a
Polymerization conditions: 3.0 ± 0.2 g 1-butene (54 mmol), 10 µmol Ni complex in 2 mL CH2Cl2, 1.0 mmol MAO; trxn = 24 h, Trxn = −40 °C. bTurnover frequency, TOF = (mol 1-butene consumed)(mol complex • time)−1. cDetermined by GPC versus polyethylene standards. dDetermined using 13 C NMR spectroscopy; see Supporting Information. eProbability factor of three consecutive 4,2-units having the same relative stereochemistry, determined using the equation [mm] = α3+(1−α)3. fDetermined by DSC. gTm not detectable. hAdditional signals are present that correspond to long chain branching. 1: R 2: R 3: R 4: R 5: R 6: R 7: R
Ar R' R
N
N Ni Br 2 R' Ar
R
= Me, R’ = H, Ar = Mesityl = Me, R' = Me, Ar = Phenyl = F, R' = Me, Ar = Phenyl = OMe, R' = Me, Ar = Phenyl = tBu, R' = Me, Ar = Phenyl = Me, R' = Me, Ar = 1-Naphthyl = tBu, R' = Me, Ar = 1-Naphthyl
Figure 1. α-Diimine nickel complexes used in this study. complex (Figure 1, complex 2) may be more effective for the regioand stereoselective polymerization of 1-butene, with the additional steric bulk potentially preserving the stereochemical information imparted by the initial insertion event Activation of complex 2 with MAO in the presence of 1-butene resulted in the production of a white, powdery material. GPC and DSC analysis revealed a polymer with modest molecular weight (12.8 kDa), controlled unimodal dispersity (1.60), and a melting transition temperature (Tm) of 77 °C (Table 1, entry 2). Analysis by 13C NMR spectroscopy revealed a microstructure composed primarily of 4,2-poly(1butene) units arising from 4,2-enchainment (Scheme 2) and high amounts of isotacticity ([mm] = 0.75). The observed tacticity is considerably higher than that reported previously for the polymerization of trans-2-butene.9
Scheme 2. Enchainment Pathways for the Polymerization of 1-Butene Using α-Diimine Nickel Catalysts. P
1,2-ins. +
LNi P
P
+
LNi
+
3
4
LNi
n
1,2-poly(1-butene)
full chain walk
1-butene
2
1
LNi
+
P
4
2
3
+
LNi P 2,1-ins. +
NiL
P
1 n
full chain walk
4,2-poly(1-butene)
LNi
+
P
4
3
2
1
n
4,1-poly(1-butene)
The addition of ortho-cumyl groups had a dramatic effect on the isoselectivity of the resulting 4,2-poly(1-butene). We believe that the steric environment created from this substitution pattern facilitates the necessary enantioface coordination event leading to stereoretentive chain isomerization. A π-stacking interaction with the ortho-aryl substituents and the acenaphthene backbone is consistently observed in
Figure 2. X-ray crystal structure of complex 5. Hydrogen atoms are omitted for clarity. Atoms drawn at the 50% probability level. the solid state for cumyl-derived complexes (Figure 2).11 We believe this π-stacking interaction persists in solution at low reaction temperature, causing the catalyst to maintain a very specific steric environment which contributes to the observed selectivity. We also cannot rule out the possibility that the catalyst structure may prevent racemization of the stereocenter after planarization during chain walking. To probe our assumptions regarding the source of selectivity for 1butene polymerization, perturbations to the ligand framework were made. Electronic variants of the ortho-cumyl nickel complexes were first studied. It was found that MAO-activated 3 with electron withdrawing para-fluoro substituents produced materials with higher molecular weight (23.3 kDa), slightly elevated Tm (80 °C), and improved isotacticity ([mm]= 0.79) compared to 2/MAO. MAO-activated 4, with electron donating para-methoxy substituents, however, produced materials with a decreased Tm (73 °C) and lower isotacticity ([mm]= 0.73). Selectivity for 4,2-enchainment also decreased slightly for 4/MAO (0.91). We also prepared and studied complex 5 with paratert-butyl substituents that are slightly more electron donating than the methyl substituents of complex 2. To our surprise, 5/MAO produced iso-4,2-poly(1-butene) with high levels of isotacticity ([mm] = 0.80) and an improved Tm of 85 °C (Figure 3). One possibility is that the tert-butyl substituents in 5 affect the ion pairing interaction of the cationic nickel complex and the MAO activator such that an improvement in selectivity is observed.12 We continued probing the system by modifying the aryl substituent of the cumyl group from phenyl (2) to 1-naphthyl (6). Complex 6/MAO produced polymer with similar tacticity to that of complex 5/MAO ([mm] = 0.80) and a similar Tm of 86 °C. We suspect the πstacking interaction between the 1-naphthyl group and the acenaphthene backbone is enhanced due to the additional π overlap, resulting in a more rigidified structure that contributes to the desired selectivity.
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Journal of the American Chemical Society After observing the enhanced selectivities for materials produced by MAO-activated 5 and 6, we attempted to increase the steric bulk of the system further by preparing complex 7, bearing both tert-butyl and 1naphthyl substituents (Figure 1). The polymer produced by 7/MAO exhibited the highest isotacticity out of all samples previously generated ([mm] = 0.84). Interestingly, despite the increase in tacticity, there was no improvement in the observed melting temperature of the resulting material (Table 1, entry 7). C C
C
mm
B
A
D
E
F
L M
m
r
m
r
r
A
F 4,1-unit
K
m
4,2-unit
B mr
J
rr
I
1,2-unit H
K
JD
L F
M
E, I
H
Figure 3. 13C NMR spectrum of iso-4,2-poly(1-butene) produced by complex 5/MAO (Table 1, entry 5). The thermal properties of iso-4,2-poly(1-butene) are influenced not only by tacticity, but also by branch composition. A small percent of ethyl branches were detected in these systems which can be installed through 1,2-insertion of 1-butene without chain walking. Ethyl branches can considerably reduce Tm and crystallinity compared to methyl branches.13 Another defect observed in this system arises from 4,1enchainment, where 2,1-insertion followed by complete chain walking produces a linear “polyethylene” segment. Due to the small amount of these units (< 5%), it is difficult to quantify its effect on the properties of the resulting iso-4,2-poly(1-butene).14 Recent studies have shown that iPP with < 5% 3,1-insertions, made with related cumyl-substituted catalysts retained most of its crystallinity, suggesting that the effect in this system may also be small.15
Table 2. Temperature and Concentration Effects on the Polymerization of 1-Butene Using 5/MAO. a entry
Trxn (°C)
conc (M)
yield (mg)
Mnb Đb Tm c [mm]d (kDa) (Mw/Mn) (°C) e
n.d.
f
1
−10
7.8
262
10.8
1.95
–
2
−30
7.8
420
23.7
1.80
66
0.53
3
−40
7.8
234
14.7
1.57
85
0.80
4
−50
7.8
173
10.6
1.81
85
0.80
5
−40
6.0
g
163
12.4
1.46
80
0.70
6
−40
9.1h
464
16.3
2.17
73
0.59
a
Polymerization conditions: 3.0 ± 0.2 g 1-butene (54 mmol), 10 µmol of complex 5 in 2 mL CH2Cl2, 100 equiv. MAO; trxn = 24 h. bDetermined by GPC versus PE standards. cDetermined by DSC. dDetermined using 13C NMR spectroscopy. eNo detectable Tm. fNot determined. g1.5 ± 0.2 g 1-butene (27 mmol). h6.0 ± 0.2 g 1-butene (107 mmol). We further studied the effects of reaction conditions on the thermal properties of the resulting polymers produced by 5/MAO (Table 2). The reaction temperature had a dramatic effect on polymer thermal properties. By increasing the temperature from −40 °C to −30 °C, the melting temperature of the resulting material dropped considerably from 85 °C to 66 °C corresponding to a significant loss in tacticity (Table 2, entry 2). Increasing the reaction temperature further to −10 °C resulted in a complete loss of crystallinity. Cooling the reaction
further to −50 °C decreased the polymerization rate and polymer yield, and resulted in materials with similar thermal properties to those produced at −40 °C (Table 2, entry 3). The effect of 1-butene concentration was also studied. Increasing the concentration from the standard 7.8 M to 9.1 M (Table 2, entry 6). resulted in higher polymer yield but a marked decrease in tacticity ([mm] = 0.59) and melting temperature (Tm = 73 °C). A decrease in 4,2-enchainment in favor of 1,2-enchainment was also observed at a higher concentration of 1-butene.16 Decreasing the concentration of 1butene to 6.0 M resulted in lower polymer yield, but also a slight decrease in tacticity ([mm] = 0.70) and melting temperature (Tm = 80 °C: Table 2, entry 5). Lower 1-butene concentrations may increase the rate of chain isomerization relative to insertion, allowing introduction of stereoerrors. Returning to our assertion of stereoselective insertion and stereoretentive chain walking, it is not immediately clear why complex 1/MAO produces iPP from propylene but atactic polymer from 1-butene. Propylene polymerization can proceed through two different mechanisms that lead to the same iPP product: (1) consecutive isoselective insertions without chain walking, or (2) consecutive isoselective insertions with full stereoretentive chain walking. To confirm the operative mechanism for propylene, we performed a study using [3-d3]propylene at −78 °C and 1/MAO. If chain walking occurs, the deuterium atoms should scramble throughout the polymer backbone. Under these conditions, however, the deuterium atoms remained on the methyl branches of the polymer, demonstrating that chain walking during propylene polymerization does not occur (Scheme 3).16 This suggests that for the polymerization of 1-butene, complex 1/MAO either does not perform the isoselective insertion of 1-butene, or tacticity is lost through racemization of the installed stereocenter during chain isomerization. Since the formation of iso-4,2-poly(1-butene) requires chain isomerization, the isotactic enchainment of 1-butene in this work represents a rare example of stereoretentive chain walking.
Scheme 3. Deuterated Propylene Study Using 1/MAO. no chain walk +
LNi-P
CD 3 1,2-ins.
cat. 1
P
+
LNi
P
+
LNi
observed
D 3C D
D 3C full chain walk
+
LNi
P D D
not observed
In summary, we performed the iso- and regioselective polymerization of 1-butene using cationic α-diimine nickel(II) complexes to produce a new polymer, isotactic-4,2-poly(1-butene). The methyl substituents on the cumyl-groups of the ligand are crucial for the isoselective insertion and stereocontrolled chain walking. Furthermore, rigidifying the catalyst through π-stacking interactions with the ligand backbone was beneficial for improving tacticity. This system allows access to isotactic polymers from a simple, inexpensive α-olefin. Additional catalyst development is currently in progress to further increase the selectivity of this unique polymerization.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures and spectral data (PDF) and crystallographic data (CIF)
AUTHOR INFORMATION Corresponding Author E-mail:
[email protected] Author Contributions
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Authors contributed equally to this work.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS Funding was provided by the Center for Sustainable Polymers, a National Science Foundation (NSF)-supported Center for Chemical Innovation (CHE-1413862). This research made use of the NMR facility at Cornell University and is supported, in part, by the NSF under the award number CHE-1531632.
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