Polymerization of α-Olefins Using a Camphyl α-Diimine Nickel

May 12, 2014 - One noteworthy example is cyclophane-based α-diimine nickel catalyst ..... in a living manner in a wide temperature range within a cer...
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Polymerization of α‑Olefins Using a Camphyl α‑Diimine Nickel Catalyst at Elevated Temperature Jun Liu, Darui Chen, Han Wu, Zefan Xiao, Haiyang Gao,* Fangming Zhu, and Qing Wu* PCFM Lab, DSAPM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China S Supporting Information *

ABSTRACT: Chiral (1S)- and (1R)-camphyl α-diimine nickel complexes were synthesized respectively with (1S)-(+) camphorquinone and (1R)-(−) camphorquinone as raw reagents and used as catalyst precursors for olefin polymerizations. It is found that the ligand chirality has no influence on catalytic activity and regioselectivity for olefin polymerizations. Ethylene, propylene, 1hexene, and 4-methyl-1-pentene polymerizations with the camphyl α-diimine nickel activated by AlEt2Cl can exhibit some living characteristics under the optimized conditions. The resultant polypropylenes and poly(1-hexene)s have significantly narrow molecular weight distributions (PDI < 1.2) in a wide temperature range, even at an elevated temperature of 70 °C. Sustainable period of the linear relationship of Mn vs polymerization time depends on temperature for propylene and 1-hexene polymerizations. Additionally, high 1,3-enchainment fraction of 45% is observed even at −60 °C for propylene polymerization using the camphyl α-diimine catalyst due to 2,1-insertion of propylene and chain walking.

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INTRODUCTION Living/controlled catalytic olefin polymerization is unsurpassed in terms of microstructure control of polymer, which allows controlled synthesis of polyolefins with precise architectures.1 Currently, living/controlled polymerization of olefins is a great challenge especially at elevated temperatures because of the processes of chain termination by β-H elimination and polymer chain transfer to cocatalyst or monomer.2−4 Moreover, short lifetime and rapid deactivation of living metal species also limit achievement of living/controlled catalytic olefin polymerization.5,6 For those reported in the literature, most living/ controlled polymerizations of olefins were conducted at subambient temperatures with short reaction time.7−10 Despite some successful applications of α-diimine nickel catalysts in living/controlled polymerizations of α-olefins such as propylene,4,7−9 1-hexene,11−13 and 4-methyl-1-pentene,14−16 living/ controlled characteristics for α-olefin polymerizations are rarely observed at elevated temperature (above room temperature) to date. One noteworthy example is cyclophane-based α-diimine nickel catalyst reported by Guan that can polymerize propylene and 1-hexene in a living/controlled fashion at 50 °C.17 Development of robust late transition metal catalysts can facilely synthesize novel polyolefin materials with different microstructures by living/controlled polymerization in combination with chain walking. In addition, late transition metal catalysts, such as α-diimine Ni and Pd, are distinctively featured with chain walking that the metal centers “walk” along the polymer chain by β-hydride elimination and isomerization process prior to insertion of next monomer. Numerous theoretical and experimental studies on the chain walking mechanism have shown that α-diimine ligand symmetry, steric bulk, and reaction conditions have significant © 2014 American Chemical Society

influence on regio- and stereoselectivity as well as microstructure of polyolefin.18−22 Generally, syndiotactic polypropylene is prevailingly produced at −78 °C using a Brookhart-type α-diimine nickel catalyst,22−24 and chain walking process is largely limited by lowering reaction temperature. Coates has synthesized a C2-symmetry α-diimine nickel catalyst with bulky chiral anilines that can polymerize propylene and other αolefins in a living fashion.9,25 This catalyst exhibits temperaturedependent regioselectivity whereby isotactic polypropylene is obtained at a low temperature of −78 °C and regioirregular polypropylene (rirPP) composed of 1,2-insertions and 1,3enchainments is produced at a high temperature of 22 °C.9 Similarly, an α-keto-β-diimine nickel catalyst in the absence of chiral environment also exhibits living characteritics for polymerization of α-olefins at low temperature and temperature-dependent regioselectivity.12,26 Although living/controlled olefin polymerizations with αdiimine nickel catalysts have been reported at low temperatures, improvement of thermal stability of α-diimine nickel catalyst and achievement of living/controlled polymerization of olefins at elevated temperature are industrially desirable. Previously, we successfully explored a type of thermostable α-diimine nickel and palladium catalysts with bulky camphyl backbone for olefin polymerizations.27−29 The camphyl α-diimine nickel catalyst is capable of polymerizing ethylene at 0−80 °C in the presence of MAO and affords branched polyethylenes with high molecular weight.27 The enhanced thermal stability is attributed to its rigid and bulky bicyclic-substituted backbone. Inspired by Received: March 3, 2014 Revised: April 25, 2014 Published: May 12, 2014 3325

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Scheme 1. Synthesis and Structure of α-Diimine Nickel Complexes with Chiral Camphyl Backbones

remarkable thermostability of camphyl α-diimine nickel catalyst, we mainly report polymerizations of α-olefins in this paper. Polymerization of propylene and 1-hexene at an elevated temperature of 70 °C can still afford the polymers with narrow molecular weight distributions (PDI < 1.2). In contrast to previous work on propylene polymerizations with an α-diimine nickel catalyst with methyl backbone, high fraction of 2,1insertion of propylene is observed for camphyl α-diimine nickel catalyst, which leads to frequent occurrence of 1,3-enchainment even at −60 °C.

transition region exhibit similar Cotton effects, suggesting that the chiral nickel complexes have similar electronic configuration and coordination sphere.33 Olefin polymerizations were performed using these two chiral nickel complexes with AlEt2Cl as cocatalyst. The polymerization results listed in Table 1 show that these two catalytic systems exhibit some living characteristics for propylene, 1-hexene, and 4-methyl-1-pentene polymerizations at 20 °C and produce narrowly dispersed polymers (PDI < 1.2). When polymerization temperature dropped to −10 °C and ethylene pressure reduced to 0.6 atm, polyethylene (PE) with a narrow molecular weight distribution (PDI = 1.19) was also obtained (entry 3 in Table 1). The comparisons of the same monomer polymerizations catalyzed by camphyl αdiimine nickel catalyst enantiomers show no real difference in activity under the same polymerization conditions (entries 1, 4, 5 vs 7, 8, 9 in Table 1). Moreover, the molecular weight, branching density, and melting point (Tm) of the same kind of polymer obtained by different chiral catalysts are basically invariable within the limits of experimental error (see Figure S3 in Supporting Information). Similarly, there was no observed difference between two chiral complexes 1 and 2 upon activation with MMAO instead of AlEt2Cl for propylene and 1-hexene polymerization (see Table S1 in Supporting Information). An experiment was further carried out by a mixture of complexes 1 and 2 in a mole ratio of 1:1 for propylene polymerization. The obtained polypropylene (entry 10 in Table 1) shows a narrow and unimodal GPC trace (Figure 2). These results demonstrate that the chirality of the backbone has no influence on polymerization of olefins, which may be attributed to a remote location of the chiral carbon to nickel metal. Nearly the same electronic configurations and coordination spheres of two chiral nickel complexes also support this claim (Figure 1b). In view of the same catalytic property of two chiral nickel complexes, (1S)-camphyl αdiimine nickel complex 1 was selected to further study α-olefin polymerization. Propylene and 1-Hexene Polymerizations. Previous reports have showed that alkylaluminum compounds as cocatalysts play an important role in olefin polymerization with α-diimine nickel catalytic system.13,14,34,35 In particular, for living polymerization, alkylaluminum compounds as potential chain transfer agents may destroy living manner of olefin polymerization. On the basis of previous study,14 AlEt2Cl was used as cocatalyst in polymerizations of α-olefins. As shown in Table 2, both catalytic activity for propylene polymerization and molecular weight of the polymer increase gradually with increasing Al/Ni mole ratio from 50 to 600 at a

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RESULTS AND DISCUSSION Effect of Chiral Camphyl Backbone. Previously, we have reported the camphyl α-diimine nickel complexes, which were synthesized by (D,L)-camphorquinone as raw reagent, as catalyst precursors for ethylene polymerization.27 Note that α-diimine nickel complex with camphyl backbone has two chiral isomers,30,31 it is thereby interesting to evaluate the effect of the chiral backbone on α-olefin polymerization. As shown in Scheme 1, two chiral reagents including (1S)-(+)camphorquinone and (1R)-(−)camphorquinone were used to synthesize the corresponding α-diimine ligands L1 and L2. Although NMR spectra of the two chiral ligands are identical, the circular dicroism (CD) spectrum of (1S)-ligand L1 is a mirror image of that of its (1R)-enantiomer L2 (Figure 1a).

Figure 1. CD spectra of camphyl α-diimine ligands (a) and nickel complexes (b) in CH2Cl2 at room temperature.

The bands at 340 and 430 nm are attributed to π−π* transitions originating from the imine bonds. Complexation of the chiral α-diimine ligands with (DME)NiBr2 (DME = 1,2dimethoxyethane) afforded two chiral camphyl α-diimine nickel complexes 1 and 2. In the CD spectra of two complexes in CH2Cl2 (Figure 1b), the bands at 530 nm with negative Cotton effect are assigned to the d−d transition, while the d−π* transitions lie near 360 and 480 nm.32 By comparison of the CD spectra of these two complexes, the signals in the d−d 3326

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Table 1. Results of Olefin Polymerizations with Chiral Nickel Catalysts (E = Ethylene, P = Propylene, H = 1-Hexene, 4MP = 4Methyl-1-pentene)a entry

cat.

monomer

time (h)

T (°C)

yield (g)

activityf

Mng

PDIg

branches/1000Ch

Tmi (°C)

1 2 3 4 5 6 7 8 9 10

b

E E E P H 4MP E P H P

0.5 0.5 0.5 1 2 2 0.5 1 2 1

20 −10 −10 20 20 20 20 20 20 20

0.93 1.30 0.14 0.16 0.26 0.23 0.91 0.17 0.27 0.17

372 520 56 16 26 5.6 364 17 27 17

225.0 390.1 84.6 53.8 98.7 24.2 215.6 57.4 97.5 55.1

1.41 1.27 1.19 1.13 1.19 1.15 1.56 1.20 1.19 1.17

67 41 40 141 65 241 71 142 64 142

43.5 71.4 71.8 30.1 68.2 j 40.1 31.3 70.1 31.0

1 1b 1b 1c 1d 1e 2b 2c 2d 1/2c

a Reaction conditions: cocatalyst AlEt2Cl, Al/Ni = 1000; solvent: toluene, total volume =20 mL. bEthylene polymerization: nNi = 5 μmol, 1.2 atm of ethylene, 0.6 atm of ethylene for entry 3. cPropylene polymerization: nNi = 10 μmol, 5 μmol of 1 and 5 μmol of 2 for entry 10; 1.2 atm of propylene. d nNi = 5 μmol; [hexene] = 1 M. enNi = 20 μmol; [4MP] = 2.4 M; total volume =10 mL. fActivity was calculated as the kg of polymer per mole of catalyst per hour, in unit of kgPO molNi−1 h−1. gMn (kg mol−1) and PDIs were determined by GPC against polystyrene standard. hBranches per 1000C, which were determined by 1H NMR spectroscopy. iBroad endotherms, determined by DSC. jNo obvious melting endotherms.

amount of cocatalyst is required to activate and stabilize the nickel active center, and AlEt2Cl only plays the role of activator rather than a chain-transfer agent even at high Al/Ni ratio. The slightly broad molecular weight distribution of the polymer obtained at low Al/Ni ratio can be ascribed to a slow activation of the catalyst at low AlEt2Cl concentration. In general, Brookhart-type α-diimine nickel and palladium catalysts for olefin polymerization are highly sensitive to temperature and prone to rapid deactivation at elevated temperature.4,7,8 Under the fixed polymerization conditions (Al/Ni = 1000 and polymerization time t = 1 h), propylene polymerizations with 1/AlEt2Cl catalyst system in a temperature range from −60 to 70 °C afford the polypropylenes with significantly narrow polydispersity (1.13−1.31). Because of slow activation of the catalyst and embedment of nickel active species in the precipitated polymer at low temperature,12,36 the PPs obtained below 0 °C have slightly broad molecular weight distribution. Additionally, the molecular weight distribution of the polymer obtained at 70 °C can be also narrowed from 1.31

Figure 2. GPC trace of the polypropylene obtained with a mixture of nickel catalysts 1 and 2 in a ratio of 1:1 at 20 °C.

fixed temperature of 40 °C, and then remain constant when Al/ Ni mole ratio further increases to 1000. It is interesting to note that narrow polydispersity of the PPs is nearly invariable with respect to the Al/Ni ratio except for a low ratio of 50 in which PDI slightly widens to 1.32. This observation indicates enough

Table 2. Results of Propylene (P) and 1-Hexene (H) Polymerizations with 1/AlEt2Cla entry

monomer

Al/Ni

T/time (°C/h)

yield (g)

activityd

Mne

PDIe

branches/1000Cf

mol %g 1,ω-enchain.

Tmh (°C)

1 2 3 4 5b 6b 7 8 9 10 11 12c 13c 14c 15c

P P P P P P P P P P P H H H H

50 200 600 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000

40/1 40/1 40/1 40/1 −60/12 −40/12 0/1 20/1 50/1 70/1 70/0.5 20/2 50/2 70/0.5 70/1

0.14 0.15 0.19 0.20 0.17 0.49 0.09 0.16 0.16 0.14 0.07 0.09 0.21 0.12 0.21

14 15 19 20 0.7 2.0 9.0 16 16 14 14 9 21 48 42

42.9 51.9 58.0 57.0 51.1 143.4 45.1 54.1 54.9 33.4 21.3 60.4 95.6 47.4 63.9

1.32 1.18 1.15 1.14 1.26 1.24 1.21 1.15 1.19 1.31 1.18 1.17 1.19 1.18 1.24

143 138 140 137 184 164 145 142 129 126 128 67 66 68 63

56 59 58 59 45 51 56 57 61 62 61 60 61 59 62

i i i 31.5 −0.5 −0.1 29.6 30.9 34.3 26.7 27.2 68.4 72.8 63.5 65.8

a Reaction conditions: nNi = 10 μmol; 1.2 atm of propylene; solvent: toluene (total volume =20 mL). bnNi = 20 μmol; mpropylene = 9 g. cnNi = 5 μmol, [1-hexene] = 0.5 M. dActivity was calculated as the kg of monomer per mole of catalyst per hour, in unit of kgPO molNi−1 h−1. eMn (kg mol−1) and PDIs were determined by GPC against polystyrene standard. fBranches per 1000C, which were determined by 1H NMR spectroscopy. g1,ωEnchainment is calculated by eq 1, ω% = 100(1 − NR)/(1 + 2R), N = (ω − 2), R = [CH3]/[CH2].25 hBroad endotherms, determined by DSC. iNot determined.

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when the polymerization is performed within 30 min at low monomer concentration of 0.5 M (entry 14 in Table 2). Microstructure Analysis. 1H NMR spectroscopy analyses have shown that the obtained PPs have reducing branching degree relative to the theoretical value (333/1000C) and the branching degree decreases with increasing temperature, which is a result of chain walking. DSC analyses also support this claim, which shows a broad melting endotherm extending to a much lower temperature (see Figure S6 in the Supporting Information).40 The precise microstructure analyses of the PPs were further studied by 13C NMR spectroscopy (Figure 5). On

to 1.18 by shortening polymerization time from 1 h to half an hour. The narrow polydispersities suggest some living features for propylene polymerization over such a wide polymerization temperature range. To further study the feature of propylene polymerization with 1/AlEt2Cl at different temperatures, the dependence of PDI on polymerization time and sustainable time of linear growth of Mn with time were determined. Figure 3 shows the

Figure 3. Plots of Mn (■) and PDI(▲) as a function of polymerization time at 20, 40, and 70 °C under 1.2 atm of propylene. Figure 5. 13C NMR spectra of the PPs obtained at different temperatures (1,2-C6D4Cl2, 110 °C).

plots of Mn vs polymerization time at 20, 40, and 70 °C, respectively. The linear growth relationship of Mn as a function of polymerization time and narrow molecular weight distribution (PDI < 1.2) can be sustained for 240 min at 20 °C, 150 min at 40 °C, and 35 min at 70 °C, respectively. This result verifies that propylene polymerization with 1/AlEt2Cl system can proceed in a living manner in a wide temperature range within a certain period of time. To the best of our knowledge, this is one of the rare examples of late transition metal catalyzed living/controlled polymerization of olefin above room temperature for such long time.17,37−39 Similarly, polymerization of 1-hexene with 1/AlEt2Cl can also produce narrowly dispersed poly(1-hexene)s over a range from 20 to 70 °C. Low monomer concentration of 1-hexene is required to yield the polymers with narrow molecular weight distribution above 50 °C. A series of parallel polymerization of 1-hexene was carried out at 20 and 50 °C and terminated at different time, and the results are presented graphically in Figure 4. Mn grows linearly with increasing monomer conversion, and the PDI values remain low. The linear relationship of Mn vs time can be sustained for 360 min at 20 °C and 120 min at 50 °C, respectively. Narrowly dispersed poly(1-hexene) (PDI = 1.18) can also be obtained at 70 °C

the basis of previous resonance assignments of PP,41−44 various branches including methyl, adjacent methyl, butyl, 2,4dimethylpentyl, and 2-methylhexyl can be safely identified (see Figure S4 and Table S2 in Supporting Information). Figure 5 also clearly shows the dependence of branch structure of the PPs obtained by 1/AlEt2Cl on reaction temperature, and the quantitative calculation results determined by 13C NMR spectroscopy are summarized in Table 3. An obvious trend is Table 3. Frequency of Various Structures per 1000 Carbon Atoms in the Polypropylenesa temp (°C)

branches/ 1000Cb

70 40 0 −40 −60

126/136 137/138 145/146 164/170 184/198

adj. Mec

Med

Bu+ e 4 3 1

20 34

89 92 130 150 164

Bu

2MeH+

Hex+ f

5 3

17 13 5

5 4 3

i

a

Blank entries mean that the structures were not detected. bBranches per 1000C, the formers were determined from 1H NMR; the latters were determined from 13C NMR. cMethyl branches in the 14−18 ppm region. dMethyl branches in the 20−21.5 ppm region. eBu+ is the butyl or longer branches and 2-EOC (end of chain) per 1000C. fHex+ are the hexyl and longer, 3-EOC.

that only short methyl branch including adjacent methyl branch can be observed below −40 °C, while long branches and 2methylalkyl branches such as 2,4-dimethylpentyl as well as 2methylhexyl can be observed at high temperatures. These branches can be reasonably interpreted on the basis of previous chain walking mechanism study.5,6 When insertion of propylene occurs without chain walking, short methyl branches are present. The existences of adjacent methyl branches unit

Figure 4. Plots of Mn (■) and PDI(▲) as a function of monomer conversion for polymerization of 1-hexene at 20 °C (1.0 M) and 50 °C (0.5 M). 3328

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ature on branching density of PP is not well consistent with that on intrinsic viscosity, strongly indicating the change of branching type of PP with an increase in reaction temperature. When reaction temperature is below −40 °C, only short methyl branches are present on PP polymer chain. Increasing temperature from −60 to −40 °C only leads to more occurrence of 1,3-enchainment arising from 2,1-insertion. Therefore, more “linear” polymer which has maximal intrinsic viscosity is formed at −40 °C. When reaction temperature is 70 °C, butyl and long branches appear in the PP polymer chain. Though branching density decreases obviously, butyl and branch-on-branch structure (2,4-dimethylpentyl and 2-methylhexyl) dominantly govern intrinsic viscosity of the PP. Obviously low intrinsic viscosity value of the PP obtained at 70 °C is a result of the decrease of short methyl branch and the increase of long branches. Generally, the difference in branch structure of the PPs obtained at various temperatures can be reflected by intrinsic viscosity of dilute solution,48,49 which is consistent with 13C NMR results. Our previous work have reported insertion mode of 1-hexene and chain walking process on the basis of 13C NMR analysis of the poly(1-hexene)s obtained by camphyl α-diimine nickel catalyst/MMAO system.28 Herein, the microstructure of the poly(1-hexene) produced by 1/AlEt2Cl was also analyzed by 13 C NMR spectroscopy (see Figure S7 and Table S4 in Supporting Information). Generally, branching type of the poly(1-hexene)s and chain walking process are nearly same as our previous report.28 A quantitative analysis of the poly(1hexene)s listed in Table 4 shows that low branching density, high fraction of 1,6-enchainment, and I(CH2)n≥6/∑ICH2 value are observed under the adopted conditions relative to our previous report (AlEt2Cl as a cocatalyst instead of MMAO),28 indicating more frequent occurrence of 2,1-insertion of 1-hexene. The higher Tm values of the obtained poly(1-hexene)s (entries 12− 15 in Table 2) also prove that the longer methylene sequences arising from higher fraction of 1,6-enchainment are present in the polymer chain. These results can be attributed to the effect of cocatalyst because of different Lewis acidities and bulkiness of organoaluminate counteranion.14,35

must arise from 2,1-insertion of propylene monomer followed by 1,2-insertion without chain walking. When nickel metal migrates forward along the newly 2,1-added propylene monomer, ethylene-type unit coming from 1,3-enchainment can be produced. When nickel metal migrates backward along the polymer chain, more long branch can be formed. It was previously reported that a C2-symmetry α-diimine nickel catalyst can afford isotactic PPs and a Brookhart-type αdiimine nickel catalyst with methyl backbone can produce prevailingly syndiotactic PP at low temperature of −78 °C.9,23,24 Differently, the camphyl α-diimine nickel catalyst still exhibits poor stereo- and regio-control for polymerization of propylene although lowering reaction temperature weakens nickel metal migration along the polymer chain. Compared with reported spectra of the PPs obtained by α-diimine nickel catalyst at low temperature,22−24 a significant difference is the occurrence of long methylene sequence −(CH2)n− (n ≥ 6) at 30 ppm, indicating successive occurrence of 1,3-enchainment arising from the 2,1-insertion of propylene. The quantitative branch analysis by 13C NMR of the PP obtained at −60 °C shows that the percentage of 2,1-insertion of propylene into nickel metal is ∼58%, and about 77% of the time it undergoes chain straightening (1,3-enchainment),42 while the rest of the time 1,2-insertion of the next monomer occurs from the secondary carbon. The fraction of 1,3-enchainment also increases with increasing temperature, indicating that high temperature is favorable for 2,1-insertion of propylene.9 Therefore, 2,1-insertion of propylene is a major pathway and becomes more frequent with increasing temperature for camphyl α-diimine nickel catalyst, which is in contrast to previous observation for Brookhart-type α-diimine nickel catalyst.23,42 On the whole, 2,1-mode of propylene insertion occurs more frequently during the polymerization with camphyl α-diimine nickel catalyst even at very low temperature, and most of the time 2,1-insertion undergoes chain straightening by 1,3-enchainment. Branch structure of polymer is closely related to dilute solution property,45−47 so dilute solution property also enables us to examine the branch structure of the PP obtained at different temperatures. The dependency of intrinsic viscosity (IV) on polymer molecular weight detected by high temperature gel permeation chromatography with a viscosity detector can directly reflect branch structure of the PP. As shown in Figure 6, a basic tendency is that the maximal intrinsic viscosity is observed for the PP obtained at −40 °C while the minimum value is found for the PP produced at 70 °C under the same polymer molecular weight. Influence trend of reaction temper-

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CONCLUSIONS α-Diimine nickel catalysts with different chiral camphyl backbones have been successfully synthesized. However, there is no observed difference between camphor-derived nickel catalyst enantiomers in olefin polymerizations. The polymerizations of propylene and 1-hexene can exhibit some living characteristic at a high temperature of 70 °C using camphyl αdiimine nickel/AlEt2Cl within a certain period of time. Unlike the previously reported α-diimine nickel catalysts, camphyl αdiimine nickel exhibits a significant chain walking ability even at low temperature of −60 °C because of more frequent occurrence of 2,1-insertion of propylene. The robust camphyl α-diimine nickel catalyst can precisely synthesize narrowly dispersed polyolefins with various branched structures by changing reaction temperature, and also introduce possibilities for preparation of novel block polyolefins.

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EXPERIMENTAL SECTION

All manipulations involving air- and moisture sensitive compounds were carried out under an atmosphere of dried and purified nitrogen with standard vacuum-line, Schlenk, or glovebox techniques. Materials. Toluene was dried over sodium metal and distilled before use. Polymerization grade ethylene and propylene were used

Figure 6. Intrinsic viscosity (IV) as a function of Mw for polypropylene obtained at −60, −40, 0, 40, and 70 °C (the dip in the curves may be a result of the presence of long-chain branch on the polymer chain). 3329

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Table 4. Frequency of Various Structures per 1000 Carbon Atoms in the Poly(1-hexene)sa temp (°C)

branches/1000Cb

1,6-enchain.c (%)

I(CH2)n≥6/ΣICH2d

Me

Et

Pr

Bu

long

20 50 70

67 69 70

59 60 61

61.6 60.1 59.5

55 56 56

− − −

− − −

10 6 5

2 7 9

Reaction conditions: cocatalyst AlEt2Cl, nNi = 5 μmol, [1-hexene] = 0.5 M. bBranches per 1000C, determined from 13C NMR. c1,6-Enchainment is calculated by eq 1, 6% = 100(1 − 4R)/(1 + 2R), where R = [CH3]/[CH2].25 dRatio of long methylene (−(CH2)n−, n ≥ 6) to total methylene, which is determined by the ratio of integral of peak at 30.00 ppm to that for all methylene signals. a

directly without further purification. 1-Hexene and 4-methyl-1-pentene were purchased from Alfa Aesar Chemical, dried over CaH2, and distilled before use. MMAO (7 wt % Al in heptane) was purchased from Akzo-Nobel. AlEt2Cl was purchased from Acros. (1S)(+)-Camphorquinone (purity 99%) was purchased from SigmaAldrich, (1R)-(−)-camphorquinone (purity >98%) was purchased from TCI. Synthesis of (1S)- and (1R)-Camphyl-Based α-Diimine Nickel Complex. The (1S)- and (1R)-camphyl-based diimine nickel catalysts were synthesized according to the previously reported procedures.27 Ligand L1: 1H NMR (300 MHz, CDCl3), δ (ppm) [major:minor = 2.6:1] major isomer 7.02−6.82 (m, 6H, Ar−H), 2.86 (m, 4H, CH(CH3)2), 2.34 (s, 1H, at camphyl), 1.84 (m, 4H, CH2 at camphyl), 0.94 (s, 6H, CH3 at camphyl), 0.77 (s, 3H, CH3 at camphyl); minor isomer 7.02−6.82 (m, 6H, Ar−H), 2.68 (m, 4H, CH(CH3)2), 2.34 (s, 1H, at camphyl), 1.84 (m, 4H, CH2 at camphyl), 0.92 (s, 6H, CH3 at camphyl), 0.75 (s, 3H, CH3 at camphyl). 13C NMR (75 MHz, CDCl3), δ (ppm): major isomer 168.80 (CN), 145.14 (C−N), 136.32, 134.96 (CAr-iPr), 123.64, 122.90, 121.98, 56.10, 50.73, 45.60, 32.44, 28.86, 27.91, 24.83, 23.12, 22.19, 20.95, 18.12, 11.45; minor isomer: 169.20 (CN), 147.32 (C−N), 134.25, 133.25 (CAr-iPr), 123.64, 122.73, 121.68, 56.10, 50.73, 45.60, 32.44, 28.57, 27.37, 24.83, 23.36, 22.61, 20.95, 18.12, 11.45. ESI-MS (m/z): calculated [M + H]+ for C34H48N2, 485.76; found, 485.4. Ligand L2: 1H NMR and 13C NMR are basically same as ligand L1 (see Figure S1 and Figure S2 in Supporting Information). ESI-MS (m/z): calculated [M + H]+ for C34H48N2, 485.76; found, 485.5. Complex 1, FAB+-MS: m/z 622, 623, 624, 625, [M − Br]+; 542, 543, 544, 545, 546, [M − 2Br]+; 484, 485, 486, [M − NiBr2]+. Anal. Calcd for complex 1 (C34H48Br2N2Ni): C, 58.07; H, 6.88; N, 3.98. Found: C, 57.01; H, 6.81; N, 3.81. Complex 2, FAB+-MS: m/z 622, 623, 624, 625, [M − Br]+; 542, 543, 544, 545, 546, [M − 2Br]+; 484, 485, 486, [M − NiBr2]+. Anal. Calcd for complex 2 (C34H48Br2N2Ni): C, 58.07; H, 6.88; N, 3.98. Found: C, 57.53; H, 6.83; N, 3.86. Propylene Polymerization. Procedure for propylene polymerization at −60 and −40 °C. A round-bottomed Schlenk flask was filled with AlEt2Cl solution in toluene (10 mmol) and 15 mL of toluene and cooled to desired temperature, and then propylene (9 g) was condensed into the toluene/AlEt2Cl mixture. A complex solution in toluene (1 mL, 20 μmol) was added to initiate the polymerization. After the reactor was vented, the reaction was quenched with methanol. Procedure for propylene polymerization above 0 °C. A calculated volume of toluene was transferred into a round-bottomed Schlenk flask containing a stirring bar, and AlEt2Cl was added in by syringe. The reaction temperature was controlled with an external oil bath in polymerization experiments. The system was brought to the desired polymerization temperature under 1.2 atm of propylene pressure, and the complex solution in toluene (1 mL, 10 μmol) was added to initiate the polymerization. The polymerization was terminated by the addition of acidic methanol. The resulting precipitated polymers were collected and treated by filtration, washing with ethanol several times, and drying in a vacuum at 60 °C. 1-Hexene/4-Methyl-1-pentene Polymerization. A calculated volume of toluene was transferred into a round-bottomed Schlenk flask containing a stirring bar, and then AlEt2Cl and the calculated monomer were added by syringe. The reaction temperature was controlled with an external oil bath in polymerization experiments. The system was brought to the desire polymerization temperature, and

the complex solution in toluene was added to initiate the polymerization. The polymerization was terminated by the addition of acidic methanol. The resulting precipitated polymers were collected and treated by filtration, washing with ethanol several times, and drying in a vacuum at 60 °C. Characterizations. Nuclear magnetic resonance (NMR) spectra of polymers were carried out on a Varian Mercury-puls 500 MHz spectrometer at 110 °C, using 1,2-C6D4Cl2 as solvent. The NMR data of the ligands were determined on a Varian Mercury-plus 300 MHz spectrometer at room temperature, using CDCl3 as solvent. The circular dichroism spectra were recorded on a JASCO J-810 spectropolarimeter in CH2Cl2 solution. ESI-MS spectra were obtained with a Shimadzu LCMS-2010A instrument. Elemental analyses were performed on a Vario EL microanalyzer. The molecular weight and the molecular weight distribution of polyolefins were determined on PLHTGPC 220 Integrated GPC/SEC System equipped with a differential refractive index detector (RI), Viscotek 220R viscometer detection system and PD 2040 binary-angle (15, 90°) light scattering detectors (λ = 658 nm) at 150 °C and 1,2,4-trichlorobenzene (TCB) with 0.0125% BHT was employed as the eluent at a flow rate of 1.0 mL/min. DSC analyses were conducted with a PerkinElmer DSC-7 system. Liquid nitrogen was used as coolant and nitrogen was used as purging gas for experiments. The DSC curves were recorded at second heating curves from −100 to +150 °C at a heating rate of 10 °C/min and a cooling rate of 10 °C/min.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra of ligands, 13C NMR of polymers and assignments, results of olefin polymerizations, DSC curves of polymers, and GPC traces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(H.G.) E-mail: [email protected]. * (Q.W.) E-mail: [email protected];. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports by National Natural Science Foundation of China (NSFC) (Projects 21174164, 51173209, 21274167, and 21374134), Technology Innovation Project of Educational Commission of Guangdong Province of China (2013KJCX0002), and the Opening Fund of Laboratory Sun Yat-Sen University are gratefully acknowledged.

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REFERENCES

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