Homo- and Heteroligated Salicylaldiminato Titanium Complexes with

Article pubs.acs.org/Macromolecules

Homo- and Heteroligated Salicylaldiminato Titanium Complexes with Different Substituents Ortho to the Phenoxy Oxygens for Ethylene and Ethylene/1-Hexene (Co)polymerization Erdong Yao, Jianchun Wang, Zhongtao Chen, and Yuguo Ma* Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: A series of homoligated (1c−1e) and heteroligated (2a−2e) salicylaldiminato titanium dichloride complexes with different substituents ortho to the phenoxy oxygens were efficiently prepared. X-ray diffraction studies on these new dichloride complexes 2b, 2d, and 2e reveal a distorted octahedral coordination of the central metal. In the presence of dried methylaluminoxane, all the complexes exhibit high ethylene polymerization productivities. Surprisingly, complex 1d incorporating an o-(trimethylsilyl)ethynyl group displays the highest activity [5.26 × 103 kg of polyethylenes (mol Ti)−1 h−1]. In ethylene/1-hexene copolymerization, the heteroligated complexes 2a−2e display improved activities and intermediate incorporation ability compared with their homoligated counterparts 1a−1f. The activity and incorporation ability for 1-hexene are highly dependent on the nature of the ortho-substituents. Among them, (trimethylsilyl)ethynyl-substituted precatalyst (1d) achieves the highest incorporation ratio (27.3 mol %), while ethynyl-substituted precatalyst (2c) achieves the highest copolymerization activity [2.89 × 103 kg of copolymers (mol Ti)−1 h−1].

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INTRODUCTION Remarkable progresses have been made in the design of nonmetallocene catalysts for olefin polymerization since the mid1990s.1 Among these candidates, the salicylaldiminato titanium complex represents a well-studied catalyst family,1d,2 which is highly active for ethylene polymerization. In particular, the ortho-fluorinated N-aryl bis(salicylaldiminato)titanium derivatives can catalyze the polymerization of ethylene and/or propylene in a “living” manner, facilitating the synthesis of a series of block copolymers with controlled structures.3 However, very few successful phenoxyimine catalysts have been reported to effectively catalyze the copolymerization of ethylene and other olefins with high activity.4 There is generally a trade-off. For example, while sterically hindered systems such as the o-tert-butyl-substituted bis(salicylaldiminato)titanium complexes are extremely productive in ethylene polymerization, they are relatively poor at incorporating bulky monomers. On the other hand, when less bulky ligand is employed, comonomer incorporation ratio improves at the expense of productivity.3c,4 Nevertheless, catalysts with high activity and © XXXX American Chemical Society

high comonomer incorporation ability are highly desirable, since they can offer the polyolefin products with an extensive wide range of physical and mechanical properties, such as polyolefin elastomers.5 It is generally known that catalyst structure has a pronounced impact on the catalytic properties. In the area of bis(salicylaldiminato) group 4 catalysts, Pellecchia disclosed that the presence of additional halogen substituents on the phenolate rings of the ligand results in an inversion of the sterospecificity of the corresponding titanium catalyst.6 Recently, Fujita found out that the phenoxyimine titanium catalysts with the o-phenyl group display remarkable catalytic properties.4,7 They can efficiently produce high-molecularweight polymers with high α-olefin incorporation, which is an exceptional behavior considering the steric bulk of the phenyl group. DFT calculations show that they can evade the steric Received: August 26, 2014 Revised: November 1, 2014

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Scheme 1. Synthetic Route of the Homoligated Complexes 1a−1f and Heteroligated Complexes 2a−2e

congestion by rotation of the o-phenyl group.7b These studies provide a new strategy to modify the properties of catalysts by introducing functional groups. So far, however, only a few functional groups have been investigated in salicylaldiminato titanium polymerization systems.7 We believe that functional groups can provide new opportunities for development of catalysts with higher catalytic efficiency and more precise control over polymer microstructures. Over the past decade, several classes of hybrid titanium catalysts have also been documented.8 For instance, the combination of pyrrolylaldiminato with salicylaldiminato ligands, which affords hybrid octahedral complexes containing two different classes of bidentate monoanionic ligands, has recently been studied. These hybrid catalysts can successfully combine the high activity of the bis(salicylaldiminato) system with the enhanced comonomer incorporation of the bis(pyrrolylaldiminato) catalysts.8a In another study, Coates used a combinatorial screening method to have successfully identified novel high activity catalysts bearing two salicylaldiminato ligands with different N-aryl groups, which can be a class of unusually active phenoxyimine catalysts for syndiospecific propylene polymerization.8f These studies have demonstrated the potential of the hybrid approach. Yet up until now, none of the heteroligated phenoxyimine complexes with different substituents ortho to the phenoxy oxygen have been reported. As substituents ortho to the phenoxy oxygen have a significant impact on both productivity and comonomer incorporation of the obtained polymers,3c,d,9 we speculate that the combination of two ancillary ligands with different substituents ortho to the phenoxy oxygen in one complex will provide catalysts with unique steric environment as well as electronic properties. Given intense investigations regarding catalyst structure and performance relationships within the family of homoligated salicylaldiminato catalysts, it is necessary to explore the catalytic properties of heteroligated catalysts. With fine-tuning one substituent in one ligand, we designed and synthesized a series of heteroligated and homoligated salicylaldiminato titanium dichloride complexes bearing different substituents ortho to the phenoxy oxygens, including some less explored substitutes, such as (trimethylsilyl)ethynyl or acetenyl group. To access a family of high activity heteroligated complexes, all these hybrid precatalysts are equipped with a sterically bulky ligand bearing t Bu ortho to the phenoxy oxygen.2 Next, the ethylene polymerization and ethylene/1-henxene copolymerization

behaviors of these catalysts were investigated. The results provide a strategy for the development of phenoxyimine catalysts that have high ethylene insertion ability and high αolefin uptake.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations with air- and/or moisture-sensitive compounds were performed under dry nitrogen using standard Schlenk techniques or in an mBraun Labstar nitrogen glovebox with a high capacity recirculator ( 2b > 2a, entries 7, 8, and 11, Table 2).3c,d,9,14 Ethylene/1-Hexene Copolymerization. The ability of complexes 1a−1f and 2a−2e to mediate ethylene/1-hexene copolymerization upon MAO activation was studied under atmospheric pressure with 1-hexene concentration at 0.7 M. Results are compiled in Table 3. All the complexes afford

Table 2. Ethylene Polymerization Resultsa entry

complex

yield (g)

activityb

Mnc

Mw/Mnc

Tmd (°C)

1 2 3 4 5 6 7 8 9 10 11

1a 1b 1c 1d 1e 1f 2a 2b 2c 2d 2e

0.121 0.438 0.505 0.876 0.116 0.846 0.557 0.604 0.669 0.782 0.662

7.3 26.3 30.3 52.6 7.0 50.8 33.4 36.2 40.1 46.9 39.7

8.0 23.7 37.0 36.7 3.9 39.7 34.7 27.6 30.6 50.0 25.3

3.02 2.00 2.39 2.87 2.36 2.52 3.15 3.50 2.70 2.61 1.87

137 140 140 139 135 139 138 139 138 138 140

Table 3. Ethylene and 1-Hexene Copolymerization Resultsa entry complex 1 2 3 4 5 6 7 8 9 10 11

a Polymerizations carried out for 5 min at 40 °C with 2 μmol of Ti and MAO as cocatalysts (Al:Ti = 250:1) in 50 mL of toluene under an ethylene pressure of 1.0 atm. bIn 105 g (mol Ti)−1 h−1. cFrom GPC calibrated with polystyrene standards. dData determined by DSC from the second melting curve.

1a 1b 1c 1d 1e 1f 2a 2b 2c 2d 2e

yield (g)

activityb

Mnc

Mw/Mnc

Tmd (°C)

He (mol %)

0.127 0.260 0.265 0.180 0.069 0.198 0.353 0.395 0.481 0.329 0.200

7.6 15.6 15.9 10.8 4.1 11.9 21.2 23.7 28.9 19.7 12.0

8.0 11.8 10.8 5.0 3.6 9.6 17.2 18.0 15.9 15.1 8.1

1.66 1.44 2.45 3.47 1.47 1.45 2.21 2.17 2.05 1.25 1.29

52 55 n/a n/a 92 116 87 96 78 64 97

15.8 15.1 22.3 27.3 9.1 3.1 6.8 5.2 8.3 14.9 5.3

Copolymerizations were carried out for 5 min at 40 °C with 2 μmol of Ti and MAO as cocatalysts (Al:Ti = 250:1) in 50 mL of toluene under an ethylene pressure of 1.0 atm. 1-Hexene concentration is 0.7 M. bIn 105 g (mol Ti)−1 h−1. cFrom GPC calibrated with polystyrene standards. dData determined by DSC from the second melting curve. e Comonomer incorporation based on 1H NMR or 13C NMR spectra;3c,10 n/a = not applicable. a

First, we investigated the ethylene polymerization properties of homoligated complexes 1a−1f. The polymerization results of the known complexes 1a, 1b, and 1f were similar to the reference (entries 1, 2, and 6, Table 2),3c and the catalytic activity increases with growing steric hindrance in these homoligated catalysts. The o-ethynyl-substituted complex 1c obtains similar modest productivity as the o-methyl-substituted complex 1b (entries 2 and 3, Table 2), which we also attributes to the similar size of groups. Surprisingly, complex 1d containing (trimethylsilyl)ethynyl ortho to the phenoxy oxygen shows higher activity than the o-tert-butyl complex 1f (entries 4 and 6, Table 2). As a general rule, increasing the steric hindrance around the metal center will slow the deactivation rate, resulting in higher activity.1d Since (trimethylsilyl)ethynyl only provides a relatively remote steric hindrance (Si−Ti distance: 6.25 Å) with respect to titanium, we reason that such a remote steric hindrance might be sufficient to inhibit deactivation. Furthermore, considering the relatively lower activity of complex 1c, the high activity of 1d is actually caused by the remote steric trimethylsilyl group. Catalysts bearing an ortho-substituted phenyl group usually display enhanced activity,4,7 but in our case, phenyl-substituted 1e shows lower activity (entry 5, Table 2). Although, the reason is unclear at the current time, we think that the π−π interaction between the o-phenyl and N-pentafluorophenyl groups may influence catalyst conformation or racemization.15 The gel permeation chromatography (GPC) curves of obtained PEs give relatively narrow polydispersities (1.8−3.5) (Table 2) and support that these precatalysts give rise to single site catalysts. The molecular weight of polymers follows a similar trend as polymerization activity.

polymers, which range from semicrystalline to amorphous materials, with high productivity [106 kg (mol Ti)−1 h−1] under the given conditions. These results show that the substituent ortho to the phenoxy oxygen has a profound influence on the copolymerization reactivity of the catalyst systems. The incorporation levels are in the range of 3.1−27.3 mol %. Their 13C NMR analyses indicate that butyl branches mostly exist in separate form in the polymers with only very few 1hexene/1-hexene dyads (Tables S9−S11). Under identical conditions, homoligated complex 1d displays the highest 1-hexene incorporation ratio (27.3 mol %), followed by complexes 1c (22.3 mol %) (entries 3 and 4, Table 3). A similar sequence is also found among heteroligated catalysts (2d, 14.9 mol %; 2c, 8.3 mol %; entries 9 and 10, Table 3). The high incorporation ratio of 1-hexene clearly demonstrates that the remote bulky trimethylsilyl group does not inhibit the coordination of 1-hexene to the metal center. These results once again testify to the superiority of the remote steric hindrance. But steric effect alone is not sufficient to explain the outstanding performance of (trimethylsilyl)ethynylsubstituted precatalysts because the incorporation levels achieved by 1c and 1d are even higher than that of complex 1a with the smallest steric hindrance. The electronically flexible properties of the ligand of 1c and 1d, due to the more E

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conjugated nature caused by the presence of the o-ethynyl group, may also be responsible for this behavior.1d,4 Again, similar to ethylene polymerization, complexes 1e and 2e with ortho-substituted phenyl shows only modest 1-hexene incorporation ratio with low activity. In all these experiments, nearly 2-fold activity is observed for heteroligated complexes 2a−2e compared with corresponding homoligated complexes 1a−1e (entries 1−6 and 7−11, Table 3), although the incorporation ratio of comonomer lies in between that of the two parent catalysts. Ligand differences may result in faster rates for both catalyst racemization and monomer insertion.8f,15 Complex 2c gives the highest activity, followed by complex 2b. The ortho-substituents in an appropriate size will allow a bulky comonomer to coordinate to the metal and meanwhile avoid the deactivation pathways by protecting the reaction site or efficiently destabilize the active species in the resting state.1d The melting points (Table 3) and glass transition temperatures of the copolymers (see Supporting Information, Table S10) suggest that they can be used as polyolefin elastomers or LLDPE.16

National Basic Research Program (2013CB933500) of the Ministry of Science and Technology of China.

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CONCLUSIONS In this work, we reported a series of heteroligated salicylaldiminato titanium dichloride complexes with different substituents ortho to the phenoxy oxygens. They can be conveniently synthesized through treatment of mono(salicylaldiminato)titanium complexes with another potassium salicylaldiminates at room temperature. These complexes are stable with respect to isomerization and ligand redistribution reactions. Moreover, hybrid precatalysts 2b, 2d, and 2e are structurally characterized by single crystal X-ray diffraction analysis. The hybrid titanium complexes effectively combine the high incorporation capability of corresponding bis(salicylaldiminato)titanium catalysts with high activity. On the other hand, functional groups have pronounced effects on the properties of catalysts. Complex 1d containing the (trimethylsilyl)ethynyl group not only gives the highest ethylene polymerization productivity but also shows the highest 1-hexene incorporation ratio (27.3 mol %). The exceptional polymerization property can be attributed to the remote steric hindrance induced by (trimethylsilyl)ethynyl group and the flexible electronic effect of alkynyl group. We hope our studies offer a new strategy for designing high productivity and high comonomer uptake catalysts in olefin polymerization by introducing new functional groups and using hybrid methods.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of ligands and catalysts, tables of crystal structure data, ethylene polymerization results, and polymer analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by National Natural Science Foundation (No. 21074004 and 91227202) and the F

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