Elsevier

Journal of Catalysis

Volume 383, March 2020, Pages 215-220
Journal of Catalysis

A comprehensive picture on catalyst structure construction in palladium catalyzed ethylene (co)polymerizations

https://doi.org/10.1016/j.jcat.2020.01.032Get rights and content

Highlights

  • Principles on catalyst construction for BPMO-Pd catalyzed ethylene (co)polymerization were fully built.

  • Steric and electronic effects and donor connectivity affect catalytic performance.

  • Efficient incorporation of a broad scope of polar comonomers into polyethylene.

  • Rational catalyst design guide with was proposed.

Abstract

Late transition metal palladium catalysts chelated by benchmark α-diimine, phosphine-sulfonate, bisphosphine-monoxide ligands are of great interest for ethylene polymerization and challenging copolymerization with polar comonomers. Compared to numerous studies on the classic α-diimine and phosphine-sulfonate palladium catalysts, the new generation bisphosphine-monoxide palladium (BPMO-Pd) system is far less investigated. In this contribution, by skillfully designing a family of BPMO-Pd catalysts bearing differed steric and electronic P(III) phosphine groups, differed steric and electronic O = P(V) phosphine monoxide groups, and different connectivity of them, a comprehensive picture on catalyst structure construction is built for ethylene (co)polymerizations. This constructed catalyst structure-polymerization property relationship paves the way for the development of more efficient late transition metal catalysts for olefin (co)polymerization.

Introduction

Weakly oxophilic late transition metal catalysts have been given much consideration in the field of functional polyolefin synthesis [1], [2], [3], [4], [5] since Brookhart discovered that α-diimine palladium catalysts could copolymerize olefin with acrylates [6], [7]. This catalytic system has recently been further developed by Guan [8], [9], [10], Chen [11], [12], Gao [13] to achieve higher thermal stability and the production of controlled branching content and molecular weight. Afterwards, the phosphine-sulfonate palladium catalysts invented by Drent et al. have received tremendous attention because this system can efficiently catalyze the copolymerization of various challenging polar monomers with olefins [14], [15], [16].

Following the vigorous study of α-diimine [17], [18] and phosphine-sulfonate palladium catalyst systems, the understanding of coordination insertion polymerization of polar comonomers with olefins has been much more profound [2]. Nozaki et al. recognized that it is possible to achieve excellent catalytic performance by mimicking the ligand electronic properties of the phosphine-sulfonate neutral palladium system, leading to the appearance of bisphosphine-monoxide (BPMO) palladium catalysts [19], [20]. These catalysts surprisingly copolymerized ethylene with a wide spectrum of fundamental polar monomers, which was unique considering their cationic active centers. The new class of versatile catalysts has subsequently experienced further modification and development by Jordan [21], Chen [22], [23], [24], Carrow and Nozaki et al. [25], [26] New phosphine/phosphine oxide donors and the linkers between them have been developed, along with improved catalytic activity and polymer molecular weight.

Despite these fantastic results, the catalytic performance of late transition metal catalysts in general still cannot meet the needs of industry production, warranting more profound catalyst optimization. Most of the previously efficient late transition metal catalysts such as phosphine-sulfonate catalysts [15], salicylaldimine [27], [28] and phosphino-phenolate [29], [30], [31], [32] neutral nickel catalyst, and Nozaki’s original BPMO-Pd catalysts [19], were all constructed on the basis of arylene (-C6H4-) backbone. The regulation of catalyst performance relies on the adjustment of the steric and electronic effect of the coordination sites. After years of investigation, the opportunity to achieve breakthrough on the basis of the usual arylene backbone become limited.

Therefore, we have recently tried to explore new backbones that span the two coordinating fragments of the bidentate ligands. We have recently reported two BPMO-Pd systems [33], [34] based on (benzo)thiophene heteroaryl backbone along with a series of alkyl or aryl P(III) and O = P(V) moiety containing electron-withdrawing or -donating substituents. Different catalytic behaviors towards ethylene (co)polymerization were observed as compared with the prototype arylene or alkylene linked BPMO-Pd systems. It is interesting to note that differed catalytic properties can be achieved by simply exchanging the position of P(III) and O = P(V) donors [33]. In this contribution, to fully explore the influence of electronic/steric effects of the donor fragments P(III) and O = P(V) and also their arrangement on the ligand backbone, we further designed BPMO palladium complexes bearing phosphonate (-P(OR)2) and phosphonic diamide (P(NR2)2) donor moieties (Chart 1). As a result, a comprehensive picture on the catalyst structure construction based on heteroaryl ligand backbone in palladium catalyzed ethylene (co)polymerization is built.

Section snippets

Results and discussion

The phosphine-dialkyl phosphonate ligands bearing benzothiophene linker were synthesized according to Scheme 1. Pd(PPh3)4-catalyzed P-C coupling reaction of 3-bromo-benzothiophene with dialkyl phosphites afforded 3-phosphonate benzothiophene in moderate yields when using ethyl and isopropyl starting materials. In the case of methyl analogue, however, the yield was too low possibly due to competing dealkylation side reactions [35], [36]. Thus, an alternative pathway involving Pd(OAc)2 and

Conclusions

In this work, we investigated the catalytic behavior of palladium complexes 1a, 2a, 3a and 4a in ethylene polymerization and copolymerization with various polar comonomers. Each of these catalysts possesses a P(o-MeO-Ph)2 phosphine donor group in common, but differs from the O = P(V) groups and the related connectivity. These catalysts can be divided into two subseries with the arrangement of O = P(V) and P(III) groups, i. e. 2-P(III)-3-O = P(V) (1a and 2a) and 2-O = P(V)-3-P(III) (3a and 4a)

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We are thankful for financial support from the National Natural Science Foundation of China (No. 21871250), the Jilin Provincial Science and Technology Department Program (No. 20190201009JC), Shaanxi Provincial Natural Science Basic Research Program-Shaanxi Coal and Chemical Industry Group Co., Ltd. Joint Fund (No. 2019JLZ-02).

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