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