Metathesis of norbornene-derivatives bearing trimethylsilyl groups using Ru-alkylidene catalysts: An experimental and computational study
Graphical abstract
Introduction
The development of new engineer polymers for industrial applications comprises one of the most fascinating topics of polymer chemistry. In gas separation processes, polymeric membranes with high permeability and controlled selectivity have been the subject of many scientific studies owing to their applications in carbon capture and storage (CCS) [1], light hydrocarbon separations and CO2 removal from natural gas. Among the approaches to obtaining suitable polymers for these applications, the use of polymers with bulky pendant groups, have been studied and reported; one of the most studied bulky groups are the silicon-containing, it has been proved that insertion of this kind of groups enhances the properties of permeability on norbornene polymeric membranes due the high free volume produced [2]. For instance, the permeation of norbornene for non-condensable gases such as H2 and CO2 increase dramatically when -Si(CH3)3 group was inserted in 5 position (Fig. 1), from 18.0 to 9.3 Barrer to 144 and 140 Barrer; respectively [3].
On the other hand, synthetic methodologies used in organic chemistry are not always applicable to polymer chemistry. This is why ring-opening metathesis polymerization (ROMP) is considered an important tool in polymer synthesis since high-performance polymeric materials can be easily obtained from functionalized cyclic molecules. On the whole, polymeric materials with a well-defined macromolecular architecture (microstructure) will have more sharply defined physical properties: melting point, crystallinity, solubility. In this regard, W- and Mo-alkylidene catalysts allow microstructure control in ROMP polymers, in contrast with homologous Ru-alkylidene catalysts such as [Cl2Ru(PCy3)2] and [Cl2NHCRuPCy3]. Despite these [[4], [5], [6], [7]], only Ru-alkylidene complexes have been widely tolerant to functional groups, allowing more freedom in the choice of functionality that can be incorporated in the monomer structure. Moreover, the microstructure of the polymers not only depends on the metallic center of the catalyst used in ROMP, but this also can be controlled by the ligand type, the solvent and the conformation of the monomers: endo/exo, syn/anti [8]. It has been reported [9] that syndiotactic polymeric material can be obtained from a mixture of syn/anti methyl substituted in the 7-position “bridge position” (Fig. 1) of norbornene monomers via ROMP in presence of Ru-alkylidene catalyst.
According to many reports on polymer synthesis via ROMP, polymerization of norbornene-derivatives with bulky pendant groups in several positions using Ru-alkylidene catalysts could result in high-molecular polymeric materials with high thermostability and good mechanical properties [[10], [11], [12], [13]]. Moreover, membranes prepared from these polymers present interesting gas transport properties (permeability, selectivity, and solubility) that can be manipulated depending on the nature of the bulky pendant group and its position in the polymer chain. Surprisingly, there are no reports of norbornene-derivatives substituted in the 7-position “bridge position” with -Si(CH3)3 or similar silicon groups in ROMP. Very likely, bridge substitutions in such norbornene-derivatives could help to avoid the lack of microstructure control given by Ru-alkylidene catalyst; leading to a new family of silylated polymer membranes for gas separation application.
The goal of this work is the synthesis and theoretical study of ring-opening metathesis polymerization (ROMP) of the conformers endo/exo, syn/anti, 7-position substituted -Si(CH3)3 cis-norbornene-2,3-dicarboxylic anhydride using Ru-alkylidene catalysts, and compare with that non-substituted dicarboxylic anhydride. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) were performed in order to elucidate the reaction pathways.
Section snippets
Materials
All starting materials were obtained from Aldrich. bis(tricyclohexylphosphine)benzylidene ruthenium dichloride (I), tricyclohexylphosphine[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium dichloride (II), [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](2-isopropoxyphenylmethylene)ruthenium dichloride (III), anhydrous 1,2-dichloroethane (DCE) and chlorobenzene (PhCl) were used as received. cis-5-norbornen-exo-2,3-dicarboxylic anhydride (1a) was prepared
Monomer synthesis
The synthesis of monomers was carried out by a Diels-Alder reaction (Scheme 1), between the dienophile specie (maleic anhydride), and the diene species: cyclopentadiene or trimethylsilyl cyclopentadiene. The monomers synthesis were carried out under same synthetic route, but different reaction conditions, monomer 1a was obtained at 180 °C in 1,2,4-trichlorobenzene as solvent following the reported route [14], and monomer 1b was obtained at room temperature in diethyl ether as solvent; giving
Conclusions
The -Si(CH3)3 substituted cis-norbornene-2,3-dicarboxylic anhydride (1b), in contrast with that of the non-substituted dicarboxylic anhydride (1a), did not undergo ROMP even under demanding conditions. The theoretical analysis revealed that this result can plausibly be explained mainly due to the absence of the π-complex (3b) in the 1b ROMP reaction pathway. Moreover, the reaction is both kinetically and energetically less favorable for 1b. Both outcomes are attributed to the steric effects of
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.
Acknowledgments
We thank to the National Council for Science and Technology of Mexico CONACYT for generous support to this research through PhD Scholarship to D. Z-S. (CVU: 417716, Scholarship holder No. 261582). Financial support is greatly appreciated from the project PAPIIT-UNAM IA207418. Also, we are grateful to Alejandrina Acosta and Victor Hugo Lemus for their assistance in NMR and EA techniques respectively techniques. We also thank Dr. Jesús Hernández-Trujillo for providing the computational resources.
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