Intramolecular Hydrogen-Bond-Based Latent Initiator for Olefin Metathesis Polymerization

H-Bonding leading to latent initiators for olefin metathesis polymerization

Ruthenium-NHC based catalysts, with a chelated iminium ligand trans to the N-heterocyclic carbene (NHC) ligand, that polymerize dicyclopentadiene (DCPD) at different temperatures are monitored using Density Functional Theory calculations to unveil the reaction mechanism, and subsequently how important are the geometrical and electronic features vs. the non-covalent interactions in between. The balance is very fragile and H-bonds are fundamental to explain the different behaviour of latent catalysts. This computational study aims to facilitate future studies of new generations of latent initiators for olefin metathesis polymerization, with the 3D and mainly the 2D Non-Covalent Interaction plots the characterization tool for H-bonds.

Reactivity regulation for olefin metathesis-catalyzing ruthenium complexes with sulfur atoms at the terminal of 2-alkoxybenzylidene ligands

For regulating the olefin metathesis (OM) activity of the Hoveyda-Grubbs second-generation complex (HG-II), the structural modification of the benzylidene ligand is a useful strategy. This paper reports the effect of a chalcogen atom placed at the end of the benzylidene group on the catalytic properties of HG-II derivatives, using complexes with a thioether or ether component in the benzylidene ligand (ortho-Me-E-(CH₂)₂O-styrene; E = S, O). Nuclear magnetic resonance and X-ray crystallographic analyses of the complex with a thioether moiety (E = S) proved the (O,S)-bidentate and trans-dichlorido coordination for the complex. A stoichiometric ligand exchange between HG-II and the benzylidene ligand (E = S) produced the corresponding complex with an 86% yield, confirming higher stability of the complex (E = S) than that of HG-II. Despite the bidentate chelation, the complex (E = S) exhibited OM catalytic activity, indicating the exchangeability of the S-chelating ligand with an olefinic substrate. The green solution color, a characteristic of HG-II derivatives, was retained after the complex (E = S)-mediated OM reactions, indicating high catalyst durability. Conversely, the complex (E = O) rapidly initiated OM reactions; however, it showed low catalyst durability. In the OM reactions conducted in the presence of methanol, the complex (E = S) exhibited higher yields than the complex (E = O) and HG-II: the S-coordination increased the catalyst tolerance to methanol. A coordinative atom (such as sulfur) placed at the terminal of the benzylidene ligand can precisely regulate the reactivity of HG-II derivatives.

Preparation and properties of an interpenetrating network polymer based on polydicyclopentadiene and phenolic resin

In this paper, phenolic resin (PF) and dicyclopentadiene (DCPD) monomers were mixed in different proportions. Under the action of a new generation of ruthenium carbene catalysts, DCPD was polymerised in situ. Polydicyclopentadiene (PDCPD)/PF interpenetrating polymer networks (IPNs) were prepared using the casting curing moulding process. The structure and properties of the prepared IPNs were characterised using Fourier infrared spectroscopy (FT-IR), apparent crosslinking degree, thermal weight loss, mechanical properties, impact resistance and scanning electron microscopy (SEM). The study results showed that the conversion of DCPD did not change with the addition of PF. But when its content exceeds 10%, the crosslinking degree of PDCPD decreases. When the PF content is 10%, the maximum bending strength of PDCPD/PF IPNs is (104.5 ± 1.3) MPa, maximum tensile strength is (74.5 ± 1.4) MPa, and maximum-notched impact strength is (4.2 ± 0.2) kJ/m2. Compared with PDCPD, the bending strength is increased by 22.7%, tensile strength is increased by 32.6%, and notched impact strength is increased by 31.3%, but the thermal stability has no major impact at this time. PF has good dispersibility and compatibility in DCPD. Due to the interpenetrating network structure of PF and PDCPD, the interpenetrating interlocking of the PF molecular chain and PDCPD crosslinked network restricts its movement. Its performance reached the optimum, and the PDCPD/PF IPNs with excellent performance was successfully prepared.

Olefin Metathesis Catalyzed by a Hoveyda-Grubbs-like Complex Chelated to Bis(2-mercaptoimidazolyl) Methane: A Predictive DFT Study

Although highly selective complexes for the cross-metathesis of olefins, particularly oriented toward the productive metathesis of Z-olefins, have been reported in recent years, there is a constant need to design and prepare new and improved catalysts for this challenging reaction. In this work, guided by density functional theory (DFT) calculations, the performance of a Ru-based catalyst chelated to a sulfurated pincer in the olefin metathesis was computationally assessed. The catalyst was designed based on the Hoveyda–Grubbs catalyst (SIMes)Cl₂Ru(=CH–o–OiPrC₆H₄) through the substitution of chlorides with the chelator bis(2-mercaptoimidazolyl)methane. The obtained thermodynamic and kinetic data of the initiation phase through side- and bottom-bound mechanisms suggest that this system is a versatile catalyst for olefin metathesis, as DFT predicts the highest energy barrier of the catalytic cycle of ca. 20 kcal/mol, which is comparable to those corresponding to the Hoveyda–Grubbs-type catalysts. Moreover, in terms of the stereoselectivity evaluated through the propagation phase in the metathesis of propene–propene to 2-butene, our study reveals that the Z isomer can be formed under a kinetic control. We believe that this is an interesting outcome in the context of future exploration of Ru-based catalysts with sulfurated chelates in the search for high stereoselectivity in selected reactions.