Grubbs Ring Opening Metathesis Mechanism

Grubbs Ring Opening Metathesis Mechanism-68
The direction of the reaction can be predicted using the Gibbs energy: The enthalpy factor makes an essential contribution to ROMP’s Gibbs energy.This fact is explained by a high strain energy of cyclic unsaturated compounds participating in the polymerization [7].The next step is a formation of a metal-cyclobutane intermediate.

The direction of the reaction can be predicted using the Gibbs energy: The enthalpy factor makes an essential contribution to ROMP’s Gibbs energy.This fact is explained by a high strain energy of cyclic unsaturated compounds participating in the polymerization [7].The next step is a formation of a metal-cyclobutane intermediate.

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As well as in any other type of polymerization, ROMP can be divided into several stages (Scheme 1).

The initiation begins with the coordination of the cycloolefins’ double bond with the metal-carbene complex.

In the history of metathesis, catalytic complexes with a well-studied structure were called “well-defined” catalysts.

The discovery of “well-defined” catalysts had significantly increased the ability of ROMP to obtain polymers that have unique properties.

Ring opening metathesis polymerization (ROMP) is a process of one or more cyclic olefins transformation to polymer catalyzed by metal carbene compounds.

Indeed, the number of double bonds both in polymer and in monomer is equal [1, 2, 3, 4].ROMP polymers are characterized by high molecular weights and a narrow MMD, as well as for products of other living polymerizations [11].Up until the mid-1970s, the information about the structure of the active center of olefin metathesis catalysts was not known.The strain energy of the cycle releasing during the decomposition of the metal-cyclobutane complex and maintaining the forward direction of the reaction.However, if the deformation energy of cycle is low, the contribution of the entropy factor into the Gibbs energy becomes more significant in comparison with a smaller enthalpy factor.The intermediate decomposes to form a new metal-carbene complex, with the growing chain being a ligand attached to the metal via a double bond.The polymerization continues until the monomer is completely reacted or an equilibrium state is reached or the reaction is terminated by the addition of a special reagent that blocks the catalyst.Cyclic olefins form polymers (copolymers) of a high tacticity in a presence of this catalyst [14].Catalytic systems prepared from compounds based on transition metals such as vanadium VCl), described by Casey and Fisher [18, 19], Katz was the first who showed the ability of a metal-carbene complex to catalyze the process of metathesis polymerization independently without additional compounds [20].Intermolecular and intramolecular chain transfers are two main side reactions of ROMP (Scheme 2).During the intermolecular transferring, the active metal-carbene complex located at the end of one macromolecule interacts with the double bond of the adjoined macromolecule, which leads to fragment exchange process.

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