oa Development of Highly Active Ring Opening Metathesis Polymerization Catalyst Systems - A New Approach for Green Catalyst Design

The aim of the green chemistry is to develop chemical products and processes having minimal use and generation of hazardous chemicals and low energy requirement. Catalytic reagents are considered to be green tools to synthesize organic molecules as they basically open an alternative synthetic route to target molecules by lowering the energy barriers of the reactions while keeping the selectivity and the yield of the reactions high. Polynorbornene - which can be synthesized by ring opening methathesis polymerization (ROMP) with Grubbs’ catalyst - is used in the automotive and appliance industries mainly as vibration and noise isolators and produced thousands of tons per year scale.

It is well known that during the catalytic cycle the reverse phosphine reassociation step competes with the subsequent alkene binding step on the coordination sphere of the catalyst, slowing the observed rate constant. One option to improve the activity of the catalyst system is rolling back the reassociation step by the lowering the free dissociated phosphine concentration in the reaction mixture. This can be achieved for example by fluorous/organic solvent biphasic catalytic systems when the dissociated phosphine has higher affinities to the fluorous phase meanwhile the fourteen-valence-electron intermediate active species and substrates are liphophilic. Thus following the dissociation the fluorophilic phosphine remains in the fluorous phase meanwhile the “activated” catalytic cycle is running in the organic phase.

Fluorous analog of Grubbs’ second generation alkene metathesis catalyst (H2IMes)((Rf8(CH2)2)3P)(Cl)2Ru(=CHPh) (Rf8 = (CF2)7CF3) has been synthesized and tested in the catalytic ring opening metathesis polymerization of norbornene at mono and biphasic condition. It was found that at the same monophasic condition the fluorous Grubbs’ second generation catalyst has essentially identical rates with the classical Grubbs’ catalyst. However, dramatic accelerations can be observed in the presence of the fluorous solvent perfluoro(methylcyclohexane) (PFMC). It is proposed that the PFMC scavenges the fluorous phosphine Rf8(CH2)2)3P (PFMC/toluene partition coefficient >99.7:<0.3), allowing norbornene to more effectively complete for the fourteen-valence-electron intermediate that begins the catalytic cycle (“phase transfer activation”). Analogous effects are observed with an 7-oxanorbornene-based N-butylsuccinimide. The molecular weights and polydispersities of the polymers produced under monophasic and biphasic conditions are comparable.