After the first generation of well-defined ruthenium metathesis catalysts were developed, most of the efforts in designing new catalysts concentrated on finding more and more reactive versions. However, more reactive catalysts are not suitable for ring-opening metathesis polymerization (ROMP) of very reactive monomers especially on a commercial scale. To solve this problem a number of latent catalysts were developed in recent years. Use of such catalysts in ROMP allows for the catalyst to be mixed with the monomer with little or no polymerization at room temperature. The latency of the catalyst allows for more time to handle the formulation or even store it for a prolonged period of time. Ideally, a latent catalyst should have no reactivity with the monomer at room temperature and then significant reactivity can be achieved when the catalyst is activated. The catalyst can be activated by chemical and physical methods including heat and light. In addition, the latent catalyst needs to have high thermal stability over time to enable a metathesis reaction to be carried out successfully at high temperatures.
The most widely applied strategy to design a latent catalyst is modifying the chelating ligand to slow down the ligand dissociation.1 Catalyst 1, with a sulfur containing chelate, was found to be fully inert at room temperature to a series of reactive exo-norbornenes and cyclooctene. However, prolonged exposure to more reactive monomers such as dicyclopentadiene (DCPD) produced polymers even at room temperature. Recently, a new Catalyst 2 containing a second chelating sulfur atom was developed in the Lemcoff group.2 This catalyst showed less than 1% conversion of DCPD after 2hrs at room temperature. Increased conversion can be achieved by raising the temperature. It’s noteworthy, that Catalyst 2 was completely inactive in ring closing metathesis reactions (RCM) even at elevated temperatures.
A different approach involving manipulation of the NHC ligand was used in the Grubbs group. Catalysts bearing a sterically hindered N-tert-butyl group on the NHC were prepared and tested.3
Catalysts 3a, b were tested for ROMP of functionalized norbornene monomers and COD (1,5-cyclooctadiene). The chloro-complex 3a was too reactive, but iodide exchange gave 3b which showed excellent latency and stability at room temperature. Quantitative conversion of functionalized exo-norbornene monomers was achieved at 85oC in THF. The reactivity of the catalyst was solvent dependent; it was found that the latency is much higher in THF than in benzene. Similar to the sulfur chelates, this complex gave no RCM of diethyl allylmalonate even at 85oC.
A lot of progress has been made in the development of new metathesis catalysts in recent years. Major efforts were focused on improving catalyst stability, functional group tolerance and improving TON and TOF. While controlled ROMP of highly active monomers still remains a challenge, the recent progress towards latent thermally stable catalysts is very encouraging.
1 Recent Review: Y. Vidavsky, A. Anaby, and N. G. Lemcoff, Dalton Trans. 2012, 41, 32.
2 Y. Ginzburg, A. Anaby, Y. Vidavsky, C. E. Diesendruck, A. Ben-Asuly, I. Goldberg, and N. G. Lemcoff, Organometallics 2011, 30, 3430.
3 R. M. Thomas, A. Fedorov, B. K. Keitz, and R. H. Grubbs, Organometallics 2011, 30, 6713.