Although their increased functional-group tolerance has made the ruthenium-based metathesis catalysts extremely attractive for use as polymerization catalysts, these catalysts have typically struggled to match the “living” polymerizations of the molybdenum- and tungsten-based catalysts. The advent of 2nd generation N-heterocyclic carbene ruthenium catalysts increased the overall activity to match other systems, but polymers generated still tend to have broad polydispersities (PDI) due to rapid rates of propagation (kp) and polymer-chain backbiting relative to rate of polymer initiation (ki). Standard techniques for increasing the ki/kp have included changing the electronic nature of coordinated phosphines , and the addition of other phosphines to the reaction mixture to generate faster-initiating catalysts in situ. An alternate course of action is the use of the “ultrafast-initiating” ruthenium catalysts reported by the Grubbs group in several publications beginning in 2002.
These six-coordinate 2nd generation ruthenium catalysts (Catalyst 2) were found to give PDIs as low as 1.04, and variation of the ratio of catalyst-to-monomer ratio gives linear control over polymer molecular weight, indicating immediate initiation of the catalyst and living polymerization. Further kinetic studies demonstrate that the kp for Catalyst 2 is actually considerably faster than that of a standard 1st generation ruthenium catalyst, but that the ki is more than ten thousand times faster, giving the ki/kp needed for narrow PDI. The living polymerization nature of the catalyst has also been demonstrated by the generation of block copolymers by sequential addition of monomer.
Whatever the method, it’s a good reminder that if you don’t like the polymerization kinetics of your metathesis polymerization, there’s always a way to change it.
 Robson D.A.; Gibson V.C.; Davies R.G.; North M. A New and Highly Efficient Grubbs Initiator for Ring-Opening Metathesis Polymerization. Macromolecules 1999, 32, 6371–3.
 Bielawski C.W.; Grubbs R.H. Increasing the Initiation Efficiency of Ruthenium-Based Ring-Opening Metathesis Initiators: Effect of Excess Phosphine. Macromolecules 2001, 34, 8838–40.
 Love J.A.; Morgan J.P; Trnka T.M.; Grubbs R.H. Angew Chem Int Ed 2002, 41, 4035.
 Choi T.; Grubbs R.H. Controlled Living Ring-Opening-Metathesis Polymerization by a Fast-Initiating Ruthenium Catalyst. Angew Chem Int Ed 2003, 42, 1743.