Metathesis catalysts can be added to the reaction mixture as a solid, a solution (methylene chloride, etc.) or as a suspension. If you’re not concerned with optimizing for the lowest catalyst loading, you can usually get away with whichever is easiest for your process. But if you have a challenging reaction or want to use low catalyst loadings, the method of catalyst addition can often have a significant impact.
It’s known that decomposition of Ru-methylidenes (intermediates in metathesis reactions involving terminal olefins) can occur by a unimolecular reaction. Especially at higher (>60 °C) temperatures, this means that you’ll have a slow but steady catalyst decomposition pathway to contend with. In these situations it’s often best to add catalyst slowly (or portionwise) to maintain a constant concentration of the active catalyst (Handbook of Metathesis, vol. 1 (Wiley, Grubbs ed.)). Another way of course to avoid methylidene-derived decomposition is to choose substrates with only internal double bonds.
A. I. Meyers and coworkers found that slow catalyst addition over 3 hours (syringe pump) improved the yields in the synthesis of spirolactams significantly (J. Org. Chem. 2001, 66, 5545).
A similar technique was used by Skaanderup and Jensen to prepare a key intermediate for the macrocyclic core of the natural product (–)-pladienolide B. A cross metathesis reaction using 20 mol% the 2nd generation Hoveyda-Grubbs catalyst in dichloromethane gave only 30% yield of the desired product, but the yield was improved to 76% simply by adding the catalyst in two portions over the course of the reaction (Org. Lett. 2008, 10, 2821).
Of course you also have to keep in mind the solubility of the catalyst. If you want to use a Grubbs-type catalyst in neat substrate or in a solvent in which it’s not particularly soluble (hexane, methanol), it’s usually best to pre-dissolve the catalyst in a small amount of a good solvent (such as toluene) to aid in dissolution. If you don’t do this, it can be tough to tell whether all the catalyst you added made it into solution.