Cause and Prevention of Palladium Mirror in Stille Coupling

I am conducting some monomer and polymer syntheses using Stille coupling or other palladium-based coupling. I usually use the combination of $\ce{Pd\_2(dba)\_3}$ and $\ce{P(o-tol)\_3}$ in Stille coupling, but sometimes use other ligand when I use other palladium-based coupling such as Buchwald-Hartwig coupling. I usually observe palladium mirror precipitated on the wall of the reaction flask and this led to unsatisfying yield or molecular weight of polymer. I have thought that this palladium mirror precipitation is due to the oxidation by oxygen, so I attributed it to the insufficient inert atmosphere. But recently, I tried some other polymerization using Stille coupling, and there was no palladium mirror at all after 48 hours, even I didn't flow Ar gas! (We don't have Schlenk line, so we alternatively flow Ar gas continuously into the reaction flask during the reaction) The reaction system only had the adaptor packed with silica gel to prevent moisture. So I rethink: Can the palladium mirror be formed because the reagents are not favourable to the Stille coupling, not the oxidation by oxygen? And in this case, does adding extra Pd catalyst and ligand help when I observe the palladium mirror during reaction?


