When varying the zirconium chloride to isopropoxide ratio (Figure and Table ), we find that with increasing chloride content in the precursor, k 1 increases, meaning that the non-catalyzed reaction is faster with more chloride. This is similar to the titania case and has likely the same origin (Scheme 1). On the other hand, the auto-catalytic rate constant k 2 does not follow a clear trend and is highest for the ZrCl 2 (O i Pr) 2 .