A method for the preparation of an organometallic zirconium precursor, comprising flowing a gas phase electron donor into a bubbler containing bis (2,2',6,6'-tetramethyl-3,5-heptanedione)Zr at a predetermined temperature, to synthesize, in-situ, an adduct represented by the following formula II:
- L.sub.x .multidot.Zr(THD).sub.4 II!
- wherein
- L is an electron donor ligand selected from the group consisting of NR.sub.3, wherein R is hydrogen or methyl, and Cl.sub.2 ;
- THD denotes 2,2',6,6'-tetramethyl-3,5-heptanedione;
- and
- x is in the range of 0.3 to 1.5 when L is NR.sub.3,
- or x is in the range of 0.5 to 2 when L is Cl.sub.2 ;
- said gas phase electron donor serving as a carrier gas and being selected from the group consisting of NH.sub.3 and Cl.sub.2.