Synthesis and Scale-up of Imino–Enamido Hafnium and Zirconium Olefin Polymerization Catalysts

New imino–enamido hafnium
and zirconium trimethyl complexes
were prepared in high yield by reacting a mixture of an imino–enamine
ligand and hafnium or zirconium tetrachloride with four equivalents
of methylmagnesium bromide in toluene. A significantly improved imine
formation reaction in the final step of the ligand synthesis was developed
that involves the reaction between the keto–enamine and a reagent
formed in situ by mixing 0.55 equivalents of titanium
tetrachloride and 5.5 equivalents of n-butylamine.
Ethylene/1-octene polymerization evaluations at 120 °C revealed
that polymerization characteristics of the imino–enamido hafnium
and zirconium trimethyl derivatives were very similar to those of
the tribenzyl analogues. A scale-up of the hafnium trimethyl derivative,
performed on a 255 g scale, was accomplished in an overall yield of
57% in four steps starting with commercially available cyclohexa-1,2-dione.
Hafnium and zirconium complexes derived from two isomeric imino–enamine
ligands each resulted in very high molecular weight ethylene/1-octene
copolymers, which differ significantly, however, in the level of incorporated
octene. These features make these new catalysts good candidates for
the preparation of olefin block copolymers via chain-shuttling polymerization.