Formation of elastomeric polypropylene promoted by the dynamic complexes [TiCl2{N(PPh2)2}2] and [Zr(NPhPPh2)4]

The homoleptic phosphinoamide complex [Zr(NPhPPh₂)₄] (1) and the bisamido complex [TiCl₂{N(PPh₂)₂}₂] (2) were prepared from ZrCl₄ and four equivalents of LiNPhPPh₂ and from TiCl4 and one equivalent of [Li(THF)N(PPh₂)₂]₂. In the solid state, the four NPhPPh₂ ligands in 1 exhibit η² coordination. The ZrN4P4 fragment is highly symmetrical and almost of D₂ symmetry. Hence, the complex is chiral, and the two enantiomers cocrystallize in the asymmetric unit. In solution, 1 exhibits signals for the six-coordinate complex [Zr(η²-NPhPPh₂)₂(η¹-NPhPPh₂)₂]. In the presence of methylalumoxane (MAO), 1 and 2 are active catalysts for the formation of high-molecular-weight elastomeric polypropylene. The formation of elastomeric polypropylene is a consequence of an epimerization mechanism of the last-inserted monomer, indicating no detachment of the growing polymer chain from the metal center during this process. Fractionation studies of all the elastomeric polymers show no atactic fractions. As corroboration for this mechanism, we have shown that these complexes are active catalysts for the isomerization and oligomerization of 1-octene, as well as for the rapid isomerization of allylbenzene to trans-methylstyrene