Asymmetric Bis(formamidinate) Group 4 Complexes: Synthesis, Structure and Their Reactivity in the Polymerization of α‑Olefins.

A series
of asymmetric formamidine ligands bearing different substituents with
various steric and electronic properties on the nitrogen of the N–C–N motif were synthesized. Group 4 bis­(formamidinate)
dimethylamido, chloride, and benzyl complexes were studied using these
asymmetric ligands and their solid-state structures and their behavior
in solution were determined. These complexes were activated with MAO
(methylalumoxane) or a combination of cocatalysts and tested in the
polymerization of ethylene and propylene. A noticeable influence of
the formamidine nitrogen substituents on the activity of the catalyst
and properties of the obtained polymers was observed. Further, a plausible
mechanism for the polymerization of propylene is presented derived
from a combination of ESR-C60 and MALDI-TOF trapping experiments
which shed light on the nature of the active catalytic species.