Elucidation of Substituent Effects in the Polymerization of Propylene Promoted by Titanium Amidinates

In polymerization reactions, the molecular weight of
the polymer
depends on the rates of monomer insertion, chain termination, and
chain transfer. Here we do present studies indicating why chelating
octahedral benzamidinate group IV complexes, which promote the polymerization
of propylene with MAO, exhibit a linear free energy relationship with
the steric Taft parameter. We correlate that this
relationship is encountered with the para substituent of the aromatic
ring, far away from the active catalytic site. A number of known events
are put together to shed light on the effect of the ancillary ligands
in the catalytic polymerization, and a new plausible mechanism for
the process is presented.