Stereoselectivity Inversion: Isospecific Propylene Polymerization Catalyzed by Rigid Cyclic Bis(phenoxyaldimine) Titanium Complexes

Precise control over
olefin polymerization, especially polymer
stereochemistry, through artful catalyst design is attractive and
highly challenging. A rigid cyclic framework was first introduced
into bis­(phenoxyaldimine) titanium catalysts to study its effects
on polymerization behaviors. For copolymerization of ethylene with
α-olefins and dienes, an improvement in comonomer incorporation
ratios was observed while the stereoselectivity and regioselectivity
of comonomers were also markedly affected through a unique steric
effect. Exceptional regio- and isoselective polymerization of propylene
was promoted by the cyclic titanium catalyst via predominately 1,2-insertions
and an enantiomorphic site control mechanism, contrary to the syndiospecific
polymerization by its acyclic analogues. The experimental results
revealed that site isomerization was effectively suppressed by ligand
cyclization, and inversions of both regio- and stereoselectivity for
propylene polymerization were achieved.