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.