Metallocene-Mediated Asymmetric Coordination Polymerization of Polar Vinyl Monomers to Optically Active, Stereoregular Polymers

Abstract

Asymmetric coordination polymerization of 13 acrylamide and methacrylate monomers of four different classes has been investigated using chiral ansa-zirconocenium ester enolate catalyst (S,S)-(EBI)Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− [(S,S)-1), EBI = C2H4(η5-Ind)2] and its enantiomer (R,R)-1. This polymerization system is built upon four advanced features of polymerization including living, stereospecific, coordination, and asymmetric core elements, thus efficiently converting prochiral N,N-diaryl acrylamides at ambient temperature to optically active, stereoregular polymers with solution-stable, single-handed helical secondary structures. Kinetic studies show that the polymerization of N,N-diaryl acrylamides by 1 proceeds via a monometallic, coordination-conjugate addition mechanism. Investigation into polymer chain-length effects on optical activity of the chiral polymers reveals two opposite trends, depending on the polymer secondary structure (i.e., helical vs random coil conformation). Examination of the polymerization scope shows that the formation of optically active poly(acrylamide)s due to solution-stable helical conformations with an excess of one-handed helicity is dictated by the sterics and rigidity of the monomer repeat units; while diaryl acrylamides can readily achieve such conformations, unsymmetrically substituted diaryl acrylamides give the chiral polymers with much higher optical activity than the symmetrically substituted ones. It is also possible for N,N-dialkyl acrylamides to lead to chiral helical polymers. Extensive asymmetric block copolymerization studies of MMA with acrylamides and other methacrylates have also been carried out, producing optically active, high-molecular-weight methacrylate-b-acrylamide stereoblock copolymers in which the acrylamide block can be either helical or nonhelical; in sharp contrast, all high-molecular-weight methacrylate-b-methacrylate di- or triblock copolymers produced by the enantiomeric catalysts 1 are optically inactive.