Mechanism and scope of stereospecific, coordinative-anionic polymerization of acrylamides by chiral zirconocenium ester and amide enolates

The mechanism and scope of the isospecific, coordinative-an ionic polymerization of acrylamides, including N,N-dimethylacrylamide (DMAA), NN-dimethylmethacrylamide (DMMA), and N-isopropylacrylamide (IPAA), using chiral ansa-zirconocenium ester and amide enolates, are reported. The zirconocenium ester enolate, rac-(EBI)Zr+(THF)[OC((OPr)-Pr-t)=CMe2][MeB(C6F5)(3)](-) [1; EBI = C2H4(Ind)(2)], effects highly isospecific and living polymerization of DMAA via a monometallic, intramolecular coordinative-conjugate-addition mechanism, with the resting intermediate during a catalytic propagation cycle being the cyclic amide enolate. The results leading to these key conclusions were derived from investigations of polymerization kinetics, polymer microstructures and chain-end groups, and block copolymerization behavior as well as modeling and isolation of the active propagating species, Specifically regarding the active species modeling, isolation, and characterization, neutral chiral amide enolate rac-(EBI)ZrMe[OC(NMe2)=CMe2](-) (2) has been synthesized and structurally characterized and its corresponding cationic complex rac-(EBI)Zr+(THF)[OC(NMe2)=C(Me)(2)][MeB(C6F5)(3)](-) (3) isolated; cation 3 is highly active for DMAA polymerization, serving as a structural model for the active propagating species. Both ester and amide enolates I and 3 are inactive for polymerizations of DMMA and IPAA; however, I equiv of IPAA or DMMA is readily added to 1 or 3, forming the eight-membered-ring cyclic amide enolates rac-(EBI)Zr+[OC((NHPr)-Pr-i)=CHCH2C(Me-2)C((OPr)-Pr-i)=O][MeB(C6F5)(3)](-) (4) or rac-(EBI)Zr+[OC(NMe2)=C(Me)-CH2C(Me-2)C(NMe2)=O][MeB(C6F5)(3)](-) (5), which correspond to the structures of the first acrylamide addition products and the resting propagation intermediates.