Stereospecificity in metallocene catalyzed acrylate polymerizations: The chiral orientation of the growing chain selects its own chain end enantioface

The mechanism of stereocontrol in the site-controlled stereospecific polymerization of methyl methacrylate (MMA) with C_2- and C _s-symmetric zirconocenes is investigated with a DFT based approach. Our model explains the experimentally observed isospecificity in MMA polymerization with the C_2-symmetric rac-C₂H ₄(Ind)₂Zr-based system as well as the experimentally observed syndiospecificity in MMA polymerization with the C _s-symmetric Me₂C(Cp)(Flu)Zr-based system. In both cases, the chiral metallocene induces a chiral orientation of the ester enolate growing chain. In turn, the chirally oriented growing chain selects its own enantioface. Comparison with the mechanism of stereocontrol operative in the case of propene polymerization by the same zirconocenes is performed to stress similarities and differences. Although analogies are expected, surprising differences also exist. The most peculiar is that the in the case of 1-olefins, the enantioselective event is selection between the enantiofaces of the prochiral monomer. Instead, in the case of acrylates, the enantioselective event is selection between the enantiofaces of the prochiral growing chain.