On the mechanism of polymerization of acrylates by zirconocene complexes, an ab initio and density functional theory MO study

RHF/3-21G ab initio, density functional theory (B3LYP/3-21G), and single-po int calculations using an effective core potential (B3LYP/G-31G*-ECP(S)//3- 21G) are performed for different mechanisms of polymerization of acrylic ac id and methyl acrylate by dicyclopentadienyl-zirconocene enolate complexes. Polymerization is considered involving a cationic zirconocene complex and a neutral zirconocene complex, respectively. In addition, a mechanism propo sed by Collins is treated which involves a neutral and a cationic zirconoce ne complex in the CC bond-forming step at the same time. Catalytic cycles c an be devised in all cases which suggest that all three mechanisms may be r ealizable under suitable renditions. However, the "bimetallic" mechanism pr oposed by Collins shows the lowest energy of activation for the propagation step. It is shown that bridging of the cyclopentadienyl rings should be im portant not only with respect to reactivity but also with respect to the po ssible catalytic cycle, Methyl methacrylate is not included in the treatmen t; i.e., the problem of tacticity in these polymerizations is not addressed .