RHF/3-21G ab initio, d. functional theory (B3LYP/3-21G), and single-point calcns. using an effective core potential (B3LYP/6-31G*-ECP(S)//3-21G) are performed for different mechanisms of polymn. of acrylic acid and Me acrylate by dicyclopentadienyl-zirconocene enolate complexes. Polymn. is considered involving a cationic zirconocene complex and a neutral zirconocene complex, resp. In addn., a mechanism proposed by Collins is treated which involves a neutral and a cationic zirconocene complex in the CC bond-forming step at the same time. Catalytic cycles can be devised in all cases which suggest that all three mechanisms may be realizable under suitable conditions. However, the \"bimetallic\" mechanism proposed by Collins shows the lowest energy of activation for the propagation step. It is shown that bridging of the cyclopentadienyl rings should be important not only with respect to reactivity but also with respect to the possible catalytic cycle. Me methacrylate is not included in the treatment, i.e., the problem of tacticity in these polymns. is not addressed.