RHF/3-21G ab initio, density functional theory (B3LYP/3-21G), and single-point calculations using an effective core potential (B3LYP/6-31G*-ECP(S)//3-21G) are performed for different mechanisms of polymerization of acrylic acid 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 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. Methyl methacrylate is not included in the treatment; i.e., the problem of tacticity in these polymerizations is not addressed.

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The polymerization of methyl methacrylate (MMA) in toluene with Cp2ZrMe2 (1) – Ph3C+B(C6F5)4− (4) as initiating system was kinetically investigated. This system can be regarded as a combination of the systems described by Collins and Soga. Only if the ratio of 1/4 was larger than 1, monomer conversions of more than 99% and polymers with number‐average molecular weights higher than 200 000 and polydispersities 1.5–1.6 were obtained. From a kinetic analysis of time‐conversion curves using the software package Predici, it can be deduced that the mechanism is consistent with that proposed by Collins et al. However, the mechanism has to be extended by incorporating a first‐order termination reaction of the active species. Rate coefficients of all elementary reactions were determined from time‐conversion curves. The kinetic investigations were accompanied by ab initio calculations, which support the assumed kinetic model.

Summary: The polymerization of methyl methacrylate (MMA) in the presence of the neutral chloro zirconocene enolates, Cp2ZrCl[OC(OMe)CMe2] (1), Me4C2(Cp)2ZrCl[OC(OMe)CMe2] (2), and Me2C(Cp)2ZrCl[OC(OMe)CMe2] (3), was investigated. None of these compounds is catalytically active on its own. They could be activated by adding Ph3CB(C6F5)4 (4); however, only if the initial concentration of enolate is higher than that of the perfluorated phenyl borate. Polymers were produced with a number average molar mass of up to 100 000 g/mol and a dispersion index of 1.1–1.3 with mixtures comprising 1 and 4 or 2 and 4. The degree of polymerization depends only on the quantity of excess enolate and not on the absolute concentration of the initiator components. In contrast, a mixture of 3 and 4 was not very active. Kinetic modeling of the systems as well as NMR spectroscopic investigations indicate that a bimetallic mechanism can describe chain growth. The activity of these catalytic systems depends on the structure of the ligand. The two zirconocenes with bridged cyclopentadienyl ligands, (2) and (3), convert MMA considerably slower. In addition to the methyl zirconocene enolates with unbridged cyclopentadienyl ligands, the corresponding chloro zirconocene enolates are also suitable for the polymerization of MMA. The latter compounds offer the advantage that preparations for the production of the initiators are comparatively simple.Structures of investigated initiators.magnified imageStructures of investigated initiators.