On the Mechanism of Polymerization of Acrylates by Zirconocene Complexes, an ab Initio and Density Functional Theory MO Study

Abstract

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.

Polymerization of Methyl Acrylate by a 2,6‐Bis(2‐benzimidazyl)pyridine Zirconium Dichloride/MAO Catalyst System

“…Results of the DFT calculation of acrylate polymerization suggest that late transition metal catalysts such as Ni or Pd polymerize the acrylate by the p-bond approach with migratory insertion [30] and early transition metal catalysts such as Zr polymerize the acrylate by the carbonyl oxygen lone pair approach with several intermediate steps. [31][32][33] Taking all these studies into consideration, it is reasonable to explain that the Zr(bbipy)Cl 2 /MAO system follows the carbonyl oxygen lone pair approach pathway (Scheme 3).…”

Polymerization of Methyl Methacrylate with Chloro Zirconocene Enolates

“…This is in agreement with the results of our theoretical calculations. [15] For such systems, these studies prefer a mechanism in which both a zirconocene cation and an enolate take part in chain growth. However, it is assumed that the applicability of this mechanism for controlling the stereospecificy of polymerization is only limited, since the location of C,C chain linkage occurs far from the metal centers.…”

“…This is also shown by the arrangement of the reaction partners in the calculated transition state ( Figure 1). [15] Therefore, the structure of ligands should only play a minor role in the spatial orientation of the reaction partners.…”

A cationic bridged zirconocene complex as the catalyst for the stereospecific polymerization of methyl methacrylate

“…Me 3 SiCl (17.24 g, 158 mmol) was added within 1 h. The reaction mixture was allowed to warm up to room temperature and stirred for 12 h. The fine yellow precipitate was filtered off, washed with toluene and dried in vacuum (8.06 g (21 mmol), 70% yield). 1 4 ] were suspended at -78 8C in 80 mL toluene. The suspension was stirred and warmed to 20 8C within 30 min.…”

“…Sustmann et al performed theoretical calculations for the Collins mechanism and monometallic alternatives and found the lowest energy of activation for the propagation via the bimetallic mechanism. [4] Soga et al reported that cationic zirconocene complexes formed from dimethylzirconocenes and [Ph 3 C] [B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 polymerize MMA when activated with alkyl zinc or alkyl aluminum compounds. [5][6][7][8][9] Both, Collins and Soga, found two or three component catalysts for the polymerization of MMA.…”