Isotactic Polymerization of Methyl Methacrylate Using a Prochiral, Zirconium Enolate Initiator

Abstract

There is considerable interest in the use of metallocene or lanthanocene complexes as initiators of polymerization of acrylate-based monomers. 1 We and others have demonstrated that C 2 -symmetric, dialkyl zirconocene initiatorsse.g. rac-en(H 4 Ind) 2 ZrX 2 (1, X ) Me)spolymerize methyl methacrylate to isotactic poly-(methyl methacrylate) (i-PMMA) by an enantiomorphic, site-control mechanism. 2 While syndiotactic PMMA can be prepared using sterically encumbered, lanthanocene initiators at low temperatures, 1a,c,d,g attempts to use C ssymmetric, dialkyl zirconocene initiatorssi.e., Me 2 C-(Cp)(Flu)ZrX 2 (2, X ) Me)sfor this purpose have been unsuccessful. 2b,cAs we have shown elsewhere, either cationic enolate or neutral enolate complexes of zirconium (the latter in concert with metallocenium activators) may serve as initiators of acrylate polymerization. 1h Mechanistic studies can be simplified using such compounds, as formation of these species in situ (from, e.g., metallocene dialkyls, ionic activators, and acrylates) can be slow and/ or inefficient.We have attempted to prepare bis(enolate) complexes of 1 or 2 in the hope of studying isospecific or syndiospecific MMA polymerization using cationic enolates derived from such species. These synthetic studies have met with very limited success; in one case the bis-(enolate) complex 1 could be prepared [X ) OC(Me)d CH 2 ] but this compound failed to effect polymerization of MMA in the presence of B(C 6 F 5 ) 3 , [Ph 3 C][B(C 6 F 5 ) 4 ], or [PhNHMe 2 ][B(C 6 F 5 ) 4 ]. 3 More recently, we have focused our attention on linked, Cp-amido complexes of zirconium, as these compounds are less sterically encumbered than their metallocene "analogues", a feature which facilitates the synthesis of bis(enolate) complexes. We report here our observations using this class of initiators for MMA polymerization.The prototypical complex 4 4 reacts cleanly with 2 equiv of stable, lithium enolate 5 (R ) O t Bu) 5 to provide bis(enolate) complex 3 as a crystalline, white solid (eq 1). 6

The Influence of the Nature of the Catalytic System on Zirconocene‐Catalyzed Polymerization of Alkyl Methacrylates

“…More recently, Gibson and collaborators reported the polymerization of MMA using the same catalytic system as Soga's group but without adding ZnEt 2 . [3] The polymer produced had a narrow molecular-weight distribution (M w / M n ¼ 1.20-1.60) but also narrow range of molecular weights (M w < 50 Â 10 3 ) arising from the much higher Full Paper: Three non-bridged dimethylzirconocene complexes; i.e., bis(Z 5 -cyclopentadienyl)dimethylzirconium (1), bis(Z 5 -tert-butyl-cyclopentadienyl)dimethylzirconium (2), bis(Z 5 -indenyl)dimethylzirconium (3), and the chiral zirconocene rac-ethylenebis(Z 5 -indenyl)dimethylzirconium (4) were synthesized and used as catalysts for the polymerization of methyl methacrylate, MMA, combined with tris(pentafluorophenyl)borate (A) or tetrakis(pentafluorophenyl)borate dimethylanilinum salt (B) in the presence of ZnEt 2 . The evolution of the polymerization characteristics; i.e., molecular weight, molecular-weight distribution, and yield were examined for the above systems.…”

Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes

“…However, contrary to the results of lanthanides, 48-B(C 6 F 5 ) 3 did not give polymers. Interestingly, 49 derived from a C s -symmetric precursor gave isotactic MMA [143]. The active species 49 is chiral at the Zr metal, and the triad distribution of the polymer was consistent with a catalyticsite control mechanism.…”

Metallocenes

“…In either mechanism, propagation requires facile decomplexation of the polymer chain carboxylate and its replacement by that of the monomer. Isotactic PMMA results when a racemic-bridged bis(indenyl) (266) or amidocyclopentadienyl half-sandwich complex is employed (267); the latter requires enantiomorphic site control in the intramolecular pathway (eq. 22).…”