Mechanistic Studies of Stereospecific Polymerization of Methacrylates Using a Cationic, Chiral ansa-Zirconocene Ester Enolate

Abstract

Mechanistic studies of isospecific polymerization of methacrylates by a cationic, chiral ansazirconocene ester enolate complex, rac-(EBI)Zr + (THF)[OC(O i Pr)dCMe2][MeB(C6F5)3] -[1; EBI ) C2H4-(Ind)2], are reported. Complex 1 effects isospecific and living (co)polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA), producing highly isotactic homopolymers and block copolymers with narrow molecular weight distributions and high initiator efficiencies. Investigations of statistical copolymerizations of MMA and BMA have yielded monomer reactivity ratios of rMMA ) 0.62 and rBMA ) 0.72 (the Kelen-Tü dõs method), indicating that the copolymer formed instantaneously has a somewhat alternating character. Studies of MMA polymerization kinetics have shown that propagation is first order in both concentrations of the monomer and the active species. The single MMA addition product, rac- (3), has been isolated from the reaction between the neutral rac-(2) and MMA; however, subsequent MMA additions do not proceed, unless the single addition product is further treated with B(C6F5)3 to generate the eightmembered-ring cyclic ester enolate species, rac-(EBI)Zr (4), which corresponds to the structure of the intermediate for the single MMA addition product of 1. All results presented here are consistent with the conclusion that isospecific polymerization of methacrylates by 1swhich models the proposed isospecific propagating species for the methacrylate polymerization by chiral ansa-zirconocene complexessis enantiomorphic site controlled and proceeds in a monometallic, intramolecular Michael addition mechanism. IntroductionPolymerization of methacrylates by group 4 metallocene and related complexes 1-7 has attracted increasing attention due to the following important attributes of this polymerization: (1) there is a paradigm shift in terms of scientific curiosity on utilizing highly active, electrodeficient transition-metal complexes for polymerization of polar monomers; (2) group 4 metallocene complexes with considerably diverse structural motifs are readily, and in many cases commercially, available, thanks to comprehensive studies of their roles in coordination polymerization of nonpolar olefins; and (3) these complexes, when used in a suitable initiating form (e.g., ester enolates), typically exhibit a high degree of control over polymerization, especially the stereochemistry of polymerization.Collins and co-workers showed that a two-component system consisting of an achiral neutral zirconocene ester enolate complex, Cp 2 ZrMe[OC(O t Bu)dCMe 2 ], as initiator, and a cationic zirconocene complex, [Cp 2 ZrMe-(THF)] + [BPh 4 ] -, as catalyst, polymerizes methyl methacrylate (MMA) in a living fashion to syndiotactic-rich poly(methyl methacrylate) (PMMA). 1j The mechanistic studies 1f,j revealed a group-transfer-type 8 bimetallic propagation involving the rate-limiting, intermolecular Michael addition of the zirconocene enolate to the activated MMA by the cationic zirconocene (Scheme 1). Both monometallic and bimet...

Robust Cross-Linked Stereocomplexes and C60 Inclusion Complexes of Vinyl-Functionalized Stereoregular Polymers Derived from Chemo/Stereoselective Coordination Polymerization

“…For instance, the polymerization of allyl methacrylate (AMA) by half-metallocene yttrium catalysts was shown to be both living and chemoselective. 11 Furthermore, a chiral C 2 -ligated ansa-zirconocenium ester enolate complex, [rac-C 2 H 4 (η 5 -indenyl) 2 3 ] − (1), 12 which is known to mediate stereospecific and living polymerization of simple alkyl methacrylates such as methyl methacrylate (MMA) 12,13 and acrylamides, 14 is not only completely chemoselective and living, but also highly isospecific, in the polymerization of polar divinyl monomers at ambient temperature. 15 The origin of the perfect chemoselectivity and high isoselectivity arises from the catalyst site-controlled coordinationaddition mechanism that dictates exclusive conjugate additions across the methacrylic double bond that is activated via coordination of the conjugated carbonyl to the cationic, chiral Zr center, thus leaving the pendant C=C bond intact.…”

Synthesis and Reactivity of Oxametallacyclic Niobium Compounds by Using α,β‐Unsaturated Carbonyl Ligands

“…The presence of the enolate ligand endows asymmetry to all of the complexes for which the 1 H NMR spectra showed an ABCD spin system for the cyclopentadienyl ring protons and methyl groups. The formation of the metallacyclic compounds 1a-3a and 2b and 3b was supported by the lowfield signals observed in the 1 H NMR spectra for the Nb-CH 2 group, shifted to δ ≈ 1.00 and 1.70 ppm from δ = 5-6 ppm in the starting free acrylate ligands, and also observed in the 13 C NMR spectra shifted to δ ≈ 70 ppm with respect to the free acrylate compounds (δ ≈ 130 ppm). Moreover, for the MA complexes 2b and 3b the signal of the internal C-H proton was observed at δ ≈ 5.0 ppm.…”

“…The solution was cooled to -40°C overnight yielding 1 as a red solid (2.00 g, 38 %). 1 Because of the low yield and solubility of this compound, we could not obtain pure samples for an adequate 13 C NMR spectrum and elemental analysis, although single crystals appropriate for X-ray diffraction studies were isolated. for 3a).…”

“…[4,8,31] We have investigated the reactivity of complexes 2a-b and 3a-b with the Lewis acids E(C 6 F 5 ) 3 (E = B, Al) with the aim of abstracting the alkyl ligand to generate a cationic enolate compound, considering that early transition-metal derivatives of this type have been used as precursors for the catalytic polymerization of acrylates. [12][13][14][15][16] Although the nucleophilicity of the oxygen atom bonded to the niobium atom could compete with that of the carbon atom, we have previously observed that alkoxido complexes of the type [Cp*TaMe 3 (OR)] reacted with these Lewis acids E(C 6 F 5 ) 3 at the Ta-C bond. [32] Furthermore, Mashima and co-workers also reported that reaction of the MMA complex [TaCp*(DAD)(MMA)] [DAD = N(R) CH=CHN(R)] with AlMe 3 produced the activation of the Ta-C bond of the coordinated MMA ligand.…”

“…The study of enolate derivatives has attracted attention because they have been proposed as the active species for polymerization of polar monomers. [11][12][13][14][15][16] Another feature of these compounds is that coordination of the asymmetric α,β-unsaturated carbonyl ligands will create two new reactive points in the molecule at the M-C and M-O bonds.…”