The synthesis and molecular structure of the first examples of monomeric lithium ester enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids, are reported. Reactions of isopropyl alpha-lithioisobutyrate (2) with suitable deaggregating and stabilizing organoaluminum compounds such as MeAl(BHT)2 (BHT = 2,6-di-tert-butyl-4-methylphenolate) in hydrocarbons cleanly generate lithium ester enolaluminate complexes such as Li+[Me2C=C(OiPr)OAlMe(BHT)2]- (3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and highly active polymerization can be achieved by using the 3/MeAl(BHT)2 propagator/catalyst pair, which is conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT)2. The structure of the added organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)2 pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator 2, and monomeric propagator 3 have been determined by X-ray diffraction studies.

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...

This paper reports the homopolymerization and block copolymerization of methacrylates by the cationic titanium methyl complex, CGCTiMe + MeB(C6F5)3 -(1, CGC ) Me2Si(Me4C5)(t-BuN)), as well as the synthesis and methyl methacrylate (MMA) polymerization of three new neutral and cationic CGC Ti ester enolate complexes. Unlike the isostructural, cationic CGC Zr methyl complex, which is inactive for polymerization of MMA, CGC Ti methyl complex 1 effects living polymerizations of MMA and BMA (BMA ) n-butyl methacrylate) at ambient temperature, producing syndiotactic PMMA ([rr] ) 82%, Pr ) 0.90) and PBMA ([rr] ) 89%, Pr ) 0.95). Sequential copolymerization using 1 and starting from either MMA or BMA produces the well-defined, syndiotactic diblock copolymer PMMA-b-PBMA with narrow molecular weight distribution (Mw/Mn ) 1.08). On the other hand, copolymerizing MMA and BMA simultaneously affords the homogeneous random copolymer PMMA-co-PBMA. Two neutral CGC Ti ester enolate complexes, CGCTiCl[OC(O i Pr)dCMe2] (2) and CGCTiMe[OC(O i Pr)dCMe2] (4), have been synthesized, and the molecular structure of 4 has been determined by X-ray crystallography. The reaction of 4 and B(C6F5)3‚THF (THF ) tetrahydrofuran) in methylene chloride readily generates the cationic CGC Ti ester enolate complex, CGCTi + (THF)[OC(O i Pr)dCMe2] [MeB(C6F5)3] -(5). All three CGC Ti ester enolate complexes have been investigated for MMA polymerization; of particular interest are the polymer characteristics (molecular weight, molecular weight distribution, and syndiotacticity) of the PMMA formed by 5, which are remarkably similar to those of the PMMA by 1, suggesting that the cationic ester enolate 5 is an appropriate model for the propagating species involved in the MMA polymerization by the alkyl cation 1. The microstructures of PMMA have been analyzed at the pentad level, and the polymerization mechanism has also been discussed.

A "one pot" synthesis of new 2,6-diimine-4-alkylpyridines is reported. The addition of 2, Pr 2 C 6 H 3 Nd C(Me)] 2 C 5 H 3 N to the preformed MnR 2 (THF) n (R ) CH 2 CMe 2 Ph, CH 2 Ph, CH 2 CHdCH 2 ), followed by hydrolysis affords a mixture of 4-alkyl 2,6-bis(imino) deriVatiVes of pyridine and 1,4dihydropyridine in a Variable ratio. Treatment of the mixture with a substoichiometric amount of CrO 3 /K 2 CO 3 in THF proVides the 4-alkyl-2,6-diiminopyridines in good isolated yields and in a highly selectiVe manner.