Biomedical subjects
Publications and source records attributed to Eugene Y-X Chen.
Unprecedented cationic zinc enolates have been generated by a novel activation route involving the amido to imino ligand transformation with B(C6F5)3, structurally characterized, and utilized as highly active catalysts for the production of high molecular weight polyacrylates at ambient temperature.
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.
The synthesis, structural characterization, and abstraction chemistry of ansa-zirconocene ester enolate complexes relevant to the isospecific polymerization of methacrylates are reported. Reactions of rac-(EBI)ZrMe(OTf) and rac-(EBI)Zr(OTf)(2) [EBI = C(2)H(4)(Ind)(2)] with 1 and 2 equiv of lithium isopropylisobutyrate in toluene produce the first examples of ansa-zirconocene mono- and diester enolate complexes: rac-(EBI)ZrMe[OC(O(i)Pr)=CMe(2)] (1) and rac-(EBI)Zr[OC(O(i)Pr)=CMe(2)](2) (2) in 89% and 50% isolated yields, respectively. The reaction of 1 with B(C(6)F(5))(3) was investigated in six different organic solvents; in THF at ambient temperature, this reaction cleanly produces the isolable cationic ansa-zirconocene ester enolate complex rac-(EBI)Zr(+)(THF)[OC(O(i)Pr)=CMe(2)][MeB(C(6)F(5))(3)](-) (3) in quantitative yield. The analogous reaction of 1 with Al(C(6)F(5))(3) in toluene, however, proceeds through a proposed novel, intramolecular proton transfer process in which propylene is eliminated from the isopropoxy group, subsequently producing a carboxylate-bridged tight ion pair rac-(EBI)Zr(+)(Me)OC((i)Pr)OAl(C(6)F(5))(3)(-) (4). In addition to standard spectroscopic and analytical characterizations for the isolated complexes 1-4, complexes 2 and 4 have also been structurally characterized by X-ray diffraction studies. Polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA) has been investigated using complexes 1, 3, and 4. Both the isolated cationic 3 and neutral 1 (the latter combined with B(C(6)F(5))(3) in situ) are highly active (10 min for quantitative MMA conversion) and highly isospecific ([mm] > 95% for PMMA; [mm] > 99% for PBMA) via enantiomorphic-site control, producing polymethacrylates with extremely narrow molecular weight distributions (M(w)/M(n) = 1.03). The aluminate complex 4, however, produces syndiotactic PMMA predominantly via chain-end control.
The reduction of ZrCl4(PR3)2 with Li powder, in the presence of a stoichiometric amount of trans-1,4-diphenyl-1,3-butadiene, affords the Zr(II) diene complexes (1) in 90-93% yields. This reaction consists of a rate-limiting step for the formation of the chloride-bridged Zr(III) dimer (2) and a fast diene-driven disproportionation of 2 to 1 and ZrCl4(PR3)2 that re-enters the reduction cycle. The reaction of 1 with Li2{Me2Si(2-Me-4-Ph-Ind)2} in toluene produces quantitatively the desired racemic, divalent ansa-zirconocene (3) that is a highly active isospecific propylene polymerization catalyst upon activation with common activators.
Representative C2v-, C2-, and Cs-symmetric metallocene complexes were effectively anchored inside the silicate galleries via a non-cation-exchange approach involving protonolysis. These intergallery-anchored metallocenium cations allow for the first synthesis of in situ polymerized nanocomposites comprising exfoliated silicate nanoplatelets homogeneously dispersed in the atactic, isotactic, or syndiotactic PMMA matrixes.
Diastereospecific ion pairs consisting of isospecific transition-metal cations and syndiospecific main-group anions promote what is termed amphicatalytic polymerization of MMA, producing PMMA with isotactic-b-syndiotactic stereomultiblock microstructures.
Stereoblock polymerization with chiral ansa-metallocene/strong Lewis acid hybrid catalysts capable of switching stereospecificity of the methyl methacrylate polymerization produces the highly stereoregular isotactic-b-syndiotactic stereoblock poly(methyl methacrylate).
In the structure of bis([N-[dimethyl(1eta5-2,3,4,6-tetramethylindenyl)silyl]cyclohexylamido-1kappaN](methyl-3kappaC)-di-mu3-methylene-1:2:3kappa3C;1:3:3'kappa3C-tris(pentafluorophenyl-2kappaC)titanium) benzene disolvate, [Me2Si(eta5-2,3,4,6-Me4C9H2)(C6H11N)]Ti[(mu3-CH2)Al(C6F5)3][AlMe(mu3-CH2)]2 or [Ti2(C21H7AlF15)2(C21H31NSi)2].2C6D6, the dimer is located on an inversion center, and the two Ti centers are linked by double Ti(mu3-CH2)Al(C6F5)3AlMe(mu3-CH2) heterocycles. The electron-deficient Ti centers are further stabilized by two alpha-agostic interactions between Ti and one H atom of each bridging methylene group.