James C. W. Chien

Ultrastructures of rabbit corneal stroma: Mapping of optical and morphological anisotropies

Abstract Pole figures of optic and morphological structures of rabbit corneal stroma have been determined. The birefringence of stroma is non-uniform, but tends to increase in the directions of nose to ear and of periphery to the vertex. There is no obvious symmetry in the local optic axes. Its direction changes from limbus to limbus. Small angle laser scattering showed stroma to have sheaf-like morphology. This texture is consistent with bundles of collagen fibrils which divide and anastomose. The average size of the scattering entities in rabbit corneal stroma is 19–23 μm with sector angles β ranging from 1 to 15°.

Alkyl-substituted indenyl titanium precursors for syndiospecific ziegler-natta polymerization of styrene

A variety of 1- and 3-substituted alkylindenes (R = H, Me, Et, tert-butyl, Me3Si) as well as 2-methylindene and 3-(methylthio)indene have been prepared in good yields. The substituted indenes were converted into trimethylsilyl derivatives via reactions of intermediate organolithium complexes with chlorotrimethylsilane. The corresponding titanium complexes, (R-Ind)TiCl3, were synthesized in excellent yield from reactions of the trimethylsilyl derivatives with TiCl4. The titanium complexes were evaluated as styrene polymerization catalysts in toluene solution when activated by methylaluminoxane. Activities increased in the order: Cp < H4 Ind < Ind < 1-(Me)Ind <2-(Me)Ind. A steep drop in activity was observed when R = Et, tert-butyl, and Me3Si, corresponding to an increase in the steric bulk of the substituent in the catalyst precursor. 1-(MeS)IndTiCl3 was found to be ineffective as a styrene polymerization catalyst. Syndiospecificities of the titanium complexes were generally very good (65–98%).

Polymer reactions—Part VII: Thermal pyrolysis of polypropylene

Abstract Polypropylene has been pyrolysed in a carrier stream of helium from 388° to 900°C in both the programmed heating and flash pyrolysis modes. The products were on-line identified and quantitatively analysed by an interfaced GC peak identification system. The first order rate constants for pyrolysis are 3·7 × 10−4 sec−1 and 4·0 × 10−4 sec−1, respectively, for atactic and isotactic polypropylene at 388°C; the corresponding overall activation energies are 56 ± 6 and 51 ± 5 kcal mole−1. The main products in decreasing yields are 2,4-dimethyl-1-heptene, 2-pentene, propylene, 2 methyl-1-pentene and 2,4,6-trimethyl-1-nonene. Also isolated, but in much smaller quantities, are: ethane, isobutylene, 4,6-dimethyl-2-nonene, 2,4,6-trimethyl-1-heptene, 3-methyl-3,5-hexadiene and methane. Propylene is the product of an unzipping reaction. Most of the other products can be accounted for by a mechanism involving first, random scission of carbon-carbon bonds to produce methyl, primary and secondary alkyl radicals, followed by intramolecular hydrogen transfer processes. Methane and ethane are formed from the methyl radicals. All the products found in high yields are derived from the secondary alkyl radicals.

Supported metallocene polymerization catalysis

Zirconocene supported on alumina or magnesium chloride exhibits modest olefin polymerization activity but not when it is supported directly on silica. The surface silanol as well as siloxane groups must be passivated with appropriate reagents. A very active and stereospecific supported catalyst was obtained by first reacting silica with methylalumoxane and bisphenol A before the impregnation of the ansa-zirconocene precursor. The main difference between a homogeneous and supported catalyst is that at the same gross amount of metallocene, the net concentration of it in the pores of a support material is several orders of magnitude greater than it exists in solution thus the rate of deactivation in the former case is correspondingly faster than in the latter. Most other supported metallocene catalysts based on supports such as zeolites, cyclodextrin, polymeric MAO, synthetic polymers, etc., are poor in olefin polymerization for this and other reasons discussed.

Polymerization with TMA‐protected polar vinyl comonomers. II. Catalyzed by nickel complexes containing α‐diimine‐type ligands

α-Diimine Ni complexes (7, 8) were used as catalyst precursors with MAO in co- and terpolymerization of ethylene/propylene/α-olefins with OH and COOH functional groups. Trimethylaluminium was used to protect the functional group of polar monomers. The presence of 5-hexen-1-ol seems to have no effect on the polymerization rate at all for the N,N′-bis(2,6-diisopropylphenyl) derivative 8 but caused activity decreases of about fivefold in copolymerization and around two times in terpolymerization for the N,N-dimesityl derivative 7. The effect levels off at higher polar comonomer concentration. This system, (7)/MAO, also incorporates well both 10-undecen-1-ol and 10-undecen-1-oic acid. The activities obtained with these α-diimine Ni complexes in co- and terpolymerization are three to twenty times higher than those obtained with group 4 Cp based complexes especially at concentrations of polar monomer in the feed higher than 80 mM.

Polymerization with TMA‐protected polar vinyl comonomers. I. Catalyzed by group 4 metal complexes with η5‐type ligands

This paper describes the use of several kinds of group IV Cp based catalyst systems, in the synthesis of co- and terpolymers of ethylene, propylene and α-olefins endowed with OH and COOH functional groups. The hydroxy monomers used were 5-hexen-1-ol (4) and 10-undecen-1-ol (5) and the carboxy monomer was 10-undecen-1-oic acid (6). The three catalyst systems used were the C 2 symmetric ansa-zirconocene (1) the in-site titanium complex (2) and the non-rigid zirconocene (3), all activated by methylaluminoxane. Trimethylaluminium was used to protect the functional group of polar monomers. The first two catalyst systems suffer similar activity loss in the presence of polar monomer whereas the third one exhibited better tolerance toward the hydroxyolefins. NMR and FTIR spectroscopies were used to characterize the polymerization products. All three catalyst systems afforded functionalized co- and terpolymers by direct polymerization of ethylene/propylene/hydroxy-a-olefins but only the catalyst system (1)/MAO displays appreciable activities for direct polymerization of ethylene, propylene and carboxy-α-olefins.

Homogeneous binary zirconocenium catalysts for propylene polymerization. II. Mixtures of isospecific and syndiospecific zirconocene systems

Isotactic polypropylene (i-PP) and syndiotactic polypropylene (s-PP) obtained with single-site C 2 - and C s -symmetric zirconocenes, respectively, activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate (3) are immiscible with one another. Physical mixture of the two PPs is macrophase separated and crystallizes very slowly as is characteristic of s-PP. A solution of C 2 - and C s -symmetric zirconocenes with 3 also produces an incompatible mixture of i-PP and s-PP. However, in the presence of an excess of a crossover agent such as triisobutylaluminum (TIBA), a new polymeric material is obtained that exhibits lower T m and ΔH f than either of the two homosteric polymers and a much faster rate of crystallization than that of s-PP or its physical mixture with i-PP. Thermal annealing of this product markedly reduces the relative amount of crystalline phase compared to the amorphous phase and atomic force microscopy finds only microphase separation but not macroscopic phase separation. Fractionation and 13 C-NMR showed the material to contain not only the homosteric i-PP and s-PP but also a stereoblock PP (i-PP-b-s-PP). The stereoblock fraction acts as a compatibilizing agent; it probably is produced by the exchange of propagating polymer chains between the syndiospecific and the isospecific sites assisted by the crossover agent.

Silylene-bridged fluorenyl-containing ligands and zirconium complexes with C1 and Cs symmetry: general synthesis and olefin polymerization catalysis

A variety of new silylene-bridged fluorenyl-containing ligands has been synthesized with good yields via a convenient synthetic route. Two dimethylsilylene-bridged (η5-indenyl)(η5-fluorenyl)(1) and (η5-cyclopentadienyl)(η5-fluorenyl) (2) zirconocene dichlorides with C1 and Cs molecular symmetry have also been prepared. Upon activation with methyl aluminoxane, the former produced a polypropylene of high molecular weight, but with low activity and isotacticity. The latter catalyzed non-stereospecific propylene polymerization without any syndiotactic tendency, but with 40 and 20 times greater ethylene and propylene activity respectively than the former catalytic system.

Nuclear magnetic resonance studies of metal substituted horse cytochrome c

Abstract The proton nuclear magnetic resonance spectra of various metal substituted derivatives of horse cytochrome c have been studied and compared to the spectra of native cytochrome c . The proteins studied were the cobalt(III), copper(II), iron(II), iron(III), manganese(III), nickel(II), and zinc(II) derivatives. Spectra of the diamagnetic cobalt(III), iron(II), and zinc(II) proteins were well-resolved and specific resonance assignments were made. All three proteins possessed a methionine ligand to the metal. The spectrum of cobalt(III) cytochrome c was investigated in some detail as this protein was used as a diagmagnetic control for iron(III) cytochrome c . Comparison of the spectra of cobalt(III) and iron(II) cytochromes c revealed that their conformations were very similar but the following conclusion could be made; the oxidation of cytochrome c is accompanied by a small conformation change.

“Anisotactic” polypropylenes produced with a zirconocene-methylalumoxane catalyst: solid state properties and microstructure

SummaryPropene was polymerized with ethylene bis(-1-indenyl)-dichlorozirconium (Et(Ind)2ZrCl2) and methylalumoxane (MAO) at 70°C. The polymer was fractionated by solvent extraction; there was no hexane insoluble material. The pentane insoluble, hexane soluble (C5iC6s) fraction contains 67% mmmm pentads. It has x-ray crystallinity of 59% comprised of 93% γ-phase, and no NMR detectable misinsertions. However, 1,3-insertion was found in the more soluble fractions.