Youngjo Kim

A Tetrameric Titanium Alkoxide as a Lactide Polymerization Catalyst

The tetrameric titanium alkoxide (MeC(CH 2 -μ 3 -O)(CH 2 -μ-O) 2 ) 2 Ti 4 (O-i-Pr) 10 (1) catalyzes the ring-opening polymerization (ROP) of lactide (LA) in toluene solution at various polymerization temperatures, and its bulk ROP at 130 °C. Compound 1 facilitated reasonably controlled polymerization characteristics via a coordination/insertion mechanism in solution, whereas the bulk polymerization products displayed broad molecular-weight distributions. The stereochemical microstructure of PLA was determined from homonuclear decoupled 1 H NMR spectroscopic studies.

Synthesis and polymerization behavior of various substituted indenyl titanium complexes as catalysts for syndiotactic polystyrene

Abstract Five substituted indenyltrichlorotitanium compounds have been synthesized and characterized by spectroscopic methods and their catalytic behavior for the polymerization of styrene was studied in the presence of methylaluminoxane (MAO) as a cocatalyst. Substituted indenyl ligands include 1,3-dimethyl, 1-methyl, 1-ethyl, 1-isopropyl and 1-(trimethylsilyl)indenyl groups. All five compounds gave extremely pure syndiotactic polystyrene, with conversion rates of at least 95%, and their catalytic properties were compared with the data obtained from the ( η 5 -indenyl)trichlorotitanium-MAO system. The catalytic activity is enhanced by less bulky and better electron-releasing substituents of the indenyl ligand.

Synthesis, structure and ethylene polymerization behavior of zirconium complexes with chelating ketoiminate ligands

Mixed ketoiminate/ketoimine/pentamethylcyclopentadienyl (Cp*) complex of zirconium, [(η5-Cp*){CH3C(O)CHC(NHR)CH3}{CH3C(O)CHC(NR)CH3}ZrCl2] (R=4-CF3Ph) (3) has been prepared in high yield by the reaction of one equivalent of 4-CF3-phenyl-β-ketoimine (1a) and one equivalent of lithium 4-CF3-phenyl-β-ketoiminate (2a) with one equivalent of Cp*ZrCl3 in Et2O. Bis(ketoiminate)zirconium dichloride complexes, 4 and 6, have been also prepared in high yield by the reaction of amine elimination of ketoimine ligands, respectively 1a and 1b, with Zr(NMe2)4 and followed by chlorination reaction with TMSCl. The X-ray crystallography reveals that the compound 3 is based on distorted octahedral geometry containing a ketoimine and a ketoiminate. The ketoiminate ligand coordinates to the zirconium as a bidentate ligand, leaving the metal center coordinatively unsaturated and thus leading to an additional binding of a ketoimine ligand to the metal to stabilize the complex 3. The zirconium complexes 3, 4 and 6 provide the moderate activity for the polymerization of ethylene in the presence of MMAO cocatalyst. Low molecular weight and high density polyethylene was obtained.

Facile synthesis of uniform large-sized InP nanocrystal quantum dots using tris(tert-butyldimethylsilyl)phosphine.

Colloidal III-V semiconductor nanocrystal quantum dots [NQDs] have attracted interest because they have reduced toxicity compared with II-VI compounds. However, the study and application of III-V semiconductor nanocrystals are limited by difficulties in their synthesis. In particular, it is difficult to control nucleation because the molecular bonds in III-V semiconductors are highly covalent. A synthetic approach of InP NQDs was presented using newly synthesized organometallic phosphorus [P] precursors with different functional moieties while preserving the P-Si bond. Introducing bulky side chains in our study improved the stability while facilitating InP formation with strong confinement at a readily low temperature regime (210°C to 300°C). Further shell coating with ZnS resulted in highly luminescent core-shell materials. The design and synthesis of P precursors for high-quality InP NQDs were conducted for the first time, and we were able to control the nucleation by varying the reactivity of P precursors, therefore achieving uniform large-sized InP NQDs. This opens the way for the large-scale production of high-quality Cd-free nanocrystal quantum dots.

New half-sandwich metallocene catalysts for polyethylene and polystyrene

Abstract A new type of half-sandwich metallocene catalysts for the polymerization of ethylene and styrene, Cp*MCl(DEAR) (DEA=(OCH2CH2)2N, R=Me, M=Ti (1); R=Me, M=Zr (2); R=n-Bu, M=Ti (3)), was prepared by the reaction of corresponding N-alkyl-N,N-diethanolamine with Cp*MCl3 in the presence of triethylamine. Compounds 1–3 are slightly air-sensitive and thermally unstable. All catalyst systems show moderate activities in ethylene polymerization and especially the 2/MMAO system gives PE with broad molecular weight distribution at all polymerization temperatures. In addition, 1/MMAO and 3/MMAO show fairly good activities in sPS polymerization in the order 3/MMAO>1/MMAO.

New half-metallocene catalysts generating polyethylene with bimodal molecular weight distribution and syndiotactic polystyrene

A new type of half-metallocene catalyst for the polymerization of ethylene and styrene Cp * M((O) 3 O 9 Si 7 (c-C 5 H 9 ) 7 ) (M= Ti (1), Zr (2)) is prepared by the reaction of (HO) 3 O 9 Si 7 (c-C 5 H 9 ) 7 with Cp * MCl 3 in the presence of triethylamine. Complex 1 is stable to heat and air but heavier congener 2 is sligtly air sensitive. The catalytic systems 1/MMAO and 2/MMAO, in which MMAO is modified methylaluminoxane show good activities in ethylene polymerization, with that of 2/MMAO greater than that of 1/MMAO. Polyethylenes with bimodal molecular weight distributions were obtained. In addition, the catalytic system 1/MMAO shows fairly good activities for the syndiospecific polymerization of styrene.

Syndiotactic polystyrene with very high molecular weight produced by sterically and electronically modified catalyst

Abstract A new type of the half-metallocene catalysts for the syndiospecific polymerization of styrene was prepared by the reaction of various kinds of trialkanolamine with Cp*TiCl3 in the presence of triethylamine. All seven compounds have a highly thermal stability and they show fairly good activities in the presence of cocatalyst MMAO in styrene polymerization. Especially, highly bulky and electronically deficient 7/MMAO system affords syndiotactic polystyrene with very high molecular weight.

Charged functional group effects on a metal–organic framework for selective organic dye adsorptions

A quaternary N-alkyl ammonium salt-containing zinc-based metal–organic framework (MOF) was synthesized and tested for selective organic dye adsorption experiments. Through covalent post-synthetic modification (PSM), the tertiary amine-containing MOF was converted into a quaternary N-alkyl ammonium salt-containing MOF under solid-state methylation conditions. The positively charged MOF was found to exclusively adsorb the negatively charged organic dye, while the positively charged organic dyes were not adsorbed. This separation experiment revealed that functional group control is an important and critical factor for molecule separations using porous materials.

Preparation of syndiotactic poly(4‐tert‐butyldimethyl‐silyloxystyrene) and poly(4‐hydroxystyrene)

Syndiospecific silyloxy-functionalized polystyrene with high molecular weight was prepared using a (pentamethylcyclopentadienyl)titanatrane/MMAO catalyst system. The resulting polymer is soluble in polar organic solvents such as THF and shows good thermal stability. In addition, the compound Ni(acac) 2 was used as a catalyst in preparing authentic atactic polymer of 4-tert-butyldimethylsilyloxystyrene under the same conditions. The chemical transformation of syndiospecific poly(4-tert-butyldimethylsilyloxystyrene) also gave a new polar polymer namely syndiotactic poly(4-hydroxystyrene) which is unattainable by traditional synthetic methods.

Increased open-circuit voltage in a Schottky device using PbS quantum dots with extreme confinement

We fabricated the PbS nanocrystal quantum dots (NQDs) based Schottky structure device (ITO/PbS/LiF/Al) with varying bandgap of NQDs from 0.8 to 2.2 eV. The open-circuit voltage increased monotonically with NQDs bandgap until 0.67 V, achieved using extremely confined, 1.5 nm sized-PbS NQDs. The power conversion efficiency reached the maximum value over 3% under AM 1.5 with NQDs bandgap of about 1.3 eV. Size-dependent photovoltaic evaluation in extreme confinement regime provides basis for efficient multi-junction solar cells composed of PbS NQDs of different sizes.