Showing papers by "Zhiyuan Zhong published in 2004"

Crystal Structure and Morphology of Poly(L-lactide-b-D-lactide) Diblock Copolymers

Abstract: The isothermal crystallization behavior of low-molecular-weight (DP 50) poly(l-lactide-b-d-lactide) diblock copolymers is reported, as studied by small- and wide-angle X-ray scattering. The symmetric diblock copolymer crystallizes in the stereocomplex form for the whole range of crystallization temperatures, while the one with a block ratio of 80/20 grows into homocrystallites. For an intermediate block ratio (66/33), the stereocomplex form is obtained at high crystallization temperatures while a coexistence region with homocrystal phase exists at low temperatures. In addition to the transition enthalpy, size effects and kinetic routes play a role in the competition between the stereocomplex and the homocrystalline forms. The lamellar spacing (long period and crystal thickness) shows a different temperature dependence for the three diblock copolymers, which is discussed in relation to the specific morphology and molecular constitution.

Controlled synthesis of biodegradable lactide polymers and copolymers using novel in situ generated or single-site stereoselective polymerization initiators.

Abstract: Polylactides and their copolymers are key biodegradable polymers used widely in biomedical, pharmaceutical and ecological applications. The development of synthetic pathways and catalyst/initiator systems to produce pre-designed polylactides, as well as the fundamental understanding of the polymerization reactions, has continuously been an important topic. Here, we will address the recent advances in the ring-opening polymerization of lactides, with an emphasis on the highly versatile in situ generated initiator systems and single-site stereoselective initiators. The in situ generated initiators including in situ formed yttrium, calcium and zinc alkoxides all have been shown to bring about a rapid and living polymerization of lactides under mild conditions, which facilitated the preparation of a variety of advanced lactide-based biomaterials. For example, well-defined di- and tri-block copolymers consisting of hydrophilic poly(ethylene glycol) blocks and hydrophobic polyester blocks, which form novel biodegradable polymersomes or biodegradable thermosensitive hydrogels, have been prepared. In the past few years, significant progress has also been made in the area of stereoselective polymerization of lactides. This new generation of initiators has enabled the production of polylactide materials with novel microstructures and/or properties, such as heterotactic (–RRSSRRSSRRSS–) polylactide, crystalline syndiotactic (–RSRSRSRSRSRS–) polylactide and isotactic stereoblock (–RnSnRnSn–) polylactide, exhibiting a high melting temperature. The recently developed polymerizations using in situ generated initiators and stereoselective polymerizations have no doubt opened a brand-new avenue for the design and exploration of polylactides and their copolymers.

Kristallstrukturen und spektroskopische Eigenschaften von 23-Phospha-1, 3-dionaten und 1, 3-Dionaten des Calciums - ein Vergleich am Beispiel der 1, 3-Diphenyl- und 1, 3-Di(tert-butyl)-Derivate = Crystal Structures and Spectroscopic Properties of 23-Phospha-1, 3-dionates and 1, 3-Dionates of Calcium - Comparative Studies on the 1, 3-Diphenyl and 1, 3-Di(tert-butyl) Derivatives

Abstract: Der Wasserstoff-Metall-Austausch zwischen Dibenzoylphosphan und Calciumcarbid in Tetrahydrofuran (THF) ergibt nach Zusatz des Liganden 1, 3, 5-Trimethyl-1, 3, 5-triazinan (TMTA) den als Kokristallisat mit Benzol isolierten zweikernigen Komplex Bis[(tmta-N, N, N)calcium-bis(dibenzoylphosphanid)] (1a). In ahnlicher Weise fuhrt die Umsetzung von Bis(2, 2-dimethylpropionyl)phosphan mit Bis(thf-O)calcium-bis[bis(trimethylsilyl)amid] in 1, 2-Dimethoxyethan (DME) mit guten Ausbeuten zum -Bis(dme-O, O)calcium-bis[bis(2, 2-dimethylpropionyl)phosphanid] (1b). Uber die Reaktion von Bis(thf-O)calcium-bis[tris(trimethylsilylmethyl)zinkat] mit dem Kohlenstoffanalogon 1, 3-Diphenylpropan-1, 3-dion (Dibenzoylmethan) bzw. 2, 2, 6, 6-Tetramethylheptan-3, 5-dion (Dipivaloylmethan) erhalt man im gleichen Solvens das einkernige Bis(dme-O, O)calcium-bis(dibenzoylmethanid) (2a) und das zweikernige (-dme-O, O)Bis[(dme-O, O)calcium-bis(dipivaloylmethanid)] (2b). Uberraschenderweise setzt sich das dabei gebildete Dialkylzink mit den 1, 3-Diketonaten 2a und 2b nicht weiter um. Schlieslich kann aus dem unsymmetrischen Liganden 2-(Methoxycarbonyl)cyclopentanon und Bis(thf-O)calcium-bis[bis(trimethylsilyl)amid] in Toluol der dreikernige Komplex 3 als Kokristallisat mit Tetrahydrofuran erhalten werden; der -Ketoester-Ligand verbruckt als Anion ausschlieslich uber die Cyclopentanon-Einheit. A hydrogen-metal exchange between dibenzoylphosphane and calcium carbide in tetrahydrofuran (THF) followed by addition of the ligand 1, 3, 5-trimethyl-1, 3, 5-triazinane (TMTA) furnishes the binuclear complex bis[(tmta-N, N, N)calcium bis(dibenzoylphosphanide)] (1a) co-crystallizing with benzene. Similarly, reaction of bis(2, 2-dimethylpropionyl)phosphane with bis(thf-O)calcium bis[bis(trimethylsilyl)amide] in 1, 2-dimethoxyethane (DME) gives bis(dme-O, O)calcium bis[bis(2, 2-dimethylpropionyl)phosphanide] (1b) in high yield. The carbon analogues 1, 3-diphenylpropane-1, 3-dione (dibenzoylmethane) or 2, 2, 6, 6-tetramethylheptane-3, 5-dione (dipivaloylmethane) and bis(thf-O)calcium bis[tris(trimethylsilylmethyl)zincate] in DME afford bis(dme-O, O)calcium bis(dibenzoylmethanide) (2a) and the binuclear complex (-dme-O, O)bis[(dme-O, O)calcium bis(dipivaloylmethanide)] (2b), respectively. Dialkylzinc formed during the metalation reaction shows no reactivity towards the 1, 3-dionates 2a and 2b. Finally, from the reaction of the unsymmetrically substituted ligand 2-(methoxycarbonyl)cyclopentanone and bis(thf-O)calcium bis[bis(trimethylsilyl)amide] in toluene, the trinuclear complex 3 is obtained, co-crystallizing with THF. The -ketoester anion bridges solely via the cyclopentanone unit.