Design of Novel Hexametallic Cartwheel Molecules from Persubstituted Benzene Compounds

The cartwheel complexes A are novel, nano-sized hexametallic species available from persubstituted benzenes C₆[3,5-(CH₂Y)₂C₆H₃] (Y=NMe₂, P(O)Ph₂, PPh₂, SPh). The molecular structure of A (Y=SPh, ML_n=PdCl) shows C₃ symmetry with adjacent radial Pd−Pd separations of 7.339(2) and 8.006(2) Å and a diametrically opposed Pd−Pd separation of 15.340(2) Å. Because of their size hexametallic species such as A are potential homogeneous catalysts in organic reactions which can be recovered by nanomembrane filtration techniques.     

Melt block copolymerization of ϵ-caprolactone and L-lactide

AB block copolymers of ϵ-caprolactone and (L )-lactide could be prepared by ring-opening polymerization in the melt at 110°C using stannous octoate as a catalyst and ethanol as an initiator provided ϵ-caprolactone was polymerized first. Ethanol initiated the polymerization of ϵ-caprolactone producing a polymer with ϵ-caprolactone derived hydroxyl end groups which after addition of L -lactide in the second step of the polymerization initiated the ring-opening copolymerization of L -lactide. The number-average molecular weights of the poly(ϵ-caprolactone) blocks varied from 1.5 to 5.2 × 10³, while those of the poly(L -lactide) blocks ranged from 17.4 to 49.7 × 10³. The polydispersities of the block copolymers varied from 1.16 to 1.27. The number-average molecular weights of the polymers were controlled by the monomer/hydroxyl group ratio, and were independent on the monomer/stannous octoate ratio within the range of experimental conditions studied. When L -lactide was polymerized first, followed by copolymerization of ϵ-caprolactone, random copolymers were obtained. The formation of random copolymers was attributed to the occurrence of transesterification reactions. These side reactions were caused by the ϵ-caprolactone derived hydroxyl end groups generated during the copolymerization of ϵ-caprolactone with pre-polymers of L -lactide. The polymerization proceeds through an ester alcoholysis reaction mechanism, in which the stannous octoate activated ester groups of the monomers react with hydroxyl groups. © 1997 John Wiley & Sons, Inc.     

Metastable liquid–liquid and solid–liquid phase boundaries in polymer–solvent–nonsolvent systems

In general liquid–liquid demixing processes are responsible for the porous morphology of membranes obtained by immersion precipitation. For rapidly crystallizing polymers, solid–liquid demixing processes also generate porous morphologies. In this study, the interference of both phase transitions has been analyzed theoretically using the Flory–Huggins theory for ternary polymer solutions. It is demonstrated that four main thermodynamic and kinetic parameters are important for the structure formation in solution: the thermodynamic driving force for crystallization, the ratio of the molar volumes of the solvent and the nonsolvent, the polymer–solvent interaction parameter, and the rate of crystallization of the polymer compared to the rate of solvent-nonsolvent exchange. An analysis of the relevance of each of these parameters for the membrane morphology is presented. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 763–770, 1997     

Synthesis and characterization of aliphatic poly(esteramide)s containing symmetrical bisamide blocks

A series of symmetrical diols were synthesised through ring-opening of lactones with different ring sizes by alkane diamines in moderate to good yields. Ring-opening of the dilactones glycolide and lactide with a diamine in equimolar amounts afforded symmetrical diols in quantitative yield. Aliphatic poly(ester-amide)s were prepared by polycondensation of N,N′-bis(caproyl)butane diamide, dimethyl adipate and 1,4-butanediol. The structure of the diamide was retained in the polymer. The polymers synthesized are semi-crystalline materials that possess ordered structures in the solid phase over almost the entire copolymer range. Depending on thermal history and copolymer composition thermal analyses is in agreement with the presence of pseudo-hexagonal crystal structures below 75°C, a fast melting recrystallization at this temperature and a triclinic structure at higher temperatures.     

Stereocomplex Mediated Gelation of PEG-(PLA)2 and PEG-(PLA)8 Block Copolymers

Stereocomplex mediated hydrogels have been prepared by mixing solutions of polymers of opposite chirality of either PEG-(PLA)₂ triblock copolymers or PEG-(PLA)₈ star block copolymers. The critical gel concentrations of the mixed enantiomer solutions were considerably lower compared to polymer solutions containing only the single enantiomer. Moreover, gel-sol transition temperatures were increased and gel regions were expanded due to stereocomplexation. Rheology measurements showed that stereocomplexed hydrogels based on PEG-(PLA)₈ have higher storage moduli compared to those based on PEG-(PLA)₂. Stereocomplexed hydrogels prepared from 13 wt% PEG-(PLA)₂ solutions in PBS showed a storage modulus of 0.9 kPa at 37 °C, while at similar conditions stereocomplexed hydrogels of PEG-(PLA)₈ showed a storage modulus of 1.9 kPa at 10 wt%.     

Gradients in valence bond theory

A gradient method for very general valence bond (VB) wavefunctions is presented. This method introduces the electronic energy as a Lagrange multiplier, and evaluates the contributions of the derivatives of the normalisation and of the first- and second-order cofactors present in the VB energy expression. The correctness of the method is illustrated with classic and breathing-orbital VB calculations on the HF molecule.     

Classifying Erdős type spaces of higher descriptive complexity

Let ${({E_n})_{n \in \omega }}$ be a sequence of zero-dimensional subsets of the reals, ?. The Erd?s type space ? corresponding to this sequence is defined by ? = {x ∈ ? ²: x _n ∈ E _n , n ∈ω}. The most famous examples are Erd?s space, with E _n equal to the rationals for each n, and complete Erd?s space, with E _n = {0} ∪ {1/m: m ∈ ?} for each n. If all sets E _n are ${G_\delta }$ and the space ? is not zero-dimensional, then ? is known to be homeomorphic to complete Erd?s space, and if all sets E _n are ${F_{\sigma \delta }}$ , then under a mild additional condition ? is known to be homeomorphic to Erd?s space. In this paper we investigate the situation where all sets E _n are Borel sets in the same multiplicative class. Many of these spaces can be linked to the Erd?s type space with all sets E _n equal to the element of that multiplicative Borel class which absorbs the class. Furthermore, we introduce coanalytic Erd?s space and we establish a link between this space and homeomorphism groups of manifolds that leave the zero-dimensional pseudoboundary invariant. The general framework that we develop gives analogous results for nonseparable Erd?s type spaces.