Copolymerization of styrene with maleic anhydride initiated by thiol compounds

Copolymerization of styrene (ST) with maleic anhydride (MAH) initiated by thiol compound was investigated under various conditions. The kinetics of copolymerization of ST with MAH initiated by p-toluenethiol (pTT) was studied in dioxane in the temperature range of 25–60°C, and the rates (R_p) of copolymerization and activation energy were determined. R_p was found to depend on [pTT],^0.6 ([ST] + [MAH])^2.7. The overall energy of activation was 10.8 kcal/mol in the temperature range of 25–60°C. A mechanism involving the formation of a complex between MAH and pTT the decomposition of which yields the initial radical is suggested.     

Graft copolymers having hydrophobic backbone and hydrophilic branches. III. Synthesis of graft copolymers having oligovinylpyrrolidone as hydrophilic branch

Graft copolymers were synthesized by the esterification reaction between acrylic copolymers and carboxyl group terminated vinylpyrrolidone oligomer using phase transfer catalysts. Acrylic copolymers were obtained by the radical copolymerization of β-bromoethyl methacrylate, chloromethylstyrene or glycidyl methacrylate with methyl methacrylate. Hydrophilic oligomers were prepared by the radical oligomerization of vinylpyrrolidone using β-mercaptopropionic acid as chain transfer agent. The degree of esterification increased with decreasing the molecular weight of oligomer and with increasing the number of potential grafting sites on polymer backbones. The water dispersibility of graft copolymers increased with increasing the nitrogen content and was therefore dependent on the branch oligomer content.     

Sintering behavior and apatite formation of diopside prepared by coprecipitation process

Sintering behavior and bioactivity of diopside, CaMgSi₂O₆, prepared by a coprecipitation process were examined for its biomedical applicability. As-prepared powder was synthesized by adding aqueous ammonia to an ethanol solution containing Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, and Si(OC₂H₅)₄ and characterized by means of TG–DTA, XRD, and TG–MS. The dried powder was X-ray amorphous and crystallized into diopside at 845.5 °C. The glass network formation by SiO₄ tetrahedra was almost completed below 800 °C. The bioactivity of the diopside prepared by sintering the compressed powder at 1100 °C for 2 h was evaluated by immersion of the sintered body in a simulated body fluid (SBF) at 36.5 °C. Leaf-like apatite particles were found to be formed on the surface of the sintered body and grew with passage of soaking time. This apatite-forming behavior in the SBF is related to the dissolution of Ca(II) ions from the sintered body in the early stage of immersion. Thus, diopside prepared by the coprecipitation process using the metal alkoxide and the metal salts was found to have an apatite-forming ability.     

Radical copolymerization of γ-methylene-Δα,β-butenolide derivative: ethyl(E)-5-OXO-2,5-dihydrofuran-2-ylidene-acetate

Radical polymerications including co- and terpolymerizations of a γ-methylene-Δ^α,β-butenolide derivative, ethyl (E)-5-oxo-2,5-dihydrofuran-2-ylideneacetate(EODY) was investigated. The monomer had no homopolymerizability but copolymerized with styrene (ST) and 1,3-cyclohexadiene to yield alternating copolymers. From IR and ¹H-NMR spectra of the copolymers, the 1,4-addition was confirmed to exclusively occur for the conjugated double bond of EODY. Terpolymerization for the system involving an acceptor monomer such as maleic anhydride, α-chloromaleic anhydride, or 7,7,8,8-tetracyanoquinodimethane in addition to ST and EODY gave the terpolymer containing about 50 mol% of ST, in spite of a high fraction of ST in the feed. It was inferred that such an apparent behavior of EODY as an acceptor monomer could be due to a resonance-stabilization of the propagating radical having EODY as a terminal unit, which is also responsible for the poor yields of the copolymers and terpolymers.     

Folding Dynamics of 10‐Residue β‐Hairpin Peptide Chignolin

Short peptides that fold into β‐hairpins are ideal model systems for investigating the mechanism of protein folding because their folding process shows dynamics typical of proteins. We performed folding, unfolding, and refolding molecular dynamics simulations (total of 2.7 μs) of the 10‐residue β‐hairpin peptide chignolin, which is the smallest β‐hairpin structure known to be stable in solution. Our results revealed the folding mechanism of chignolin, which comprises three steps. First, the folding begins with hydrophobic assembly. It brings the main chain together; subsequently, a nascent turn structure is formed. The second step is the conversion of the nascent turn into a tight turn structure along with interconversion of the hydrophobic packing and interstrand hydrogen bonds. Finally, the formation of the hydrogen‐bond network and the complete hydrophobic core as well as the arrangement of side‐chain–side‐chain interactions occur at approximately the same time. This three‐step mechanism appropriately interprets the folding process as involving a combination of previous inconsistent explanations of the folding mechanism of the β‐hairpin, that the first event of the folding is formation of hydrogen bonds and the second is that of the hydrophobic core, or vice versa.     

Hybridization energies of double strands composed of DNA, RNA, PNA and LNA

The electronic properties of double strands composed of trimeric LNA, PNA, DNA and RNA single strands were investigated by density-functional molecular orbital calculations. The computed hybridization energies for the double strands involving PNA or LNA are larger than those for DNA-DNA and RNA-RNA. The larger stability is attributed to the presence of a larger positive charge of the hydrogen atoms contributing to the hydrogen bonds in the PNA-DNA and LNA-DNA double-strands. These results are comparable to the experimental finding that PNA and LNA single strands display high affinity toward a complementary DNA or RNA single strand.     

Crystal structure, thermal behavior and enzymatic degradation of poly(tetramethylene adipate) solution-grown chain-folded lamellar crystals

Solution-grown chain-folded lamellar single crystals of poly(tetramethylene adipate) (PTMA) were prepared from a dilute solution of 2-methyl-1-propanol by isothermal crystallization. PTMA crystals were hexagonal-shaped and polyethylene decoration of the crystals resulted in a "six cross-sector" surface morphology and showed that the average direction of chain folding is parallel to the crystal growth planes of [110] and [010]. Chain-folded lamellar crystals gave well-resolved electron diffraction diagrams corresponding to all the equatorial reflections of the X-ray fiber diagram obtained from stretched PTMA melt-quenched film (beta structure). The unit cell parameters of the beta structure of PTMA were determined as a = 0.503 nm, b = 0.732 nm and c (fiber axis) = 1.442 nm with an orthorhombic crystal system. The fiber repeat distance is appropriate for an all-trans backbone conformation for the straight stems. The setting angle, with respect to the a axis, is +/-46 degrees for the corner and center chains. Thermal behavior of lamellar crystals has been investigated by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM). The lamellar thickness at the edges of the crystal increased after thermal treatment with taking the molecular chains into recrystallization parts; the holes then opened up at the thickening front of the crystal. The morphological changes of lamellar crystals after enzymatic degradation by Lipase type XIII from Pseudomonas sp. and water-soluble products were characterized by TEM, AFM, gel permeation chromatography, high performance liquid chromatography and fast atom bombardment mass spectrometry. The degradation progressed mainly from the edges of the lamellar crystals without decreasing the molecular weights and the lamellar thicknesses. The central portion of single crystals was often degraded by enzymatic attacks. This result combined with thermal behavior indicates that the loosely chain-packing region exists inside the single crystal, and that molecular chains in this region have higher mobility against thermal and enzymatic treatments.     

Thermosensitive and redox-active polymers: Preparation and properties of poly(N-ethylacrylamide-co-vinylferrocene) and poly(N,N-diethylacrylamide-co-vinylferrocene)

Thermosensitive and redox-active polymers were prepared by copolymerization of N-ethyl- or N,N-diethylacrylamides with vinylferrocene (VFc). LCST (lower critical solution temperature) of the aqueous copolymer solution was decreased by increasing the ferrocene content in the copolymer. The oxidation of ferrocene led to a significant increase in LCST due to the transition from hydrophobic to hydrophilic character of the ferrocene moiety in the copolymer. The ferrocene content in the copolymer increases with increasing differences between the LCST's of the oxidation and reduction states. The transition could be made reversible by redox reaction using L -ascorbic acid as an oxidant and cerium sulfate as a reductant. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1967–1972, 1997