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Won Seok Lyoo - One of the best experts on this subject based on the ideXlab platform.
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High molecular weight syndiotacticity-rich poly(vinyl alcohol) gel in aging process
Colloid & Polymer Science, 2000Co-Authors: Jewon Choi, Won Seok Lyoo, H. D. GhimAbstract:The properties of the aged gels of high molecular weight syndiotacticity-rich poly(vinyl alcohol)s (HMW S-PVAs) with different syndiotactic Diad (s-Diad) contents were investigated. HMW S-PVA gels with s-Diad content of 61.5% and 58.2% showed the rapid increases of the syneresis and the turbidity from the early stage of aging time, which is ascribable to the phase separation, while that with s-Diad content of 55.7% did not. From the morphological study, it was confirmed that the phase separation in HMW S-PVA gel with s-Diad content of 61.5% occurred without the liquid-liquid phase separation in sol state, whereas both the liquid-liquid phase separation in sol state and the subsequent phase separation in gel state occurred in the case of HMW S-PVA gel with s-Diad content of 58.2%. On the other hand, HMW S-PVA gel with s-Diad content of 55.7% showed neither the liquid-liquid phase separation in sol state nor the phase separation in gel state in the long period of time. It was also confirmed from wide angle X-ray diffractogram that the crystallization was accompanied by the phase separation in gel state in the aging process of PVA gel. However, the crystallization was hindered by the fast network formation at the initial stage of time. Later the syndiotacticity promoted the crystallization. The tensile modulus of HMW S-PVA gel with higher syndiotacticity increased more significantly with time.
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Effect of stereoregularity on the properties of syndiotactic-rich ultrahigh molecular weight poly(vinyl alcohol)/dimethyl sulfoxide/water gel
Macromolecular Chemistry and Physics, 1999Co-Authors: Jin Hyun Choi, Won Seok LyooAbstract:Syndiotactic-rich ultrahigh molecular weight poly(vinyl alcohol) (UHMW S-PVA) gels having syndiotactic Diad (S-Diad) contents of 61.5, 58.2, and 55.7% and similar molecular weights were prepared in dimethyl sulfoxide (DMSO)/water solution. The gelation temperature of UHMW S-PVA/DMSO/water solution decreases with an increase in syndiotacticity. A great difference in the gel melting temperature (T gm ) was not found between the gels having an S-Diad content over 58.2%. However, a significant decrease of T gm was revealed in the gel having an S-Diad content of 55.7%. The enthalpy of junction fusion and the number of segments participating in the junction of the UHMW S-PVA gel increases with syndiotacticity, and they are remarkably higher than the values of atactic PVAs reported in previous papers. The higher the syndiotacticity of the UHMW S-PVA gel, the higher is the dynamic storage modulus. The elasticity of the PVA gel with an S-Diad content over 58.2% was recovered under cooling after heating, whereas almost no recovery was found in the gel having an S-Diad content of 55.7%. The PVA gel having an S-Diad content over 58.2% is opaque and exhibits a phase separated appearance in optical morphology, while the PVA gel with an S-Diad content of 55.7% is almost transparent and phase separation does not occur. A highly microporous and close structure was observed for the surface of the UHMW S-PVA gels.
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structure of poly vinyl alcohol microfibrils produced by saponification of copoly vinyl pivalate vinyl acetate
Macromolecules, 1998Co-Authors: Won Seok Lyoo, John Blackwell, Han Do GhimAbstract:High molecular weight specimens of syndiotacticity-rich poly(vinyl alcohol) (PVA) have been prepared by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). VPi and VAc were copolymerized using the low-temperature initiator 2,2‘-azobis(2,4-dimethylvaleronitrile) and saponified in KOH/methanol/water with vigorous stirring, i.e., under shear. The PVAs obtained had number-average degrees of polymerization in the range (5.6−16.5) × 103 and syndiotactic Diad (S-Diad) contents of 52.8−61.5%. The highest molecular weight and S-Diad content were obtained for specimens derived from homopoly(VPi) prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymers. The structures of these PVAs show a consistent sharp transition between 55 and 56% S-Diad content. Specimens with lower S-Diad contents have shapeless morphologies, but at 56% S-Diad content the polymers were fibrous, with a higher degree of crystallinity and or...
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Structure of Poly(vinyl alcohol) Microfibrils Produced by Saponification of Copoly(vinyl pivalate/vinyl acetate)
Macromolecules, 1998Co-Authors: Won Seok Lyoo, John Blackwell, Han Do GhimAbstract:High molecular weight specimens of syndiotacticity-rich poly(vinyl alcohol) (PVA) have been prepared by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). VPi and VAc were copolymerized using the low-temperature initiator 2,2‘-azobis(2,4-dimethylvaleronitrile) and saponified in KOH/methanol/water with vigorous stirring, i.e., under shear. The PVAs obtained had number-average degrees of polymerization in the range (5.6−16.5) × 103 and syndiotactic Diad (S-Diad) contents of 52.8−61.5%. The highest molecular weight and S-Diad content were obtained for specimens derived from homopoly(VPi) prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymers. The structures of these PVAs show a consistent sharp transition between 55 and 56% S-Diad content. Specimens with lower S-Diad contents have shapeless morphologies, but at 56% S-Diad content the polymers were fibrous, with a higher degree of crystallinity and or...
Han Do Ghim - One of the best experts on this subject based on the ideXlab platform.
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structure of poly vinyl alcohol microfibrils produced by saponification of copoly vinyl pivalate vinyl acetate
Macromolecules, 1998Co-Authors: Won Seok Lyoo, John Blackwell, Han Do GhimAbstract:High molecular weight specimens of syndiotacticity-rich poly(vinyl alcohol) (PVA) have been prepared by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). VPi and VAc were copolymerized using the low-temperature initiator 2,2‘-azobis(2,4-dimethylvaleronitrile) and saponified in KOH/methanol/water with vigorous stirring, i.e., under shear. The PVAs obtained had number-average degrees of polymerization in the range (5.6−16.5) × 103 and syndiotactic Diad (S-Diad) contents of 52.8−61.5%. The highest molecular weight and S-Diad content were obtained for specimens derived from homopoly(VPi) prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymers. The structures of these PVAs show a consistent sharp transition between 55 and 56% S-Diad content. Specimens with lower S-Diad contents have shapeless morphologies, but at 56% S-Diad content the polymers were fibrous, with a higher degree of crystallinity and or...
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Structure of Poly(vinyl alcohol) Microfibrils Produced by Saponification of Copoly(vinyl pivalate/vinyl acetate)
Macromolecules, 1998Co-Authors: Won Seok Lyoo, John Blackwell, Han Do GhimAbstract:High molecular weight specimens of syndiotacticity-rich poly(vinyl alcohol) (PVA) have been prepared by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). VPi and VAc were copolymerized using the low-temperature initiator 2,2‘-azobis(2,4-dimethylvaleronitrile) and saponified in KOH/methanol/water with vigorous stirring, i.e., under shear. The PVAs obtained had number-average degrees of polymerization in the range (5.6−16.5) × 103 and syndiotactic Diad (S-Diad) contents of 52.8−61.5%. The highest molecular weight and S-Diad content were obtained for specimens derived from homopoly(VPi) prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymers. The structures of these PVAs show a consistent sharp transition between 55 and 56% S-Diad content. Specimens with lower S-Diad contents have shapeless morphologies, but at 56% S-Diad content the polymers were fibrous, with a higher degree of crystallinity and or...
Yoshio Okamoto - One of the best experts on this subject based on the ideXlab platform.
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Stereocontrol in Radical Polymerization of N,N-Dimethylacrylamide and N,N-Diphenylacrylamide and Thermal Properties of Syndiotactic Poly(methyl acrylate)s Derived from the Obtained Polymers
Polymer Journal, 2000Co-Authors: Weihong Liu, Tamaki Nakano, Yoshio OkamotoAbstract:Free-radical polymerization of N,N -dimethylacrylamide (DMAA) and N,N -dephenylacrylamide (DPAA) were carried out under various conditions. DMAA gave an isotactic-rich polymer in most cases, whereas DPAA gave a syndiotactic polymer. The polymerization stereochemistry was also affected by reaction solvents including toluene, tetrahydrofuran, and fluoroalcohols. It is notable that the stereochemistry of the polymerization depended on monomer concentration: a lower monomer concentration led to a higher isotacticity for DMAA polymerization and a higher syndiotacticity for DPAA polymerization. This effect was explained by assuming that there exist two types of growing radicals having different stereoselectivities. By optimizing the aforementioned reaction conditions, poly(DMAA) with dial isotacticity ( m ) of 77% (polymerization in toluene at −78°C at [M]_0=0.1M) and poly(DPAA) with Diad syndiotacticity ( r ) of 93% (polymerization in THF at -98°C at [M]_0=0.22M) were obtained. Stereoregular poly(methyl acrylate)s were derived from the poly(DPAA)s. Thermal property of the poly(methyl acrylate)s with different tacticities was studied by DSC.
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Stereospecific Polymerization of N,N-Diphenylacrylamide
Polymer Journal, 1996Co-Authors: Kei Shiohara, Shigeki Habaue, Yoshio OkamotoAbstract:N , N -Diphenylacrylamide (DPAA) was polymerized with anionic and radical initiators in toluene and tetrahydrofuran. The tacticity of the obtained polymers (poly(DPAA)) was unable to be determined directly by NMR spectroscopy. Therefore, poly(DPAA) was carefully converted to poly(acrylic acid) by solvolysis in a mixture of concentrated sulfuric acid and methanol (60:40; by volume) at 90°C for 24 hours, followed by methylation with diazomethane. Under suitable conditions, the transformation was performed without change (racemization) of tacticity of the polymers. Diad tacticity of the derived poly(methyl acrylate) was determined by ^1H NMR spectroscopy. Anionic polymerization using butyllithium in toluene afforded a polymer rich in meso Diad ( m =0.95), and that in tetrahydrofuran racemo Diad ( r =0.66). Radical polymerization in toluene afforded a polymer with the highest racemo Diad content ( r =0.85).
Shuji Matsuzawa - One of the best experts on this subject based on the ideXlab platform.
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Influence of Syndiotacticity on Poly(vinyl alcohol)–Iodine Complex Formation
Polymer International, 1997Co-Authors: Hiroji Noguchi, Hiroshi Jodai, Shoji Tamura, Shuji MatsuzawaAbstract:The extent of colour development in an aqueous poly(vinyl alcohol) (PVA)–iodine complex solution increased with increasing syndiotacticity up to a certain syndiotactic content, then decreased with increasing syndiotacticity. The complex formation depended on the content of the syndiotactic pentad sequence length in the PVA molecules. The most suitable content of the syndiotactic pentad for forming complexes was around 12% in the case of reaction at 30°C. A 0·5% solution of syndiotactic PVA (S-PVA) with an syndiotactic Diad (s-(Diad)) content of 63·8% changed into a sol, whereas that with an s-(Diad) content of 58% did not change after standing for 2h at 30°C, and the colour due to the iodine complex in the latter was deeper than in the former. The gel prepared from the 0·5% solution of S-PVA with an s-(Diad) content of 63·8% was hardly coloured. In the solutions containing two kinds of s-PVA having different syndiotacticities, additivity for the extent of colour development did not hold, and the extent was lower than the sum of that of the components. The decrease in the extent of colour development at higher syndiotacticity was due to the formation of microgels. The absorption maximum shifted to a longer wavelength with increasing syndiotacticity. The length of polyiodine increased with increasing syndiotacticity. © of SCI.
Yoshiharu Iwabuchi - One of the best experts on this subject based on the ideXlab platform.
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Oxidation of Alcohols to Carbonyl Compounds with Diisopropyl Azodicarboxylate Catalyzed by Nitroxyl Radicals
2016Co-Authors: Masaki Hayashi, Masatoshi Shibuya, Yoshiharu IwabuchiAbstract:A nitroxyl-radical-catalyzed oxidation of alcohols using diisopropyl azodicarboxylate (Diad) as the terminal oxidant is reported. A variety of primary and secondary alcohols including aliphatic, benzylic, and allylic alcohols are efficiently oxidized to their corresponding aldehydes and ketones without overoxidation to carboxylic acid. 1,2-Diols are oxidized to hydroxyl ketones or diketones depending on the amount of Diad used
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oxidation of alcohols to carbonyl compounds with diisopropyl azodicarboxylate catalyzed by nitroxyl radicals
Journal of Organic Chemistry, 2012Co-Authors: Masaki Hayashi, Masatoshi Shibuya, Yoshiharu IwabuchiAbstract:A nitroxyl-radical-catalyzed oxidation of alcohols using diisopropyl azodicarboxylate (Diad) as the terminal oxidantis reported. A variety of primary and secondary alcohols including aliphatic, benzylic, and allylic alcohols are efficiently oxidized to their corresponding aldehydes and ketones without overoxidation to carboxylic acid. 1,2-Diols are oxidized to hydroxyl ketones or diketones depending on the amount of Diad used.