摘要:

AbstractWe present an overview of the synthesis, physical characterization and catalytic properties of polymer‐supported reagents and consider the major advantages and drawbacks in the use of polymeric supports for synthesis and catalysis. The most important development in recent years appears to be the characterization of polymeric supports by thermal analysis, advanced spectroscopic and scattering methods, and electron microscopy. As a result, the mechanism of catalytic activity is better understood; the challenge remains to prepare supported catalysts with good mechanical properties, and high chemical stability at elevated temperatures, and to prevent the loss of often pricey catalysts such as rhodium during numerous catalytic cycle

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470101

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe reactions of the mildly electron‐poor olefin methyl acrylate (MA) with three electron‐rich substituted hydrocarbon 1,3‐dienes, namely 2,3‐dimethyl‐1,3‐butadiene (DMB), 4‐methyl‐1,3‐pentadiene (MPD) and isoprene (IP) were investigated in 1,2‐dichloroethane in the presence of various amounts of zinc chloride at different temperatures. ZnCl2complexes with the ester group of MA and increases the MA's electron‐poor character and therefore its tendency towards reactions with electron‐rich dienes. In the absence of ZnCl2, no reaction occurs between the investigated dienes and MA under the used reaction conditions. Both alternating copolymers and [4+2] cycloadducts formed spontaneously in the presence of ZnCl2even at room temperature. The structure of the diene and its nucleophilicity control the product distribution. For the DMB and IP systems, raising the reaction temperature and increasing the ZnCl2concentration enhance the overall reaction rate and in most cases also favor the cycloaddition over the copolymerization. IP is less reactive than DMB. With MPD only copolymer is formed because the two terminal methyl groups retard the concerted [4+2] cycloaddition. These results are in agreement with a postulated mechanism involving competition between the formation of a π‐allyl 2‐hexene‐1,6‐diradical, capable of initiating free radical copolymerization, and concerte

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470102

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractVarious methods are reviewed that have been developed in recent years to produce uniform polymer beads with diameter larger than 1 μm. Methods such as emulsifier‐free emulsion polymerization, suspension polymerization (including non‐standard polymerization devices and a method of activated swelling), dispersion polymerization, precipitation polymerization, and other techniques are covered. The discussion focuses on the mechanism of bead formation and variables influencing the properties of the beads. Potential applications of monosized beads in both industry and research are descr

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470103

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe static and dynamic scattering properties of multicomponent polymer mixtures are examined starting from the theoretical formalism based on the random phase approximation (RPA) for multicomponent polymer mixtures in the presence of a low molecular weight solvent. The results are compared with the light scattering data reported recently by Strazielle and coworkers [1] for polystyrene/polydimethylsiloxane/polymethylmethacrylate/toluene mixtures. The present paper is an extension of the work published recently by these authors [2]and considers data obtained from several concentrations of PMMA. It also includes a discussion of the dynamic properties which are obtained by quasi‐elastic light scatterin

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470104

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractInfrared spectroscopy is used to study the influence of the polarization of the irradiating light and the substrate surface treated with polyimide on the reorientation process of dye‐containing liquid‐crystalline side‐group copolymers. The depolarization of the linearly polarized light on brushed polyimide surfaces affects the reorientation process and as a consequence the irradiation direction becomes the new order axis after irradiation. Relaxation processes are observed after short irradiation times, since a new stable equilibrium state has not been reached. In contrast, complete reorientation, achieving changes in order parameter of 0.9, is observed after long irradiation times. The system is then stabilized in the new orientational distribution and no relaxation processes occur. The relaxation processes can be explained by a combination of matrix response and thermal cis‐trans back relaxation of the azo

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470105

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractWe studied the peculiarities of the glass transition in semicrystalline polymers by simultaneous temperature‐dependent small angle X‐ray scattering experiments and dielectric relaxation studies on ethylene‐vinylacetate statistical copolymers. Properties of the β‐relaxations, which represents the glass transition of these systems, change systematically with the comonomer content and the temperature. The broadening of the dielectric loss spectra, indicative of large variations in the sequence mobility in the amorphous regions, increases with the crystallinity, and on approachingTgalso explicitly with decreasing temperature. While the first effect is due to the special structure of the disordered regions, where the presence of fixed entanglements and loops results in motional restrictions, the temperature effect nearTgmay reflect the increase in the correlation length of the

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470106

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractX‐ray methods have been used to investigate the structure of the hard domains in polyurethane elastomers based ontrans, trans‐dicyclohexylmethane 4,4′‐diisocyanate (HMDI) with butanediol as the chain extender. The crystalline hard segments become oriented on stretching the films at room temperature, although the degree of crystallinity remains relatively low. The ordering is improved considerably by annealing the films for several hours at 130–190°C: the fiber diagram contains 25 sharp Bragg reflections which are indexed by a triclinic unit cell with dimensionsa=5.1 Å,b=10.2 Å,c=37.5 Å, α=115.2°, β=84.9°, and γ=94.2°. The cell contains dimer units of two chains which are probably staggered along thecaxis direction in thebcplane. In the samples annealed for shorter periods of time, a second less abundant crystal structure is observed that coexists with the first. This second structure is also triclinic with a more extended fiber repeat ofc=41.3 Å and is slowly converted to the contracted form on further annealing. Molecular models show that in the extended form the butanediol units have the all‐transconformation, whereas in the contracted form they probably have thetg+tg−tortg−tg+tconformations, which have lower potential energies than the all‐transform. The chains in both the extended and the contracted conformations are able to form a network of intermolecular hydrogen bonds, and in this regard the structures are very similar to those reported previously for the analogous diphenylmethane diisocyanate (MDI)‐based elastomers. Thus the higher melting points for the HMDI‐based hard segments are most likely due to hydrophobic interactions between the dicyclohexyl methanes that are stronger than those between the diphenyl meth

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470107

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractTwaron fiber is spun from the polymer poly(paraphenylene terephthalamide), PPT, and has a crystallinity of 82%. Wideangle X‐ray diffraction, combined with the multipeak resolution procedure, was applied to study the changes in the structure after annealing the fiber in the temperature range of 100–675°C. The structural parameters included crystallinity (crystalline order), microparacrystal (mPC) size, and netplane spacing. A thermogravimetric analysis (TGA) was made to determine the loss in mass of the fiber in the temperature range in which the annealing study was carried out.Crystallographically‐disordered (amorphous) powder of the PPT polymer was prepared by grinding crystalline Twaron fibers, which had a crystallinity of 87%, in a vibratory ball mill for a period of 64h. A study was then made to determine the extent to which the original crystallinity or mPC size is restored after annealing the powder in the temperature range of 100

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470108

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractChain‐length‐dependent π electron delocalization is analyzed in pristine oligofurans and polyfuran by means of Fourier transform Raman spectroscopy. Neutral polyfuran shows a large frequency dispersion with conjugation length and it has not yet reached saturation at the level of the hexamer. In spite of such a large π electron delocalization the polymer seems to show an unexpectedly good stability in air at room temperature. Regio‐defined poly(2,5‐difurans) could be good candidates for technological dev

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470109

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY

ISSN:0323-7648    

DOI:10.1002/actp.1996.010470111

出版商:Akademie Verlag GmbH    

年代:1996

数据来源: WILEY