摘要:

AbstractAn overview of the synthesis of optically active polymers due to main‐chain chirality based on the configurational arrangement in cyclopolymerization and cyclocopolymerization is presented. The template having chiral twist transmitted its chirality to the main chain in the intramolecular cyclization of divinyl monomers to form the enantiomeric racemo cyclic units. The asymmetric cyclocopolymerization produced the polymers which were optically active even after cleaving the template. The synthesis of optically active polymers from achiral monomers using a slight amount of chiral initiator was achieved by the enantioselective cyclopolymerization. The cyclopolymerization capable of controlling stereochemistry of the cyclic constitutional units was one of useful means for asymmetric polymerizatio

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481101

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractMolecular motions in low molar mass (46200 g mol—1) poly(diethylsiloxane) (PDES) are studied by2H NMR. From DSC measurements the existence of the crystalline phases α1, β1and α2, β2, as well as a pronounced mesogenic phase αm, is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25°. The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70° within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main ch

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481102

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe individual impact of different degradation mechanisms occurring during composting on the degradation of poly(3‐hydroxybutyrate‐co‐6%‐3‐hydroxyvalerate) [poly(3HB‐co‐6%‐3HV)] have been assessed in simulated tests. Biotic hydrolysis withAspergillus fumigatushad the largest impact on the breakdown of poly(3HB‐co‐6%‐3HV) during composting conditions, with small changes in molecular weight and large weight loss. The major degradation products in the biotic environment were 3‐hydroxy butyric acid and 3‐hydroxy valeric acid. Abiotic hydrolysis during composting conditions (sterile water pH 7 at 60°C) has minor or no effect during such short times as 21 days. During abiotic hydrolysis both monomers and dimers were idientified while the abiotic thermo‐oxidation of poly(3HB‐co‐6%‐3HV) showed no degradation at all. The quantitation of degradation products and copolymer composition analysis by1H NMR indicate that the abiotic hydrolysis is not random. Crotonic and 2‐pentenoic acid which is produced at 100°C in air was not produced at 60°C during 298 days. We propose that this temperature is too low to promote the well‐known six‐member transition that produces crotonic acid, 2‐pentenoic acid and oli

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481103

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractWe have measured the dielectric relaxation of the polymeric glass‐former 1,4‐polybutadiene in the glassy and supercooled liquid state. The loss spectra ϵ″ of the β‐relaxation are measured between the lower detection limit atT= 125 K andT= 225 K>Tβ, withTβ≈ 210 K being the α‐β‐merging temperature. The related storage data ϵ′(ω) indicates that the β‐process is a signature of supressing the higher frequency polarization with increasing temperature, rather than giving rise to additional polarizability at frequencies below the average relaxation frequency of the β‐process. This observation, as confirmed by data obtained foro‐terphenyl and salol, contradicts the previous understandings of the mechanism underlying the Johari—Goldstein‐type β‐process in glasses. A tentative link between the β‐process and high f

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481104

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractWater‐soluble diblock copolymers (DBCPs) with a polyelectrolyte block are accessible by sequential anionic polymerization oftert‐butylmethacrylate (TBMA) and ethylene oxide (EO), followed by polymer analogous conversion of the PTBMA into poly(methyacrylic acid) (PMAA) blocks. These materials are highly efficient dispersants for oxide ceramic powders in aqueous media. A series of block copolymer samples withMnranging from 1300 to 38 900, and (EO:MAA) block length ratios from 0.5 to 11.7 were prepared with polydispersities close to 1.2. The influence of overall molecular weight and block length ratio, pH and ionic strength on the stability of aqueous α‐Al2O3suspensions was investigated by sedimentation and adsorption experiments, surface plasmon resonance (SPR) and electrokinetic measurements. The copolymers are capable of stabilizing alumina suspensions by a combination of specific adsorption of one block on the particle surface and a shielding effect provided by the nonadsorbing block. In addition, the adsorption of the negatively charged PMAA block on the oppositely charged alumina surface reverses the electrophoretic potential of the oxide particles, a process which is strongly pH dependent. With respect to the powder dispersing efficiency, an optimum was found when the DBCP consisted of a short PMAA anchoring and an approximately tenfold longer PEO stabilizer block with an overall molecular weight of about 5000. Alumina suspensions with the DBCP added were highly dispersed and rather stable against salt addition in a much wider pH window than block‐copolymer‐free s

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481105

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractLiquid crystal poly[oxy(1,2‐butene)oxycarbonyl‐1,4‐phenyleneoxycarbonyl‐1,4‐phenylenecarbonyloxy‐1,4‐phenylenecarbonyl], PTOBEE, has been synthesized in a different way than the previously reported nematic and it has been obtained as cholesteric, although synthesized from a racemic mixture of the 1,2‐butanediol involved. Stereoselective solubilization of one enantiomer during its decantation in toluene, at the reaction temperature, explains why this chiral polymer phase appears. Evidence of this could be precisely obtained when a white solid recrystallized within the same toluene after the filtration of the polymer, being identified as (—) PTOBEE with a value of [α] 25589= −2.33 [

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481106

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractA soluble copolymer formed by thiophene and 3‐octylthiophene has been obtained by using nickel catalyst polymerization. The nearly alternated structure allows the formation of a highly conjugated system although the copolymer microstructure is not regioregular.1H‐NMR, GPC characterization and pyrolysis experiments support the alternated structure of the copolymer. Moreover, theoretical calculations indicate that the average angle between adjacent monomeric units has been reduced, in agreement with the observed shift towards lower energy of the electronic spectra of this alternated struct

ISSN:0323-7648    

DOI:10.1002/actp.1997.010481107

出版商:Akademie Verlag GmbH    

年代:1997

数据来源: WILEY