摘要:

AbstractThe adduct (3) from 2 moles ofsec‐butyllithium with 1,3‐bis(1‐phenyl‐ethenyl)benzene has been prepared and evaluated as a hydrocarbon‐soluble, dilithium initiator for butadiene and styrene polymerization in cyclohexane or benzene solution. Under high vacuum conditions with purified reagents, bimodal molecular weight distributions are observed for polybutadienes withMn<150 × 103g/mol and for polystyrenes withMn<50 × 103g/mol; monomodal distributions are observed only for higher molecular weights. Addition of tetrahydrofuran ([THF]/[Li]= 14 to 32) or preformed lithiumsec‐butoxide ([LiOBu]/[3]= 1·1) produces narrow, monomodal molecular weight distributions even atMn= 26·2 × 103g/mol polybutadiene and atMn= 10 × 103g/mol for polystyrene. Added lithiumsec‐butoxide is the preferred additive since high 1,4‐polybutadienes are obtained. These results provide an explanation for the contradictory results reported previously; under less rigorous experimental conditions, oxygen and hydroxylic impurities can form lit

ISSN:0959-8103    

DOI:10.1002/pi.4990240402

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractImmobilization was carried out of the lactate dehydrogenase (LDH) from rabbit muscle (EC 1.1.1.27), cross‐linked through the bifunctional reactive glutar‐aldehyde on to nylon tubing (1 m long, 53cm2internal surface area). Immobilized LDH inactivation kinetics are of first order (t1/2= 3·6 years,k= 5·4,e−4day−1to 5°C). The smaller effect of pH on activity than in the case of LDH in solution can be explained on the basis of limitation to proton diffusion towards the support. A limiting effect to free external diffusion of the substrate towards and products from the support was also observed, an effect which seems to determine the effective kinetic behaviour of immobilized LDH. The apparent optimum temperature is centred around 40°C, observing a clear inactivation (thermal denaturation) above this temperature. In the temperature range studied (10–40°C), the co‐existence was seen of a kinetic control accompanied by another control, involving diffusional transport of substrates and products, on the global activity of the immobilized enzyme. This makes the Arrhenius profiles curvilinear. Both graphic and statistical non‐linear regression analysis of the kinetic data—rate,v, versus substrate concentration [S]—carried out under conditions in which the diffusional limitations can be considered negligible (high recirculation flow rate), permitted investigation of the intrinsic kinetic behaviour of immobilized LDH. In this sense, it can be deduced that the rate equation to which these data seem to be fitted is of the polynomial quotient type in [S]of minimum degree 2:2. Although the diffusional limitations have a marked effect on the type of global kinetics shown by immobilized LDH, temperature was not found to affect itsv[S] behaviour. The experimental evidence obtained thus indicates that the rate equation in the 10‐40°C temperature range continues to be a rational equation of a

ISSN:0959-8103    

DOI:10.1002/pi.4990240403

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIonic polymer cements, formed by the reaction of metal oxides or ionleachable glass powders with various polyacids, have found widespread use in dentistry. Commonly, the polyacid employed is a homopolymer or copolymer of acrylic acid. An alternative ionic polymer cement system has recently been developed, based on phosphonic acid polymers. In this study, cement formation between polyvinylphosphonic acid, PVPA, and a range of metal oxides has been examined, including an assessment of hydrolytic stability and the relative stabilities of the Mn+‐PVPA bonds formed. An infrared spectroscopic technique has been used to determine the relative extents of the reactions between PVPA and the various metal oxides.It is found that cements based on PVPA are far more reactive than those based on the polycarboxylic acids, and that the hydrolytic stability of the cements is greater. This greater resistance to water is probably due to the nature of the binding of the cations to the polyanio

ISSN:0959-8103    

DOI:10.1002/pi.4990240404

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe synthesis and thermodynamic evaluations of a series of random copolyesters based on terephthalic acid, methyl hydroquinone and 1,4‐butane diol are presented. The copolyester composition was varied by altering the relative concentrations of methyl hydroquinone and 1,4‐butane diol. The copolyesters display thermotropic transitions. The enthalpy changes during mesophasic transition display a minimum corresponding to equimolar composition. The copolyester with 75 mol% of spacer diol exhibited the lowest nematic transition temperature (190‐6°C). At this composition the nematic mesophase was most stable (δTN= 115‐6°C). ΔHN‐1and ΔSN‐1decrease with increasing rigid diol content. The isotropization temperatures were almost comparable since the largest mesogenic residues are similar in aspect ratio. These residues are required to melt prior t

ISSN:0959-8103    

DOI:10.1002/pi.4990240405

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractSeveral equation models were investigated to find the relationship between temperature (T). number of ethylene oxide (EO) units (n) or the hydrophile‐lipophile balance (HLB) and the surface and thermodynamic properties of some ethoxylated alkylphenol‐formaldehyde polymeric nonionic surfactants. These properties include critical micelle concentration (CMC), free energy of micellization (ΔGmic), surface tension at CMC (7CMC), effectiveness (γCMC) and efficiency (pC20) of surfactant to reduce the surface tension of water. The values of the ratio CMC/C2(π = 20)were also considered.The linear multiple regression technique was employed to determine the parameters of the equations and to choose the best forms with the highest values ofR2andF‐ratio which reflect the goodness and the reliability of

ISSN:0959-8103    

DOI:10.1002/pi.4990240406

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractWood flour and fibres do not bond well to polyolefins. Fillers or reinforcing agents for polyolefins based on these materials need to be treated before incorporation. This paper describes the property improvements obtained by treating papers, wood flour and wood fibres with zircoaluminate coupling agents before their forming into composites with polyolefins. The zircoaluminates were found to be effective coupling and hydrophobing agents in this system at low concentrations.

ISSN:0959-8103    

DOI:10.1002/pi.4990240407

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractPalladium complexes of polyvinyl pyridine coated silica gel have been prepared and used as hydrogenation catalysts for alkenes and alkynes under ambient conditions. The kinetics of hydrogenation for a few substrates have been studied and a possible mechanism proposed. The kinetic and mechanistic aspects of the hydrogenation of the substrates, effect of additives on the rate of hydrogenation, selectivity of the catalyst and recycling efficiency of the catalyst are presented in this paper.

ISSN:0959-8103    

DOI:10.1002/pi.4990240408

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0959-8103    

DOI:10.1002/pi.4990240410

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0959-8103    

DOI:10.1002/pi.4990240412

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0959-8103    

DOI:10.1002/pi.4990240415

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY