摘要:

AbstractThe classical problem of chain‐packing polarity in native cellulose fibres is approached by electron‐diffraction analysis in fibrils of the algaValonia ventricosa. Meridional reflections up to the 12th layer line were registered and compared with the intensities calculated for one‐dimensional parallel and antiparallel models of the structure, projected upon thec‐axis. The results are compatible with cellulose molecules packed in parallel fashion, and with a stagger of the central chain with respect to that at the origin of +2,80 or +2,39 Å. Some consequences of the parallel structure with regard to the biogenesis of cellulose are d

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911001

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractFourteen novel side‐chain polysiloxanes2and their alkene precursors1containing methylene‐cyclohexane, cyclohexane, benzene, bicyclooctane,3‐oxobicyclooctane, bicyclohexane and biphenyl moieties in the mesogenic core, and with (CH2)nspacer lengths ofn= 3, 6 and 11 are reported. The order of efficiency of these cyclic moieties in promoting liquid‐crystal properties in the polymers is the same as that observed for the precursor alkenes, indicating that similar qualitative structural rules govern mesophase type and thermal stability in both monomeric and macromolecular systems. The apparent occurrence of smectic C properties for two of the polymers is particularly inte

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911002

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe tungsten‐carbene complex was used to initiate the metathesis polymerization ofsyn‐andanti‐7‐methylbicyclo [2.2.1] hept‐2‐ene (Msand Ma). Two kinds of experiment were performed:(i) the reactions were followed at low monomer/initiator ratios by1H NMR spectroscopy at 220–250K; (ii) polymers and copolymers of Msand Mawere prepared at room temperature and their microstructure analysed by13C NMR spectroscopy. The propagating tungsten‐carbene species derived from both Macould be detected. Some rate constants were evaluated for various steps in the mechanism. A polymer of Mswas prepared for the first time, containing 90%transdouble bonds and probably isotactic. After hydrogenation this polymer exhibits two first‐order transitions, with DSC maxima at 179 and 230°C. Hydrogenated atactic polymers of Maand of norbornene also exhibit first‐order transitions. Conditions are described for the preparation of block and tapered block co

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911003

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe anionic polymerization oftert‐butyl acylate (tBuA) initiated bytert‐butyl α‐lithioisobutyrate was investigated in THF at 25°C. The individual oligomers were isolated and characterized by UV, IR and NMR spectroscopy. The distribution of products was determined in the range of reaction times 0,02s≤t≤ 1800s. The oligomerization is extremely fast. Even after 0,02 s at ‐30°C, no residual monomer could be detected. After short reaction times the reaction mixture almost exclusively consists of linear oligomers oftBuA. This indicates that there is no termination reaction by Claisen condensation during polymerization. Nevertheless, the molecular weight distribution is rather broad (M̄w/M̄n= 2,15). The very large amount of dimer observed is an indication of its low reactivity. A two‐state mechanism is proposed to account for the high polydispersity. Only after longer reaction times side products are formed by Claisen condensation (“back‐bitting”). The termination product of the trimer is an eno

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911004

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe molecular dynamics technique to simulate the behavior of polyethylene crystals, described earlier, is used to study the creation and mobility of rotational isomers of a polyethylene‐like chain in a crystal environment. Both a “gross defect” (formed by perturbation of the cartesian coordinates) and a “kinetic defect” (formed by placing a specific amount of kinetic energy that excites a pure local torsion) are added to the central chain of a polyethylene crystal. The initial dynamic instability quickly forms a more stable, but more or less static defect that, subsequently, moves somewhat along and between neighboring chains, and finally, gets quenched on a time scale of a few picoseconds. Increasing the temperature leads from a highly correlated state to a state where defects are created and quenched statistically (condis state). Increasing the energy input for defect generation leads to the creation of different static defects. The onset of conformational disorder is independent of the initial perturbation. There is a correspondence of the observed defects to the defects proposed for polyethylene based on molecular mechanics calculations, but lifetimes are relativ

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911005

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractA direct method of introducing tetraalkylphosphonium ions into polyacetylene (PA) is described. The maximum doping levels achieved with reducing agents having various redox potentials were determined. The electrical and magnetic properties of resulting doped products were assessed. The thermal stability of this product was monitored by electrical conductivity measurements. An indirect ion‐exchange technique was devised to insert the phosphonium ion into PA. A similar method was found useful for the preparation of reduced β‐carotene, a soluble analogue of PA, containing tetraalkylphosphonium counterions. The stability of the resulting compound was also stu

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911006

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractRacemic lactide was polymerized with various initiators containing Zn and Al, and the structures of the obtained polymers were determined by means of13C NMR spectroscopy. The extent of transesterification reactions was calculated on the basis of the analysis of the methine carbon signal in the polymer13C NMR spectra. Three groups of initiators can be distinguished in view of their influence on transesterification: ZnCl2exhibits the strongest transesterification activity, ZnEt2and ZnEt2/Al(OiPr)3are among those of medium activity, and Al(acac)3does not show transesterification activity at all.

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911007

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractNew synthetic routes for styrene derivatives having both trimethylsilyl and hydroxyalkyl groups were investigated. Reactions between oxirane (5a) and chemical species present in an equilibrium system resulting from 4‐(trimethylsilylmethyl)styrene (1s) and lithium diisopropylamide (2) in tetrahydrofuran give an addition product (8a) between2and5apreferentially. By using alkyl‐substituted oxiranes such as ethyloxirane (5c) andtert‐butyloxirane (5e), a 1 : 1 addition reaction of1swith the oxiranes takes place predominantly to form 1‐alkyl‐3‐trimethylsilyl‐3‐p‐vinyl‐phenyl‐1‐propanol (8) homologues. From results of studies on the reaction between2and several oxiranes, it was concluded that the selective formation of8homologues was attained by suppression of reactions between oxirane and2owing to steric hindrance between alkyl substituents of oxirane and isopropyl groups of2. The reactivity of8in polymerization reaction was also estimated on the basis of spectroscopic measurements.8was found to have a reactivity similar to that of 4‐(trimethy

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911008

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractBlock copolymers poly(methyl methacrylate)‐block‐polystyrene‐block‐poly(methyl methacrylate) and polystyrene‐block‐poly(tert‐butyl methacrylate) were prepared by anionic polymerization and were studied in dilute solution as well as in the bulk state. The viscometric study revealed that the block copolymers exhibit a conformational “transition” occurring in a narrow temperature range. Films were obtained with the same copolymers by casting at different temperatures, chosen below and above the “transition”. From the investigation of these films by differential scanning calorimetry, using the entropy relaxation method, and by dynamic mechanical measurements, it was shown that the block copolymers exhibit a rather segregated structure if they are obtained below the “transition” temperature, and that they adopt a quasi‐Gaussian conformation (implying a better miscibility of the two kinds of blocks) if the solvent is evaporated above t

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911009

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractHetero‐arm star copolymers are constituted of a central core carrying branches of two kinds. The two‐stage synthesis of such polymer species involves an arm‐first step and a core‐first step: A linear anionic living polystyrene precursor was used to initiate the polymerization of a small amount of divinylbenzene, yielding star‐shaped polystyrene bearing living sites inside the cores; in a second stage these sites were used to initiate the polymerization of another monomer — butyl methacrylate —, more electrophilic than styrene. This second step results in the formation of a new set of branches originating from the same polydivinylbenzene cores. The star polystyrenes, and the hetero‐arm star copolymers were characterized adequately, and their molecular weights were found to be in satisfactory agreement with expectation. The presence of some remaining linear precursor in the star polymer samples is discussed thoroughly on the basis of a detailed computerized analysis of the size exclusion chromatograms of both the star polystyrene samples and the hetero‐ar

ISSN:0025-116X    

DOI:10.1002/macp.1990.021911010

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY