bluuebunny/crossref_metadata_2025 huggingface dataset:
The ionic polymerizations of methyl glyoxylate
Authors: Vairon J. P. , Muller E. , Bunel C.
Published: March 2011

Methyl glyoxylate has been polymerized in CH2Cl2 solution either cationically or anionically to give the corresponding polyacetal. Thermodynamic and kinetic results of the polymerizations initiated by CF3SO3H, BF3OEt2, and NEt3 are reported here. Monomer conversion was followed by UV and active centers concentrations were determined through phosphorus end‐capping. A ceiling temperature of 26°C was observed for Meq=l mol.H (Tc = 109°C for bulk) with ΔH°=−29.5 kJ.mo−1 and ΔS°=−99 J.mo−1.K−1. Both initiations were found instantaneous and quantitative, and no termination was observable within the time scale of the reaction (second to hour). Cationic propagation appears to take place essentially on free ions (kp+ ≈ 60 l.moH.s−1 at −20°C) in equilibrium with a larger amount (Kd ≈ 6.7.10−6 mol.−1) of much less reactive ion‐pairs (kp± ≈ 0.16 ± 0.03 1.mo−1.s−1). A stopped‐flow device was used to follow the much more rapid anionic polymerization (kp,app ≈ 8 ± 2.103 1.mo−1.s−1 at −20°C).
13C NMR sequence determination and modelling of polypropylene oils
Authors: Cheng H. N.
Published: March 2011

Propylene can be polymerized using a variety of catalysts under different reaction conditions. In the general case, the polymer may exhibit both stereochemistry (tacticity) and regiochemistry (normal and inverted propylene enchainment). When the molecular weight is low, chain ends also become important. The NMR spectra of the resulting polymers can be very complex. In this work, the spectra are studied with the help of analytical (model fitting) and synthetic (computer simulation) approaches in conjunction with a tetrapolymerization model. The approaches described herein are applied to regioirregular polypropylene, particularly low‐molecular‐weight irregular polypropylene samples (polypropylene oils) made with homogeneous catalysts. Complete spectral assignments and analyses are provided for these oils. Studies of polymer main chain structure provide information on polymer chain propagation; analyses of chain ends provide information on initiation, chain transfer, and termination.
Structural investigations on polymer latices by small‐angle x‐ray scattering
Authors: Ballauff Matthias
Published: March 2011

An overview is given on recent studies of latices by small‐angle x‐ray scattering (SAXS). It is demonstrated that SAXS‐studies performed at different contrast (contrast variation) allows the full elucidation of the fine structure of the particles down to a size‐scale of a few nanometers. Examples are given including studies on core‐shell latices composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) as well as of carboxylated latices.
Quasielastic light scattering, mutual diffusion and viscoelasticity of polymer solutions
Authors: Wang C. H. , And Z. Sun , Zhang X. Q.
Published: March 2011

Quasielastic light (QEL) scattering studies of semidilute solutions of polystyrene in benzene and benzene plus diethyl phthalate mixed solvents have been carried out. Comparisons are made between experiment and theoretical results. It is found that the relative amplitude of the viscoelastic modes depends on the coupling parameter β (defined in the text). For the solutions with β=0, no viscoelastic component is observed in the QEL scattering spectrum, consistent with the theoretical prediction. The amplitude and a relaxation time distribution of the viscoelastic component of the QEL scattering spectrum of PS in the mixed solvent are found to depend on time. Steady state values are obtained only after the solution is equilibrated for more than a month. Time evolution of the QEL spectrum has been investigated.
Stereochemistry of the initiation step in Ziegler‐Natta catalysts containing dialkyl propane diethers: A tool for distinguishing the role of internal and external donors
Authors: Sacchi Maria Carmela , Forlini Fabrizio , Tritto Incoronata , Locatelli Paolo
Published: March 2011

The results of a study on a new generation of MgCl2 supported Ziegler‐Natta catalysts that contain dialkyl propane diethers as internal donors and produce highly stereospecific polypropene even without any external donor are presented. The results concerning three different catalysts containing 2,2‐diisobutyl‐1,3‐dimethoxy propane (DIBDMP), 2,2‐dicyclopentyl‐1,3‐dimethoxy propane (DCPDMP) and 2‐ethyl,2‐butyl‐1,3‐dimethoxy propane (EBDMP) as internal donors are compared. The same donors have also been studied by using them as external donors with a catalyst contaning diisobutyl phthalate as internal donor. The present results, along with previously obtained findings on traditional supported Ziegler‐Natta catalysts, permit a general comprehension of the role of internal and external donors.
Functionalized polyolefins prepared by the combination of borane monomers and transition metal catalysts
Authors: Chung T. C.
Published: March 2011

This paper summarizes the new developments in the functionalization of polyolefins by borane monomers and transition metal catalysts. It is becoming clear that the borane groups are stable to transition metal catalysts and borane groups in polyolefins are valuable intermediates which not only can be quantitatively converted to a wide range of functional groups but also can be used as initiator for free radical graft‐from copolymerizations. Some interesting functionalized polymers, such as polyethylene (PE) and polypropene (PP) with ‐OH, ‐NH2 and halide groups, and graft copolymers of polypropene‐graft‐poly(methyl methacrylate) (PP‐g‐PMMA), polypropene‐graft‐polycapolactone (PP‐g‐PCL) and PE‐g‐PMMA, have been synthesized with controllable compositions and molecular microstructures. Most of them would be otherwise very difficult to prepare by the other existing methods.
The roles of oxidation state, Lewis acid cocatalyst and solvent polarity in activating titanium compounds for olefin polymerization
Authors: Eisch John J. , Pombrik Sofya I. , Shi Xian , Wu Shu‐Ching
Published: March 2011

The polymerization activity of the following three catalyst systems towards ethylene has been investigated and compared: 1) methyltitanocene chloride‐Me2AlCl 2) η3‐allyltitanocene‐Me2AlCl and 3) titanium dichloride tetrahydrofuranate‐Me2AlCl. The first two catalysts formed homogeneous phases and produced linear polymer; the last catalyst functioned in a heterogeneous medium and formed crosslinked polymer. The titanium(IV) catalyst was about 30 times more active than the titanium(III) system and the titanium(II) catalyst on MgCl2 could be made 8 times more active than the titanium(IV) system. A novel mechanism is proposed to explain the behavior and activity of the titanium(II) system.
Highly isospecific, immobilized zirconocene catalysts supported on chemically modified SiO2
Authors: Soga Kazuo
Published: March 2011

Most of the supported metallocene catalysts reported so far have been devised to immobilize metallocenes on the solid surface utilizing the ionic interactions between metallocenes and surface active sites. Whereas, we have been challenging to prepare more rigidly immobilized catalysts by using the chemically modified SiO2. The SiO2‐supported zirconocene catalysts thus prepared could be activated by ordinary trialkylaluminiums like Al(i‐C4H9)3 to give highly isotactic polypropene (PP) with high melting temperature (Tm) and number‐average molecular weight (Mn). This paper summarizes the results obtained so far.
New results in the isotactic polymerization of propene promoted by homogeneous metallocene catalysts
Authors: Busico Vincenzo , Cipullo Roberta
Published: March 2011

In this presentation, some mechanistic aspects of isotactic propene polymerization in the presence of homogeneous Group IV metallocene catalysts are analyzed in the light of new (and still largely unpublished) experimental results. In particular, the regiospecificity of catalyst systems based on two typical C2‐symmetric ansazirconocenes (i.e. rac‐ethylenebis(1‐indenyl) ZrCl2 and rac‐dimethylsilylbis(1‐indenyl) ZrCl2; co‐catalyst, methylalumoxane) is discussed comparatively.

An influence of propene concentration on the ratio between 2,1 and 1,3 monomer enchainments and surprisingly ‐ on the stereoregularity of the produced polymers is pointed out. Tentative kinetic explanations of the observed polymerization behaviours are also proposed.
Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler‐Natta catalysis
Authors: Guerra Gaetano , Corradini Paolo , Cavallo Luigi , Vacatello Michele
Published: March 2011

The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler‐ Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene‐based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non‐bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back‐skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.
Polymers of α‐ and β‐substituted acrylates with controlled structures
Authors: Hatada Koichi , Kitayama Tatsuki , Ute Koichi , Nishiura Takafumi
Published: March 2011

Isotactic and syndiotactic living polymerizations of methacrylates with t‐C4H9MgBr and t‐C4H9Li‐R3Al (Al/Li≥2), respectively, were utilized to prepare highly stereoregular block and random copolymers, stereoblock PMMAs, highly branched star polymers with stereoregular arms, stereoregular PMMA macromonomers with methacryloyl functions and stereoregular comblike and graft polymers derived therefrom. A combination of t‐C4H9Li and bis(2,6‐di‐t‐butylphenoxy) methylaluminum was found to be an efficient initiator for heterotactic living polymerization of methacrylates in toluene at −78°C; e.g. ethyl methacrylate gave a polymer with mr content of 87%. Polymerization of triphenylmefhyl crotonate (TrC) with fluorenyllithium (FILi)/N,N,N′,N′‐tetramethylethylenediamine in toluene at −78°C gave a threodiisotactic polymer with narrow MWD, whose stereochemistry was confirmed from the x‐ray analysis of the pentamer of methyl crotonate (MeC) derived from the TrC pentamer and 1H NMR spectral comparison of the pentamer and the poly(MeC). The poly(TrC) prepared with FILi/(S,S)‐(+)‐2,3‐dimethoxy‐1,4‐bis(dimethylamino) butane also gave a threodiisotactic polymer which showed optical activity due to the one‐handed helix. Polymerization of t‐butyl crotonate was also discussed in some detail.
Old and new problems in polymer stereochemistry
Authors: Farina Mario
Published: March 2011

After a short historical description, the stereochemistry of linear macromolecules is discussed in modern terms, namely stereogenicity, chirotopicity, cryptochirality, incremental optical activity, etc. The last part of the communication is devoted to a similar analysis carried out on stereorigid metallocene catalysts. It has been shown that only five classes of catalysts exist, each characterized by a different chirotopicity of the catalytic site. For class V catalysts four different statistical models of increasing complexity have been developed; they can be easily reduced to simpler cases using symmetry arguments.
Effective cross‐sections for molecular aggregation in external orienting fields
Authors: Ziabicki Andrzej , Jarecki Leszek
Published: March 2011

Effects of an external orienting field and orientational cross‐section for molecular association on the kinetics of crystal nucleation and critical temperature of crystallization is discussed. Dipole and quadrupole effects of an electric (magnetic) field and quadrupole effects of hydrodynamic potential of uniaxial flow are considered. Critical temperature of crystallization, free energy barrier of nucleation and nucleation rates are orientation‐dependent. Smaller orientational cross‐sections result in stronger anisotropy of the nucleation rates.
Thermal relaxation of poled non‐linear optical sidechain polymers: A new, semi‐empirical model
Authors: van Der Vorst Cornells P.J.M. , van Gassel Rob A. P.
Published: March 2011

Electro‐Optic relaxation of a poled, Non‐Linear Optical sidechain polymer with Tg 140°C, containing 4‐dimethylamino‐4′‐nitrostilbene (“DANS”) in the sidechains, has been studied at 120°C with and without annealing at the same temperature. The time‐dependence of the decaying EO coefficients r(t) shows a strong departure from the classical single‐exponential Debye model, especially in the unannealed samples. This departure is attributed to physical ageing, slowing down the orientational relaxation of the sidechains. The Debye model with r(t)‐r(0). exp ‐t/τ] is modified semi‐empirically by introducing a time‐dependent characteristic Debye relaxation time τ(t). Of several trial expressions, one is selected which fits the relaxation data. This is τ(τ)‐τi+C.tb
Elasticity of solid polymers as a result of thermal motions
Authors: Gusev Andrei A. , Zehnder Marcel M. , Suter Ulrich W.
Published: March 2011

ABSTRACT The elastic constants Cik of solids are related to the amplitude of the strain fluctuations which can be simulated atomistically, thus offering a possibility of determining the Cik values of the solid of interest based on a proposed atomistic arrangement and a given interaction between the atoms. We show that the approach is practical by calculating the elastic constants of crystalline (orthorhombic) polyethylene, crystalline (monoclinic) isotactic polypropylene, and glassy bisphenol‐A‐polycarbonate.
Applications of time‐resolved anisotropy measurements to the study of molecular mobility and order in polymer systems
Authors: Soutar Ian , Swanson Linda
Published: March 2011

The applications of luminescence anisotropy (especially time‐resolved measurements) in polymer science are discussed. In this brief overview of the area, particular attention is paid to investigations of polymer dynamics and ordering in dilute solution and aqueous dispersions. The account is illustrated with examples from recent studies in the authors' laboratories.
New developments in the synthesis of block and graft copolymers via anionic and cationic methods
Authors: Rempp Paul , Franta Emile
Published: March 2011

Ionic living polymerization methods are well suited for the synthesis of macromolecules of known structure, molecular weight and composition, and exhibiting low fluctuations in composition and in molecular weight. They were the first to allow the synthesis of well characterized block and graft copolymers. The present paper discusses a number of recent developments concerning these syntheses based on anionic or cationic living polymerization.
Concentration polydispersity in polymer gels
Authors: Geissler Erik , Horkay Ferenc , Hecht Anne‐Marie
Published: March 2011

For swollen polymer networks there is no generally accepted relation between the macroscopic osmotic properties and the scattering behaviour. Detailed information on the relationship between these properties can, however, be deduced from complementary measurements of osmotic and elastic behaviour, small angle neutron and X‐ray scattering (SANS and SAXS) and quasi‐elastic light scattering. We describe such an investigation in two types of networks, differing in the mechanism of cross‐linking.

The SANS spectra yield information on the structure, which is generated both by the dynamics of the system and by long range static constraints. The former, arising from thermodynamic concentration fluctuations, governs the macroscopic osmotic and elastic moduli of the swollen network.

The static superstructure in the gel reflects local variations in the cross‐link density. The resulting concentration polydispersity, <δφ2>/φ2, is determined by the details of the cross‐linking procedure. Its concentration dependence as a function of gel swelling can be expressed in terms of the same macroscopic osmotic and elastic variables as those that govern the thermodynamic properties of the gel.
Inorganic‐organic composites containing mixed‐oxide phases
Authors: Mark J. E. , Wen J.
Published: March 2011

Several novel fillers, specifically silica‐titania, silica‐zirconia and silica‐alumina mixed oxides, have been successfully precipitated into poly(dimethylsiloxane) (PDMS) networks using a sol‐gel approach. The resulting filled networks were found to have very good mechanical properties. In comparison with networks filled only with silica, these materials had good extensibilities as well as high strengths. Filler particle diameters were generally several hundred Angstroms, but were found to decrease with increase in crosslink density of the networks. The distributions of particle size were relatively narrow, and there was very little particle aggregation. The presence of in situ silica‐titania and silicazirconia mixed oxides can also increase the onset temperature for degradation of the PDMS. Swelling equilibrium measurements indicated good bonding between these mixed‐oxide fillers and the PDMS. The presence of the PDMS was found to shorten gelation times in these systems, and to improve the impact strengths of the composites.
Fluorescence studies on volume phase transition in poly(acrylamide) and poly(N‐isopropylacrylamide) gels
Authors: Horie Kazuyuki , Asano Makoto , Hu Yuxin
Published: March 2011

The changes in microenvironments during the volume phase transition of poly(acrylamide) and poly(N‐isopropylacrylamide) gels induced by pH change or the change in solvent composition were studied by using dansyl or pyrenyl fluorescent probes.
Modelling the elastic behaviour of real chains in polymer networks
Authors: Stepto Robert F.T. , Taylor David J.R.
Published: March 2011

Monte‐Carlo simulations of the elastic behaviour of PDMS networks, using a realistic (R‐I‐S) network‐chain model, are able to reproduce experimentally observed deviations from Gaussian network behaviour in uniaxial extension. The finite extensibility of the network chains is shown to cause non‐affine deformation of the mean‐square network‐chain end‐to‐end distance, even at moderate sample deformations (λ≈1.5). An increase in the proportion of fully‐extended chains with increasing macroscopic strain gives rise to a steady decrease in the rate of network free‐energy change, causing a reduction in the network modulus at moderate macroscopic strains. There is no need to invoke a transition from affine to phantom chain behaviour as deformation increases.
Low ‐ frequency raman modes in crystalline chain molecules
Authors: Viras Kyriakos G.
Published: March 2011

Low ‐ frequency Raman modes of substituted derivatives of n ‐ alkanes and oligo(oxyethylene)s are discussed. The major features in these modes arise from whole molecule longitudinal vibrations. The effects of end group, chain length and temperature on the frequencies of these modes are described. The frequencies of the longitudinal vibrations are interpreted in terms of chain models.
Macromolecules in ordered media. II. A fluorescence study of the polymer‐liposome association
Authors: Porcar Iolanda , Gómez Clara M. , Codoñer Armando , Soria Vicente , Campos Agustín
Published: March 2011

The interactions of poly (4‐vinyl pyridine) with dimyristoylphosphatidic acid‐based liposomes were studied in heterogeneous solution systems. The polymer was dispersed in aqueous buffer solutions of small unilamellar vesicles and after a suitable period of incubation to raise the equilibrium, the experiments has been conducted as a function of polymer concentration and temperature. According to the measurements of the steady‐state fluorescence spectra, changes in the intensities as well as shifts in the emission peak wavelength, has served as raw data to accounting for the polymer‐vesicle interactions. Quantitative evaluation of the interaction has been done using a simple associating model with some approximations. The key parameters evaluated were the amount of polymer bound and the number of charged lipids on a membrane site as well as the number of effective charges of poly (4‐vinyl pyridine) chains at the liposome interface.
Characterization by Raman microspectrometry of residual compressive stresses induced by resin curing in single filament composite samples
Authors: Masse P. , Cavrot J. P. , Depecker C. , Laureyns J. , Escaig B.
Published: March 2011

The present work concerns monofilament composite samples used in the pull‐out test to quantify the effect of different fibre surface treatments on the adhesion of a high modulus polyethylene in a polyester resin. Optical observations revealed local deformations induced by a compressive stress generated in the fibre by the resin shrinkage during the curing. The sensitivity of the polyethylene Raman peak at 1130 cm−1 to elastic strain has been used to characterize the local compression state still present in the fibre after the curing. The results show residual stresses varying along the fibre embedded length up to a level depending on the applied surface treatment i.e. on the degree of adhesion between the fibre and the resin. This is consistent with the localization and the morphologies of shear bands observed in the samples. More the Raman microspectrometry has been used to determine the tensile stress profile along the embedded part of the fibre when its free part is stretched. Corrected of the initial compression state this profile represents the stress transfer effectively realized from the fibre to the resin for different adhesion conditions. Its knowledge is very useful to clarify the interface damage and pull‐out mechanisms.
Spectral biodiagnostics of tissues with fiber optics
Authors: Afanasyeva N. , Artjushenko V. , Lerman A. , Plotnichenko V. , Frank G. , Neuberger W.
Published: March 2011

The synergy of novel MIR‐fibers with FTIR‐spectroscopy opens a new horizon in applications of spectroscopy in 4 − 16 μm range in vivo medical diagnostics. The innovated bare core and core‐clad MIR‐fibers are produced by hot extrusion method from the crystals of silver halides solid solutions. These non‐toxic and non‐hygroscopic fibers are characterized by low optical losses: 0.1 − 0.5 dB/m nearby 10μm, and high flexibility: Rbending > 10–100 fiber diameters. Some prototypes of accessories for FTIR‐spectrometers, based on commercially available MIR‐fibers for several areas of promising applications are developed. Non‐invasive medical diagnostics in vivo: cancer detection and glucose content measurements in whole human blood is possible. The new family of evanescent fiber sensors is started for the direct in‐process applications. Fiberoptic Evanescent Wave FTIR Spectroscopy (FEWS) for cancer diagnosis in human organs is exemplified in this paper, and their great potentiality is demonstrated.
Counterion effects in the anionic polymerization of methyl methacrylate: Mechanistic relevance to GTP
Authors: Warmkessel Jeffrey , Kim Jungsoo , Quirk Roderic P. , Brittain William J.
Published: March 2011

The room temperature polymerization of methyl methacrylate (MMA) by the cesium or tetrabutylammonium salt of 9‐methylfluorenide proceeds in an uncontrolled fashion. To understand these results, we have studied the structure of the propagating chain‐ends using polymer microstructure and stopped‐flow FT‐IR spectroscopy of model compounds. Deprotonation of methyl isobutyrate with the cesium salt of 1,1,4,4‐tetraphenylbutane dianion afforded the corresponding cesium enolate (U=1645 cm−1). The deprotonation of methyl 2,2,4‐trimethyl‐4,6‐dicarbomethoxyhept‐6‐en‐1‐oate (the trimeric ester of MMA) was accompanied by a rapid (2–4 s) intramolecular cyclization. Comparison of group transfer polymerization (GTP) to anionic polymerization reveals that the counterion is important but probably not the sole mechanistic feature responsible for the living behavior of GTP.
Multi‐arm star polyisobutylenes: 2. The effect of synthesis conditions on the structure of star PIBs
Authors: Marsalkó Timea M. , Majoros István , Kennedy Joseph P.
Published: March 2011

Multi‐arm star polyisobutylenes (*‐(PIB)n) have been prepared by the “arm‐first” method. This synthesis was accomplished by adding various linking agents (“core builders”) such as p‐ and m‐divinylbenzene (DVB) and p‐ and m‐diisopropenylbenzene (DIB) to living PIB® charges and thus obtaining a crosslinked aromatic core holding together a corona of well‐defined arms. The products were characterized in terms of overall arm/core composition, molecular weight, and molecular weight distribution (M̄w/Mn). The effect of reaction conditions (time, [linking agent]/[PIB], arm molecular weight) on the kinetics of the star formation and star structure were investigated. The multi‐arm star nature of *‐(PIB)ns was proven directly by determining the molecular weight (by light scattering) of the intact products, selectively destroying the aromatic polyDVB (or polyDIB) core (“core‐destruction”), and finally determining the molecular weight of the surviving aliphatic PIB arms. The synthetic strategy, overall kinetics, and observations during the preparation of star‐PIBs were discussed. Among the critical parameters that determine product structures are the rate of crossover PIB⊕ + DVB (or DIB) → PIB‐DVB⊕ (or PIB‐DIB⊕), the concentration of the linking agent DVB (or DIB), and the molecular weight of the PIB arm. Evidence for the formation of higher order stars (“secondary”, etc.) by star‐star‐ coupling has been presented.
Kinetic study of the living cationic polymerization of isobutylene using t‐Bu‐m‐DCC/TICL4/2,4‐dimethylpyridine initiating system
Authors: Storey Robson F. , Choate Kim R.
Published: March 2011

Kinetics of the living cationic polymerization of isobutylene, initiated by the system t‐bu‐m‐DCC/TiCl4/2,4‐dimethylpyridine (2,4‐DMP), were studied as a function of concentration of the various components of the initiation system, solvent polarity, and presence of the protic acid scavenger, 2,6‐di‐tert‐butylpyridine (DTBP). Under a variety of conditions, the effective number of growing chains in a given polymerization remained constant and Mn increased linearly with monomer conversion. The system was found to yield an essentially homogeneous reaction mixture in hexanes/methyl chloride cosolvents, with only a small amount of precipitate, probably 2,4‐dimethylpyridinium salts resulting from proton scavenging by the tertiary amine. It was found that increasing [TiCl4] strongly increased the rate while increasing [2,4‐DMP] weakly decreased the rate. Evidence of a retardation of the polymerization rate by the soluble TiCl4:2,4‐DMP complex was observed. The addition of DTBP as a protic acid scavenger, with or without 2,4‐DMP, very weakly decreased the rate of polymerization. Increasing the fraction of methyl chloride in the solvent mixture caused an increase in the rate of polymerization. All of the results were consistent with a propagation mechanism in which an equilibrium exists between dormant and ionized, active chain ends.
Living and stereoselective polymerization of [2.2]‐paracyclophan‐2‐ENE derivatives
Authors: Miao Yi‐Jun , Sun Benjamin J. , Bazan Guillermo C.
Published: March 2011

Paracyclophene based monomers can be polymerized in a living fashion using the alkylidene initiator Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2. The cis‐specific nature of the polymerization is critical, since small amounts of trans olefin in the backbone renders the material insoluble. These polymers have complex photophysical behavior, probably a consequence of the close proximity of chromophores along the polymer backbone. Polymerization of 9‐(tert‐butyldimethylsilyloxy)‐[2.2]‐paracyclophan‐1‐ene produces a new precursor material which furnishes PPV under remarkably mild conditions.
Living block copolymerization of phenyl vinyl sulfoxides and characterization of a stereogradient polymer
Authors: Buese Mark A. , Zhang Yingchao
Published: March 2011

The polymerization of phenyl vinyl sulfoxide in blocks of increasing enantiomeric excess was carried out. Portions of the polymer were removed after each charge of monomer was consumed. The poly(phenyl vinyl sulfoxide)s were characterized by gel permeation chromatography and nuclear magnetic resonance spectroscopy. Reduction of the poly(phenyl vinyl sulfoxide)s to poly(phenyl vinyl sulfide)s permitted assignment of the polymer stereochemistry and verification that stereogradient polymers were formed.
Liquid crystal dimers and oligomers: Experiment and theory
Authors: Luckhurst Geoffrey R.
Published: March 2011

Liquid crystal dimers and trimers exhibit a range of unusual properties which are fascinating in their own right and which also parallel those found for semi‐flexible main‐chain liquid crystal polymers. This close similarity suggests that dimers and oligomers should form good model systems for their polymeric counterparts and evidence is presented in support of this view. The transitional properties of a range of dimers, all containing the cyanobiphenyl mesogenic group, are presented to illustrate their dependence on the length and parity of the spacer as well as the nature of its link to the mesogenic groups. It is shown that the odd‐even effect thought to characterise dimers docs not always occur and can also vanish when rod‐like mesogens are replaced by disclike groups. Theories, of varying degrees of sophistication, are presented with which to understand the transitional behaviour of the dimers. Of particular interest is a generic model which seems to represent the essential features of the liquid crystallinity of dimers. An especially important characteristic of liquid crystals is their orientational order. The results of NMR studies of this for dimers are described and shown to lead to a novel model of the molecular organisation in the nematic phases of dimers. This simple picture is found to be in accord with the more complete molecular field theories. Finally, the behaviour of liquid crystal trimers is compared with that of dimers, this comparison suggests that much of the essential physics for the polymeric systems is contained within that of the dimers. However, trimers do allow the introduction of novel structural features not possible within dimers. The behaviour of such systems should provide a uniquely valuable test bed for molecular theories of liquid crystal oligomers.
Structure and dynamics of liquid‐crystalline polymers with different molecular architectures
Authors: Spiess H. W.
Published: March 2011

The structure and dynamics of liquid‐crystalline polymers with different molecular architectures as revealed by multidimensional NMR Is described. Specific examples discussed Include semiflexible, rigid‐rod, calamitic, and discotic systems. Structural aspects addressed are the determination of high rank order parameters, the micro‐order in frozen liquid crystals and the packing of flexible side chains. Local dynamics In the glassy state, conformational dynamics and collective phenomena such as the development of convective director structures are discussed. It is demonstrated that advanced NMR spectroscopy yields highly specific information not available from conventional scattering, microscopic and relaxation techniques.
The spatial inhomogeneity of rodlike polymeric systems subject to simple shear flow
Authors: Ding Jiandong , Yang Yuliang
Published: March 2011

A novel simulation approach has been developed by the combination of Brownian dynamics and Lebwohl‐Lasher nematogen potential, by which we have made preliminary investigation on the spatial inhomogeneity of nematic rodlike polymeric system with planar surface anchoring and subject to simple shear flow. Some rheological behaviors such as the damped oscillation of stresses with time or the negative first normal stress difference at moderate shear rate find good agreement with previous experimental observations reported in literature. The distribution function of rod orientation takes on “breathe‐like” time‐evolution and is verified not Gaussian‐featured. The layered director, order parameter, shear stress and first normal stress difference have also been calculated by statistics, and most importantly, all of these statistic quantities show spatial distribution in different layers and corresponding distribution functions are time‐dependent. Therefore, our simulation implies the complicated pattern formation and pattern evolution in this system. It is, hence, very meaningful to further study the spatial inhomogeneity of rodlike polymers under shear flow extensively both in theory and in experiment.
Liquid crystalline polymers: From self‐organization to functions and applications
Authors: Anderle Klaus , Wendorff Joachim H. , Chemie Fachbereich Physikalische , Fur Material Wissenschaft Wissenschaftliches Zentrum
Published: March 2011

Liquid crystals derive their unusual properties and their broad range of applications from their unique spatial and orientational order giving rise to specific symmetries, to strong anisotropies with respect to macroscopical properties and to a strong coupling to external fields. We have studied for liquid crystalline polymers modes of inducing strong modifications of the anisotropic optical properties locally in solid films by light. A storage process is described in this contribution which is based on light‐induced trans‐cis‐trans‐isomerization reactions of azobenzene chromophores attached to a polymer backbone as side groups in liquid crystalline polymers. The chromophores are able to rotate in the glassy state if subjected to linearly polarized light: the azobenzene units approach a saturation orientation which is perpendicular to the polarization direction of the light. The contribution discusses the molecular mechanism of this process as well as possible applications.
Charge carrier mobilities in liquid crystalline mesomorphic poly(DI‐n‐Alkylsilylene)s; influence of backbone conformation
Authors: van Der Laan Garrelt P. , De Haas Matthijs P. , Frey Holger , Möller Martin
Published: March 2011

The charge transport properties of a series of symmetrically substituted mesomorphic poly(di‐n‐alkylsilylene)s are studied using the pulse‐radiolysis time resolved microwave conductivity (PR‐TRMC) technique. The observed conductivities for these polymers could be correlated with different backbone conformations present both in the crystalline solid phase and in the liquid crystalline mesophase. The transition from the solid phase to the mesophase is accompanied by a disordering of the silicon backbone that results in a decrease of the conductivity of up to two orders of magnitude. The charge carrier mobilities found varied from 5×10−5 m2 /Vs for the all‐trans conformation in the solid phase to 6×10−7 m2 /Vs for the disordered backbone conformation in the mesophase. The anisotropic radiation‐induced conductivity observed for aligned poly(di‐n‐hexylsilylene) samples demonstrate that charge carrier migration takes place preferentially in the direction of the polymer backbone.
The effect of aluminium compounds in the copolymerization of ethylene / α‐olefins
Authors: Katayama Hiroaki , Shiraishi Hiroyuki , Hino Takahiro , Ogane Takuya , Imai Akio
Published: March 2011

Two types of copolymerizations were found with the catalyst system Cp2ZrCl2/MAO. One was a less exothermic reaction, and another was a more exothermic one. The former provided a polymer with a single composition distribution because of a single cationic active site. The latter provided a polymer with a bimodal composition distribution, indicating the existence of two active sites. The two active sites were investigated through the comparison of MAO systems and alkylaluminium / borate systems. We speculated that the two active sites were the normal cationic site and the active site where the interaction between a cationic metallocene and an aluminium compound was strong.
The preparation of aluminoxane from trimethylaluminium at a defined surface of deeply cooled ice
Authors: Winter Hergen , Schnuchel Wolf , Sinn Hansjörg
Published: March 2011

The controlled hydrolysis of trimethylaluminium (TMA) on a defined surface of cooled ice to the corresponding aluminoxanes provides good yields of a toluene soluble product. The reaction was conducted in a specially designed reactor with a rotating scraper. The product properties are independent of reaction temperature between −28 and −40 °C and scraping speed, but can be varied with the extent of hydrolysis.

A series of reactors was built for the investigation of this reaction system, the biggest ones producing between 0.5 and 1 kg of aluminoxane solution per day. The safety and ease of use was improved with each subsequent reactor design.
Phase separation of methylalumoxane with diethyl ether
Authors: Hagendorf Wiebke , Harder Annette , Sinn Hansjörg
Published: March 2011

After the addition of diethylether to a solution of methylalumoxane in toluene, phase separation can be observed. The investigation of several phase separation experiments led to an idea of the possible composition of methylalumoxane that could be consisting of a basic element and differently associated trimethylaluminium.

Investigations of the system methylalumoxane–diethylether–trimethylaluminium show that diethylether is stronger complexed at methylalumoxane than with trimethylaluminium. A second model of the structure of methylalumoxane allows an attempt to explain the phenomenum of phase separation.
An approach to phase behaviour in polymers
Authors: Keller A. , Wills H. H.
Published: March 2011

Aspects of phase transition will be surveyed such as are direct consequences of straightforward thermodynamic considerations yet are not normally taken count of in traditional treatments of phase behaviour even if, as is here shown, they can assume special significance in polymers. The central theme will be the common experience that in most materials, but especially in polymers, the state of ultimate thermodynamic stability is hardly ever attained, consequently that we are dealing with metastable states, the issue of metastability thus becoming the main theme of the present treatise. This metastability can manifest itself either through the phase transformation, even in the thermodynamically stable phase, not reaching completion and/or that phases other than those of ultimate stability appear first often taking on a dominant role. The above two strands are being followed through first separately and finally, with the example of polymer crystallisation, in combination. In the course of it all a variety of phase transformations and numerous potentially intriguing consequences are being touched upon. These are encompassing aspects of liquid‐liquid phase separation, liquid‐crystal formation and crystallisation, the intervention of glass transition, the significance of supercooling, the effect on and influence of the morphology and the connection and interrelation between thermodynamic stability (including metastability) and the rates of the phase transformation, with passing comments on gels and also on some aspects of chain conformation in solution. In the case of the most extensively covered item of crystallisation attention is being drawn to the possibility of stability inversion with phase (here crystal) size. The latter, in terms of ‘Phase Stability Diagrams’, offers a new approach to crystal growth, which in the case of polymers, creates both new perspectives and opens up the possibility of treating so far largely disconnected aspects of the subject within a unifying framework. The survey ends with reference to recent work on amorphisation through pressure pointing to new aspects of material behaviour and to some new and possibly surprising variants of otherwise familiar phase diagrams.
Living polymerization with well‐defined alkylidene catalysts
Authors: Schrock Richard R.
Published: March 2011

Alkylidene complexes of the type Mo(CH‐t‐Bu)(NAr)(OR)2 (Ar = 2, 6‐diisopropylphenyl; OR = O‐t‐Bu, OCMe2(CF3), OCMe(CF3)2, etc.) can be prepared from a “universal precursor”, Mo(CH‐t‐Bu)(NAr)(triflate)2(1, 2‐dimethoxyethane),1 which can be prepared in three high‐yield steps from ammonium dimolybdate.2 These complexes serve as initiators for the ring opening metathesis of norbornenes and substituted norbornadienes,3 or for the polymerization of acetylenes to give polyenes,4 all in a living manner. The organic polymer can be cleaved upon treatment with an aldehyde. Norbornenes that contain a variety of organic or inorganic functionalities can be polymerized to give essentially monodisperse homopolymers and block copolymers. Recently chiral catalysts have been prepared that will polymerize norbornenes and norbornadienes stereoselectively to give all cis isotactic polymers.5 Molybdenum catalysts also have been found that will cyclopolymerize dipropargyl derivatives or substituted phenylacetylenes in a living manner.6 The stereospecific synthesis of all cis, or all trans, tactic polynorbornadienes and polynorbornenes will be discussed in detail, including a proof of tacticity.7
New supramolecular architectures based on hydrogen bonding
Authors: Zimmerman Steven C.
Published: March 2011

Heterocyclic compounds containing two and three adjacent hydrogen bond donor and acceptor sites in all possible arrangements were synthesized (see generalized structures shown below) to study and use their “base‐pairing” capabilities. With two adjacent donor and acceptor groups there are three possible arrangements and these form two types of complexes (DA·AD and AA·DD). Three adjacent hydrogen bond donor and acceptor sites can be arranged in six different ways and these form three different complexes (ADA·DAD, AAD·DDA, and AAA·DDD).
New industrial fluoropolymer science and technology
Authors: Smart Bruce E. , Feiring Andrew E. , Krespan Carl G. , Yang Zhen‐Yu , Hung Ming‐H. , Resnick Paul R. , Dolbier William R. , Rong Xiao X.
Published: March 2011

Teflon® AF, a family of copolymers of perfluoro‐2,2‐dimethyldioxole with tetrafluoroethylene, and Cytop®, a ring‐cyclized homopolymer of CF2=CFO(CF2)2CF=CF2 are the first commercial amorphous perfluoroplastics which combine high optical clarity and solubility with outstanding chemical, thermal and electrical properties. The processes for making these materials are described and recent structure‐property studies that reveal dramatic substituent effects on polymer Tg and related properties are reviewed. The results of some initial fundamental kinetic studies on fluorinated free‐radical cyclizations that relate to efficient cyclopolymerization of fluorinated dienes and the design of new ring‐containing fluoroplastics are discussed.
New designs for sensory materials: Conducting polymers displaying molecular recognition
Authors: Swager Timothy M. , Marsella Michael J.
Published: March 2011

The integration of molecular recognition elements into conducting polymers holds great promise for the formation of new types of sensory materials. We describe some of our recent results in this area which focus on producing polymers which are responsive to alkali ions and electron deficient organic molecules.
Influence of cell wall polysaccharides on fruit juice technology
Authors: Will Frank , Dietrich Helmut
Published: March 2011

Readily processed clear fruit juices contain variable amounts of colloidally dissolved pectic substances. Separation of fruit juice polysaccharides by anion‐exchange‐chromatography yielded mainly arabinogalactans (type II) and rhamnogalacturonans. The addition of small amounts of the residual pectins to colloid free juices resulted in a up to 60 % reduction of the flux rates in micro‐ and ultrafiltration systems.
Biological functions of synthetic polysaccharides
Authors: Kobayashi Kazukiyo
Published: March 2011

Structurally well‐defined polysaccharide derivatives have been synthesized via ring‐opening polymerization of bicyclic anhydro sugar derivatives and via enzyme‐catalyzed polymerization using phosphorylase. This article describes the following four synthetic polysaccharides, (a) Octadecylated amphiphilic polysaccharides, (b) Regiospecifically fluorinated polysaccharide, (c) Comb‐shaped branched polysaccharide, (d) Graft copolymers having pendant polysaccharide chains. None of these polysaccharides could be prepared by direct chemical modifications of natural polysaccharides. The synthetic polysaccharides are useful as tools to elucidate structure‐function relationships and exhibit various novel biofunctional properties.
Enzymatic polymerization to polysaccharides of well‐defined structure
Authors: Shoda Shin‐Ichiro , Kobayashi Shiro
Published: March 2011

Various poly‐ and oligosaccharides having a well‐defined structure have successfully been synthesized via enzymatic polymerization of glycosyl fluoride monomers. All of the reaction proceeded under perfect control of regio‐ and stereochemistry. The relative direction of the synthetic cellulose (parallel or antiparallel) could be controlled in vitro in the enzyme‐promoted propagation system, and based on these findings, a new concept “allos‐selectivity” has been proposed.
Novel polymeric excipients for colon‐targeting
Authors: Hirsch Stefan , Schehlmann Volker , Kolter Karl , Bauer K. H.
Published: March 2011

Oral drug delivery to the colon is of interest for the local treatment of colonic diseases and for the systemic delivery of drugs. Our goal is to develop coating materials that are stable in gastric and small intestinal fluids and degradable in the colon by bacterial polysaccharidases. For testing colon‐degradability an in vitro model was developed. A number of polysaccharide derivatives were synthesized and tested: almost completely ethylated or acetylated galactomannans, polyurethanes with galactomannan sections, crosslinked galactomannans and dextranesters of fatty acids. Generally, degradability decreased with increasing degree of substitution. Especially crosslinked galactomannan and dextranesters with a low degree of substitution or crosslinking, respectively, are promising materials. Using an industrial coating technique tablets were coated with crosslinked galactomannan and the enzymatically induced in vitro dissolution of a model drug was investigated.
Recent developments in the science and technology of high modulus flexible polymers
Authors: Ward Ian Macmillan
Published: March 2011

Recent progress in basic science and technological innovation is described for three areas of research in the IRC at Leeds University; melt spun high modulus polyethylene fibres, hydrostatic extrusion and die drawing.
Molecular reinforcement and compatibility in rod/coil polymer blends
Authors: Eisenbach Claus D. , Fischer Karl , Hofmann Jörg , Macknight William J.
Published: March 2011

Compatibility between a true rod‐like and a coil‐like polymer could be achieved by preparing ionomer blends of rigid polydiacetylenes with functional or ionic side groups and carboxylated or sulfonated polystyrene or sulfonated polyester‐ureaurethanes. Microphase separation was prevented through ionic contact between the blend components. The miscibility on a length scale of the rigid‐rod polymer repeating unit was proven by infrared spectroscopical, dynamic mechanical and differential scanning calorimetry analysis. The molecular reinforcement of the ionomer blends containing only a few weight percent of the reinforcer polymer amounted up to 1 order of magnitude compared to the matrix polymer as reflected from Young's modulus.
Model tests for wear measurements in plastics processing
Authors: Mennig Günter
Published: March 2011

In plastics processing different mechanisms of wear are observed. In view of the complex nature of the tribological systems wear can be studied systematically by model tests only. These must be developed in a manner that they coincide directly with the practical process. The youngest DKI model test, the adhesive wear tester, is described and results for various metal pairings are presented.