bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermotropic liquid crystal polyesters derived from 4,4′‐biphenyldicarboxylic acid and oxyalkylene spacers
Authors: Bello Antonio , Pereña José Manuel , Pérez Ernesto , Benavente Rosario 
Published: March 2011

AbstractSeveral polyesters derived from bibenzoic acid and oxyalkylene glycols have been prepared and the ability of formation of mesophases was studied. The phase behaviour was investigated by means of differential scanning calorimetry, wide‐angle X‐ray diffraction and real‐time variable‐temperature small‐angle X‐ray diffraction. A common feature of all the studied polyesters, with ether groups in the spacer, is the inhibited ability for developing three‐dimensional structures, in contrast to the polymers with all‐methylene spacers. Moreover, the latter polymers show a monotropic mesophase behaviour but the former exhibit an enantiotropic transformation. Smectic structures of different types have been detected: The polyester derived from 3,3'‐oxybis(1‐propanol) produces a smectic SA structure, while the polyester from triethylene glycol leads to a smectic SC mesophase. The influence of the spacers on the dynamic mechanical relaxations of these polybibenzoates has been studied and discussed.
Oxidative degradation of water ‐ soluble polyethers by HO• and HO2• generated photochemically: Ft‐ir studies
Authors: Kaczmarek H. , Lindén L.Å. , Rabek J. E. 
Published: March 2011

AbstractTwo water ‐ soluble polyethers, poly(ethylene oxide) and poly(methyl vinyl ether) were UV irradiated in hydrogen peroxide (H2O2) solutions. It has been shown that HO• and HO2• radicals accelerate oxidative degradation of these polymers. Hydroxy (HO•) and probably hydroperoxy (HO2•) radicals can abstract hydrogen from methylene groups in both polymers. As a result of further oxidative reactions, different carbonyl, hydroxy and hydroperoxy groups are formed. A mechanism for the formation of these groups has been proposed.
Controlled copolymer structures via anionic/insertion polymerization of cyclic carbonates
Authors: Höcker H. , Keul H. 
Published: March 2011

AbstractCyclic carbonates are polymerized in a ring‐opening fashion by means of anionic and insertion type initiatiors. Different hexacyclic carbonates are copolymerized in a random way when both monomers are added to the initiator simultaneously. Block copolymers are obtained when the monomers are added in sequential way.With anionic initiators cyclic carbonates are polymerized preferentially as compared with lactones while insertion initiators are less selective.A special case is the preparation of copolymers of carbonates with lactams. When the polycarbonate is formed first the lactam unit is inserted into carbonate bond to form urethane and ester bonds. Thus, eventually an alternating polyesterurethane is formed with negligible amount of carbonate, amide, and urea units.Moreover, the anionic ring‐opening polymerization of cyclic carbonates may be combined with vinyl polymerization. In the case of group transfer polymerization a transformation of the active site to the non‐metallic anionic initiation of the cyclic carbonate occurs.
The ionic polymerizations of methyl glyoxylate
Authors: Vairon J. P. , Muller E. , Bunel C. 
Published: March 2011

AbstractMethyl 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

AbstractPropylene 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

AbstractAn 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

AbstractQuasielastic 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

AbstractThe 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

AbstractThis 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

AbstractThe 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

AbstractMost 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

AbstractIn 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

AbstractThe 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

AbstractIsotactic 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

AbstractAfter 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

AbstractEffects 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

AbstractElectro‐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

AbstractABSTRACT 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

AbstractThe 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

AbstractIonic 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

AbstractFor 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

AbstractSeveral 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

AbstractThe 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

AbstractMonte‐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

AbstractLow ‐ 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

AbstractThe 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.