bluuebunny/crossref_metadata_2025_split huggingface dataset:
Anionic polymerization methods: An efficient tool for the synthesis of tailor‐made macromolecules
Authors: Rempp Paul , Lutz Pierre 
Published: March 2011

AbstractThe criteria of livingness, as applied to anionic polymerizations, are discussed. A number of restrictions have to be taken into consideration, if anionic polymerizations are to be applied to the synthesis of model polymers, Several examples of synthesis of tailor‐made polymers via anionic polymerization are presented, namely end‐functionalizations, chain extensions, end‐to‐end cyclizations, block copolymer synthesis, grafting processes, methods to make star polymers. In each case the reliability of the methods is discussed thoroughly.
Comparison of living polymerization mechanisms. Acrylates and carbocationic polymerization
Authors: Pugh Coleen , Matyjaszewski Krzysztof 
Published: March 2011

AbstractThe concept of a living polymerization is critically discussed. A system ranking various classes of “livingness” is proposed, and the importance of determining the real values of ktr/kp and kt/kp ratios is expounded. New living systems, including carbocationic polymerization and group transfer polymerization of acrylates are compared with classic ionic systems. The mechanism of propagation and the nature of the true active species are similar in both new and classic polymerizations. The role of various components which improve the “livingness” of the polymerizations is discussed and explained by dynamic equilibration between dormant and active species and suppression of side reactions.
Structures of (ester) enolates and related compounds
Authors: Boche Gernot , Harms Klaus , Marsch Michael 
Published: March 2011

AbstractThe side reactions in the anionic polymerization of (meth)acrylates initiated by organolithium compounds raise the question about favorable interactions between organolithium and carbonyl compounds in general and in this polymerization reaction. The results of several crystal structures of lithium(sodium) (ester) enolates without or with other molecules clearly demonstrate the favorable formation of homo‐ and mixed aggregates, respectively. It is also shown how different gegenions (ZnBr+, R4N+, etc.) influence the overall structure and the structure of the anion.
Kinetics and stereoregulation in the isotactic‐specific and living polymerization of methyl methacrylate
Authors: Kitayama Tatsuki , Fujimoto Nobutaka , Hatada Koichi 
Published: March 2011

AbstractKinetic studies on the isotactic‐specific living polymerization of methyl methacrylate with t‐C4H9MgBr were carried out in toluene at −78°C and the kinetic orders with respect to the monomer and initiator were found to be unity. Propagation rate in the early stage of polymerization and the initiation rate were studied by kinetic measurement and analysis of oligomer distribution. The rate constants for the propagation of the unimer (k1), dimer (k2), and trimer anions (k3) were found to be in the following order; k1 >> k2 > k3. 13C NMR spectra of the oligomer anions indicated the coordination of penultimate and antepenultimate ester groups to Mg counterion. Possible mechanism for stereoregulation in the early stage of polymerization was discussed.
Poly(ethylene oxide) gel as a novel polymerization medium anionic polymerization of methyl methacrylate
Authors: Tsvetanov Ch.B. , Novako Ch.P. , Dotcheva D.T. , Gitsov I. 
Published: March 2011

AbstractCrosslinked poly(ethylene oxide)‐(PEO‐N) is used as a novel medium for the anionic polymerization of methyl methacrylate (MMA) initiated by t‐BuOK and ethyl‐α‐lithioisobutyrate (α‐LiEtIB) in toluene. Comparative studies with linear poly(ethylene oxide)‐(PEO‐L) are performed as well. It is found that PEO‐N effectively binds both initiators, and the polymerization process takes place mainly in the gel phase. PEO‐N accelerates the polymerization process initiated by t‐BuOK enabling the formation of high‐molecular‐weight polymers with high yields. Part of poly(methyl methacrylate)‐(PMMA) remains in the gel particles yielding semi‐interpenetrating networks with amphiphilic properties. PEO additives do not influence profoundly the course of the polymerization, initiated by α‐LiEtIB. The influence of PEO‐N on the proceeding of the polymerization is discussed in some detail.
Thermolytic behaviour of polybutadienyllithium and polystyryllithium in ethylbenzene at high concentrations
Authors: Schué F. , Aznar R. , Sledz J. , Nicol P. 
Published: March 2011

AbstractThe polymerization of butadiene and styrene with lithium and lithium alkyls has been extensively studied. The kinetics and mechanism of the pyrolysis of alkyllithium compounds has also been investigated thoroughly. The mechanism of these elimination reactions is conveniently elucidated by manometric methods, and mechanistic interpretations have been proposed. However, there are less published accounts of the thermal stability of polymer‐lithium systems. There is a need to have kinetic data describing the thermal termination of “living” polymers at lithium concentrations approaching those of commercial polymerization systems. This paper will discuss the thermal stabilities of concentrated solutions in ethylbenzene of polystyryllithium and polybutadienyllithium.
Mechanism of anionic (ionic) polymerization of acetylene monomers
Authors: Kobryanskii Valery M. 
Published: March 2011

AbstractPolymerization of phenylacetylene in presence of butyllithium has been studied. It is shown that the process of polymerization is accompanied by the appearance and growth of the ESR signal. The model of anionic polymerization is suggested according to which formation of low‐molecular‐weight polyphenylacetylene is caused by the destruction of the active centers by means of electron transfer from active center to the conjugated chain. The assumption is made that electron transfer from conjugated chain to an active center may also be a fundamental reaction limiting the chain length in cationic polymerization of acetylene monomers. The kinetic scheme of ionic polymerization of acetylene monomers with consideration of electron transfer is analyzed and molecular weight distribution functions are obtained with good agreement between the calculated parameters and experiment. It is shown that one of the ways of obtaining high‐molecular‐weight polyacetylenes in ionic polymerization is the formation of donor‐acceptor complexes with a polyene chain in the process of the chain growth. The formation of complexes exchanges parameters of electron structure of the polyene chain and decreases the rate of the electron transfer reactions.
Mechanistic aspects of group transfer polymerization
Authors: Quirk Roderic P. , Ren Jie , Bidinger Gregory 
Published: March 2011

AbstractThe experimental evidence supporting the involvement of enolate anions in group transfer polymerization(GTP) is reviewed. The results of silyl group exchange studies between living silyl ketene acetal‐ended oligomers under typical GTP conditions are discussed. It is concluded that the observations of significant amounts of silyl group exchange in the presence of polymerizing monomer are not consistent with the originally proposed “associative mechanism” based on the GTP Criterion which precludes intermolecular silyl group exchange.
Mechanism of GTP: Can all of the available data be accommodated?
Authors: Webster Owen W. 
Published: March 2011

AbstractGroup transfer polymerization (GTP)a of acrylic monomers is a living system mediated by a trialkylsilyl capped growing chain end. The fact that it operates at temperatures as high as 100° C differentiates GTP from living anionic polymerization, which at best operates at 25° C for hindered methacrylates. To accommodate all of the mechanistic data available it appears that at least two mechanistic pathways are required: an associative process for mild nucleophilic catalysts and a dissociative process for strong nucleophilic catalysts.
Polymer devolatilization: State of the art
Authors: Astarita Gianni , Maffettone Pier Luca 
Published: March 2011

AbstractThe current state of the art on polymer devolatilization is reviewed. After a brief description of existing processes, two aspects of the problem are taken into consideration: modeling and optimization of actual processes, and physicochemical data needed for analysis. It is shown that, while many of the available models concentrate on the fluid mechanics involved and on mass transfer, heat transfer may well be the rate determining phenomenon. Values of the relevant physicochemical parameters may range over several orders of magnitude within any given process, and the availability of reliable data over the ranges of interest is in general significantly less good than would be needed for reliable analysis.
Numerical evaluation of a method for determining extensional viscosity of fluids using contraction flow analysis
Authors: Davies A. R. , Farah I. M. , Rides M. , Thomas K. 
Published: March 2011

AbstractWe report an evaluation of the use of Binding's method (Ref 11) for determining extensional viscosity values from capillary die rheometry entrance pressure drop data. A constitutive relation defining the fluid's rheological behaviour is first postulated, from which the pressure drop in a contraction region is calculated by numerical simulation for a range of flow rates. Extensional viscosity estimates are then made from the simulated pressure drop data using Binding's method, and these estimates are compared with the exact extensional viscosity determined from the constitutive relation. A highly accurate augmented spectral method is used to perform the numerical simulation. The Binding estimates obtained are good for certain types of fluid, but poor for others. A simple empirical correction of the Binding method, however, is found to give good agreement in all the cases studied.
Influence of the chemorheology of toughened epoxy matrices on the processing behavior of high performance composites
Authors: Kenny Jose M. , Oplicki Marica 
Published: March 2011

AbstractThe rheological and kinetic behavior of a new commercial toughened epoxy matrix is characterized through a complete experimental program. The kinetic behavior of the thermosetting resin is described by applying a new empirical model, accounting for autocatalytic and difussion control effects. The rheological model uses the kinetic information and describes the matrix viscosity as a function of temperature and degree of reaction using only three fitting parameters. The complete model is verified through calorimetric and rheological tests performed at different temperatures (isotherms) or heating rates (dynamic tests); or under more complex conditions simulating the processing conditions during autoclave lamination. Finally the chemo‐rheological model is considered as a part of a master model describing the complete processing behavior of thermoset matrix composites. The use of the master model in control and optimization algorithms is also illustrated.
Extrusion computations in three dimensions
Authors: Tanner R.I. , Beverly C.R. 
Published: March 2011

AbstractThe present paper considers the problem of predicting extrudate shapes from asymmetrical dies for Newtonian and non‐Newtonian fluids. The flow is fully three‐dimensional and an exploration of finite elements and boundary elements is made with a view to finding accurate, stable and economical schemes. A number of finite elements are compared and we conclude that some of the Fortin elements are most useful on the grounds of computational overhead and solution accuracy. These have been used to investigate some symmetrical (square dies) and asymmetrical (unequal lip) planar and general L‐shaped die flows for Newtonian fluids. The question of predicting regions of plug flow is addressed for materials with a yield stress in the three‐dimensional case. Finally, we show that an unconstrained extrudate may, in absence of gravity, describe a helix in space as the most general result.
A flexible simulation code for microwave curing of polymers
Authors: Acierno D. , Masi P. , Pierro V. , Pinto I. M. 
Published: March 2011

AbstractWe describe a modular simulation code for microwave curing of polymers. The exposed item geometry can be either 3D or 2D (extruded draw pieces), the item can be multi‐component and contain metal insertions as well. The applied microwave field can be either resonant or traveling wave, with or without mode‐stirring. The output of the code is in the form of cross‐linking degree and temperature maps. Utilities are included to input the problem's geometry and to display the outputs.
Prediction of rheological properties in polymer colloids
Authors: Mewis J. , Haene P. D' 
Published: March 2011

AbstractThe rheological properties of concentrated suspensions, containing sterically stabilized polymer colloids, are reviewed. It is shown that the various rheological characteristics are strongly interrelated in such systems. They deviate from Brownian hard sphere behaviour, but these deviations are closely related to the interparticle potential and other colloidal characteristics like particle diffusivity. As a result the rheological properties can be predicted from the colloidal characteristics and vice versa. This is demonstrated by means of available data on well characterized model dispersions. In the experiments volume fraction, particle size, suspending medium and temperature have been changed. Data reduction schemes and scaling laws for these parameters are discussed.
Crystallization kinetics in relation to polymer processing
Authors: Brucato V. , Piccarolo S. , Titomanlio G. 
Published: March 2011

AbstractPhase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non‐isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.
Morphology‐rheology relationships in incompatible blends based on thermotropic liquid crystalline polymers
Authors: Incarnato L. , Nobile M. R. , Acierno D. 
Published: March 2011

AbstractIn this paper the effect of the inclusion of two different thermotropic liquid crystal polymers (TLCP) on the processing of a polyetherimide (ULTEM 1000) is studied.
Chiral liquid crystalline polymers: Recent issues and perspectives
Authors: Chiellini Emo , Galli Giancarlo , Cioni Fabio , Dossi Eleftheria 
Published: March 2011

AbstractThe syntheses and structures of chiral liquid crystalline polymers (LCPs) are briefly reviewed with emphasis on the established structure‐property correlations. The most recent advances and future trends in their potential applications in optics and electrooptics are also outlined, with particular reference to the ferroelectric, electroclinic, piezoelectric, and nonlinear optical properties of chiral LCPs.
Orientation of crystalline polymers by non‐classical means
Authors: Lotz Bernard , Wittmann Jean‐Claude 
Published: March 2011

AbstractCrystalline polymers can be oriented by epitaxy on various substrates, including low molecular weight, usually aromatic compounds and polymers oriented by epitaxy and by mechanical means (strain induced polyolefin films and friction‐deposited PTFE layers). The resulting oriented films are thin, but display unusual morphologies with lamellae standing edge‐on and, often, single crystal structure. They are adequate investigation materials for structure determination, show enhanced or maximized orientation‐dependent properties, and may contribute to mechanical strengthening in composite polymer films.
Molecular modelling of melt rheology
Authors: Marrucci G. 
Published: March 2011

AbstractThe main assumptions and concepts used in molecular modelling of melt rheology are briefly reviewed. Because rheological phenomena take place over time scales which correspond to motions of large segments of the chains, models need only account for the long‐range structural aspects of the chain, while the local details of the chemical structure get lumped into a few adjustable parameters. The structural aspects which must be accounted for in a direct way are discussed, and a concise overview of the corresponding modelling is made.
Kinetics of network formation via free‐radical mechanisms — Polymerization and polymer modification
Authors: Zhu S. , Hamielec A.E. 
Published: March 2011

AbstractThe study of polymeric networks synthesized via free‐radical polymerization and/or prepolymer modification have become an attractive research area of both fundamental and commercial interest. Recently, a research/development program was initiated to investigate the fundamentals and applications of polymer networks. The following aspects are being emphasized: the elucidation of crosslinking mechanisms and kinetics, the characterization of network microstructures, and the development of kinetic gelation models. This paper reports recent activities towards these goals.
Linear‐temperature programmed pyrolysis of thermoresistant polymers ‐ mass and FTIR spectrometries I. Experimental; aromatic polyethers and polyetherketones
Authors: Hummel Dieter O. , Neuhoff Ulrich , Bretz Anke , Düssel Heinz‐Jürgen 
Published: March 2011

AbstractDuring thermal decomposition, most of the thermo‐resistant polymers leave a considerable fraction of the original mass behind, consisting mainly of a carbonaceous material. The gases evolved are, to the largest part, low‐molecular (H2O, CO2, CO, NH3, HCN, hydrocarbons); they are formed during polycondensation processes. The rate of heating strongly influences the degradation mechanism and thus the composition of the gaseous pyrolyzate. Rapid heating i.v. to the maximum temperature chosen frequently favors the formation of heavy fragments. These can be identified by field‐ionization or low‐voltage mass spectrometry; they usually allow conclusions on the composition of the original polymer.Slow, temperature‐controlled heating allows rearrangement and polycondensation reactions in the polymer. A study of the processes going on in the residue during temperature‐controlled heating is important with regard to the behavior of thermoresistant polymers under real conditions.An elegant method of continuous residue analysis is FTIR spectrometry. A heatable IR cell designed in our institute allows the study of both gaseous pyrolyzate and solid residue in separate experiments. The time needed for the measurement of one spectrum (from 20 scans) was 36 s. This, with a heating rate of 5 K/min, corresponds to a temperature resolution of 3 K/spectrum.Results are given for aromatic polyethers and polyetherketones.
Submolecular dynamics of amorphous polymers probed by two‐dimensional infrared (2D IR) spectroscopy
Authors: Noda Isao , Dowrey A. E. , Marcott C. 
Published: March 2011

AbstractTwo‐dimensional infrared (2D IR) spectroscopy was used to probe the submolecular dynamics of atactic polystyrene. 2D IR is a powerful analytical technique especially suited for the elucidation of localized motions of polymer segments. In 2D IR, a polymer sample is excited by a small‐amplitude oscillatory strain at a frequency in the acoustic range. The fluctuation of IR dichroism signals resulting from the strain‐induced reorientation of electric dipole‐transition moments is monitored with a time‐resolved spectrometer. Spectra defined by two independent wavenumber axes are constructed by applying a correlation analysis to such signals. The 2D spectra provide detailed information about the local dynamics of submolecular constituents of the system. From the sign of cross peaks in the synchronous 2D IR spectrum of glassy polystyrene, it is shown that the main‐chain backbone of polystyrene reorients in the direction of applied strain. Cross peaks in an asynchronous 2D IR spectrum reveal highly localized reorientational motions of phenyl side groups occurring more or less independently of the main‐chain realignment. In the glassy state, the phenyl ring tends to fold back along the main‐chain, indicating that there exists a highly constrained local distortion of side groups during deformation.
Micro‐Raman study of the transition front in uniaxially stretched semicrystalline polymers
Authors: Pastor J. M. , Jawhari T. , Merino J. C. , Fraile J. 
Published: March 2011

AbstractThe transition front of the neck between the isotropic and oriented region in uniaxially stretched polyvinylidene fluoride (PVDF) and polypropylene (PP) is analyzed with a high spatial resolution by micro‐Raman spectroscopy. The variation of the microstructure, i.e. change in the degree of crystalline modification in the case of PVDF and in the orientation of the PP chains in function of the strain rate, is correlated with other parameters associated with the drawing response of these polymers and the temperature rise during deformation. The results here described can be understood through the clear increase of temperature that was detected in the neck due to the heat generated by the deformation work of the drawing process.
NMR spectroscopy and relaxation studies on liquid‐crystalline side‐chain polysiloxanes
Authors: Hempel Günter , Wobst Matthias , Israel Gunter , Schneider Horst 
Published: March 2011

AbstractTwo polysiloxanes with mesogenic side chains were investigated with polarization microscopy, DSC, 1H and 13C NMR. From proton NMR the global order parameter of one sample (the other does not orient itself in the magnetic field of 2.1 T) and some information about phase transitions was obtained. 13C NMR yielded selective order parameters with respect to different atomic positions. The order parameter varies slightly also within the mesogenic unit, but much stronger within the spacer (odd‐even effect). With the help of T1ρ experiments some molecular motions had been detected and characterized (main‐chain motion, over‐head rotation of the mesogenic unit, methyl rotation, side‐chain rotation).
Rheo‐optical fourier‐transform infrared spectroscopy of polymers 16 orientation and strain‐induced conformational regularity of poly(dimethylsiloxane) networks during cyclic elongation and recovery
Authors: Besbes S. , Bokobza L. , Siesler H. W. 
Published: March 2011

AbstractTwo SiO2‐filled poly(dimethylsiloxane) networks with different molar masses Mc (22000 g/mol and 33000 g/mol) between the junctions have been investigated by simultaneous mechanical and Fourier‐transform infrared (FTIR) spectroscopic (rheo‐optical) measurements to monitor the structural changes in cyclic elongation‐recovery procedures. Higher polymer chain orientation was observed at corresponding elongations for the low‐Mc network, but due to the higher extensibility appreciable strain‐induced conformational regularity occurred only in the high‐Mc material. Differences with reference to unfilled model networks are pointed out.
Ring‐opening polymerization and copolymerization of cyclic carbonates with a variety of initiating systems
Authors: Höcker H. , Keul H. , Kühling S. , Hovestadt W. , Müller A. , Wurm B. 
Published: March 2011

AbstractThe ring‐opening polymerization of cyclic monomers is a field of long tradition. More recently, however, under the pressure to recycle polymers the generally undesired capability of ring‐closing depolymerization has gained a new aspect. As an example, the anionic ring‐opening polymerization of six‐membered cyclic carbonates is discussed, the alcoholate being the active species. The latter is also able to initiate the ring‐opening polymerization of lactones such as ϵ‐caprolactone and pivalolactone. On the other hand, monofunctional and bifunctional alcoholates of polytetrahydrofuran can be used as initiators for the anionic ring‐opening polymerization of cyclic carbonates to produce AB‐ and ABA‐block copolymers. The use of other initiators than alcoholates on the basis of Al, Zn, and Sn is discussed for both the polymerization and the depolymerization reaction.