bluuebunny/crossref_metadata_2025_split huggingface dataset:
A few considerations on some catalysts for olefin polymerization
Authors: Luciani Luciano , Milani Federico , Zannetti Roberto , Di Noto Vito 
Published: March 2011

AbstractAmong many precursors and catalysts for alpha‐olefins polymerization, one seems to be particularly interesting, because it has not been completely clarified yet.We refer especially to precursors obtained via reaction between Mg‐alkyls and SiCl4.The products of this reaction are not well known; in fact, under some operating conditions, a special form of MgCl2 is obtained, showing x‐ray diffraction peaks in the angular region lower than 15° (2 theta), which corresponds to the 5.9 A interplanar spacing, characteristic of alpha‐MgCl2.Under other conditions, MgCl2 is obtained in the well known and strongly disordered delta structure.By employing these precursors, some catalytic systems for alpha‐olefins polymerization have been prepared.In this paper, the peculiar aspects of these precursors and catalysts are described, particularly focusing on the correlation between structure and performances in ethylene and propylene polymerization.
Ethylene polymerization kinetics with heterogeneous Ziegler‐Natta catalysts
Authors: Kissin Y. V. 
Published: March 2011

AbstractEthylene polymerization kinetics was examined using a supported Ti‐based catalyst activated by AlEt3. A computer‐based technique for deconvoluting the GPC curves into constituent Flory components was developed. The kinetic analysis showed that the catalyst contains five populations of active centers. Each population has different formation and deactivation rates, produces polymers of different molecular weights and is differently affected by hydrogen.
Structure‐activity and structure‐selectivity correlations in metallocene‐based catalysts for α‐olefin polymerization
Authors: Burger Peter , Hortmann Kai , Brintzinger Hans‐Herbert 
Published: March 2011

AbstractZirconocene complexes carrying substituents at various positions of their ring‐bridged ligand frameworks were studied as catalysts for methylalumoxane‐activated propene polymerization. Effects of different substitution patterns on catalyst activities and on chain lengths and stereo‐ and regio‐regularities of the polymers produced are correlated with geometrical features of the complexes, e.g. with parameters describing their structural rigidity or the aperture and obliquity of their coordination gaps.
Hemiisotactic poly(propylene) through propene polymerization with the ipr[3‐mecpflu]zrcl2/mao catalyst system: A kinetic and microstructural analysis
Authors: Herfert Norbert , Fink Gerhard 
Published: March 2011

AbstractThrough polymerizing propene by means of the catalyst system iPr[3‐MeCpFlu]ZrCl2/MAO it was possible for the first time to generate hemiisotactic poly(propylene); the microstructure of this polymer is characterized by an alternating sequence of monomer units with equal and statistical configuration. iPr[3‐MeCpFlu]ZrCl2 is one of the new developed stereorigid ansa‐metallocene catalysts which by means of the structure of the bridged η‐ligand system are considered to give access to the description and calculation of the behaviour of the stereospecifity. In the present paper the α‐olefin polymerization was investigated with the iPr[3‐MeCpFlu]ZrCl2/MAO catalyst system by means of kinetics and product analytics; parameter were the [Al]:[Zr] ratio, the monomer concentration, the polymerization temperature, the polarity of the solvent as well as the chain length of the starting α‐olefin. The order of the polymerization reaction concerning the propene concentration was found to be 1,3; this broken order indicates a complex reaction. Concerning the dependency of the polymerization rate on the [Al]:[Zr] ratio a bell‐shaped curve was found; obviously, at very high [Al]:[Zr] ratios the complex MAO compound inhibits the active zirconocene centre. The microstructure of the poly(propylene)s generated in toluene at 25°C corresponds with the expected hemiisotactic structure. On the contrary, the microstructure of the poly(propylene)s generated in toluene at higher polymerization temperatures and in the polar solvents o‐dichlorobenzene and methylene chloride at 25°C shows considerable deviations from a hemiisotactic structure. Under these polymerization conditions the stereospecifity of the iPr[3‐MeCpFlu]ZrCl2/MAO catalyst can only be satisfyingly explained with isomerization reactions of the active zirconocene species between two insertion steps. These isomerization reactions without monomer coordination occur preferably starting from the unstereospecific state of the catalyst. This interpretation is confirmed by 1‐butene and 1‐hexene polymerization experiments.
The role of non‐coordinating anions in homogeneous olefin polymerization
Authors: Siedle A. R. , Lamanna W. M. , Newmark R. A. , Stevens J. , Richardson D. E. , Ryan M. 
Published: March 2011

AbstractWe present an overview of homogeneous olefin polymerization catalysts in which chain initiation and propagation involves a metallocenium ion (e.g. Cp2ZrMe+) that can be produced by equilibrium or irreversible CH3− transfer. The Kaminsky catalyst is an example of the former. Complexes of (C6F5)3B with water, alcohols, mercaptans, silanols and oximes are strong acids which can protonate dialkylmetallocenes to produce weakly‐coordinated metallocenium salts that are hydrocarbon soluble and which produce high molecular weight poly‐1‐olefins. Polymer molecular weight is correlated with the steric and electronic effects of the ligands attached to the metal.
A new mechanism for the transition metal catalyzed copolymerization of conjugated dienes with olefins: The double coordination mechanism
Authors: Robert P. , Spitz R. 
Published: March 2011

AbstractWe have focused our interest on the ethylene‐ butadiene copolymerizations catalyzed with Ziegler‐Natta type catalysts. The catalysts used are MgCl2 supported titanium catalysts. The main features of these copolymerizations (stereospecificity of the butadiene enchainments, multiblock structure, gel formation) cannot be rationalized on the basis of the Cossee mechanism. We propose a new model: The double coordination mechanism. It explains all the phenomena observed. Experimental confirmations will be presented in this paper.
Recent advances in the neodymium catalysed polymerisation of 1,3‐dienes
Authors: Wilson David J. 
Published: March 2011

AbstractThe benefits of using a homogeneous neodymium‐based catalyst for the industrial “high cis” polymerisation of 1,3‐butadiene are underlined.Preformed homogeneous catalysts for the “high cis” polymerisation of 1,3‐butadiene based on Nd(carboxylate)3/diisobutylaluminium hydride/tbutyl chloride have been examined. The effects of changing (a) the order of catalyst component addition, (b) the carboxylate component and (c) the halogen component, on catalyst homogeneity, activity and polymer characteristics have been examined.
Kinetics and mechanisms in anionic ring‐opening polymerization
Authors: Penczek Stanislaw , Duda Andrzej 
Published: March 2011

AbstractRecent advances in the anionic ring‐opening polymerization (AROP), including covalent (pseudoanionic) polymerization, are reviewed. Thermodynamics, kinetics, and mechanisms of AROP are discussed, covering mostly polymerization of oxiranes, lactones and cyclic siloxanes as monomers. The following general problems of AROP are discussed: anionic polymerizability, thermodynamics ‐ particularly of the monomers exhibiting low ring strain, chemistry of initiation, structures and reactivity of active species. New phenomena, particularly polymerization with reversibly aggregating species are analyzed in more detail. Chain transfer to polymer ‐ the major side reaction ‐ is analyzed quantitatively, by introducing the selectivity parameter β, expressed by the ratio kp/ktr. This parameter has been determined for the anionic and pseudoanionic polymerization of ϵ‐caprolactone.
Tailoring of polyacrylates with combined initiators
Authors: Vlček Petr , Lochmann Lubomíar 
Published: March 2011

AbstractThis article summarizes possibilities of tailoring polyacrylates and polymethacrylates prepared by anionic polymerization initiated with combined (mixed) initiators especially those containing alkali metal tert‐alkoxides as stabilizing additives. First part gives a short overview of side reactions in anionic polymerization of polar vinyl monomers and the ways of solving the problem. Second part is devoted to the anionic polymerization of acrylic and methacrylic esters initiated with mixed initiators. Examples of synthetic methods are given for the preparation of special polymeric products based on acrylates, and the necessity of using different polymerization conditions for individual acrylic esters is demonstrated with respect to branching of the ester alkyl group.
Highly stereospecific protonation of stereoregular living PMMA anions with several alcohols
Authors: Ute Koichi , Asada Takeshi , Miyatake Nobuo , Hatada Koichi 
Published: March 2011

AbstractLiving and highly isotactic poly(methyl methacrylate) (PMMA) anion (M̄n = 2.5 × 103) prepared with t‐C4H9MgBr as an initiator was protonated with phenol in toluene at −78°C. The reaction was stereospecific toward meso addition, and the meso/racemo ratio at the chain‐end of the resultant polymer was 89/11. Addition of 1,4‐dioxane to the living isotactic PMMA anion in toluene at −78°C remarkably reduced the viscosity of the system, and protonation of the PMMA anion with phenol in the presence of 1,4‐dioxane enhanced the meso‐specificity to 94%. On the other hand, the protonation reaction of the living syndiotactic PMMA anion (M̄n = 2.5 × 103), which was generated by t‐C4H9Li/(n‐C4H9)3Al in toluene at −93°C, with t‐butanol was found to be 97% racemo‐specific. These highly stereospecific protonation reactions of the stereoregular PMMA anions were in contrast to the protonation of the anions with methanol or benzyl alcohol which was almost non‐stereospecific.
Novel synthesis of high molecular weight polymers with extremely low polydispersity by the unique initiation properties of organo‐lanthanide complexes
Authors: Yasuda Hajime , Yamamoto Hitoshi , Takemoto Yasunori , Yamashita Masahiro , Yokota Kiyohiko , Miyake Shigenobu , Nakamura Akira 
Published: March 2011

AbstractOrganolanthanide(III) complexes such as |(C5Me55)2SmH|2 and (C5Me5)2SmMe(THF) were found to initiate the living polymerizations of methyl methacrylate (MMA) to give high molecular weight polymers (M n > 500 × 103) with extremely low polydispersity (M w/Mn = 1.04). The syndiotacticity increased up to 95.2% by lowering the temperature to −95°C. The molecular structure of the 1:2 adduct of (C5Me5)2SmH with MMA determined by X‐ray method indicates that this intermediate assumes the 8 membered ring conformation where the Sm atom is bound to MMA in an enolate form and the ester of penultimate MMA is coordinated to the metal. Based upon these results, an anionic coordination mechanism has been proposed for the present reaction. Organolanthanide(II) complexes also exhibit high activity and proceed the living polymerizations. Organolanthanide(III) complexes also initiate the living polymerizations of lactones such as ϵ‐caprolactone and δ‐valerolactone. The stoichiometric reactions indicate that real active species assumes the alkoxylanthanide(III) form.
Well‐defined polyisocyanates via organotitanium(IV) catalyzed living polymerizations of isocyanates
Authors: Patten Timothy E. , Novak Bruce M. 
Published: March 2011

AbstractThe development of living organotitanium(IV) catalyzed polymerizations is presented. A living polymerization of alkyl isocyanates was developed using TiCl3OCH2CF3, I, as a catalyst, through which large quantities of well‐defined (in terms of molecular weight and polydispersity) polyisocyanates are obtainable under ambient conditions. η5‐CpTiCl2OCH2CF3 (Cp = cyclopentadienyl), II, is also an excellent catalyst for the polymerization of isocyanates and, unlike I, will polymerize monomers containing Lewis basic functional groups in the side chain. Compound II was used to synthesize poly(allyl isocyanate) and poly(2‐isocyanatoethyl methacrylate). Both polymers were found to be soluble, in contrast to previous reports, and spectroscopic characterization showed that no crosslinking of the side chains had occurred.
Recent developments in the field of star‐shaped polymers
Authors: Rein David , Rempp Paul , Lutz Pierre Joseph 
Published: March 2011

AbstractThe interest for star‐shaped polymers arises from their unique structure and enhanced segment density. This paper will review the different methods of preparation for star‐shaped polymers. Two cases of special interest will be considered in greater detail: the core‐first method which gives access to end‐functionalized star‐shaped polymers and the three‐step method which leads to the so called hetero‐star copolymers with rather specific properties.
Near‐ and supercritical fluid solvents for living anionic polymerizations
Authors: Desimone J.M. , Maury E.E. , Lemert R.M. , Combes J. R. 
Published: March 2011

AbstractNear‐ and supercritical fluids are investigated as solvents for anionic polymerizations of 1,3‐dienes. Solvent strength (solubility parameter) of mixtures of light hydrocarbons containing cyclohexane is shown to be adjustable using solvatochromic probe methods. The results indicate that these mixtures have solubility parameter values that are lower than conventional liquid solvents and represent a regime of solvent strength that has remained unexplored for ionic polymerizations. The anionic initiator 3‐methyl‐1,1‐diphenyl‐pentyllithium (MDPPL) was shown to be soluble in supercritical ethane and that its ultraviolet/visible absorption maximum was sensitive to systematic density changes of the supercritical fluid induced by pressure profiling. These results were extended to the anionic polymerization of isoprene in butane/cyclohexane mixtures.
New consideration of the mechanism of chain transfer to monomer in anionic polymerization
Authors: Arest‐Yakubovich Alexander A. , Litvinenko Galina I. 
Published: March 2011

AbstractA new kinetic scheme for chain transfer to monomer in the anionic polymerization of hydrocarbon monomers is presented. The scheme agrees with generally accepted views on the chemical mechanism of carbanionic reactions better than the one used previously. It is suggested that the most probable path of the transfer reaction is the proton abstraction from the side group of the monomer, the terminal double bond of the monomer molecule remains unchanged, and therefore the intermediate species can participate in succeeding reactions as a macromonomer. The discrepancy between the predictions of the proposed scheme and of the previous one concerning the molar characteristics of polymers are discussed and the ways to establish the true mechanism of transfer in particular systems are suggested.
Principles and design of living cationic polymerization of vinyl monomers: The nature of the growing species based on in‐situ 1H NMR analysis
Authors: Sawamoto Mitsuo , Higashimura Toshinobu 
Published: March 2011

AbstractThis paper focuses on in‐situ 1H NMR analysis of model reactions that are directed to clarify the nature of the growing species in the following three classes of living cationic polymerization based on the stabilization of the growing carbocations with nucleophilic counteranions and added salts:(a) Vinyl ethers/HCl/SnCl4/added nBu4N+Y‐;(b) Vinyl ethers/HX/ZnCl2 (salt free);(c) Styrene/HX/SnCl4/added nBu4N+Y‐ (X and Y: halogen) Through the NMR analysis, system (a) provides evidence for the generation of carbocations from adducts [CH3CH(OR)Cl] of vinyl ethers and hydrogen chloride and also for the suppression of such ionic species by the added salts. For systems (b) and (c) the spectroscopic observation demonstrated rapid counteranion exchanges in the growing species, indicating that upon reacting with a monomer the living end assumes an ionic character.
A survey of kinetic and mechanistic similarities in living polymerization systems
Authors: Iván Béla 
Published: March 2011

AbstractIn the light of recent discoveries in the field of living polymerizations it seems inevitable to reconsider our views on these polymerization systems. This paper surveys the kinetic and mechanistic similarities in living polymerizations, and analyses and compares chain transfer dominated nonliving polymerizations and living systems to conclude on the nature of propagating species, shelflife and livingness. Some recently raised specific problems are also summarized and discussed. It has been found that most of the living polymerizations known to date, such as living anionic, cationic ring opening, group transfer, carbocationic, ring opening metathesis, Ziegler‐Natta, free radical and immortal polymerizations, exhibit the characteristics of quasiliving polymerization, i.e., an equilibrium exists between propagating (active) and inactive (dormant) species. On the basis of this finding and a comparison between mechanistic and kinetic models of quasiliving and ideal living polymerizations, it is suggested that the former is the general phenomenon, and ideal living polymerization is a subclass of quasiliving polymerizations.
Termination processes in group transfer polymerization
Authors: Brittain William J. , Dicker Ira B. 
Published: March 2011

AbstractTwo major termination processes in the GTP of methacrylates and acrylates have been studied. For methacrylates, intramolecular cyclization is the dominant, if not sole, termination reaction. Both cyclization and O/C‐silyl isomerization of the chain end are observed for acrylates. Oligomerization experiments were used to study the relative rates of cyclization and propagation and the influence of temperature, catalyst and dp. Under typical GTP conditions for methacrylates, cyclization is at least two orders of magnitude slower than monomer addition.
The effect of hydrostatic pressure on the flow properties of various polymers
Authors: Baker F. S. , Thomas M. 
Published: March 2011

AbstractA novel device to fit on the exit of the ACER capillary extrusion rheometer has been designed to allow the controlled application of hydrostatic pressure to the body of the fluid under test. The rheological properties of the fluid can be examined over a wide range, ambient to 200MPa. This paper will describe the equipment and the effect of hydrostatic pressure and temperature on the flow properties to the various polymer melts, PC, ABS, PP and HDPE.
The correlation between residual stresses and delamination behavior of polymer coatings
Authors: Farris RICHARD J. , Goldfarb JAY , Maden MICHELE A. 
Published: March 2011

AbstractThe determination of residual stresses in thin polymer coatings is important for the prediction of failure due to delamination and cracking. If the internal tensile stresses exceed the cohesive strength of the solid coating, cracking may occur which can result in coating ‐ substrate delamination. This is especially important in the coatings and electronics industries in which coating reliability is central to the performance of a product. There are three primary causes of residual stresses in coatings, 1.) Chemical reactions in which a condensation molecule is evolved and given off during processing, 2.) Evaporation of solvent or carrier solution during drying, and 3.) Mismatch in coefficients of thermal expansion between the substate and the coating.
Impregnation, flow and deformation during processing of advanced thermoplastic composites
Authors: Savadori A. , Cutolo D. 
Published: March 2011

AbstractThermoplastic matrix composites are of increasing technical interest. This new generation of materials is not only comparable to conventional thermosetting systems in terms of performance and durability, but also affords considerable incentive in weight reduction and cost‐effective manufacturing. A careful description of the different intermediate material forms is presented in this paper along with their advantages and inherent drawbacks in processing and the manufacturing steps which follow. The micro‐mechanics of flow and deformation which, however, have to occur are partly different from that known from thermosetting composites and metallic materials. Therefore, the flow and micro‐deformation mechanisms, such as resin percolation, transverse flow and inter as well as intra‐ply slip will be discussed in detail along with their importance to get optimum consolidation of the final components.
Low band‐gap oligomers and polymers
Authors: Müllen Klaus , Scherf Ullrich 
Published: March 2011

AbstractThe relationship between electronic structure and chemical stability is discussed for three different classes of low band‐gap oligomers and polymers. In oligo‐ and poly[n]acenes as well as in poly‐(arylenemethide)s, the low band‐gap character is connected with a high chemical reactivity due to an energetically favored rearomatization of olefinic or quinonoid substructures, respectively. For double‐stranded oligo‐ and poly(rylene)s, the stability is dramatically increased. Therefore, rylenes appear especially promising in the field of low band‐gap materials.
Liquid‐crystalline polymers for fibres
Authors: McIntyre J.E. 
Published: March 2011

AbstractRecent advances in the development of liquid‐crystalline polymers for fibres are reviewed. The discussion of lyotropic systems concentrates upon poly(p‐phenylene terephthalamide) and its copolymer with 3,4′‐oxydiphenylene terephalamide units. The discussion of thermotropic systems identifies specific problems restricting the development of fibres and suggests some possible means of improvement.
Solid state NMR of a polymer composite and of a polymer blend
Authors: Veeman W.S. 
Published: March 2011

AbstractAlthough nuclear magnetic resonance may not seem the technique of choice to study interfaces between components in a polymer composite or polymer blend because of its inherent low sensitivity, for certain systems solid state NMR techniques can emphasize the signals from these interfaces. In case of a composite material with an inorganic filler (particles, fibers) cross‐polarization or dipolar dephasing techniques from protons in the organic matrix or in the interphase to nuclei in the filler can be used to selectively observe the nuclei in the surface of the filler. Any changes at the filler surface caused by the presence of the matrix or coupling agent can then be detected. An example of glass reinforced nylon modelcomposites is discussed.The same techniques can also be used to study interphases in polymer blends when one of the components contains a NMR nucleus that is not present in the other component. As an example the blend poly(vinylidene fluoride)‐poly(methyl methacrylate) is studied and it is shown that such techniques can provide very detailed information about the miscibility at a molecular scale.
Electron microscopy and quantitative polymer morphology
Authors: Bassett D. C. 
Published: March 2011

AbstractThe lamellar nature of semi‐crystalline polymers can now be studied systematically using transmission electron microscopy, especially following permanganic etching. In combination with thermal analysis and selective extraction followed by molecular characterisation, this approach allows the distribution of lamellae, thermal stability and molecular constitution to be mapped in real space. Extension of this methodology has allowed the extraction of individual melt‐crystallized lamellae from bulk polymer. Their study is adding to understanding of fundamental processes of crystal growth.