bluuebunny/crossref_metadata_2025_split huggingface dataset:
Block and graft copolymers of 2‐oxazolines
Authors: Schulz Rolf Christian. , Dworak Andrzej 
Published: March 2011

AbstractThe cationic ring‐opening polymerization of 2‐oxazolines is known to be initiated by alkyl halides, Lewis acids and esters of strong acids. The polymerization proceeds by a living mechanism. Numerous block and graft copolymers have already been described. Recently it was found that chloroformates (R‐O‐CO‐CI) are also useful initiators. The mechanism of the initiation and propagation is discussed. This type of initiators allows the synthesis of different novel two‐block and three‐block copolymers, star‐shape polymers, and a graft copolymer with a polyvinylacetate backbone.
Synthetic strategies to well‐defined polymers by ionic polymerization
Authors: Faust Rudolf 
Published: March 2011

AbstractThe living cationic polymerization and sequential copolymerization of isobutylene and styrene has been achieved. Polymethylmethacrylate‐g‐polyisobutylene graft copolymers have been prepared by the combination of living cationic and group transfer polymerization.
Kinetics and mechanism of group transfer polymerization of N‐butyl acrylate catalyzed by Hgl2/(CH3)3Sil in toluene
Authors: Zhuang Rugang , H. E. Müller Axel 
Published: March 2011

AbstractThe group transfer polymerization (GTP) of n‐butyl acrylate (nBuA) using 1‐methoxy‐1‐(trimethylsiloxy)‐2‐methyl‐1‐propene (MTS) as an initiator, mercuric iodide (HgI2) as a catalyst in toluene at room temperature gives a very good control of molecular weight and narrow molecular weight distribution. (M̄w/M̄n < 1.2). Kinetic studies in this system reveal that this reaction is rather slow, half‐lives being in the range of hours. The kinetic order of the apparent rate constant of propagation with respect to initiator and catalyst concentrations were found to be near to unity. However, the first‐order time‐conversion plots exhibit considerable induction periods. Upon addition of trimethylsilyl iodide (TMSI) the induction periods vanish and the rates are increased by two orders of magnitude. The reaction is first‐order with respect to TMSI indicating that it takes part in the reaction by activating the catalyst. The complex from HgI2 and TMSI acts as a nucleophilic catalyst, similar to oxyanions in the GTP of MMA in THF. It is proposed that the induction periods are due to the formation of TMSI from MTS and HgI2.
Sequence analysis and kinetic study of the polymerization of 1,4‐bis(hydroxydimethylsilyl)benzene with sir2Cl2
Authors: Newmark Richard A. , Babu Gaddam N. 
Published: March 2011

Abstract29Si NMR rate studies of the bulk reaction of 1,4‐bis(hydroxydimethylsilyl)benzene (BHB) with dichlorodimethylsilane, dichloromethylphenylsilane, and dichlorodiphenylsilane show a very rapid kinetic equilibrium giving aryl‐SiMe2Cl groups in place of the aryl‐SiMe2OH of the BHB and OH groups in place of Cl in the dichlorosilane. This is followed by a slow condensation polymerization of the 1,4‐bis(chlorodimethylsilyl)benzene with the hydroxy terminated disilane groups. The very rapid initial rates could be followed with 29Si NMR using the INEPT pulse sequence despite the very long 29Si relaxation times. Condensation polymerization is complete within minutes for BHB + SiMe2Cl2, but hours and days are required for the SiMePhCl2 and SiPh2Cl2, respectively, at +22°C. Initial reaction kinetics in the presence of Cr(acac)3 are not identical to those obtained using the INEPT pulse sequence and no relaxation reagent.
Chemically resolved solid state NMR images of polymers
Authors: Cory D. G. , Miller J. B. , Garroway A. N. 
Published: March 2011

AbstractPolymers are generally spatially heterogeneous, either in terms of morphology or as blends of various components. Solid state NMR imaging provides a means of characterizing both the chemical/morphological composition and its spatial variation. Here we discuss multiple‐pulse line‐narrowing approaches to acquiring high resolution NMR images and how these sequences can be modified so as to be sensitive to the chemical composition of a sample.
NMR imaging of elastomeric polymers
Authors: Koenig Jack L. 
Published: March 2011

AbstractWe have used NMR imaging to determine the dispersion in crosslinking density in complex elastomers both along the chains as well as spatially. Our work has established that the ordinary industrial accelerator systems generate a broad distribution of crosslink densities in rubber articles due to the poor mixing of the solid accelerator/sulfur recipe in the raw rubber. This high inhomogeneity in elastomeric network has an adverse effect on the properties of the elastomer system. In particular, the variation in crosslink density leads to substantial differences in the coefficients of thermal expansion and high internal stresses in the rubber article.
Peels of buckyballs
Authors: Löffler Matthias , Schlicke Benedikt , Schirmer Heike , Schlüter Arnulf‐Dieter 
Published: March 2011

AbstractThe polymer analogous aromatization of double‐stranded Diels‐Alder polymers to give fully unsaturated, all‐carbon ladder polymers by means of dehydration and dehydrogenation is described. The polymers obtained were characterized by solid state 13C NMR spectroscopy.
Speed control for cyclodextrin rings on polymer chains
Authors: Wenz Gerhard , Keller Bruno 
Published: March 2011

AbstractThe inclusion of monomeric α,ω‐diamines and related polymers by cyclodextrins was investigated by 1H NMR spectroscopy. As a result we found, that for both monomers and polymers the rate of the inclusion process largely depends on the degree of protonation of the amino groups of the guest.
Supramolecular formation of triblock copolymers in polar/nonpolar solvents
Authors: Chu Benjamin , Wu Guangwei 
Published: March 2011

AbstractTriblock copolymers could form supramolecules in either polar or nonpolar solvents at appropriate concentration and temperature ranges or in the presence of additives. The association properties and the structure of supramolecules of PEO‐PPO‐PEO and PPO‐PEO‐PPO (PEO and PPO refer to poly(oxyethylene) and poly(oxypropylene), respectively) triblock copolymers in xylene and/or water were investigated by using light scattering, small‐angle neutron scattering, and small‐angle X‐ray scattering. The association process of aqueous solution or water‐rich ternary systems was entropy driven and temperature played an important role. The additive, e.g., water in the oil‐rich ternary system, played a very important role on the micellization of PEO‐PPO‐PEO, e.g., Pluronic L64, in xylene. The micelles had a core‐shell structure and the micellar shell was rather heavily solvated. At high copolymer concentrations, large aggregates with a lamellar structure was formed and the amount of large aggregates increased with increasing copolymer concentration before gel formation.
Comparison of the supramolecular structures formed by a polymethacrylate with a highly tapered side chain and its monomeric precursor
Authors: Kwon Yong Ku , Danko Craig , Chvalun Sergei , Blackwell John , Heck James A. , Percec Virgil 
Published: March 2011

AbstractInsight into the supramolecular structure formed by a polymethacrylate with a highly tapered side chain is obtained from parallel X‐ray analysis of oriented fibers of the polymer and its monomeric precursor. The polymer is poly(2‐{2‐[2‐(2‐methacryloyloxyethoxy)ethoxy]ethoxy}ethyl 3,4,5‐tris(p‐dodecyloxybenzyloxy)benzoate) (abbreviated to 12‐ABG‐4EO‐PMA); the monomeric precursor is the hydroxy‐terminated side chain 2‐{2‐[2‐(2‐hydroxyethoxy)‐ethoxy]ethoxy}ethyl 3,4,5‐tris(p‐dodecyloxybenzyloxy)benzoate (12‐ABG‐4EO‐OH). The polymer and precursor both form ordered solid state structures that are converted to columnar hexagonal liquid crystalline (φh) phases at approximately 40°C and 50°C, respectively. The ordered solid state structures consist of ordered hexagonally packed cylindrical columns, in which the monomer units are probably packed with helical symmetry. For the polymer at 25°C, the column diameter is 60.4Å with an axial repeat of 5.03Å containing eight monomer units. For the precursor at 25°C, the column diameter is reduced to 53.5Å, probably due to the absence of the polymer backbone from the center of the column, and the axial repeat is doubled to 10.04Å. The X‐ray data are compatible with a tighter winding of the monomers in a helical structure, but otherwise suggest that there are common features in the stacking of the aromatic groups in the two structures.
Rheology of textured rodlike polymeric systems under shear flow as revealed by brownian dynamics simulation
Authors: Yang Yuliang , Ding Jiandong 
Published: March 2011

AbstractA Brownian dynamics simulation algorithm has been developed in this paper to study the rheology of rodlike polymers under shear flow. Through the introduction of the Lebwohl‐Lasher potential and consideration of the nearest‐neighbor interaction without mean‐field approximation, the spatial inhomogeneity of the director field, stresses as well as the multiple‐domain effect are automatically included. By this new algorithm, the transient rheological behaviors of textured and untextured (uniformly aligned) rodlike polymeric systems are investigated. The damped oscillations of stresses are reproduced and the negative first normal stress differences are found at the moderate shear rate, all of which agree with the experimental observations in literature very well. Our simulation also demonstrates that the textured initial states only affect the transient responses of stress etc. at early shear stage and that the multiple domains formed during shear flow started from both textured and untextured initial states are essentially the same. The multiple domains are, therefore, the inherent structures resulting from the instability of shear flow, which are not sensitive to the initial textures, and the so‐called multiple domains are conceptually different from the rest textured structures.
Raman spectroscopy of poly[BIS(P‐toluene sulfonate) of 2,4‐Hexadiyne‐1,6‐DIOL] crystals at high pressure
Authors: Webster Simon , Batchelder David N. 
Published: March 2011

AbstractThe resonance Raman spectrum of crystalline poly[bis(p‐toluene sulfonate) of 2,4‐hexadiyne‐1,6‐diol] has been studied as a function of applied pressure to 50 kbar. The Raman bands are observed to shift to higher frequency and change in relative intensity. The origins of these effects are discussed. One of the Raman bands is observed to split into two bands of approximately equal intensity at high pressure. This effect is attributed to correlation field splitting within the unit cell and is possibly the first such observation in a conjugated polymer.
The impact of the discovery of stereoregular polymers in macromolecular science
Authors: Corradini Paolo 
Published: March 2011

AbstractThe paper recalls some aspects of the fascinating history of the discovery by Giulio Natta and his coworkers of the stereospecific polymerization of α‐olefins and of the determination of the structure of synthetic stereoregular polymers.The most relevant recent advances in the comprehension of the mechanisms regulating their synthesis with heterogeneous supported catalysts and with the newly discovered homogeneous metallocene‐based catalysts are briefly outlined.
Stereoregularity and regioregularity of active centers in propene polymerization
Authors: Kashiwa Norio , Kojoh Shin‐Ichi 
Published: March 2011

AbstractThe relation between stereoregularity of active centers on a MgCl2/TiCl4 catalyst and functions of inside donor (ID) and outside donor (OD) was investigated in the case of ethyl benzoate (EB)/methyl p‐toluate (MPT) as an ID/OD pair. The results indicate that stereregularity depends merely on the amount of MPT supported on the catalyst.On the other hand, regioregularity of active centers was investigated with a MgCl2/TiCl4/dioctyl phthalate(DOP)‐Et3Al/diphenyldimethoxysilane(DPDMS) catalyst system. Regio‐irregular inserted units were detected only in end groups of PP. It indicates that regio‐irregular insertion leads to dormant centers with respect to propene insertion, though such centers are active for hydrogen transfer.
A mechanistic approach to MgCl2 supported stereospecific propene polymerization: A new model of active sites
Authors: Guyot A. , Spitz R. , Journaud C. , Eisenstein O. 
Published: March 2011

AbstractA precise description of the active site for heterogeneous stereospecific catalysts for propene polymerization has not been achieved, taking into account the effect of all the components of the catalytic system. A new model is proposed based on the following facts: the active Ti is directly bonded to the support MgCl2; it is bridged with an Al atom of the alkylaluminium; the latter is complexed with the external Lewis base; the more efficient Lewis bases are known to be dialkoxy dialkyl (or aryl) silane containing at least one bulky alkyl group; it is suggested that the two alkoxy groups are complexed, one with the aluminium atom, the second with the titanium atom. The structure of such a site has been built using a Chem 3D software and starting from an octahedral Ti and a bipyramidal Al. Both are bridged with the polymer chain. The silane is located so that each oxygen atom is just above each of the two metal atoms; the bulky alkyl group covers the vacancy of the Ti octahedre being in agostic interaction with the Ti. Such a tight geometry imposes the isospecific coordination of the monomers before its insertion in the polymer chain. The model is flexible enough to allow for the transition state, the insertion of the incoming monomer and the rotation of the new bridge, the driving force being the agostic interaction. The successful replacement of the silane with a diether with 3 carbons atoms in between the oxygen atoms is easily explained by the model, as well as the effect of the structure of the alkoxy and of the alkyl groups of the silane.
The Influence of the growing polymer chain on the catalytic complex in homogeneous ziegler‐natta polymerization: Density functional calculations on model systems TiCI2R+ and ZrCl2R+
Authors: Farina Mario , Fusco Roberto , Longo Luca 
Published: March 2011

AbstractThe influence of the growing chain on the electronic structure of soluble Ziegler‐Natta catalysts was investigated through quantum‐mechanical calculations for model systems MCI2R+ with M = Ti, Zr and R = CnH2n+1 where n = 1–7. The theoretical results show an increase in the energy of the Lowest Unoccupied Molecular Orbital (LUMO) of the cation and a decrease of the transition metal charge in the early stages of polymerization. This behaviour could explain the experimentally observed initial acceleration of the monomer insertion rate as reported in literature for ethylene polymerization with TiCp2CH3Cl·AlCl2CH3 system.
New cationic group 4 metal alkyl complexes
Authors: Tjaden Erik B. , Jordan Richard F. 
Published: March 2011

AbstractCationic group 4 metal alkyl complexes containing tetradentate Schiff base ligands, (acen) Zr(R)+ and (F6‐acen) Zr(R)+, are prepared by protonolysis of suitable neutral dialkyl precursors. These complexes display electrophilic behavior and are moderately active ethylene polymerization catalysts in the presence of 1 molar equivalent of AlR3.
New polyolefin elastomers from metallocenes
Authors: Galimberti Maurizio , Martini Emilio , Piemontesi Fabrizio , Sartori Franco , Camurati Isabella , Resconi Luigi , Albizzati Enrico 
Published: March 2011

AbstractIn this paper, new polyolefin elastomers, obtained from metallocene based catalytic systems, are presented. The potentialities of metallocenes for the preparation of polyolefin elastomers are discussed with reference to the traditional vanadium and titanium based catalysts. The role played by a Ziegler ‐ Natta catalyst, either stereospecific or aspecific, is discussed.
Metallocene ion pairs: A direct insight into the reaction equilibria and polymerization by 13C NMR spectroscopy
Authors: Tritto Incoronata , Sacchi Maria Carmela , Locatelli Paolo , Li San Xi 
Published: March 2011

AbstractThe reaction equilibria of Cp2Ti13CH3Cl and Cp2Ti(CH3)2 with AlMe3 (TMA) and/or methylaluminoxane (MAO) have been investigated by 13C NMR. Several adducts have been identified. A study of the 13C 90% enriched ethylene polymerization in an NMR tube in the presence of the above catalytic systems, in the most experimentally significant conditions, and a comparison of the NMR data with the catalytic activity have been made as well. It has been shown that: i) some species are side products, inactive for addition ethylene polymerization; ii) active cation‐like species such as Cp2TiMe+Cl·[AlMeO]n‐ and Cp2TiMe+Me·[AlMeO]n‐ are formed in titanocene‐MAO systems. Concerning the role of AlMe3, contained in MAO solutions, it has been shown that: a) AlMe3 is mainly bound to MAO; b) if some “free” AlMe3 exists in solution it is not the actual cocatalyst in the metallocene‐MAO based catalytic systems; c) the amount of AlMe3 influences either active or inactive species.
Cationic zirconium benzyl complexes as catalysts for olefin polymerization: A comparison among dicyclopentadienyl, monocyclopentadienyl and Cp‐free derivatives
Authors: Pellecchia Claudio , Pappalardo Daniela , van beek§ Johannes A. M. 
Published: March 2011

AbstractCationic zirconium complexes of formula [Cp2ZrBz]+, [Cp'ZrBz2]+, or [ZrBz3]+ (Cp = η5‐C5H5, Cp' = Cp or η5‐C5Me5, Bz = CH2C6H5) have been tested as catalysts for the polymerization of ethylene and α‐olefins. The catalyst performances have been compared with regard to polymer productivities, molecular weights and stereoregularities, and tentatively correlated to the structure of the ionic active species.
Mechanism of syndiotactic‐specific polymerization of styrene
Authors: Zambelli Adolfo , Pellecchia Claudio , Proto Antonio 
Published: March 2011

AbstractPerusal of literature data and some new results concerning syndiotactic‐specific polymerization of styrene suggest a reaction mechanism accounting for the steric control. Key features of the proposed mechanism are stereorigid ηn coordination of the growing chain end and diastereoselective coordination of the monomer imposed by direct interactions with the ancillary ligand of the metal complex, a pseudotetrahedral chiral Ti(III) or Zr(III) cation, which inverts its configuration after every syndiospecific insertion step.
Influence of monomer structure on chemo‐ and stereoselectivity of 1,3‐diene polymerization
Authors: Ricci Giovanni , Porri Lido , Giarrusso Antonino 
Published: March 2011

AbstractMAO/CpTiCl3 is an active catalyst for the polymerization of various types of 1,3‐dienes. Butadiene, (E) ‐ and (Z) −1,3‐pentadiene, (E) −2‐methyl‐1,3‐pentadiene and 2,3‐dimethylbutadiene yield, at room temperature, polymers with a cis‐1,4 or a mixed cis/1,2 structure. 4‐Methyl‐1,3‐pentadiene and (E,E) −2,4‐hexadiene give, respectively, a 1,2 syndiotactic and a trans‐1,4/1,2 polymer. MAO/CpTiCl2·2THF and MAO/(CpTiCl2)n are less active than the CpTiCl3 catalyst, but give the same type of polymers. A change of stereospecificity with temperature was observed in the polymerization of (Z)‐1,3‐pentadiene: a cis‐1,4 isotactic polymer was obtained at +20°C, and a crystalline 1,2 syndiotactic polymer at −20°C. This effect was attributed to a different mode of coordination of the monomer, which is cis‐η4 at +20°C and may be trans‐η2 at −20°C. Results obtained with catalysts from CpTi(OBu)3 and Ti(OBu)4 are reported for comparison. An interpretation is given of the formation of cis‐1,4 isotactic poly(2‐methylpentadiene) and of 1,2 syndiotactic poly(4‐methylpentadiene), as well as of syndiotactic polystyrene.
Copolymerization kinetics of ethylene and carbon monoxide in the presence of palladium complexes
Authors: Gennadii P. Belov , Oleg N. Golodkov , Zinaida M. Dzhabieva 
Published: March 2011

AbstractThe copolymerization kinetics of ethylene and carbon monoxide has been studied for the catalysts: Pd(C5H7O2)2‐P(C6H5)3‐p‐CH3C6H4SO3H in the medium of acetic acid (system A), and Pd(CH3COO)2‐CF3COOH ‐ (C6H5)2P(CH2)nP(C6H5)2(system B), where n=1−6, in the medium of methyl alcohol. The cationic palladium(II) ‐diphosphine complexes: (Dppp)2Pd(CF3COO)2 (I) and [(Dppp) Pd(μ‐OH)2Pd(Dppp)] (CF3COO)2 (II) have been synthesized and investigated by IR, electrospray mass spectrometry and elemental analysis methods. A comparative study of the copolymerization kinetics in the presence of these complexes and system B has been carried out.
Catalysis and the unipol® process in the 1990s
Authors: Karol Frederick J. 
Published: March 2011

AbstractDevelopments in the UNIPOL® process for polyethylene, FLEXOMERS®, EPR/EPDM, and polypropylene continue to demonstrate the broad versatility and scope of the process. The potential of catalyst technology to provide dramatic new opportunities in the gas phase remains high. Selection of the appropriate catalyst with control of ligand environment at the active site will provide the basis for improved process operations and unique product opportunities.
Statics and dynamics of dense polymer systems studied by monte carlo simulation
Authors: Binder Kurt 
Published: March 2011

AbstractMonte Carlo simulations of coarse–grained models of macromolecules offer a unique tool to study the interplay between coil conformations, thermodynamic properties, and chain configurational relaxation and diffusion.Two examples are discussed where the chain conformation strongly differs from a gaussian coil: (i) collapsed chains in a bad solvent, where anomalous diffusion occurs in the Rouse limit and the relaxation time increases at least with the third power of chain length. (ii) Expulsion of a chain from a semidilute polymer brush. The initially stretched chain contracts to a gaussian coil and the center of mass moves outward with constant velocity until it reaches the region of the “last blob” where crossover to diffusive behavior occurs.
The dynamics of polymer melts as seen by neutron spin echo spectroscopy
Authors: Ewen Bernd , Richter Dieter 
Published: March 2011

AbstractUsing the neutron spin echo spectroscopy, the internal segmental diffusion of chain molecules in polymer melts and concentrated solutions was studied. These investigations show that beyond a characteristic length dt and after a cross over time τe(dt) the segmental diffusion of the single chains is strongly impeded and deviates from the Rouse dynamics. dt is polymer specific and depends on the temperature as well as on the polymer concentration.Within the framework of the reptation concept, where dt is identified with the mean distance between intermolecular entanglements or with the tube diameter, the microscopically determined dt‐values agree quite well with those derived from related macroscopic measurements of the plateau modulus. A similar good agreement is also found with respect to the segmental friction coefficients obtained either from the Rouse regime of the NSE spectra or from Theological data of corresponding short chain systems, where entanglements are not yet effective.