bluuebunny/crossref_metadata_2025_split huggingface dataset:
Multicomponent free‐radical polymerization in batch, semi‐ batch and continuous reactors
Authors: Hamielec A. E. , MacGregor J. F. , Penlidis A. 
Published: March 2011

AbstractA practical methodology for the computer modelling of multicomponent free radical polymerizations for solution and emulsion systems has been developed. The approach is general providing a common model framework which is applicable to many comononer systems. Model calculations include conversion of the monomers, multivariable distributions of concentrations of monomers bound in the polymer chains and molecular weights, long and short chain branching frequencies, chain microstructure and crosslinked gel content when applicable. Diffusion‐controlled termination and propagation reactions are accounted for using the free‐volume theory. When necessary, chain‐length‐dependent diffusion‐controlled termination may be employed.Kinetic parameters have been measured and are available for the following binary comonomer systems: styrene/acrylonitrile, p‐methylstyrene/acrylonitrile, p‐ methylstyrene/styrene, p‐methylstyrene/methyl methacrylate and styrene/butadiene. Various comononer systems will be used to illustrate the development of practical semi‐batch and continuous reactor operational policies for the manufacture of copolymers with high quality and productivity.It should be mentioned that these comprehensive polymerization models may be used by chemists and chemical engineers to reduce the time required to develop new polymer products and advanced production processes for their manufacture.
Continuous free radical polymerization in disperse systems
Authors: Reichert K. H. , Moritz H. U. 
Published: March 2011

AbstractContinuous free radical polymerization in disperse systems was performed in a laminar stirred tubular reactor which is a novel reactor type with narrow residence time distribution in the laminar flow regime. The residence time distribution of such a reactor can be modelled by very simple dimensionless equations containing the axial and radial Reynolds numbers of the reactor at operating conditions. In such a reactor of pilot plant scale the continuous free radical polymerization of acrylamide in emulsion and of vinyl acetate in suspension was studied with respect to conversion and performance of reactor. For modelling the conversion of polymerization the segregation and the dispersion model were used and the results compared with experimental data. Both are suitable models for modelling the upper and lower limit of conversion of micro or macro mixed polymerizations. The vinyl acetate polymerization was also investigated with respect to the breadth of the molecular weight distribution of the polymer formed in batch, continuous segregated, and continuous micro mixed reactors. Models were developed employing a complex kinetic scheme including transfer reaction to polymer and terminal double bond polymerization. Simulation results showed that the molecular weight distribution order in the different reactor types is not fixed, but a function of reactant concentrations and importance of chain branching. The simulations are compared with experimental data.
Free‐radically initiated polymerisations for reaction injection moulding (RIM): Some observations on polymer blends involving poly(methyl methacrylate) as one of the components
Authors: Armitage P. D. , Coates P. D. , Johnson A. F. , Hill S. , Hynds J. , Leadbitter J. , Myktiuk J. 
Published: March 2011

AbstractThe rapid growth of interest in reaction injection moulding (RIM) and reinforced reaction injection moulding (RRIM) has focused attention on polymerisation reactions which can generate useful solid polymeric mouldings with moulding cycle‐times of less than three minutes. Free‐radically initiated polymerisations have long been used in reactive processing, for example, in the casting of acrylic sheet materials and for the cure of polyester resin, but the overall polymerisation times for these reactions has been relatively long. Adapting existing free‐radical polymerisations for RIM and RRIM technology raises many interesting chemical, kinetic and thermodynamic problems. We have been exploring the use of radical processes for very fast processing techniques such as RRIM using a novel computer controlled and monitored RRIM machine developed in our laboratories. Both polyester and acrylic based materials have been reactively processed to give 2–4 kg plaque mouldings in cycle‐times of less than two minutes. Of particular scientific interest are polymer blends and interpenetrating polymer network systems (IPNs) (where one of the concurrent polymerisations is initiated by a free radical mechanism) since these systems give considerable scope in the design of product properties through composition. The principles of reaction injection moulding are briefly described together with some experimental results obtained for an acrylate/polyurethane composite material given emphasis to fundamental aspects of radical polymerisation reactions raised by the need for very rapid and complete conversion of the methyl methacrylate component.
Reactions of carbocations with carbanions through coordination and single electron transfer. Heterolysis and homolysis of carbon‐carbon bonds
Authors: Arnett E. M. , Whitesell L. G. , Amarnath K. , Cheng J. P. , Marchot E. 
Published: March 2011

AbstractCarbocations, carbanions and free radicals are the most important reactive intermediates in organic chemistry and the majority of bond‐forming and bond‐breaking reactions involve their participation at some stage. We will review briefly our studies, over the past five years, of the direct coordination reaction between carbocations and carbanions to form covalent bonds in solution. We have gathered considerable thermodynamic and kinetic information for this reaction.Particular emphasis will be placed on recent work on the measurement and analysis of heterolysis energies. Heterolysis is the reverse of the coordination reaction and we have obtained heterolysis energies in solution for over 160 bonds by measuring the heats of reaction between several resonance‐stabilized carbocations and a variety of carbanions, oxyanions, nitranions a thiophenoxide ions in solution. The results are correlated by very simple equations so that heats of heterolysis may be predicted with considerable precision from readily available data.Carbocations and carbanions can also react by electron transfer to form free radicals. We will describe recent experiments which demonstrate that under appropriate conditions all three types of reactive intermediates can coexist at equilibrium in solution with the electrons shuttling between the three different species.
Identification of some carbocationic active species: Structure and stability relationship
Authors: Sigwalt P. , Matyjaszewski K. , Moreau M. 
Published: March 2011

AbstractThe nature and the stability of the various types of propagating species involved in cationic polymerization of ethylenic monomers are examined. After a short discussion of the evidence for and against the presence of covalent species in styrene polymerization, recent data about the identification of carbocationic active centres involved in the polymerization of para‐substituted α‐methyl‐styrenes are given. Carbenium ions were generated in methylene chloride either by reaction of the p‐substituted cumyl chloride with SbF5 or by reaction of the monomer with trifluoromethanesulfonic acid (HA). They are stable around −70°C, but some of them give secondary reactions at higher temperature. The various types of species were identified by 1H NMR correlated with UV spectroscopy.For the p‐tert‐butyl derivative, monomeric cations remain stable at −70°C (and even at higher temperature) while oligomeric ones are less stable, giving indanic derivatives. With p‐methoxy‐α‐methylstyrene, the monomeric, dimeric and trimeric cations are stable at −63°C but give indanic derivatives at −40°C and above. For both monomers, monomeric cations show a single maximum at 350 or 367 nm with a high absorption coefficient (≁ 30000). In the presence of an excess of monomer, a bathochromic shift of this maximum (≁ 15 nm) is observed, together with the appearance of a new (smaller) maximum around 480 nm. The presence of these two maxima could be correlated with that of “isomerized” species, some of which could be identified by NMR. The possible occurrence of a similar situation with other monomers and particularly with p‐methoxystyrene is discussed.
Mechanisms of cationic polymerizations of lactones
Authors: Kricheldorf Hans R. , Sumbél Movita‐Verena 
Published: March 2011

AbstractIt is demonstrated by means of 1H NMR spectra in combination with IR spectra that the reaction of δ‐valerolactone with an alkylating reagent results in alkylation of the exocyclic oxygen. The propagation of the resulting cyclic dioxocarbenium ion involves cleavage of the alkyl‐oxygen bond. Initiation and propagation mechanisms induced by Lewis acids such as boron bromide, aluminum bromide, titanium(IV) and zirconium (IV) bromide, tin(IV) bromide and zinc bromide are discussed. It is demonstrated by means of IR and NMR spectroscopy that complexation of the lactones at the exocyclic oxygen is followed by cleavage of the alkyl‐oxygen bond and halogen transfer, so that ω‐bromocarboxylic acids are obtained (after hydrolytic work up). In the case of aluminum, tin and zinc the intermediately formed metal‐oxygen bond initiates rapid polymerization via a nonionic insertion mechanism which is characterized by cleavage of the acyl‐oxygen bond.
Cationic polymerization and copolymerization of lactams and lactam derivatives
Authors: S̄ebenda Jan 
Published: March 2011

AbstractLactams and lactam derivatives may be polymerized by various types of processes, but only the cationic mechanisms provide suitable pathways for the polymerization of all types of lactam monomers. Therefore, the cationic polymerization is predestinated for the synthesis of their copolymers. Random copolymers can be obtained even if the rates of homopolymerization of comonomers differ by several orders of magnitude, either by transacylation of block copolymers, or in a new process designated as polymerization with transient chemical activation. This process is operative e.g. in the copolymerization of an unsubstituted and an N‐acylated lactam. It implies N‐acylation of the unsubstituted lactam by the polymer of the N‐acylated one, followed by the incorporation of the activated monomer and subsequent deacylation of the incorporated monomer unit.
Developments in the design and applications of novel thermal and photochemical initiators for cationic polymerization
Authors: Crivello J. V. , Lee J. L. , Conlon D. A. 
Published: March 2011

AbstractSince their discovery in our laboratories as novel photo and thermal initiators for cationic polymerization, diaryliodonium and triarylsulfonium salts have been the object of considerable investigation in our laboratories as well as in a number of other academic and industrial research groups. Recent work in our laboratory has focused on several major themes which will be the topics of this presentation. These include; the synthesis of new photoinitiators with higher photosensitivity, the preparation of polymer bound photoinitiators, the preparation of new cationically polymerizable monomers with high reactivities and lastly, the use of onium salt induced photo and thermally induced cationic polymerization in a number of practical applications.
The role of sterically‐hindered pyridines in cationic polymerization
Authors: Gandin Alessandro , Martínez Armando 
Published: March 2011

AbstractA brief survey is given of the syntheses, physico‐chemical properties and reactivity of sterically‐hindered pyridines (SHP). Then a more detailed analysis is devoted to the specific role of these peculiar Lewis bases in the elementary steps of cationic polymerization. The existing literature is critically reviewed and new results are presented and discussed. The unique features of hindered pyridines and their usefulness in both mechanistic studies and practical issues related to cationic polymerization are emphasized.
Cationic reactivation polymerization of vinyl monomers
Authors: Heublein G. , Spange S. , Ulbricht A. 
Published: March 2011

AbstractThe oationio polymerization of strong π‐donor monomers has been investigated, where initiation takes place with triphenylmethylium picrate and picric acid, respectively. In the case of p‐methoxystyrene the polymerization runs via aggregated species, where monomer and picric acid stabilizes the propagating intermediate state. The transfer‐suppressed polymerization oan be reactivated after monomer consumption. The mechanism of IBVE polymerization runs similar, however, by side reactions double bonds can be introduced into the polymer chains and chains can be terminated by water under formation of aldehyde groups. NVC polymerization proceeds via freely solvated ions, which results in relative broad MWD. The polymerization of NVC and p‐methoxystyrene initiated with cationogenes involving nucleophilic counterions, leads in the presence of AEROSIL to increased molecular weights and narrow distribution, in comparison with the polymerization under same conditions in solution.
Characterization of the active centers in the cationic polymerization of p‐substituted α‐methylstyrenes
Authors: Lenz Robert W. , Faullimel Jean G. , Jonte J. Michael , Fisher Danilella J. 
Published: March 2011

AbstractPolymer tacticity, molecular weight and molecular weight distribution were used as the basis for characterizing the type of active centers present in the cationic polymerization α‐methylstyrene monomers. The affect of reaction solvent polarity and the resonance effects of para‐substituents were used to deduce the nature of, and changes in, the ion pair character of the carbocation active center. For this purpose, the polymer yield, tacticity and molecular weight parameters were determined in the cationic polymerization reactions of p‐chloro and p‐fluoro‐α‐methylstyrene. The dielectric constant of the reaction medium was varied by using mixtures of methylene chloride and either hexane, a non‐solvent for the polymers, or toluene, a good solvent for the polymers. For both solvent systems, the yields and molecular weights of the polymers from both of the monomers were strongly effected by reaction time and solvent composition. Also, for both monomers, tacticity did not change with reaction time, and for the p‐fluoro monomer tacticity was constant with changes in reaction solvent composition in both mixed solvent systems. In contrast, the tacticities of the polymers from the p‐chloro monomer decreased with increasing amounts of the non‐polar solvent in the mixture. The tacticity effects induced by reaction solvent changes are interpreted on the basis of the electronic effects of the para‐substituents on changing the ion pair character of the carbocation active center. The effect of solvent composition on polymer yield and molecular weight may be a function of changes in the rates of initiation, propagation and monomer transfer, with the latter two also determined by the ion pair character of the carbocation active end group.
Technical developments in polyoxymethylene from an industrial point of view
Authors: Cherdron Harald , Burg Karlheinz , Kloos Friedrich , Reske Eckart , Sextro Günter 
Published: March 2011

AbstractThe attempt is made to indicate the future industrial developments in the field of polyoxymethylenes. As far as the process development is concerned, the development of a dehydrogenation route from methanol to formaldehyde or trioxane without the formation of water would be an economical breakthrough. The major product developments are in the area of copolymers with defined sequence length, of blockcopolymers, of blends and of fibrids.
Application of isobutylene telechelics in vulcanizing systems
Authors: Musch R. 
Published: March 2011

AbstractThe application of the “inifer technique” introduced by J.P. Kennedy et al. for the preparation of polyisobutylene (PIB) with two reactive end‐groups results in a new range of functionalized polymers. These products can be blended with polymers to improve the network structure. This presentation deals with the preparation of PIB‐telechelics, their characterization and their application in combination with polychloroprene. During the mixing step we observed an improved processibility of polychloroprene based on the plastifying efficiency of these telechelics. The vulcanization conditions are described under which vulcanization with improved technical properties can be achieved, in summary, the addition of 10–30 parts PIB improves the low temperature flexibility, the weathering and ageing resistance of the polychloroprene vulcanizates. The use of these blends in various fields of application is explained.
Cyclic oligomer formation in the copolymerization of isoprene with isobutylene
Authors: Kuntz I. , Powers K. W. , Hsu C. S. , Rose K. D. 
Published: March 2011

AbstractThe formation of trace amounts of cyclic oligomers in the copolymerization of isoprene with isobutylene has been studied using mass spectrometry and NMR. Two major products are detected with empirical formulas of C13H24 and C21H40. The C13H24 material is identified as 1‐isopropenyl‐2, 2,4,4‐tetramethylcyclohexane. The C21H40 substance is postulated to be a structurally related cyclohexane derivative. The mechanism for the production of such materials is discussed.
Activated esters in the cationic polymerization of styrenes
Authors: Matyjaszewski Krzysztof 
Published: March 2011

AbstractCationic polymerization of styrenes initiated by protonic acids can proceed via covalent intermediates. Calculations of the charge distribution in these species show low probability of transfer reactions. Macromolecular and model covalent species in the perchloric acid initiated polymerization of styrene have been directly observed by NMR. Esters formed with triflic acid are very reactive and participate in Friedel‐Crafts alkylation at −78 °C. Acetates become active only in the presence of protonic and Lewis acids. 1‐Phenylethyl trifluoroacetate activated with the parent acid reacts with styrene faster than the acid does. The racemization of the optically active ester is faster than its incorporation indicating an ionic mechanism of propagation in this system. Activation by Lewis acids proceeds by the reversible formation of complexes. Some systems yield polymers with low polydispersity.
Telechelic tetrahydrofuran
Authors: Kress H.‐J. , Stix W. , Heitz W. 
Published: March 2011

AbstractCondensation polymers with block structure are stable only if one of the constituent blocks is stable against reactions like transesterification, transamidation, etc. This stable block must have a pure CC chain which may be interrupted by ether, sulfide or sulfone linkages. It must be strictly bifunctional in order to be no limiting factor for the molecular weight in the synthesis of blockcopolymers.If the two functional groups are located at both ends of the polymer chain, we name this product a telechelic. The characterization of telechelics needs the determination of the functionality. Any end group analysis will give only average values. An efficient chromatographic method is necessary to guarantee that bifunctionality is caused by one polymer homologous series.A number of commercial telechelic polytetrahydrofuranes are strictly bifunctional as shown by GPC1). Two functional end groups can be introduced either by bifunctional initiation and killing the living end groups by a proper nucleophile. The disadvantage of this method is that one mole of an expensive initiator is needed to produce one mole of polymer. The second method to introduce functional end groups uses a proper transfer agent. Only catalytic amounts of initiator are necessary in this case.
Properties of copolymer latexes prepared in inverse microemulsions
Authors: Candau Françoise , Collin Dominique , Kern François 
Published: March 2011

AbstractThe structural and rheological properties of poly(acrylamide‐co‐sodium acrylate) latexes prepared in inverse nonionic microemulsions have been investigated. The formation of stable and optically transparent microlatexes is strongly related to the acrylate content in the feed due to a salting‐out effect of the ethoxylated surfactant. This salting‐out effect determines the optimum HLB value of the blend. The dimensions of the particles are rather low (d < 100nm) with a narrow distribution. The inverse microlatexes which consist of water‐swollen copolymer particles dispersed in an isoparaffinic oil show a newtonian behavior up to high volume fractions (⋍ 50%) and a shear‐thinning behavior for volume fractions above 50%. The presence of charges in the copolymer leads to an apparent close‐packing volume fraction lower than that of 64% predicted for a random packing of hard spheres.
Prediction of the primary particle size distribution in vinyl chloride polymerization
Authors: Kiparissides Costas 
Published: March 2011

AbstractIn the suspension polymerization of VCM insoluble polymer particles are formed inside the vinyl chloride monomer droplets. The nucleation, growth and aggregation of these particles are responsible for the internal morphology and associated properties (i.e. surface area, pore size distribution) of the particulate PVC product. The objective of the present study is to develop a microscopic particle morphology model that can predict the dynamic behaviour of the primary particle size distribution in terms of the controlling process variables (i.e. temperature, initiator concentration, etc.) by formulating a population balance equation to follow the time variation of the primary particle density function from the point of nucleation of basic particles to the formation of particle aggregates. It is shown that model predictions of the primary size distribution are in very good agreement with experimental observations.
Chemical composition distribution of styrene‐methyl acrylate emulsion copolymers
Authors: van Doremaele G.H.J. , van Herk A. M. , German A. L. 
Published: March 2011

AbstractThin layer chromatography with flame ionisation detection (TLC/FID) was applied to determine the chemical composition distribution (CCD) of styrene‐methyl acrylate (S‐MA) emulsion copolymers. The reliability of this method was determined using a well defined high conversion S‐MA solution copolymer. The influence of conversion and monomer water ratio on the emulsion copolymer CCD could successfully be described by a model that uses r values determined in solution, and takes into account the monomer partitioning. The samples were prepared in the presence of 1‐dodecanethiol in order to reduce the molecular mass of the copolymer formed. Not only the conversion part of the total copolymer CCD but also the instantaneous (statistical) part appeared to be important.