Selectivity in polymerization

Liu L, Rozenman M, Breslow R. Hydrophobic effects on rates and substrate selectivities in polymeric transaminase mimics. J. Am. Chem. Soc. 2002 124 12660-12661. 

Liu, L, M Rozenman and R Breslow (2002). Hydrophobic effects on rates and substrates selectivity in polymeric transaminase mimics. Journal of the American Chemical Society,... 

Cis/Trans Selectivity in Polymerizations Catalyzed by Imido Molybdenum Alkylidene Complexes... 

Control of enantiomorphic selectivity in polymerization of the substituted oxiranes can lead to controlled-structure polymers, the properties of which will range from crystalline thermoplastics to amorphous elastomer precursors such as are used as soft segments in polyurethanes. Crystallizable sequence distributions in highly controlled-structure polymers can lead to thermoplastic elastomers and/or to elastomers that will stress-crystallize that is, crystallize on stretching as does natural rubber (79). 

ILs are low-melting salts that are attractive for a number of applications as they are relatively nonvolatile, nonflammable, environmentally benign, and exceptionally thermally stable [18], In addition, there are numerous combinations of cations and anions that can be used to produce ILs, and thus chemical and physical properties of ILs can be tuned, which is needed to design an energy-efficient liquid absorbent for CO2 capture. The mechanism for CO2 capture in ILs is often based on physisorption and involves a weak association between the IL and CO2 molecules [19], Once the CO2 has been removed from the gas mixture, it can be released from the ILs (which would be reused) by either a decrease in pressure or an increase in temperature [18], While the viscosity of ILs minimizes solvent loss from the gas stream, this attribute also limits mass transfers, and they often suffer from low rates of absorption. To overcome these shortcomings and increase the capacity of simple ILs, amine-functionalized ILs have been developed, which allow higher rates of sorption to be achieved at pressures relevant to flue streams [ 19,20]. A number of reports have also demonstrated high CO2/N2 selectivity in polymerized ILs, which exhibit enhanced CO2 solubility relative to the monomeric ILs [21]. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

In this section, the reactions undergone by radicals generated in the initiation or chain transfer processes are detailed. Emphasis is placed on the specificity of radical-monomer reactions and other processes likely to take place in polymerization media under typical polymerization conditions. The various factors important in determining the rate and selectivity of radicals in addition and... 

Table 4.11 Kinetic Parameters for Propagation in Selected Radical Polymerizations in Bulk Monomer...

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). These groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8.2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

This study may act as a guide in the selection of initiators with desired half-life and activation energy for reducing batch time in polymerization reactors. 

Flotation is certainly the major separation method based on the surface chemistry of mineral particles. It is, however, not the only method. Selective flocculation and agglomeration may be mentioned as other methods used commercially to a limited extent. The former is for hematite, while the latter is for coal and finely divided metallic oxide minerals. Both processes use the same principles as described for flotation to obtain selectivity. In selective flocculation, polymeric flocculants are used. The flocculants selectively adsorb on the hematite, and the hematite floes form and settle readily. Thereby separation from the sili-... 

Palmer, C.P. and Tanaka, N., Selectivity of polymeric and polymer-supported pseudo-stationary phases in micellar electrokinetic chromatography, /. Chromatogr. A, 792, 105, 1997. 

It has proved to be very useful, providing both qualitative and quantitative information derived from mathematical processing of UV/VIS spectra. The principles of derivative spectrophotometry were discussed [15,16]. Obviously, derivatisation of spectra does not provide any additional information to that acquired during the measurement, but allows for easier interpretation. In particular, the possibility of resolving overlapping peaks makes derivative spectrophotometry a valuable tool for multicomponent analysis. Typically, derivative spectrophotometry is useful for the simultaneous determination of two additives in polymeric materials with very closely positioned absorption maxima. In quantitative analysis, derivative spectrophotometry leads to an increase in selectivity. 

Table 8.1 shows some selected inorganic components in polymeric matrices. The broad variety of elements contained in polymers may be classified into three product-oriented categories ... 

For the analysis of organic additives in polymeric materials, in most cases, prior extraction will be necessary. Depending on the nature of the additive, many different approaches are employed. These include soxhlet extraction with organic solvent or aqueous media, total sample dissolution followed by selective precipitation of the polymer leaving the additive in solution, assisted extraction using pressurised systems, ultrasonic agitation and the use of supercritical fluids. In trace analysis, solid phase extraction (SPME) from solution or solvent partition may be required to increase the analyte concentration. 

Based on the extraordinary selectivity in hydrosilylation reactions when an alkyne competes with other groups for a silicon-bonded active hydrogen, further derivatisation can be carried out. The hydrosilylation of 2-methyl-3-butynol, which works very well with polymeric siloxanes, gives hydroxyal-kenylsilicon compounds - a l-silylalkenyl/2-silylalkenyl mixture from cis-addition across the triple bond. Elimination of water from the tert. alcohol produced, catalyzed by traces of a strong acid, results in isoprenylic siloxanes in more than 90 % overall yield (Eq. 8). 

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