Polymers, steric composition

The a-TiCla treated in this way gives reproducible results for the kinetic behavior, the molecular weights, and the steric composition of the polymer. 

Polymerization of Propylene to Isotactic Polymer. Independence of the Molecular Weight and of the Steric Composition from the Polymerization Time... 

In a recent work (48), the influence of the variation of some factors, concerning the polymerization, on the steric composition of the polymer has been studied subjecting the obtained raw polymer to subsequent extractions with the following series of solvents employed at their boiling point ether, n-heptane and n-octane. 

Furthermore, this method enabled us to evaluate separately the number of the conventional active centers from which, respectively, each of two particular fractions of polymer, having different steric composition, is originated an amorphous one, ether soluble, and another one (ether extraction residue) containing stereoblock and isotactic chains. 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

In this paper, we use both numerical and analytical self-consistent field (SCF) models to isolate a robust scheme for creating stable dispersions from polymers and clays that are immiscible. Specifically, we show that adding a small fraction of end-functionalized polymers to a melt can lead to the formation of exfoliated structures, where the clay sheets are uniformly dispersed within the polymer matrix. Our findings reveal that the ideal pol eric candidates for creating stable exfoliated composites are those that would constitute optimal steric stabilizers for colloidal suspensions. This prediction provides new design criteria for fabricating polymer/clay composites with the... 

Because model colloids tend to have a ratlier well defined chemical composition, elemental analysis can be used to obtain detailed infonnation, such as tlie grafted amount of polymer in tire case of sterically stabilized particles. More details about tire chemical stmcture can be obtained using NMR techniques (section B1.13). In addition, NMR... 

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

This relatively small catalog of molecular shapes accounts for a remarkable number of molecules. Even complicated molecules such as proteins and other polymers have shapes that can be traced back to these relatively simple templates. The overall shape of a large molecule is a composite of the shapes associated with its inner atoms. The shape around each inner atom is determined by steric numbers and the number of lone pairs. 

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

Various halogenated polymers are used as the polymeric initiator for ATRP, for example, vinyl chloride-vinyl chloroacetate and styrene-p-chloromethylstyrene copolymers, bromi-nated butyl rubber, and chlorosulfonated polyethylene [Bomer and Matyjaszewski, 2002 Coskun and Temuz, 2003], (The vinyl chloride units have much lower reactivity for ATRP compared to the vinyl chloroacetate units because of steric reasons.) The density of grafting is adjusted by varying the copolymer composition. 

One problem encountered in the field is the apparent irre-producibility of the results of different workers, even those in the same laboratory. This is particularly the case with molar mass distribution and steric triad composition. The explanation of these apparent inconsistencies comes with the realization that the mechanisms are eneidic and the polymer properties are primarily determined by independent active centres of different reactivities and stereospecificities whose relative proportions are set at the initiation step, which is completed in the first seconds of the polymerization. The irreproducibilities arise from irreproducibilities in the initiation step which had not been thought relevant. Ando, Chfljd and Nishioka (12) noted that these rapid exothermic reactions tend to rise very significantly above bath temperature (we have confirmed this effect) and allowed for this in considering the stereochemistry of the propagation reaction. However our results show that the influence on the initiation reactions can have a more far-reaching effect. 

The effect on the chain configuration of the polymers was just as drastic. t-Butylmagnesium bromide in toluene solution at 230 produced a polymer of steric triad composition i h s 1 1 1 if initiated at the melting point of toluene, and i h s s 99 0 1 if initator solution at 230 was mixed with monomer solution at 230 K. 

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