Tacticity dependence

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

For disubstituted ethylenes, the presence and type of tacticity depends on the positions of substitution and the identity of the substituents. In the polymerization of a 1,1-disubstituted ethylene, CH2=CRR, stereoisomerism does not exist if the R and R groups are the same (e.g., isobutylene and vinylidene chloride). When R and R are different (e.g., —CH3 and —COOCH3 in methyl methacrylate), stereoisomerism occurs exactly as in the case of a monosubstituted ethylene. The methyl groups can be located all above or all below the plane of the polymer chain (isotactic), alternately above and below (syndiotactic), or randomly (atactic). The presence of the second substituent has no effect on the situation since steric placement of the first substituent automatically fixes that of the second. The second substituent is isotactic if the first is isotactic, syndiotactic if the first substituent is syndiotactic, and atactic if the first is atactic. 

There are significant differences in rate and polymer tacticity, dependent on the nature of the transition metal compound and organo-metal compound (Tables 12 and 13). 

As shown in the data above, the two different Friedel-Crafts catalysts, TiCl and SnCl, with the same cocatalysts gave polymers with either the same or different tacticities depending on the monomer involved. For the p-chloro, p-bromo, and p-methoxy monomers, polymers with the same syndiotactic triad contents were obtained with both TiCl and SnCl catalysts. In contrast, with... 

Nuclear magnetic relaxation parameters give important information on molecular motion and have become more and more familiar to and inevitable for NMR users after the introduction of the Fourier transform (FT) method to NMR technology. The relaxation parameters are also of essential importance in adjusting data acquisition conditions in FT-NMR measurements to obtain quantitative data. Section 6 covers the reliability of spin-lattice relaxation time (7)) and the nuclear Overhauser enhancement (NOE) factor of polymers in solution and the tacticity dependence of H- and 3C-7) values of vinyl polymers in solution. 

In these types of polymer reaction, quantitative conversion is a definite requirement otherwise, there would be a possibility that the tacticity of the derived polymer differs from the original one owing to the tacticity dependence of the reaction. 

Tacticity dependence of H- and 13C- 7) values of vinyl polymers in solution... 

The tacticity dependence of the C-T values was also observed for the methine carbons of polypropylene and poly(1-butene). Smaller Tx values were observed for the syndiotactic polymers as compared with the corresponding values for the isotactic polymers. The difference is small but significant and tends to be larger when the comparison is done on the correlation times determined with the log x distribution model.318... 

PREFERENTIAL ADSORPTION. POLYMER MOLECULAR WEIGHT AND TACTICITY DEPENDENCE OF PREFERENTIAL ADSORPTION... 

It is important for a company to recognize the importance of the sourcing strategy. It can be either strategic or tactical depending on the type of item being purchased. For example, if the item being purchased is a critical... 

Tacticity a property that describes the steric order of a polymer chain. Polymers with a unique way of coupling the monomer units are called isotactic. If the coupling of monomer units varies, but in a regular way, the polymers are called syndiotactic. Atactic polymers are polymers with an irregular steric structure. Polystyrene, for example, can adopt all types of tacticities, depending on the process of synthesis. 

As far as the stereoregularity is concerned, studies of various types of initiation show that methacrylates could be polymerized to give as well as isotaetie, syndiotaetie atactic polymers. Numerous physical properties are tacticity dependent for example, the rate of water absorption is higher for syndiotaetie than for isotactic polymer [97], the transition temperatures of liquid crystalline methacrylic polymers can be specifically influenced [160-162], and the miscibility of polymer blends is changed [163-165]. In general, the stereoregularity depends on the solvent used, the initiator, and the reaction temperature. Reviews have provided an overview concerning analysis, properties and reactivities of polymers with respect to their tacticity [97,166,167]. 

Polymerization of this monomer with the above mentioned catalysts led to ring-opened polymers of various configurations and tacticities depending on the catalyst employed and the reaction parameters [7] [Eq. (69)]. 

Specific deterrence is aimed at specific groups and involves a myriad of tactics, depending on the group. In the context of DWI specific deterrence is directed most often at repeat offenders and young drivers, typically under the legal minimum drinking age. 

What else are you doing outside of an event to make your presence known Consider a mix of the following tactics (depending on your budget) ... 

This chapter is focused on the conformational analysis of medium and large rings and its implementation in asymmetric synthesis. The concept is commonly referred to as macrocyclic stereocontrol, and its applications in dia-stereoselective synthesis were formulated and extensively studied in the mid-1980s. As a synthesis tactic, it addressed deficiencies in the available asymmetric synthetic methods of the time. Beyond its tactical use, it has had wider implications in the evolution of synthesis strategies for the construction of complex molecules. In the period since its enunciation, countless new methods, which have largely supplanted the tactical dependence on macrocyclic stereocontrol, have appeared on the scene. However, the basis for its strategic use remains attractive, and the synthesis of complex structures can benefit substantially from its clever, timely implementation. 

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