Tacticity limiting cases

TABLE 7. Some limiting cases of tactic polymers made by ROMP... 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

Tacticity is required for the synthesis of crystalline thin polysilane films used for optical and semiconductor devices. Modern synthetic routes allow control over the conformation and tacticity of polysilane molecules used as precursors for thin layers of photoresists, photoconductors and nonlinear optical phases in complex semiconductor and (opto)electronic devices. These properties can be exploited only if the synthesis method ensures a minimal level of contamination, especially with oxygen and metals, and special care is taken to limit electronic-grade polysilanes to a level of contamination on the order of a few ppm in the case of oxygen and in the ppb range for metals. The reactivity of polysilane toward oxygen has forced placing the devices in a helium environment during measurement procedures.36... 

A low degree of tacticity is obtained because these monomer-orienting forces are quite weak. Thus, polymer stereoregularity is achieved only with certain suitable monomers and at low temperatures. Increased tacticity can be achieved in some cases by using monomer-orienting forces other than the catalyst or the polymer end groups, but these have rather limited utility (canal complexes, solid state polymerizations, etc.). Simple polymerization systems fall outside the scope of this review and are not discussed further. 

If the value of this probability parameter is a <= 1, an essentially iso tactic structure is obtained. If, on the other hand a 0, almost all the monomer units in the chain are in syndiotactic placements. If the polymer is capable of crystallization and the crystallization takes place under equilibrium conditions, then the limitation of this model is that a small melting point depression implies also a high degree of percent crystallinity. Although there are a number of systems, for example stereoregular methyl methacrylate (2, 8), in which this is true and this model is valid, this is not the case for polymers of propylene oxide from different catalysts that we discuss in another chapter (1). 

The reason for their attitude is clear. The Japanese dyestuffs industry was still developing. It suffered from a lack of production lines in some principal dyes such as synthetic indigo, as well as from limited areas of export. The Japanese side was therefore afraid that it would tie its own hands if it concluded a comprehensive agreement. It preferred instead to conclude a series of partial agreements and to aim at developing new products while expanding its production capabilities and widening its export areas from China to Indochina and British India. The tactics of I.G. Farben as well as those of the three-party cartel were entirely different from those of the Japanese side. The Western firms wanted to contain the Japanese production capabilities. I.G. Farben s explanation to the Swiss companies mentioned above may, therefore, have concealed its own real aims. What is certain, in any case, is that I.G. Farben endeavored to obtain the consent of the Swiss companies. 

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