Polymerization centers isospecific

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

From the above it is clear that the Cp and k values reported in the preceding section are only average values which do not reflect the real situation, although they are quite useful in understanding certain phenomena. The active species not only consist of isospecific and non-specific centers in the case of the propylene polymerization but, rather, by a plurality of species having different reactivities, which cannot be completely identified by kinetic studies or by catalyst poisoning. 

Moreover, application of the above law to the formation rates of isotactic and atactic fractions showed that the overall rate equation is the result of two equations characterized by different values of kA (200 1 mol-1 for the isospecific centers and 40 1 mol 1 for the non-specific centers). Thus, the kinetic behavior of the polymerization was rationalized on the basis of a two-center polymerization model. Furthermore, based on an approximate estimate of the partition function of the transition state involving propagating chain and coordinated monomer, monomer insertion was proposed as the rate determining step. 

However, the comparative data on (Table 1) and the stereoregularity of polymer fractions (Table 6) for one- and two-component catalysts based on titanium chlorides indicate that the cocatalyst does not influence the reactivity and stereospecificity of the propagation centers. Its effect on the overall polymerization rate is apparently due to the change in the total number of active centers and the ratio of isospecific and non-stereospecific centers. 

The vast majority of papers reporting stereoselective epoxide polymerization focus on isospecific propylene oxide polymerization. For clarity, this chapter is organized by the type of metal of the catalyst active center. The three most commonly used metals for discrete stereoselective epoxide polymerization catalysts are aluminum, zinc, and cobalt, and research using these metals forms the foundation of this chapter. 

Various models of catalytic centers and of monomeric unit addition mechanism have been proposed to interpret the isospecific polymerization of oc-olefins with Ziegler-Natta catalytic systems [285-293]. For the oc-olefins the combination of x-ray diffraction and IR analysis showed very early that the polymers obtained with the Ziegler catalytic system... 

The international symposium on Recent Developments in Olefin Polymerization Catalysts was held in Tokyo in October 1989. This volume includes 38 i>apers fi"om the 31 lectures and 18 posters presented at the symposium, which covered the following topics Overview of super-active homogeneous and heterogeneous catalysts, kinetic profile of olefin polymerization including copolymerization, characterization of catalysts and polymers, methods for the determination of active center concentration, role of Lewis bases on the catalyst isospecificity, polymerization mechanisms, and synthetic pathways for functionalized polyolefins. We believe the contents are well balanced between fundamental research and application as well as between homogeneous and heterogeneous catalyst systems. 

Catalysts which are isospecific for the polymerization of propylene, butene-1, etc., can also be used for the preparation of copolymers of these olefins with ethylene and with each other. There is much evidence to indicate that the chiral specificity of the catalyst is retained during these reactions [72-76], Thus, the spectra of ethylene(e)-propylene(P) copolymers prepared with an isospecific catalyst system are much simpler than those prepared with a nonspecific catalyst [75]. Studies on copolymers of propylene with c-enriched ethylene prepared with an isospecific catalyst indicate that the propylene-ethylene-propylene triads have a single type of configuration, which is probably meso. This is indicated by the fact that only two resonances are observed for ethylene units in P-E-P triads. Furthermore, there is no evidence for the presence of ethylene units flanked by two tertiary carbon atoms (due to inversions of propylene units) in the copolymers. In contrast, copolymers prepared from nonspecific or syndiospecific catalysts have been shown to contain ethylene units centered in both meso- and racemic- PEP... 

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