Ziegler s method

In contrast to Ziegler s method (see Chapter 2), this reaction can be affected with aryl halides bearing no ort/jo-coordinating substituents. 

Natta, in collaboration with the Italian company, Montecatini, extended Ziegler s method to propene. He found that polymerization occurs almost entirely head to tail and that crystalline, stereoregular products can result. Using EtjA1 in conjunction with TiCl (active species TiClj) isotactic polypropene is formed in which all the methyl groups are all on the same side of the carbon backbone (Fig. 12.9). With Et AlCl/VCl, syndiotactic polypropene results in which the configurations at the carbon atoms alternate along the chain. 

Mathis and Francois have described the preparation of cumyl barium and strontium in THF by an adaptation of Ziegler s method. Very conveniently, they included details of the synthesis of the precursor, cumyl methyl ether. Nakh-manovich and Arest-Yakubovich have investigated the kinetics of the polymerization of styrene by organo-alkaline earth initiators in THF they report that the rate increases in the sequence Ba < Sr < Ca. A study of stability was made by Mathis et al., who found that the slow decomposition of e oHlicarbanionic polystyryl barium and strontium in THF is due to protonation by the solvent. An important, and unexpected, feature is that the termination is not random - partially deeomposed polymer is devoid of monocarbanionic content. 

By using Ziegler s colorimetric method 178) to determine the dissociation constant of 2-triphenylmethyl and its dimer, the dissociation constant of the 2,4,6-triphenyl-phenoxyl dimer under various conditions has been measured The values presented in Table 9 were found by evaluating the extinction coefficients of the maximum at 530 nm of the radical depending on the concentrations 133 ... 

A variant of the above method is the nitrone-olefin cycloaddition, exploited for the synthesis of shikimic acid [349] and for the synthesis of the carbocyclic nucleoside neplanocin [350]. RCM has been recently exploited in Ziegler s synthesis of cyclophellitol [351]. This synthesis encompasses a number of steps described throughout this chapter, including chain extensions at Cl, at C6 and branching by Michael addition followed by carbocyclization. 

A number of block copolymers prepared with Ziegler-Natta catalysts have been reported however, in most cases the compositions may include significant amounts of homopolymer. The Ziegler-Natta method appears to be inferior to anionic polymerization for synthesizing carefully tailored block copolymers. Nevertheless, bock copolymers of ethylene and propylene (Eastman Kodak s Pofyallomers) have been commercialized. Unlike the elastomeric random copolymers of ethylene and propylene, these are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene. They also contain homopolymers in addition to block copolymers. 

In Ziegler s ETS method based on DFT, the M-H2 bond energy can be decomposed into steric and orbital interaction terms plus a term for the energy... 

One good feature of the Ziegler-Nichols closed-loop method is that it can be learned more quickly than starting with trial and error alone. There is a procedure to be followed, and the pattern of sustained cycling is easy to recognize. The Ziegler-Nichols method is often completely acceptable for tuning control loops that respond quickly, for example, liquid flow rate control loops that respond with an ultimate peak-to-peak time period (UTP) of 5 to 15 s. 

PP fibers are the most recent synthetics to achieve commercial importance. They appeared on the fiber market in the early 1970s and represent by now the fourth major class of fibers besides the other traditional synthetic fibers polyamides (PAs), polyesters (PESs), and polyacrylics. Their starting point was the application of Ziegler and Natta s method for the production of isotactic PPs, which can be processed into films and fibers. Montecatini applied for the patents in 1954 [1, 2],... 

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