Isotactic regulation

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

Table 16. Isotactic polypropene waxes prepared by [Me2Si(2Me-4tBu-Cp)2]ZrCl2/MAO (I), [Me2Si(IndH )2]ZrCl2/MAO (II) compared to waxes produced by conventional catalysts via molecular weight regulation by hydrogen (III) or visbreaking of PP random copolymers (IV) or homopolymers (V) [155]...

In my opinion, an active centre of alkene polymerization in the liquid phase is not a single chemical entity to be visualized by a single (and simple) chemical formula. Probably a set of compounds, of complexes with variable composition, a dynamic system where the effects of individual components are mutually complementary or overlapping is really in play. The same macroscopic effect (centres of equal activity and iso-specific regulating ability) can be obtained with various starting organometals and donors. In such a system, subsystems may exist each of which is externally manifested as an individual active centre (rapid or slow, isotactic, with a tendency to transfer or termination, or living, etc.) [225],... 

Isotactic poly(butene-l) is produced commercially with three-component coordination-type catalysts. It is manufactured by a continuous process with simultaneous additions to the reacticMi vessel of the monomer solution, a suspension of Ti l2-Al l3, and a solution of diethyl aluminum chloride [84], The effluent containing the suspension of the product is continually removed from the reactor. Molecular weight control is achieved through regulating the reaction temperature. The effluent contains approximately 5-8% of atactic polybutene that is dissolved in the liquid carrier. The suspended isotactic fractions (92-98%) are isolated after catalyst decomposition and removal. The product has a density of 0.92 g/cnf and melts at 124—130° . 

Hercules discovered the role of hydrogen as a molecular weight regulator (180). Esso improved performance hy using AICI3 in TiCls solid solution instead of pure TiCls (181). Mitsubishi increased isotacticity to 92-94% by adding an electron donor, such as carboxylic acid ester, to the TiCls (182). 

The helix is the most common regulated higher order stmcture of macromolecules. Many sophisticated functions and intricate biological activities of biomacromolecules largely depend on their well-ordered helical stmctures. Since the discovery of helical stmcture of isotactic polypropylene," various types... 

Syndiotactic polypropylene is formed by the nickel(II)-diimine complex 67 (M=Ni) at low temperature ([rrrr]=0.80 at -78 °C,0.65 at 0 °C) [204,205]. Polymerization proceeds by 1,2-insertion and the stereochemistry is regulated under chain-end control. On the other hand, isotactic polypropylene can be prepared using the iron complexes 68 (M=Fe [mmmm]=0.55-0.67 at -20 °C) despite the low molecular weight of the polymer [206]. Polymerization proceeds via a 2,1-insertion mechanism by chain-end control. 

Inclusion polymerization of various monomers in PHTP is similar to urea and thiourea The polymerization was initiated by 7-ray irradiation which gave highly regulated polymers for example, the polymerization of 1,3-tra/i.s-pentadiene gave the l,4-/ran5-isotactic polymer [66]. 

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