Stereoregulation

For the Cossee-Arlman mechanism to operate, migration of the vacant site back to its original position is necessary otherwise, an alternating position is offered to the chemisorbed monomer and a syndiotactic polymer would result. This implies 

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The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

Since the coordination almost certainly involves the transition metal atom, there is a resemblance here to anionic polymerization. The coordination is an important aspect of the present picture, since it is this feature which allows the catalyst to serve as a template for stereoregulation. 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

These polymers could not be crystallized, despite their apparent stereoregularity, probably because of the sterically-hindered character of the chains. It was proposed by Farina and Bressan62 that the chain growth was stereoregulated by the optically active anion of the ion-paired chain carrier. Further studies63 showed that the first portion of the polymer produced in a given reaction always possessed a less regular structure than later portions, unless the reaction was started in the presence of previously prepared polymer. This observation was interpreted as evidence for the pre-... 

Further work on these systems would obviously be very welcome, considering the implications of such stereoregulated mechanisms to biological systems, and in view of the rather unexpected findings reported lately66,67 which add new interest to this field. 

Marks TJ, Stern CL, Chen YXC (1997) Very large counteranion modulation of cationic metallocene polymerization activity and stereoregulation by a sterically congested (pefluoroaryl) fluoroaluminate. J Am Chem Soc 119 2582-2583... 

Acyclic stereocontrol has been a striking concern in modern organic chemistry, and a number of useful methods have been developed for stereoregulated synthesis of conformationally nonrigid complex molecules such as macrolide and polyether antibiotics. Special attention has therefore been paid to the aldol reaction because it constitutes one of the fundamental bond constructions in biosynthesis. 

The major obstacle confronting the implementation of such biomimetic syntheses has been associated with the stereochemical aspects of the aldol process. Over the past few years considerable progress has been made in the development of stereoregulated aldol condensations. This chapter attempts to survey this aspect of the topic. For a more general treatment of the subject the reader is referred to several other excellent reviews (1). 

The versatility of such stereoregulating systems is demonstrated in the polymerization of 1,3-butadiene where all four of the potential structures, isotactic-1,2-, syndiotactic-1,2-, trans-, A-, and cis-, A-, can be synthesized in relatively pure form using different catalyst systems. 

The new soluble stereoregulating polymerization catalysts require three features ... 

FIGURE 5.1 Proposed mechanism for soluble stereoregulating catalyst polymerizations. 

Natta, a consultant for the Montecatini company of Milan, Italy, applied the Zeigler catalysts to other vinyl monomers such as propylene and found that the polymers were of higher density, higher melting, and more linear than those produced by the then classical techniques such as free-radical-initiated polymerization. Ziegler and Natta shared the Nobel Prize in 1963 for their efforts in the production of vinyl polymers using what we know today as solid state stereoregulating catalysts. 

Ultralinear polyethylene (ULPE) has recently become available through the use of soluble stereoregulating catalysts. Along with a decreased amount of short-chained alkyl branching, ULPE has a narrower molecular weight spread. 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

Discuss advantages of the soluble stereoregulating catalysts in comparison with the Natta-Zeigler catalysts. 

How are the difficulties associated with the use of solid state catalysts overcome by the use of soluble stereoregulating catalysts ... 

By 1953, Karl Ziegler and Giuilo Natta discovered a family of catalysts that allowed the introduction of monomer units onto growing polymer chains in an ordered manner. This allowed the synthesis of rubberlike polymers with greater strength and chemical stability in comparison with similar polymers made without the use of these stereoregulating catalysts. 

With the advent of the soluble stereoregulating catalysts, so-called older polymers have been synthesized with additional control over the structure giving products with enhanced strength and dimensional stability. Amorphous PS is relatively brittle, requiring a plasticizer to allow it to be flexible. The use of soluble stereoregulating catalysts allowed the synthesis of sPS with a I m of about 270°C and a Tg of about 100°C with good solvent and chemical resistance. DOW commercialized sPS under the trade name Questra in 1997. It is used in specialty electrical and under-the-hood automotive applications. 

In 1997, Dow introduced sPS under the trade name Questra. The production of Questra is based on relatively new technology and science involving soluble stereoregulating catalysts that produce PS, which has a fixed and repeating geometry as each new styrene monomer unit is added to the growing PS chain. Targeted areas include medical, automotive, and electronic applications. 

Several other produces have been developed based on the relatively new soluble stereoregulating catalysis systems. Index, an ethylene-styrene interpolymer, was introduced in 1998... 

Polymer hypothesis Stereoregulation of polymer structure Organization of polymer chains Synthesis on a solid matrix Polymer structure and control at interfaces Conductive polymers... 

Busico, V., R. Cipullo, F. Cutillo, G. Talarico, and A. Razavi, Macromol. Chem. Phys., 204, 1269 (2003a). Busico, V., R. Cipullo, F. Cutillo, M. Vacatello, and V. V. A. Castelli, Macromolecules, 36,4258 (2003b). Busico, V., R. Cipullo, N. Friederichs, S. Ronca, and M. Togru, Macrolmolecules, 36, 3806 (2003c). Bywater, S., Carbanionic Polymerization Polymer Configuration and the Stereoregulation Process, Chap. 28 in Comprehensive Polymer Science, Vol. 3, G. C. Eastmond, A. Ledwith, S. Russo, and... 

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