Stereoregulation, polymerization

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

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

The present review describes recent developments on in vitro polymer production using an isolated enzyme as catalyst via nonbiosynthetic pathways (enzymatic polymerization). Beyond the in vivo relationship of the key-and-lock theory, in vitro catalysis of enzymes allowed structural variation of monomers and polymers, leading to not only natural polymers but unnatural polymers including new useful materials. In many cases, enzymes catalyzed highly enantio-, regio-, and chemoselective as well as stereoregulating polymerizations to produce a variety... 

Of the three polymerization types radical chain, ion chain, and stereoregulated polymerization, the latter has become the most important, above all in polymerization of the higher oc-olefins. 

In 1953, Ziegler s work then inspired Giulio Natta of Milan to apply these insights to the stereoregulated polymerization of propylene. This work led to discovery of isotactic, syndiotactic, and atactic PP as well as to research into their structural property differences. The CH3 groups can be arranged in various different orders along the carbon chain, a property termed tacticity [3]. 

The following forms of polypropylene are differentiated in the stereoregulated polymerization of this polymer ... 

Montagnoli, G Pini, D. Lucherini, A. Ciardelli, R Pino, P. Quantitative aspects of the stereoselectivity in the stereoregulated polymerization of racemic a-olefins by Ziegler-Natta catalysts. Macromolecules 1969, 2, 684-686. 

The second case is that of polymers obtained from pro-chiral monomers and whose optical activity is the result of a stereoregulating polymerization process. The two examples shown below illustrate this situation ... 

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). 

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... 

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. 

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. 

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. 

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... 

Our study concerns the polymerization of 2-VP, Initiated by organomagneslum derivatives of the type R Mg R, In hydrocarbon solvents. In order to obtain a detailed knowledge of the stereoregulating mechanism, we focused this study on the determination of the nature and of the structure of active centers. 

Each of the fluorinated catalysts has an optimum temperature-range for stereoselective polymerization. At the lower temperatures, the rate of propagation and yield of polymer decrease dramatically. At higher temperatures, the molecular weight of the polymer produced becomes lower, presumably because chain transfer or termination processes increase in importance. At still higher temperatures, the stereoregulation is lost, and the low-d.p. polymer produced has a mixed, anomeric configuration. 

Results from polymerizations of various types of homologs of monomers indicated that, in general, the requirement imposed on the catalyst to realize the stereoregulation was severest for the monomer having the smallest side group, i.e., the lowest homologs. Therefore, acetaldehyde and propylene oxide were most extensively investigated. 

Fig. 1. The proposed model of stereoregulation in the propagation process of isotactic polymerization of acetaldehyde by organoaluminum catalyst [Natta...

We should like to consider a molecular mechanism of stereoregulation in the stereospecific polymerization of aliphatic monoaldehydes by carrying out a molecular model examination based on the structure, chemical behavior and catalytic behavior of the catalyst. 

In this mechanism, the stereoregulation in the propagation reaction is realized faithfully by the steric effect acting between the incoming monomer and the growing end monomeric unit, both of which have interactions with an aluminum atom. The penta-coordinate aluminum atom of the catalyst, which is similar to that shown in the dimeric acetaldehyde-catalyst complex, plays an important role as an intermediate compound or as a transition state. In principle, an identical mechanism may be applied to the polymerization by the dialkyl-aluminum monoalkoxide catalyst. 

Poly[o-phthalaldehyde] was reported to be soluble in organic solvents, in contrast with isotactic polyacetaldehyde. This solubility behavior afforded us a good chance to test the stereoregulating capacity of our catalyst in the polymerization process. 

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