Stereoregulation polymers

As reported in 1959 at the Symposium on Stereoregulated Polymers in Boston (I), it is possible to combine three molecules of butadiene into 1,5,9-... 

There is a large amount of literature and many patents in this area, as well as many good reviews and books [8,9,10,11,12,13,14,15,16,17,18,19]. The recent review by Coates [10] describing stereoselective polymerization overlaps considerably with this chapter, and is recommended for consultation. In this chapter, metallocene-catalyzed olefin polymerization is discussed, focusing on the synthesis of stereoregulated polymers. The aim of this review is not to be a complete survey the outline and some recent topics in polymerization of propylene, higher a-olefins, styrene, acrylate esters such as methyl methacrylate (MMA), 1,3-butadienes, and cycloolefins will be described. Polyethylene is one of the most important commercially manufactured polymers. The homopolymer, as well as the copolymer with ethylene and other olefins, is an important subject in the polyolefin industry. However, it will be only briefly mentioned because the stereochemistry is less involved. 

The ease of ligand design in metallocene catalysts has made possible novel stereoregulated polymers that have not been seen with conventional Ziegler-Natta catalysts. Hemi-isotactic polyolefins, and isotactic-atactic block polymers are examples that will be discussed in Sect. 2.5. 

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] ... 

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-... 

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. 

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. 

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... 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

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. 

Sequences of sterically identical steps lead to isotacticity of the product. Active sites with two vacancies or with one vacancy and a loosely bound chlorine characteristic of p-TiCl3 with a chainlike structure are nonstereospecific and result in polymers with increased atacticity.306 Molecular mechanics studies on the basis of this model lent additional support for the asymmetric active center being responsible for stereoregulation.341 It was concluded that the main factor determining... 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Investigation of the stereoregulation and polymerization mechanism using optically active monomers and polymers... 

ZIEGLER-NATTA POLYMERIZATION. Polymerization of vinyl monomers under mild conditions using aluminum alkyls and TiCL lor other transition element halide) catalyst to give a stereoregulated, or tactic, polymer. These polymers, in which the stereochemistry of the chain is not random have very useful physical properties. 

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