Polymerization, stereoregular

By enantioselective polymerization polymer chains, each containing only one configurational kind of monomeric unit, are produced from a mixture of stereoisomeric monomer molecules. The number of kinds of polymer chain generated therefore equals the number of various stereoisomers in the monomer mixture. In the course of propagation, the enantiomeric composition of the polymer and unreacted monomer remains identical to the intial composition. When optically active monosubstituted cyclic monomers are polymerized, stereoregular polymers are formed with both isotactic polyR and polyS chains... 

Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

Stereoregular polymerization. This is also taken up in Chap. 7. 

Stereoregular polymerizations strongly resemble anionic polymerizations. We discuss these in greater detail in Chap. 7 because of their microstructure rather than the ionic intermediates involved in their formation. 

The statistical nature of polymers and polymerization reactions has been illustrated at many points throughout this volume. It continues to be important in the discussion of stereoregularity. Thus it is generally more accurate to describe a polymer as, say, predominately isotactic rather than perfectly isotactic. More quantitatively, we need to be able to describe a polymer in terms of the percentages of isotactic, syndiotactic, and atactic sequences. 

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

Rate of polymerization. The rate of polymerization for homogeneous systems closely resembles anionic polymerization. For heterogeneous systems the concentration of alkylated transition metal sites on the surface appears in the rate law. The latter depends on the particle size of the solid catalyst and may be complicated by sites of various degrees of activity. There is sometimes an inverse relationship between the degree of stereoregularity produced by a catalyst and the rate at which polymerization occurs. 

Rubber Modifiers. Derivatives of furan and tetrahydrofurfuryl alcohol are used in the polymerization of synthetic mbber to control stereoregularity and otherproperti.es (149,150). 

Eastman Chemical has utilized a unique, high temperature solution process for propylene polymerization. Polymerization temperatures are maintained above 150°C to prevent precipitation of the isotactic polypropylene product in the hydrocarbon solvent. At these temperatures, the high rate of polymerization decreases rapidly, requiring low residence times (127). Stereoregularity is also adversely affected by high temperatures. Consequentiy, the... 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

Stereoregular Polymerization. Chemists at GAF Corporation were first to suggest that stereoregularity or the lack thereof is responsible for both nontacky and crystalline or tacky and amorphous polymers generated from IBVE with BF2 0(C2H )2, depending on the reaction conditions (22,23). In addition, it was shown that the crystalline polymer is actually isotactic (24). Subsequentiy, the reaction conditions necessary to form such polymers have not only been demonstrated, but the stereoregular polymerization has been extended to other monomers, such as methyl vinyl ether (25,26). 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

After brief discussion of the state-of-the-art of modern Py-GC/MS, some most recent applications for stixictural and compositional chai acterization of polymeric materials are described in detail. These include microstixictural studies on sequence distributions of copolymers, stereoregularity and end group chai acterization for various vinyl-type polymers such as polystyrene and polymethyl methacrylate by use of conventional analytical pyrolysis. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Before coordination polymerization was discovered by Ziegler and applied to propene by Natta, there was no polypropylene industry. Now, more than 10 ° pounds of it aie prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in chemistry Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular- polymerization. 

Even more important is the stereoregular catalytic polymerization of ethene and other alkenes to give high-density polyethene ( polythene ) and other plastics. A typical Ziegler-Natta catalyst can be made by mixing TiCU and Al2Eti in heptane partial reduction to Ti " and alkyl transfer occur, and a brown suspension forms which rapidly absorbs and polymerizes ethene even at room temperature and atmospheric pressure. Typical industrial conditions are 50- 150°C and 10 atm. Polyethene... 

Stereoregular polymerization of ethene and propene by catalysts developed by K. Ziegler and by G. Natta (shared Nobel Prize 1963). 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

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