Polyolefins syndiotactic

Heterogeneous Emulsion dispersion Suspension Precipitation Solid catalyzed Vinyl polymers (styrene, MMA, PVC) Vinyl polymers (styrene, MMA, PVC) Vinyl polymers (PVC, PAN, PVDC) Polyacetals, vinyls Polyolefins, syndiotactic polystyrene Polyamides, solid state polymerization... 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

Inspired by the design of metallocence catalysts, there has been research into the use of C2-symmetric Ni a-diimine catalysts for the preparation of polyolefins with stereoregularity. Such catalysts were shown to afford higher degrees of isotacticity in polypropylenes as compared to the standard C2v-symmetric catalysts, which afford mostly syndiotactic polymer [106], Coates and coworkers have studied... 

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

An analysis of the ionic factors for the polymerization of dienes to cis and trans structures is possible in the same way as for isotactic mono-enes. The mechanism which controls the steric structure of poly 1,4 dienes is parallel to that we have already seen for the mono-olefins. Roha (2) listed the catalysts which polymerize dienes according to the polymer structures produced. It was shown that the highly anionic as well as the highly cationic catalyst systems with increasing ionic separation produced trans-poly-1,4-dienes. This is analogous to the production of syndiotactic polyolefins. 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

Metallocenes are very versatile catalysts for the production of polyolefins, polystyrene and copolymers. Some polymers such as syndiotaetic polypropene, syndiotactic polystyrene, cycloolefin copolymers, optically active oligomers, and polymethylenecycloalkenes can be produced only by metallocene catalysts. It is possible to tailor the microstructure of polymers by changing the ligand structure of the metallocene. The effect and influence of the ligands can more and more be predicted and understood by molecular modeling and other calculations. 

The Ziegler-Natta catalysts that are used most industrially are solids that are suspended in the reaction medium. The polymerization reaction that is favored depends on the catalyst components and on their state of aggregation and other details of their preparation. Heterogeneous catalytic systems appear to be necessary for the production of isotactic polyolefins, but soluble catalysts have been used for the synthesis of polyethylene and syndiotactic polypropylene. 

A further breakthrough was the synthesis of enantiomeric sterorigid ansa-metallocenes by Brintzinger and co-workers [33] and the discovery by Ewen [34] that such racemic metallocene/methylalumoxane systems generate isotactic polypropylene. It was further found that the metallocene structure determines the polymer structure [35-37]. Again, with these compounds polyolefins such as syndiotactic polypropylene become available on a large scale [38]. Indeed, metallocene/methylalumoxane catalysts offer new prospects for olefin oligomers and polymers [39 2],... 

An isotactic structure is one in which the optically active centers of the repeat units all have the same absolute stereochemistry (G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio, J. Am. Chem. Soc. 1955, 77, 1708) in a syndiotactic polymer, neighboring units have opposite stereochemistry. If an isotactic polyolefin is drawn in its extended conformation, it will have all its substituents pointing in the same direction. If an isotactic polyketone is drawn in its extended conformation, the substituents will alternately point up and down. 

Shiomura, T. Uchikawa, N. Asanuma, T. Sugimoto, R. Fujio, I. Kimura, S. Harima, S. Akiyama, M. Kohno, M. Inoue, N. Metallocene-catalyzed Syndiotactic Polypropylene Preparation and Properties. In Metallocene-based Polyolefins Preparation, Properties and Technology, Scheirs, J., Kaminsky, W., Eds. Wiley Chichester, 2000 Vol. 1, p 467. 

The original, simplest polyolefins, polyethylene and polypropylene, continue to dominate the scene, even after two decades, to such an extent that no other polyolefin even appears on the production charts. Nevertheless, a great many (we may assume all) available olefins have been tested, and many have been found capable of being converted to stereoregular polymers. As was mentioned above, poly(l-butene) and poly(4-methy1-1-pentene) are being offered commercially and may be expected to achieve significant volume in the future. Isotactic and syndiotactic polystyrene are of much theoretical interest (26) but are not yet commercial products. 

Polytransoctanamer (PTO) has been used as a high performance elastomer and in blends with commodity and engineering resins. Polyoctadecene (POD) blends with PP are thermochromic. The most interesting are the metallocene-type polycyclic polyolefins, e.g., polycyclopentene or polynor-bornene, either syndiotactic or isotactic with T =400-600°C. 

Recovery of properties may be based on specific polymers and their blends (viz. syndiotactic polystyrene, polyesteretherimide, some blends of polyphenyleneether) that have been shown to maintain high performance for at least 5-10 re-extrusions. It has been also shown that properly stabilized resins (e.g., polyolefins) are able to maintain performance for at least five re-extrusions [Herbst et ah, 1997]. 

Just as polydienes and polyolefins have been modified in solution, they have also been surface-modified. For example, polar functional groups have been introduced to syndiotactic 1,2 polybutadiene surfaces by photolytic addition of thiols, as well as treatment with aqueous permanganate. Such si ace functionalization leads to changes in wettability, as evidenced by contact angle measurements. Furthermore, the surface functionalization can be reconstructed upon heating. In turn, polyolefins have been surface modified by oxidation, chlorination, sulfonation, etc. °... 

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