Tacticity atactic

Very little research has been done on the relations between glass transition temperatures and tacticity. Atactic and isotactic poly(styrenes) almost always have the same glass transition temperatures, and this is also the case for at- and it-poly(methacrylate). The glass transition temperature of it-poly(methyl methacrylate) (42° C), on the other hand, is distinctly lower than that of the atactic product (103°C). 

For instance, the Tg of PP depends strongly on its tacticity. Atactic PP has a Tg—19 C and is used in bitumen and hot-melt adhesive formulations, while for isotactic PP, it is approximately -8°C. The latter is a tough, semicrystalline plastic used in packaging films. The Tg of syndiotactic form of PP can be as high as ISO C Sophisticated catalyst systems even aUow synthesis of polymers that have ordered blocks of atactic materials and isotactic materials on the polymer chains. 

Tacticity % atactic (can be produced in isotactic form from its butyl ester) Miller, M L O Donnell, K Skog-man, J, J. Coll. Sci., 17,649-59, 1962. 

As an alternative to the electrophific functionalization of PS, the iridium-catalyzed borylation of aromatic C—H bonds to functionalize PSs with different tacticities (atactic, isotactic, and syndiotactic Fig. 24A) was reported. Because the borylation did not occur at the C—H bonds of sp carbons, it did not affect the tacticity of PSs. Similar to the case of molecular arenes, this C—H borylation of PS was achievable with bis(pinacolato) diboron (B2pin2) or pinacolborane (HBpin) and generated a mixture of meta- and para-substituted borylated PSs. However, the ratio of the two isomers was different than that of isopropylbenzene because a phenyl ring was attached to every other carbon in the PS backbone, the meta C—H bonds of PS were more sterically hindered than the para C—H bonds. Thus, while the ratio of meta- and para-substituted isomers in the C—H borylation of isopropylbenzene was 7 3, the C—H borylation of PS under identical... 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Figure 1.2 Sections of polymer chains of differing tacticity (a) isotactic (b) syndiotactic (c) atactic.

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). 

Microstructure. Interest in PVP microstmcture and the potential for tacticity has been reviewed (39,40). PVP generated by free radicals has been shown to be atactic except when polymerization is conducted in water. In this case some syndiotacticity is observed (40). In the presence of syndiotactic templates of poly(methacryhc acid) (or poly(MAA)), VP will apparentiy polymerize with syndiotactic microstmcture, although proof is lacking (41—45). The reverse, polymerization of MAA in the presence of PVP, affords, as expected, atactic poly(MAA) (46,47). 

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Figure 31.1 Isotactic, syndio-tactic, and atactic forms of polypropylene.

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

The algorithm also incorporates tacticity control for vinyl chains. The random number generator is used to choose between d- and l-versions of the transformation matrix. A single parameter controls the relative probability of d- and l-residues. The poly(p-fluorostyrene) results presented here are for atactic (stereochemically irregular) chains. 

PP synthesized using TiCl4/Et3Al is mostly isotactic, but two minor fractions are also produced. One is a soluble, atactic PP, whilst the other fraction is a partially crystalline, elastomeric stereoblock of iso- and a-tactic PP sequences.98 Elastomeric PP may also be prepared using the ansa-titanoccnc complex, (26), (although this catalyst does undergo rapid deactivation).99 Stereoblock formation was attributed to an equilibrium mixture of slowly interconverting isospecific and aspecific catalyst sites. Other stereoblock PP materials have been prepared via chain transfer between two catalysts of different stereoselectivities.101,102... 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

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