Tacticity isotactic

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

The second example is the stereoregularity displayed by monosubstituted vinyl polymers of olefins. As we saw earlier, these types of polymers can occur in three forms of tacticity isotactic, syndiotactic, and atactic. Isotactic and syndiotactic polymers possess stereoregular structures. Generally these polymers are rigid, crystallizable, high melting, and relatively insoluble. On the other hand, atactic polymers are soft, low melting, easily soluble, and amorphous. 

Asymmetrical monomers such as propylene and styrene form polymers with different tacticity. Isotactic and syndiotactic chains have regular molecular structures and are generally semicrystalline, while atactic polymers are amorphous because of the random placement of the monomer imits in the chain. The tacticity distribution may be modeled as a comonomer distribution, where the different monomer orientations during chain insertion (meso or racemic) are treated as pseudo-comonomer t5T)es. Therefore, at least in principle, Stockmayer s distribution can be used to approximate the tacticity distribution of homopolymers. 

Tacticity % isotactic 25-29, heterotactic 47-51, and syndiotacic 22-27 in radical polymerization syndiotactic (mainiy orthorhombic), isotactic (tetragonai) ... 

The first extensive lUPAC publications on stereochemistry in high polymers were published in the early 1960s, and subsequently in 1966 as a single article. In addition to more conventional polymer names, e.g., polyethylidene and polypropylene, the -amer nomenclature was introduced in 1952." lUPAC basic definitions relating to stereochemistry, e.g., tacticity, isotactic polymer, etc., were published in 1974. ... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is 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.

Use zero-order Markov statistics to evaluate the probability of isotactic, syndio-tactic, and heterotactic triads for the series of p values spaced at intervals of... 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

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). 

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. 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

The isotactic form of propylene has better physical and mechanical properties than the three tactic form mixture (obtained from free radical polymerization). Isotactic polypropylene, in which all of the stereo cen-... 

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

Tacticity of VIA A is influenced by solvent,90 9 the presence of amines (Table 8.3)90 and complcxation. PYIAA appears more isotactic when formed in a nonhydrogen-bonding solvent,90 95 Polymerization of MAA in CHCh in the presence of 14 or 15 also yields a more isotactic polymer90 Polymerization of zinc complexes of MAA also yields more isotaclic polymers,9 >... 

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 ... 

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