Tacticity isotactic chain

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... 

C NMR spin-lattice relaxations in solid PMMAs of different tacticity were studied by CP/MAS NMR spectroscopy.324-328 The relaxation proceeds more slowly in isotactic PMMA than in syndiotactic PMMA even in the solid state this is because of the faster reorientation of the a-CH3 group in the isotactic chain.135 The nC-T times of the a-methyl carbons of PMMAs with different tacticities were measured over a wide range of temperatures by cross polarization, magic-angle spinning, dipole dephasing (CP/DD)... 

The tyqrical structure of metallocene catalyst molecules is reminiscent of that of enzyme molecules, which also catalyze the uniform s5mthesis of a biopolymer molecule. Metallocenes are also capable of determinate control of polymerization reactions of a given tacticity and chain length, for example the synthesis of isotactic polypropylene with a uniform molar mass [4,5],... 

The second generalisation is that the relaxation times of comparable main-chain atoms will increase, at all temperatures, as more conformations become energetically available to the chain. This may lead to a modest dependence of Ti upon local tacticity. In vinyl polymers, there are indeed several reports of somewhat longer values for isotactic chain segments relative to syndio-tactic segments. These are discussed later in more detail with respect to RIS theory (section 4.5). 

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. 

Stereospecific behavior of the catalyst site is related to the chirality of the surface sites of the solid TiCls. Models by Corradini can explain a number of observations—the type of tacticity errors along a predominantly isotactic chain, stereospecificity of the initiation reaction, and the maintenance of isotacticity after the insertion of ethylene monomer in the chain (241). Furthermore, since violet TiCls and MgCl2 solid-state crystal structures are similar, Corradini s model relates well to both TiCls and MgCl2-supported catalysts. 

Figure 44 Schematic depictions of tacticity (a) isotactic (b) syndiotactic (c) atactic (d) primarily isotactic chain, with one stereo-alteration (e) primarily atactic chain.

Abe et ah have studied the conformational characteristics of a series of poly[oxy(-l-alkylethylenes)]. In the first paper a comparison of the statistical weights of various conformations in poly(propylene oxide), deduced from the solution properties of isotactic and atactic chains, were compared with conformational weights derived from a potential-energy surface, and an anomeric effect identified with a discrepancy between the two. Flexible side-chains influence the main chain in a manner which is expressible in the statistical weight matrices. The dimensions of these chains are strongly influenced by their tacticity. Polymer chains containing both sulphur and oxygen heteroatoms have received the attention of... 

Uryu and Alyiiruk [113] concluded that both structural (head-to-head and tail-to-tail linkages) and steric (syndiotactic dyads) irregularities occur in otherwise isotactic chains and their abundance apparently increased as the melting point and the chain length of the fractions decreased. Various other workers have discussed the application of C-NMR spectroscopy to tacticity studies on PPO [103,116-119]. 

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. 

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 ... 

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. 

The tacticity or distribution of asymmetric units in a polymer chain can be directly determined using NMR spectroscopy and infrared (IR) spectroscopy and has been studied for a variety of polymers. Figure 5(a) and 5(b) show the proton NMR spectra [26,27] and IR spectra [28,29], respectively, for the two stereoisomers of poly(methyl methacrylate) (PMMA), syndiotactic and isotactic PMMA. These two structures in a polymer like PMMA give rise to different signatures in both the techniques. In the case of the NMR spectra [26,27], the... 

PMMA can polymerise to form three different chains, depending on the tacticity of the chiral carbon in the chain. The isotactic (i-) and the syndiotactic (s-) forms are shown in Figure 4. 

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],... 

As for the stereochemistry, for the case of complete cyclization, besides the usual tacticity (possibly, isotactic or syndiotactic, referred to relative configurations of equivalent stereogenic carbons of subsequent monomeric units), the cis or trans configuration of the 1,3-cycloalkane rings which are present in the polymer main chain also has to be considered.70,74... 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

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