Chain syndiotactic

In dilute solution, the dependance of LnRj on solvent polarity for copolymers is definitely measurable, but it is significantly reduced with respect to that of the model compounds, by a factor of about 3 for a predominantly syndiotactic chain bearing keto-2-picolyl functions in the form of isolated units (DSm = 0.129,... 

Figure 10.23. Syndiotactic, chain-end controlled polystyrene catalyst...

The theory of strain birefringence is elaborated in terms of the RIS model as applied to vinyl polymer chains. Additivity of the polarizability tensors for constituent groups is assumed. Stress-birefringence coefficients are calculated for PP and for PS. Statistical weight parameters which affect the incidences of various rotational states are varied over ranges consistent with other evidence. The effects of these variations are explored in detail for isotactic and syndiotactic chains. 

The RIS model, coupled with the Flory matrix method, is applied to the calculation of the unperturbed mean-square end-to-end distance in polylcyclohexene sulphone) as a function of several parameters. The calculations are performed for atactic, isotactic and syndiotactic chains the tacticity arises from the two possible ways, D and L, in which the rings can be attached to the main chain, assuming that the C—C bonds are all in the trans conformation, as indicated by dielectric measurements. 

On the basis of x-ray and infrared studies we may now accept the syndiotactic chain configuration for crystalline PVC. If, in addition, we accept the proposed crystal structure [Natta and Corradini (154)), it is possible to base the symmetry analysis of the spectrum on the unit cell instead of just on a single chain, as was done in the earlier work [Krimm and Liang (101)). The results of this analysis are given here. The unit cell, shown in Fig. 10, has the following symmetiy elements E, Cs2(a), 6(b). C (c), [Pg.123]

With conventional polymerization processes, atactic chains are predominantly formed for the formation of isotactic and syndiotactic chains a special catalyst system is required, e.g. Ziegler-Natta catalysts. Such a process is called stereospecific polymerization. It enables the manufacture of, i.a., technically usable PP and also unbranched PE (see 4.1). The newest development is the metallocene katalyst it enables the building-up of chains-to-measure with very high degrees of chain regularity also the manufacture of syndiotactic polystyrene is technically possible in this way (see Qu. 2.47). 

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

The CH3 groups can be arranged on the same side of the chain (called an isotactic chain) as shown above, can alternate (called a syndiotactic chain) as shown below,... 

Figure 6.24. Illustration of the two limiting cases of perfect stereoregularity, for a vinyl-type chain backbone. One diad (pair of -CXR- groups) is shown. X and R denote atoms or groups attached to backbone carbon atoms, with X R. The thick lines represent bonds coming out of the plane of the paper, while the dashed lines represent bonds going below the plane of the paper. X s and the R s point above and below this plane, respectively, for the isotactic chain. They alternate regularly in direction (handedness) for the syndiotactic chain. The limit of total lack of stereoregularity (an atactic chain) occurs when the isotactic and syndiotactic diads are equal in number on a polymer chain and are distributed completely randomly over its length.

If polymers of a-alkenes are regarded as infinite chains in their most symmetrical zigzag conformation, only atactic polymers can be chiral. Infinite isotactic polymers have a mirror plane along their chain and numerous mirror planes perpendicular to the chain. Infinite syndiotactic chains of ac-alkene polymers contain mirror planes in all tertiary carbon centers perpendicular to the chain. For convenience, the model of infinite polymer chains can be replaced by analogous cyclic systems (cyclopropanes for triades, cyclobutanes for tetrades, etc.). 

Syndiotactic chain a polymer chain in which the substituent groups such as CH3 are arranged on alternate sides of the chain. 

In most NMR spectra of stereoregular polymers, the key factor for the analysis is the methylene group in the polymer chain. In an isotactic molecule, the two methylene protons are not identical, and therefore, two resonances must be observed. For a syndiotactic chain, the two protons in the methylene group are identical by symmetry, so only one resonance is observed [27]. 

The spectra of stereoregular polymers show a single sharp line for each chemically distinct carbon because, within each type of chain, each monomer residue is identical. However, the chemical shifts for isotactic and syndiotactic chains are not the same. 

These words are based on the Greek roots iso- = same, a- = not, tactos = form. Isotactic chains contain exclusively mm monomer placements syndiotactic chains contain exclusively mr or rm placements, while atactic chains contain 25% rr, 25% mm and 50% mr placements. Special catalysts are necessary to produce isotactic polypropylene, which is highly crystalline, whereas atactic polystyrene is produced without the use of special catalysts. It is possible to produce isotactic polystyrene and atactic polypropylene, but these have very limited markets. 

The reverse is true for the syndiotactic chain in which the substituents are all located on one side. 

Steric factors play an important role. A high is obtained when large pendant groups attached to the ehain restrict its internal rotation, and bulky pendant groups tend to impede crystallization, except when arranged regularly in isotactic or syndiotactic chains. 

More generally, isotactic means that all repeat units have the same handedness and syndiotactic means that the handedness alternates along the chain. Alternate chemical repeat units along the syndiotactic chain are mirror images of each other. In the translationally symmetric conformations shown in fig. 4.2 the translational repeat unit for the syndiotactic chain is two chemical repeat units, whereas it is only one for the isotactic chain. 

---