Syndiotactic polystyrene/atactic

Hong, B.K., Jo, W.H., and Kim. J. (1998) Miscibility of syndiotactic polystyrene/ atactic polystyrene blends by crystallization kinetics and enthalpy relaxation. Pdymer, 39, 3753. 

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,... 

Remarkable differences in the G values of F12 were also recorded in atactic and syndiotactic polystyrene. The tacticity itself was also affected by the irradiation. Fligh-temperature irradiation with y-rays caused the reduction of isotacticity of isotactic PMMA... 

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). 

Polystyrene (PS), CAS 9003-53-6, is typically found in atactic and amorphous form. Isotactic and syndiotactic polystyrenes can be synthesized, but they are not used in practice since there are few advantages in properties compared to the atactic form. The reticulation of the carbon chains in polystyrene is frequently done using in the polymerization process a certain proportion of 1,4-divinyibenzene or less often of 1,3-divinylbenzene. The resulting polymer in the case of crosslinking with 1,4-divinylbenzene has the idealized structure shown below ... 

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

Althought this decomposition mechanism is not yet well-defined (homolytic cleavage, reductive dimerization. hydride formation, or other mechanisms), it is practically meaningful. Styrene polymerization by CpTiMea/FAB carried out in the low-temperature regime (below the decomposition temperature) produces only atactic polystyrene, possibly via a car-bocationic polyaddition. However, at higher polymerization temperatures (above the decomposition temperature), highly syndiotactic polystyrene can be obtained by coordinative 2,1-polyinsertion. ... 

Polystyrene is a linear polymer that, in principle, can be produced in isotactic, syndiotactic, and atactic forms. The commercial product or general-purpose polystyrene is atactic and as such amorphous isotactic polystyrene is more brittle and more difficult to process than atactic polystyrene. It is therefore not of commercial interest. 

Other symbols occasionally used in the literature include (br) for branched materials and (iso), (syndio), and (a) for isotactic, syndiotactic, and atactic structures respectively. No symbol appears to exist for mechanical blends, although these materials are obviously important. Where necessary the symbol -m- will denote a mechanical blend, for example, poly(styrene-m-butadiene) for a mechanical blend of polystyrene with polybutadiene. 

The mutual repulsion between substituents may cause some displacement. As a result, the plane of symmetry is bent in the form of a helix. This occurs also in biopolymers (double-helix of deoxyribonucleic acid (DNA)). Different stereoisomers have different mechanical and thermal properties. For example, atactic polystyrene is an amorphous polymer whereas syndiotactic polystyrene is a crystalline substance. The chemical design of macromolecules determines their properties as extent of crystallization, melting point, softening (glass transition temperature), and chain flexibility which in turn strongly influence mechanical properties of the materials. 

Give a definition of and illustrate by examples on polystyrene, isotactic, syndiotactic, and atactic arrangement of the macromolecules. This should include Fischer and Newman projections. 

Polystyrene that is manufactured by free-radical polymerization is atactic. Isotactic polystyrene formed with Ziegler-Natta catalysts was introduced commercially in the 1960s, but failed to gain acceptance. Syndiotactic polystyrene is now being produced commercially. 

How are atactic and syndiotactic polystyrenes prepared commercially Describe and explain. 

Polystyrene is an example of a commercial polymer whose properties depend on whether it is syndiotactic or atactic. 

Syndiotactic polystyrene, as generally polymerized with monocyclopentadienyl catalysts, has a high percentage of rrrr pentad structure. The nmr chemical shift for the phenyl-1 carbon and backbone methylene carbon are approximately 145.3 and 44.9 ppm, respectively (1,2). In general these polymers are found to be greater than 99% pnre in syndiotactic structure as defined by nmr. sPS can be further purified of any atactic polystyrene (aPS) developed during the polymerization procednre by extraction with solvents such as methyl ethyl ketone (MEK). 

Apart from Ti(IV), titanium complexes of other oxidation states (Ti(III) and Ti(II)), such as Ti(acac)3 (acac = acetylacetonate) and TiPh2, also show syndioselective polymerization activity and produce syndiotactic polystyrene. As for the Ti(0) complex, Ti(bipy)3 (bipy = 2,2 -bipyridine), only atactic polymer is produced (Table 14.1, entries 10-12). ... 

Titanium complexes with acetylacetonate (acac) ligands were first tested for the polymerization of styrene by Ishihara et al." and Zambelli et al When activated with MAO, Ti(acac)2Cl2 and Ti(acac)3 can produce syndiotactic polystyrene with low activities, whereas their zirconium analogues only afford atactic polystyrene. Hu and coworkers studied a series of MAO activated 1,3-diketonato titanium complexes (Figure 14.11, 53-57) and found that their activities increased in the following... 

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