Isotactic, atactic, and syndiotactic

Stereochemistry. The field of organic chemistry devoted, to three-dimensional spatial arrangements of molecules. Deals with stereoisomers, compounds having identical chemical formulas but different spatial arrangement of their atoms, such as geometric (cis/trans) isomers and optical (isotactic, atactic, and syndiotactic) isomers. 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg= — 108°C) and trans(T = — 18°C) or polyisoprene cis Tg = —73°C) and trans T = —53°C). An example of tacticity variation is polyfmethyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

The thermal properties of isotactic, atactic and syndiotactic PPBMA are illustrated by the D.S.C. traces in Figure 3. None of these samples showed a melting endotherm. Thus even a syndiotactic configuration (extent of tacticity 90%) is not able to promote efficient packing of the side groups for these polymers. 

Figure 3.1 The 25-MHz spectra of three preparations of polypropylene isotactic, atactic, and syndiotactic. (From Bovey, F.A., Chain Structure and Conformation (f Macromolecules, Academic Press, New York, 1982. With permission.)...

In polymethyl methacrylate (PMMA), heat capacity studies confirm that T is a substantial function of tacticity (T = 45°, 100°, 115°C fo the isotactic, atactic and syndiotactic forms respectively) and that AC at T. and C itself are different for the stereo-isomers, (Fig. 3,4 ) (6). ... 

Carbon-13 spectroscopy is particularly effective in the study of paraffinic polymers because the chemical shifts are very sensitive to structure, being spread over a ca. ppm range as compared only about 2 ppm for protons. In Fig. 5 are shown the spectra of isotactic, atactic and syndiotactic polypropylenes. Although all three carbons, the CH2 ot-CH, and CH3, are responsive to configuration, the CH3 carbon is clearly the most sensitive. 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene ... 

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... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

An expression for M for an atactic aryl vinyl polymer generally cannot be written in closed form, because the nonzero values of kin> permit a make-and-break form of energy migration. However, if kJn,/kD syndiotactic triads by taking kr>M — 2 koMmeJO kDM,m o + koM.dii and 2khM,di. respectively. The value of M for the polymer is then obtained by averaging over the triad population of the polymer. 

In sterically random propagation, the probability of meso diad generation Pm in each addition is equal to 0.5, and the ratio of triads in the chain is i h s = = 1 2 1. In an atactic polymer, the diads m and r are statistically distributed in the chain. Natta [81] called the simplest stereoregular chains isotactic —mmmm— and syndiotactic —rrrr—. Monomers with both a and / carbons unsymmetrically substituted can form diisotactic polymers... 

In general, atactic polymers tend to be soft, amorphous material, whereas the more regular structures of isotactic 54 and syndiotactic 55 polymers permit chains to lie closer together, and accordingly these polymers have a greater tendency to be crystalline. The terms atactic, isotactic and syndiotactic are quite general and are used for other similarly substituted polymers. 

Figure 2. Variation of melting temperature with crystallization temperature for atactic isotactic (O),and syndiotactic (l ) PPBA...

M = Li, K, and Cs), in combination with either liCl or Et2Zn. Moreover, syndiotactic-rich, isotactic-rich, and atactic polymers could be obtained by turning the initiator system. The atactic and syndiotactic-rich polymers were both soluble in water, whereas the isotactic-rich polymers (m > 69%) were insoluble in water. The atactic polymers showed Tc-values of around 32 °C, similar to those of the polymers obtained by radical polymerization, while a Tc-value of 37 °C was observed in a syndiotactic-rich polymer (r = 83%). 

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