Atatic polymers

Polymers in which groups are arranged randomly in space over the chain are known as the atatic polymer. 

Atatic polymers - The arrangement of the asymmetric carbon atoms in the polymer chains are completely random (random d and T) and no proper sequence is maintained. An example is atactic polypropylene. 

PP has lower specific gravity than other plastic materials. Having broad resistance to organic chemical ingredients, they are used in consumer products. The total environmental impact of PP and other thermoplastic materials is less than traditional materials in life-cycle analysis. Commercial PP is a complex mixture of varying amounts of isotactic, syndiotac-tic, and atatic polymers with a given MWD. 

In addition to identihcation of unknowns, NMR can be used for conformational and stereochemical analyses. This includes the determination of tacticity in polymers, that is, whether the side chains are arranged regularly (isotactic and syndiotactic) or randomly (atatic) along the polymer backbone. Fundamental studies of bond distances from dipolar coupling and molecular motion from relaxation time measurements are used by physical chemists and physical organic chemists. In biology and biochemistry, the use... 

Figure 5- A plot of flie CED for a number of different polymers as a function of the temperature. The polymer types are indicated in the legend where the abreviations have the following meaning. PE polyethylene- PEG poly(ethylene oxide) aPP, atatic polypropylene PMMA, poly(methyl mefliacrylate)- PDMS polyfdimethyl siloxane) PTMeO, poly(tetra-methylene oxide) PPG, polvfpropvlen e oxideV PHFP, poly(hexafluoro propylene) PHFPO, polyfhexafluoro propylene LiS 3GT, polyt piylene t PCPT, polycaprolactone PS, polystyrene PEKK,...

Most polyacrylates, especially the atatic and syndiotactic polyacrylates, are amorphous due to the irregular array of pendant ester groups, which prevent the parallel alignment of the polymer backbones. The T s of the different polyacrylates vary from subambient temperature to values above 100 °C. The T and the melting temperature for some polyacrylates are listed in Table 2. 

The gelation mechanism as a result of phase separation is schematically illustrated in Fig. 84 for solutions of monodisperse polymer. Upon cooling a diluted solution, as presented, for example, by point A, phase sq>aTation will take place as soon as the temperature crosses the binodal at point B. The rate of the phase separation process depends on the presence of nuclei as long as the spinodal curve (not illustrated in Fig. 84) is not reached. When the mint C is reached, two phases will develop Ci, the polymer poor phase and C2, the polymer rich phase. In principle, in the long run two clearly distinct pha wiU be created the solvent rich phase is clear, whereas, in neral, the polymer rich phase will be turbid due to the presence of solvent rich droplets which, even after months, are not phase parated due to the high viscosity of the polymer rich phase. 

It has previously been shown for PS, poly (vinyl acetate), and atatic polypropylene that the shift factor of the terminal relaxation or the viscosity aT,n has a weaker temperature dependence than do the softening dispersion ar.s (Fig. 2.11) and the local segmental relaxation ar,a (Figs. 2.19 and 2.20). Therefore, in practice the shift factors ut used to obtain master curves for polymers by time-temperature superposition are actually combinations of the individual shift factors of the several different viscoelastic mechanisms. At low temperatures, aj is principally determined by the shift factor of the local segmental mode aT,a- With increasing temperature, aj is principally determined sequentially by the shift factors of the sub-Rouse modes, r,sR. the Rouse modes, modes in the rubbery plateau, and, finally, the terminal modes, aT,r,- Hence, it is not correct to assume that aj describes the temperature dependence of any or all of the viscoelastic mechanisms in a polymer. 

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