Macromolecules atactic

When studying a polymer, one should first determine its solubility. This is very characteristic for macromolecules and can serve as an early means of characterization. Important examples are the recognition of crosslinking, the separation and distinguishing between tactic and atactic macromolecules and the identification of copolymers. Moreover, solubility is a prerequisite for most physical measurements. 

Isotacficmacromolecnle - Atactic macromolecule, essentially comprising only one species of repeating unit which has chiral or prochiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. [8]... 

T acticity - The orderliness of the succession of configurational repeating units of a macromolecule or oligomer molecule. In atactic macromolecule essentially all the configurational repeating units are identical with respect to directional sense. See Configurational repeating unit, Isotactic, Syndiotactic. [8]... 

Thus, in vinyl polymers, only atactic macromolecules can be chiral in the first model with infinite length chain. The atactic and the syndiotactic macromolecules with an even number of monomer residues are chiral in the finite chain model with identical end groups, whereas all isotactic, syndiotactic and atactic chains have a chiral structure for the last model with different end groups [1,2]. 

Arai, T, Sawatari, N., Yoshizaki, T., Einaga, Y. and Yamakawa, H. (1996) Excluded-volume effects on the hydrodynamic radius of atactic and isotactic oligo- and poly(methylmethacrylate)s in dilute solution. Macromolecules, 29, 2309-2314. 

Macromolecules, 35, 7110 (2002). Segmental Dynamics of Atactic Polypropylene as Revealed by Molecular Simulations and Quasielastic Neutron Scattering. 

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et al. (Macromolecules, 1983. 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, it appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. 

Figure 9. 90 MHz C NMR spectnim of atactic poly(vinyl alcohol) (148). From Ovenall, D.W. Macromolecules 19 4, 17, 1458. Copyright (1984) American Chemical Society.

The atactic polymers, as well as the linear head-to-tail vinyl polymers consisting of macromolecules having different terminal groups, obtained by the known polymerization processes, are in general optically inactive owing to intramolecular compensation. 

G. D. Smith, R. L. Jaffe, and D. Y. Yoon, Macromolecules, 26, 298 (1993). Conformations and Order in Atactic Poly(vinyl chloride) Melts from Molecular Dynamics Simulations. 

Figure 5.12 Phase diagram of atactic polystyrene in nitro-propane. The theta temperature is 0 — 200 K. (Reprinted with permission from Tan et al.. Macromolecules 16 28. Copyright 1983 American Chemical Society.)...

Greenfield, M. L., and Theodorou, D. N., Geometric analysis of diffusion pathways in glassy and melt atactic polypropylene. Macromolecules 26, 4561 (1993). 

An atactic polymer is a regular polymer with macromolecules composed of a certain number of statistically distributed configurational units. The constitutional unit is a type of atom or group of atoms composing the macromolecule (e. g. —[CH2—CHPhJ- or —[CHPh— in polystyrene). The configurational unit is a constitutional unit with one or several stereoisomer-ic centres. These definitions would require a more detailed explanation. In this volume they will only rarely be used, the stereochemistry of jjolymers is a special branch of macromolecular chemistry. More information can be found in the original literature [2]. 

In this paper, we report the first extensive sub-ambient VT-MAS 13C T j and Tj data on macromolecules. The emphasis of the study was placed on isotactic poly(propylene)(PP) and atactic poly(methylmethacrylate)(PMMA) as they represent semi-crystalline and glassy polymers, respectively. Specifics of the investigation were directed to the issue of elucidating sidechain and backbone motions from the high frequency relaxation experiments. 

Blends of atactic poly(methyl methacrylate) with poly(ethylene glycol), PMMA/PEG, were reported miscible [Colby, 1989]. Their rheology, PMMA/PEG = 50/50 and 80/20 at T = 160-210°C, was studied in a dynamic shear field [Booij and Palmen, 1992]. By contrast with homopolymers, the blends did not follow the time-temperature superposition. The deviation was particularly large at low temperatures. The reason for the deviation is most likely based on the different temperamre dependence of the relaxation functions. The authors concluded that in miscible blends, the temperature dependence of the relaxation times of individual macromolecules depends on composition. This leads to different degrees of mutual entanglement and hence the rubber plateau moduli. 

---