Molecular Weight Dependency of Tacticity

Molecular weight dependence of tacticity and copolymer composition... 

Section 7 introduces a recently developed on-line coupled gel permeation chromatography (GPC) and NMR system, in which a 500-MHz H NMR spectroscopy is used as a detector for GPC. The system facilitates evaluation of molecular weight dependence of polymer characteristics such as tacticity and comonomer composition and determination of molecular weight without using standard polymer samples. 

The molecular weight dependence of polymer structures such as tacticity or copolymer composition can also be easily evaluated by on-line GPC/NMR. Data on the molecular weight dependence of polymer structures are very useful for understanding the mechanism of polymerization or the properties of polymers, and are usually collected by the fractionation of the polymer and the subsequent structural analysis of each fraction. However, this method requires a lot of time and rather a large polymer sample. The on-line GPC/NMR method is very useful for this purpose it needs only a few hours and a very small sample (0.5 1.0mg). 

It is worth noting that a single sample with a broad MWD is enough to make the calibration curve if the number of end-groups per chain is well-defined. Online detection by H NMR offers not only an absolute calibration curve but also information on the molecular weight dependency of some polymer properties such as tacticity or copolymer composition at the same time. Thus, efforts... 

The on-line GPC/NMR technique is also useful for studying the molecular weight dependence of polymer properties such as tacticity and copolymer composition. This type of information is very important for an understanding of the mechanism of polymerization. The polymerization of MMA by anionic initiators often involves multiple active species with different reactivities and stereospecificities. For example, Grignard reagent exists in the Schlenk equilibrium ... 

Figure 1 presents the dyad tacticity determined from 300 MH NMR (5) versus the Tg measured by DSC for the poly(alkyl a chforoacrylates) studied. All values are corrected to "infinite" molecular weight as follows. The molecular weight dependence of Tg for a linear polymer is given by the Flory-Fox relationship... 

It is known that the physical properties of a polymer depend not only on the type of monomer(s) comprising it, but also on the secondary and tertiary structures, i.e., the stereochemistry of the linkage, the chain length and its distribution, its ability to crystallize or remain amorphous under various conditions, and the shape or distribution of the shapes of the chain in the crystalline and amorphous states. Through advances in polymer chemistry, in most cases polymers can be designed with specific properties. Control of the microstructure, e.g., the tacticity and molecular weight distribution of vinyl polymers, has been the focus of a number of papers in the last two decades. 

Polymer physical and chemical properties also depend upon the R group, the molecular weight, and the tacticity of the polymer. 

This section treats the flow behavior of SPS and particularly focuses on the temperature and molecular weight dependencies. The subject of this section is limited to the flow behavior of SPS in the molten state above its melting point. The effects of the tacticity on the rheological behavior in the amorphous state are discussed by comparing SPS with atactic polystyrene (APS) and isotactic polystyrene (IPS). 

The dn/dc can show a dependence on polymer tacticity (41) and molecular weight, but these effects usually are minor relative to that of polymer composition (40). Also, the... 

The refractive index is an important quantity for characterizing the structure of polymers. This is because it depends sensitively on the chemical composition, on the tacticity, and - for oligomeric samples - also on the molecular weight of a macromolecular substance. The refractive indices (determined using the sodium D line) of many polymers are collected in the literature. In order to characterize a molecule s constitution one requires knowledge of the mole refraction, Rg. For isotropic samples, it can be calculated in good approximation by the Lorentz-Lorenz equation ... 

Xab appears here as a universal parameter. However, it was found experimentally to depend on a number of factors [30-33] such as temperature, molecular weight, composition, inter-monomer distance (and therefore on the scattering vector Q), isotopic constitution, tacticity, microstructure, etc. These dependencies are shortcomings of the crude RPA description. The scattered intensity (macroscopic cross section d (Q)/d 2) is given by ... 

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