Sequence length configurational

It is basically a fractionation process that depends not only on molecular size, but also on chemical composition, stereo-configuration, branching, and crosslinking. For multicomponent systems, fractionation with different ion polymolecularity, chemical heterogeneity and sequence length distribution, solubility or elution fractionation is of primary importance. Therefore, gel permeation chromatography or size exclusion chromatography is used as an important tool for the characterization of PBAs. 

When measuring vinyl polymer tactlclty, one prefers the longest complete n-ad distribution available as well as the translated simplest comonomer distribution, possibly m versus r. An alternative exists to the m versus r distribution In the form of number average or mean sequence lengths. If any vinyl homopolymer Is viewed conceptually as a copolymer of meso and racemic dyads, mean sequence lengths can be determined for continuous runs of both meso and racemic configurations (32), that Is,... 

In addition to viewing a vinyl homopolymer conceptually as a copolymer of meso and racemic dyads, one may also consider the mean sequence length of "like" configurations (M). In this Instance, the polymer chain Is seen as a succession of different lengths of co-orlented configurations from one to "n", the longest sequence of like configurations, that Is,... 

As was true in the case of mean sequence lengths for meso and racemic dyads, the necessary relationships can be used to develop corresponding equations for any particular n-ad distribution. A favorable point concerning the concept of "like" configurations is that it attaches a physical significance to the racemic distribution. 

Mean sequence lengths can also be used to write average configurational structures, that is. 

Note Configurational sequences comprising two configurational units are termed diads, those with three such configurational units triads, and so on. In order of increasing sequence lengths they are called tetrads, pentads, hexads, heptads, octads, nonads, decads, undecads, etc. 

We have carried out a preliminary study (61) of methyl methacrylate-styrene and methyl methacrylate-a-methylstyrene copolymers which indicates that high resolution NMR spectroscopy of solutions of copolymers can yield valuable information concerning both sequence lengths and stereochemical configurations of the monomer units. At the present stage of this work, however, it is not possible to disentangle sequence and configurational effects satisfactorily, and very much remains to be done. 

In proton spectra of polymers knowledge of the exact values of the coupling constants and the chemical shifts makes it possible to simulate the experimental spectra by computer calculations. In this way it is possible to interpret quantitatively the complex spectra of many homopolymers and copolymers in terms of configuration (or "tacticity") and monomer sequences (copolymers). The obtained sequence lengths can be directly related to the polymerisation mechanism. 

The sequence lengths of configurational y-ads are defined and treated analogously to constitutional 7-ad sequence lengths (see Section 2.4.5). Thus, the number average sequence length of isotactic sequences is... 

A configurational or tactic sequence consists of at least one tactic linkage and, hence, at least two monomeric units. The number of successive linkages of a like nature is defined as the sequence length. The transition from an iso- and a syndiotactic sequence involves a heterotactic triad The number-average sequence length L of all iso- and syndiotactic sequences of a polymer is thus given by the inverse of the mole fraction of all heterotactic triads ... 

Infrared and Raman spectroscopy (considered together) Molecular identification determination of chemical functionality chain and sequence length quantitative analysis stereochemical configuration chain conformation. (d,e)... 

In fig. 4 the well known polypropylene microstructures and those we found in our experiments are compared. In isotactic polypropylene nearly all tertiary carbon atoms have the same configuration. Typical se-quence lengths are about 50 or more. Atactic polypropylene shows a completely statistical distribution of the configuration of the tertiary C-atoms. The isotactic sequence length is two. 

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