Conversion effect, chain structure

Table 18. Effect of Conversion on Chain Structure of Polyisoprene ...

Several assumptions were made in using the broad MWD standard approach for calibration. With some justification a two parameter equation was used for calibration however the method did not correct or necessarily account for peak speading and viscosity effects. Also, a uniform chain structure was assumed whereas in reality the polymer may be a mixture of branched and linear chains. To accurately evaluate the MWD the polymer chain structure should be defined and hydrolysis effects must be totally eliminated. Work is currently underway in our laboratory to fractionate a low conversion polydlchlorophosphazene to obtain linear polymer standards. The standards will be used in polymer solution and structure studies and for SEC calibration. Finally, the universal calibration theory will be tested and then applied to estimate the extent of branching in other polydlchlorophosphazenes. 

The effect of conversion on the structure of an asphaltene molecule has been reported to depend on the operating conditions and on the presence or not of a catalyst. The effect of thermal processing reaction of a vacuum residue resulted in the selective cracking of the aliphatic or naphthenic side chains of the molecule, leaving the highly condensed aromatic core structure almost intact (see Fig. 16) [116]. 

Effect of Conversion. The effect of increasing degrees of conversion on the chain structure in polymerization of undiluted isoprene is shown clearly in Table II. As can be seen, the extent... 

In this connection, a recently proposed theory (16) to explain the effect of lithium concentration on polyisoprene chain structure deserves mention. This theory is based on a proposed competition between the rates of chain propagation and isomerization of the chain end, which presumably changes from the cis-1,4 to the trans-1,4 configuration. Although this theory may have some merit, it cannot account for the resiilts demonstrated in Tables II, III and V above, i.e., the absence of any effect of temperature or degree of conversion, both of which would strongly affect the propagation rate, but would not be expected to influence the chain-end isomerization rate. It is far more likely, therefore, that the effects on chain structure described above are due to subtle effects of these reaction parameters on the structure and reactivity of the carbon-lithium bond complex at the active chain end. 

One alternative to partial reduction for selectivity is active site isolation chemically by modification of solid state structure. The zig-zag chain structure of V2O5 (Fig. 14) provides inherent structural isolation, which can be enhanced by the addition of alkali metal oxide. The dependence of conversion to selective product on K2O/V2O3 ratio is similar to the effect of reduction (Fig. 15), and gives a maximiam at 0.25 K/V. Bifunctional systems, in which the components individually have very low activity, are among the most efficient catalysts. 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

The conversion rates of individual components in a commercial LAS mixture are dependent on the molecular structure. For example, the length of the alkyl chain is positively correlated with the primary degradation rate, and as such, isomers with the phenyl substituted at central positions are degraded more slowly than other isomers [79,80]. Both effects are a direct consequence of the enzymatic attack on the hydrophobic moiety. The relation between surfactant structure and the biodegradation has been termed as Swisher s distance principle which, in summary, describes that an increased distance between... 

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