Molecular weight averages solvents

Ziegler-Natta type catalysts can generate a very high cis-1,4 stmcture (>90%), which is the choice polymer for tires. It is made to specifications similar to SBR, ie, molecular weight average of 100,000—200,000, Mooney viscosity 50, and od-extended. Lithium catalysts on the other hand yield variable chain stmctures, depending on the solvent used, ie, mixed stmctures of cis-1,4 and trans-1,4 and 1,2. These polymers are generally ia the lower... 

Anionic polymerization Narrow molecular weight distribution Limited chain transfer reactions Predictable molecular weight average Possibility of forming living polymers End groups can be tailored for further reactivity Solvent-sensitive due to the possibility of chain transfer to the solvent Can be slow Sensitive to trace impurities Narrow molecular weight distribution... 

These two types of molecular weight averages are representative of the type called absolute methods, in that well-established thermodynamic equations can be used to convert the experimental data directly into a value of the molecular weight. However, some other methods require calibration. The most important of these indirect methods involves a measurement of the intrinsic viscosity. This quantity is a measure of the extent to which a polymer molecule increases the viscosity of the solvent in which it is dissolved. The viscosity method can be calibrated to yield a viscosity-average molecular weight, defined by... 

In effect, different calibration curves are necessary for different polymer solvent pairs, or even the same polymer/solvent pair if the microstructure of the polymer is different to that of the standards that are available (Figure 12-41). Sometimes, molecular weight averages based upon linear polystyrene standards are reported, but this can obviously lead to large errors. 

In order to obtain accurate and precise molecular-weight averages, the column temperature, as well as the difference of both temperatures, the solvent reservoir and the column oven, must be maintained. 

The GPC equipment is fairly expensive and prone to failure, and the actual experiment is slow, labor intensive, and requires dissolution of the PP at high temperatures in solvents like xylene and trichlorobenzene. Thus, other methods to estimate the molecular weight have been developed. The most popular one is termed the MFR test, and it gives a number that is easily correlatable to the average. Most HPP products are sold with MFR numbers ranging from 0.2 to 45, and these correspond to MwS from 1,000,000 down to 100,000. Note that the molecular weight averages are inversely proportional to MFR numbers. 

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