Answer to Question 1
If the MWD is broad, the short molecules will melt before the long molecules and serve as lubricants for the entire plastic mass, even before complete melting occurs. This results in some residual entanglement among the long molecules which gives tensile strength to the melt. Processes that require melt strength, such as extrusion and blow molding, are thus improved in processability by broad MWD. On the other hand, narrow MWD will result in a sharper melting point and a lower viscosity melt conditions that are favored in injection molding. The cooling of the polymer is also faster with a narrow MWD to further improve injection molding processability.
Answer to Question 2
In isotactic polypropylene all of the methyl groups are on the same side of the tertiary carbon. This reduces the intermolecular interference between these groups and allows the molecules to pack tightly together, thus forming crystals.
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