polypropylene tacticity

I can't say it as I haven't a clue what isotactic, atactic or syndiotactic means!!!

I'm off to Google >>>>>>>>>>>>>>>>>>>

John.

Isotactic, the methyl groups are all on the same side of the polymer chain. Atactic, there is a random placement. For syn-, the methyl groups alternate regularly back and forth across the chain. PP is normally predominantly isotactic, which causes the rotation of the extended polymer chain by one-third of a turn (120 degrees) per monomer unit. The potential energy barriers which retard rotation around the C - C bond in the chain runs from ~ 0.2 to 1.2 kJ/mole, similar to molecular adhesion forces. Isotactic PP has an unusually high Tm due to the low entropy of fusion, which is due to the chain stiffness, which results from the high rotational energy barrier of moving the methyl groups of neighboring monomer units past each other, if the polymer chain were to rotate. This causes the helical coiling that you mention.