Question

Polypropylene has a degree of crystallinity that is dependent on its tacticity. Why?

Answer 1

Polyolefins with side chains have stereocentres at every other carbon. With so many stereocentres, the stereochemistry can be complex.Mechanical and optical properties of polypropylene are influenced my its degree of crystallinity. The degree of crystallinity is, in turn dependent on the tacticity of the chain. Polypropylene has three tactic forms.

Atactic: Random orientation

Isotactic: All stereocentres have same orientation. Isotactic polypropylene are usually semicrystalline and often form a helix configuration.

Syndiotactic: Alternating stereochemistry

Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule.The practical significance of tacticity rests on the effects on the physical properties of the polymer. The regularity of the macromolecular structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder.

The irregular structure of the atactic version makes it impossible for the polymer chains to stack in a regular fashion. The result is that, whereas syndiotactic polypropylene is a semicrystalline material, the more common atactic version cannot crystallize.