Stereoregularity, Isotactic, Syndiotactic, and Atactic
Quick Stereoregularity is concerned with the spatial arrangement of the atoms and groups of atoms in the repeating units of a polymer molecule.
Isotactic is an arrangmenet in which the odd atom groups are all on the same side.
Syndiotactic is an arrangement in which the atom groups alternate on opposite sides.
Atactic is an arrangment in which the groups are randomly along either side.
Details
Stereoregularity is concerned with the spatial arrangement of the atoms and groups of atoms in the repeating units of a polymer molecule. An important aspect of stereoregularity is the way the atom groups are located along the chain for a polymer that has one of the H atoms in its mers replaced by some other atom or atom group. Polypropylene is an example; it is similar to polyethylene except that CH3 is substituted for one of the four H atoms in the mer. Three tactic arrangements are possible as shown:
Isotactic
The odd atom groups are all on the same side.
Syndiotactic
The atom groups alternate on opposite sides.
Atactic
The groups are randomly along either side.
The tactic structure is important in determining the properties of the polymer. It also influences the tendency of a polymer to crystallize. Polypropylene can be synthesized in any of the three tactic structures. In isotactic form, it is strong and melts at 175°C (347°F); the syndiotactic structure is also strong, but melts at 131°C (268°F); but atactic polypropylene is soft and melts around 75°C (165°F) and has little commercial use.
