Computer graphics representation of a section of a polypropylene molecule, (CH3CHCH2)n, a plastic produced by the catalytic polymerization of propylene (an alkene hydrocarbon). Polypropylene possesses a methyl group (CH3) that is attached to alternate carbon atoms in the long chain of the molecule. Different plastic properties arise depending on the way in which these methyl groups are arranged - the stereochemical configuration, which depends on choice of catalyst, temperature and solvent. A random distribution of methyl groups produces a soft, atactic polymer. If all the methyl groups point in the same direction the plastic is much harder and described as isotactic.