Although polyethylene can twist and turn in random ways, the most stable form is a linear one with the carbon backbone oriented as shown in the following figure:
The solid wedges in the figure indicate bonds from carbon that come out of the plane of the page; the dashed wedges indicate bonds that lie behind the plane of the page.
(a) What is the hybridization of orbitals at each carbon atom? What angles do you expect between the bonds?
(b) Now imagine that the polymer is polypropylene rather than polyethylene. Draw structures for polypropylene in which (i) the CH3 groups all lie on the same side of the plane of the paper (this form is called isotactic polypropylene); (ii) the CH3 groups lie on alternating sides of the plane (syndiotactis polypropylene); or (iii) the CH3 groups are randomly distributed on either side (atactic polypropylene).Which of these forms would you expect to have the highest crystallinity and melting point, and which the lowest? Explain in terms of intermolecular interactions and molecular shapes.
(c) Polypropylene fibers have been employed in athletic wear. The product is said to be superior to cotton or polyester clothing in wicking moisture away from the body through the fabric to the outside. Explain the difference between polypropylene and polyester or cotton (which has many -OH groups along the molecular chain) in terms of intermolecular interactions with water.
