Although polyethylene can twist and turn in random ways, the...

Although polyethylene can twist and turn in random ways, the most stable form is a linear one with the carbon back-bone 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 CH $_{3}$ groups all lie on the same side of the plane of the paper (this form is called isotactic polypropylene), (ii) the $\mathrm{CH}_{3}$ groups lie on alternating sides of the plane (syndiotactic polypropylene), or (iii) the $\mathrm{CH}_{3}$ 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 $-\mathrm{OH}$ groups along the molecular chain) in terms of intermolecular interactions with water.

Although polyethylene can twist and turn in random ways, the most stable form is a linear one with the carbon back-bone 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 CH $_{3}$ groups all lie on the
same side of the plane of the paper (this form is called isotactic polypropylene), (ii) the $\mathrm{CH}_{3}$ groups lie on alternating sides of the plane (syndiotactic polypropylene), or (iii) the $\mathrm{CH}_{3}$ 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 $-\mathrm{OH}$ groups along the molecular chain) in terms of intermolecular interactions with water.

Key Concepts

sp3 Hybridization and Tetrahedral Geometry

Each carbon atom in both polyethylene and polypropylene is sp3 hybridized, meaning that the carbon forms four sigma bonds arranged in a tetrahedral geometry with bond angles of approximately 109.5°. This hybridization is fundamental in understanding the three-dimensional structure and flexibility of the polymer backbone.

Tacticity in Polymers

Tacticity refers to the stereochemistry of the side groups attached to the polymer backbone. In polypropylene, the arrangement of the CH3 groups can be isotactic (all on the same side), syndiotactic (alternating sides), or atactic (randomly distributed). The tacticity greatly influences the polymer’s physical properties, such as crystallinity and melting point.

Crystallinity and Melting Point

The degree of crystallinity in a polymer is significantly affected by the ordering of the side groups. Isotactic polymers, which have a regular arrangement of side groups, pack more efficiently, resulting in higher crystallinity and higher melting points. In contrast, atactic polymers, due to their random structure, exhibit lower crystallinity and lower melting points. These structural differences influence the thermal and mechanical properties of the polymer.

Intermolecular Interactions and Polymer-Water Interactions

Intermolecular interactions in polymers influence properties like moisture absorption. Polypropylene is nonpolar due to its lack of polar functional groups, leading to weak interactions with water and enhancing its capacity to wick moisture away from the body. In contrast, polyester and cotton contain polar -OH groups that form strong hydrogen bonds with water, resulting in higher moisture retention. This difference is crucial in selecting materials for athletic wear where moisture management is important.

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