Stephen Lee

Olefin

Olefin is made of chains of monomers called propylene, and is thus also known as polypropylene. Polypropylene serves as both a plastic and a fiber, and is an oil-based synthetic material, as it is made from a by-product of gasoline refinement. Structurally, polypropylene is a linear hydrocarbon, similar to polyethylene. The monomer propylene has a chemical formula of C3H6, with a molecular weight of 42. A diagram of the molecular orientation of propylene is shown below. 

- - - CH2-CH - - - H H
| \ /
CH3 or - - - C = C - - - 
/ \
H CH3

Figure 1: Microstructure of propylene monomer

Polypropylene would be made by linking additional propylenes by their carbon atoms, as indicated by the dotted lines in the preceding diagram. Many different forms of polypropylene can be manufactured using various lengths of propylene chains.

Polypropylene can be made in two forms: isotatic, where all the methyl groups are aligned on one side of the chain, or atatic, where the methyl groups are placed randomly on both sides of the chain. Each has unique properties, as the isotatic chains form crystals and are strong, whereas the atatic chains are highly amorphous, becoming soft and rubbery. When blocks of isotatic and atatic polypropylene can be created in the same polymer chain, a strong, pliable elastomeric polypropylene is formed.

Polypropylene can be made from propylene monomers through either Ziegler-Natta polymerization or metallocene catalyis polymerization. Ziegler-Natta polymerization is a method of vinyl polymerization which allows the manufacturer to create polymers that are either isotactic or syndiotactic (methyl groups alternate) using a particular transition metal catalyst (typically TiCl4 for isotactic, VCl4 for syndiotactic) along with an aluminum based co-catalyst to begin the production of the polymer chain.

When the two catalysts combine, they form a complex with one empty orbital left to be filled. The propylene monomer fills the empty orbital, and then migration occurs, resulting in a shift in the positions of the electrons so that an orbital is once again empty, ready for another propylene to fill it.

Currently, it has not been determined why or how this migration occurs, but its existence has been confirmed.

Physical properties of polypropylene
Polypropylene is relatively lightweight, and has high tensile, flexural, and compressive strengths. Propylene creates a soft fiber which has lesser resiliency and texture retention than other synthetics, such as nylon, resulting in a shorter life expectancy of the fiber, but polypropylene can be made at a significantly lesser cost. Polypropylenes low moisture absorption rate and its relatively high melting temperature make it ideal for use in transferring hot liquids or gases. Because polypropylene is water resistant, water-based dyes cannot be used to add color. Therefore, pigment is added to the polypropylene during the manufacturing process. Polypropylene is used in a wide variety of products in various forms. As a plastic, polypropylene is used to make dishwasher-safe food containers (as it has a high melting point), sinks and ducts, and laboratory floors (as it is highly resistant to acids and alkines). As a fiber, polypropylene is used in indoor-outdoor carpeting, door mats, and automobile upholstery. Polypropylene can also be found in some rather uncommon products. For example, polypropylene is used to make diving suits. The unique properties of the thermoplastic elastomer, including elasticity/flexibility, quick dry time, abrasion resistance, low water absorption and retention, and thermal properties, make polypropylene an ideal choice for use in diving suits.