Session 4:

Polymerization of Substituted Olefins


Question:

Besides polyethylene, what other kinds of polyolefin polymers are possible?


Text:

While polyethylene has proven to be an extremely versatile and useful plastic, its properties are not ideal for all applications. For this reason, chemists have investigated the polymerization of other olefin monomers, particularly monomers that possess a substituent group other than hydrogen on one of the olefinic carbon atoms. The polymers that result from these reactions do, in fact, possess different physical properties from those of LPDE and HDPE and have found important applications. For example, the polymerization of propylene, which is identical to ethylene except that one hydrogen substituent has been replaced by a methyl (CH3) group, yields "polypropylene" (Scheme 6).

propylene

polypropylene

Scheme 6

This material has a higher melting point (160-170 oC), higher tensile strength, and greater rigidity than polyethylene. It is also less permeable than polyethylene to liquids and gases. Polypropylene has many uses, including synthetic rope, automotive parts like battery casings, thermal underwear, and the ubiquitous plastic stackable chairs.

Other important polymers that are generated from substituted olefin monomers include polyvinylchloride (PVC) and polystyrene (Scheme 7).

vinyl chloride

polyvinylchloride

styrene

polystyrene

Scheme 7

The chlorine substituent groups on the PVC polymer chain make it more fire resistant than polyethylene or polypropylene. They also increase the force of attraction between individual polymer chains, giving rise to a harder plastic, which is used in pipes, house siding, toys, and furniture. A copolymer of vinyl chloride and vinylidene chloride (CH2=CCl2) is used to make cling wraps like Saran. The same increase in the force of attraction between polymer chains that gives PVC its hardness also gives thin films of Saran its tendency to stick.

Polystyrene is a glassy polymer with sparkling clarity. It is hard but also rather brittle. Because polystyrene melts around 90 oC, it can be easily molded into familiar items likes audio cassette boxes. Interestingly, "styrofoam" cups are also fabricated from polystyrene. To make styrofoam, polystyrene beads are mixed with a low-boiling liquid hydrocarbon, placed in a mold, and heated. As the hydrocarbon liquid vaporizes, it expands the polymer, causing the polystyrene to take the shape of the mold. So many bubbles are trapped in the polystyrene that it becomes a light, opaque foam. It is also an excellent insulator, which makes it ideal for applications in the food service industry.


Hands-On Activity:

Using Cochrane's Molecular Models, make a model of polypropylene. Compare it to a model that your neighbor has constructed. Are they identical?


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