Polymers - Structure and Response to Heat

Let"s begin by noticing some important real-world characteristics of polymers. While they all contain molecules with very long chains, there are some important differences between the properties of different types of polymers. Most polymers are formed into the desired shapes after softening or melting by heating. Some, like the familiar polyethylene and polystyrene, may be melted and reshaped again and again. These are called thermoplastic polymers.

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Others char or burn when reheated. These are called thermosetting polymers. Examples include Bakelite and vulcanized rubber. The structural difference between these polymers is that the thermosetting polymers have crosslinks between the chains and the thermoplastic polymers do not. When a thermoplastic polymer is heated the chains are free to move past each other making the sample less rigid and eventually melting it. This cannot happen with a thermosetting polymer, since its chains are locked together by the cross links. The energy from the heat must eventually go into breaking bonds which leads to decomposition of the polymer.

This sles-grizzlys-catalans.orgatic view suggests the difference:

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In poly(vinyl chloride) the repeating unit comes directly from the end-to-end linking of many vinyl chloride molecules. A molecule from which a polymer is made is called a monomer. Each vinyl chloride monomer molecule contributes a CH2 group joined to a CHCl unit by a single bond. This single bond is a remnant of the double bond which joined those groups in the vinyl chloride molecule. This is just what happens in an addition reaction of an alkene. We"ll see how an addition reaction leads to such polymers in a few paragraphs.

Repeating units can also be made from two monomers. Here"s an example:

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The monomers are a dicarboxylic acid (terephthalic acid) and a dialcohol, also called a diol (ethylene glycol). We immediately recognize that these monomers can make an ester, so that we expect our polymer to be linked by ester functional groups. The resulting polymer is called polyethylene terephthalate and is the common polyester of plastic pop bottles and polyester fabrics.

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The most important monomers for this process are ethylene (which makes the polymer polyethylene) and substituted ethylenes like vinyl choride (polyvinyl chloride), styrene (phenylethylene, polystyrene), methyl methacrylate (Plexiglas), and acrylonitrile (cyanoethylene, acrylic fibers). Table 15.1, p 427 in Brown lists the structures of these monomers, from which you can deduce the structures of the polylmers.

We might ask about the orientation of attack of a radical on a substituted ethylene (vinyl) monomer. It has been shown that the orientation is the same as it is for an electrophilic addition, that is, the free radical attacks the less substituted of the two alkene carbons so as to produce the new free radical at the more substited carbon. Here"s an example for styrene: