How many repeating units in a polymer

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.

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.

We noticed crosslinking earlier when we saw how the disulfide crosslink formed by oxidation of the SH group in cysteine was important in maintaining protein structure. Since polymers are made by linking together many identical small molecules, there are repeating units in polymers. 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 CH 2 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. We notice that the repeating unit is linked to the rest of the chain by amide functional groups, and that the repeating unit contains an amide group.

We can deduce the structrure of the monomers by imagining the compounds which might be used to make the amide group. Approximate ranges for macromolecules, low polymers, high polymers:.

Classification of polymerization reactions:. Descriptions of polymer physical properties:. IUPAC system: identify repeat unit, starting with highest priority atoms in chain and putting substituents on lowest number positions. Linear polymers that include rings cyclolinear polymers. Orientation and configuration in polymer chains 1. Head-to-tail orientation of monomer units predominates.

Orientation and stereochemistry at double bonds. Note - natural rubber is head-to-tail: "The Isoprene Rule".

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