Polymers

 

 

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A polymer or macromolecule is a substance having large molecules consisting of repeating units. Some are found in nature (e.g., natural rubber, polysaccharides, proteins, nucleic acids). Others are man-made (synthetic polymers).

 

POLYMERIZATION

Synthetic polymers are prepared by a polymerization process. Polymerization is a chemical reaction in which small molecules (monomers) are joined together to form a polymer.

A chain polymerization is a chain reaction involving some sort of active centre (e.g., a radical). In a chain polymerization, repeat units are added to the growing polymer one at a time. Usually, the reaction does not involve elimination of a small molecule, so it may be called an addition polymerization.

A step polymerization proceeds by a series of individual reactions. In a step polymerization, any suitable combination of molecular species present in the reaction mixture can react (monomer+monomer, monomer+dimer, dimer+dimer, etc.). Often the reaction involves elimination of a small molecule such as water, in which case it may be called a condensation polymerization.

 

GLASSES AND RUBBERS

Glassy polymers are hard, brittle materials. Rubbery polymers are readily deformed and will recover their original shape (elastic deformation). Both glasses and rubbers are amorphous (there is no long-range order). A glass becomes a rubber on heating to above its glass transition temperature. A glass transition is different to a melting point. At a melting point, there is an abrupt change in volume, but at the glass transition there is only a change in the slope of the volume versus temperature curve. The glass transition temperature depends on the time-scale of the experiment.

 

POLYMERS OF INTRINSIC MICROPOROSITY

Most polymers have single bonds in their backbone, about which rotation can occur. Consequently, the polymer molecules can change their shape (conformation) when in the rubbery state or in solution. A polymer of intrinsic microporosity (PIM) does not have any single bonds in the backbone about which rotation can occur, and so cannot undergo large-scale conformational change without breaking bonds. PIMs are glassy polymers that do not show a glass transition below their degradation temperature.

 

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Polymers come in a great variety of forms. They may be oily liquids, deformable rubbers, brittle glasses, or tough semi-crystalline materials.