ORGANIC MACROMOLECULES
| Contents for this page | Related topics |  |
|---|---|---|
|
Macromolecules and synthetic polymers Thermoplastics Thermosets Additional questions |
The polymer industry - 1 The polymer industry - 2 Polysaccharides and proteins Nucleic acids |
|
| Learning Outcomes | ||
| After studying this section, you will understand what is meant by (a) a "macromolecule", and (b), a "synthetic polymer". | ||
A MACROMOLECULE is simply a molecule with a large relative molecular mass, Mr. How large is large? Anything from Mr > 500 to several millions. Generally speaking though, we use the term to describe organic synthetic polymers (discussed in this topic) made by laboratory and industrial processes, and certain classes of large biological molecules (discussed in another topic), that are made by and in the cells of living organisms.
Macromolecules of both types are of tremendous importance. On the one hand, the materials we call "plastics" are macromolecules, while on the other, biological macromolecules are the very basis of living processes.
A POLYMER is a macromolecule whose structure displays repeating structural units. For example, the well-known plastic called "polythene" or "polyethylene" is a large molecule that has the -CH2- unit repeated many times. Nylon has -NH(CH2)6NHCO(CH2)4CO- as its repeating unit. By varying the type of repeating units and controlling the size of the polymer, chemists have come up with large numbers of useful plastic materials that are in daily use.
Synthetic polymers can be classified on the basis of how they were chemically produced as being either ADDITION POLYMERS (formed by a process called ADDITION POLYMERISATION) or CONDENSATION POLYMERS (resulting from CONDENSATION POLYMERISATION). Addition polymers have uninterrupted chains of carbon atoms, while in condensation polymers, the carbon chains are interrupted by oxygen or nitrogen atoms. Each type is discussed in the Grade 12 Section on Chemical System (see the links to the Polymer industry in the menu above).
Another mode of classifyng synthetic polymers is based on how they behave at elevated temperatures. In this way, we divide synthetic polymers into two groups, the THERMOPLASTICS and the THERMOSETS. These are discussed below.
These plastics have different physical properties that are dependent on temperature. At a temperature below the GLASS TRANSITION TEMPERATURE, the material is glassy and brittle. As the temperature rises, it becomes softer and shows it typical plastic character. Increasing the temperature still further causes the plastic to become more deformable and softer, until it starts to melt to a viscous liquid. These changes are reversible. The transitions described above do not take place at definite temperatures, such as in pure organic crystalline solids, but rather over temperature ranges.
Addition polymers such as polythene, polyvinyl chloride and polystyrene are typical thermoplastics. Since these materials can be melted and recast, they are able to be recycled.
Thermosets, (also known as THERMOSETTING PLASTICS) are plastics that are converted to a final form by a curing process. This curing process may be achieved by heat, irradation with light of certain wavelengths, by the addition of substances such as organic peroxides (
) that act as catalysts, or a combination of these.
Once a thermoset has been cured ("set"), it cannot be melted, as heat decomposes the material before it reaches its (very high) melting point. For this reason, thermosets are cured in moulds that will establish the final shape of the objects, or alternately, they are applied in a stable liquid or semi-liquid form, and cured by the addition of a catalyst or by shining ultraviolet light on it. An example of this is the "resin" used for making fiberglass, and various adhesives known as "epoxy glues". Since thermosets do not melt without decomposing, these polymers cannot be recycled. Commonly, the final product consist of long polymer chains that are CROSS-LINKED by covalently attaching a group of atoms between them. The two examples below will serve as illustration.
Vulcanised rubber is a thermoset that combines desirable properties such as elasticity and recovery from strain, apart from durability. These properties are introduced into natural or synthetic rubber by a process called VULCANISATION.
Natural rubber, obtained from Hevea species, is a natural polymer consisting of long chains made up of so-called isoprene units (right): |
 |
If this polymer is heated with sulphur, CROSS-LINKS are introduced between polyisoprene chains, to form a three-dimensional lattice (shown in two dimensions here on the right). This is the so-called vulcanisation process, which gives a host of useful rubber products, such as motor-car tyres. The sulphur crosslinks may be of different lengths, but in the diagram only two sulphur atoms are shown. |  |
|
Epoxy resins are normally made on the spot by mixing a "protopolymer" with a "hardener", such as a diamine. In the diagram on the left, the structure of these molecules have been simplified, but the key to the reaction is that the protopolymer has two reactive epoxy groups, while the diamine has two reactive amino (-NH2) groups. |
When these compounds react, cross-linking occurs to form a thermoset with a structure such as the one shown below. Epoxy glues work in this way, where the two components are in separate tubes, and mixed just before use.
