Polymers are large molecules. Fortunately, they are not random collections of atoms; if this were the case, they would be impossible to study. Polymers are built up from smaller molecules (monomers), and therefore possess a characteristic chain structure consisting of multiple repeat units that are related chemically, as for the example of polystyrene, shown below:
Note that all three representations are equivalent. The lower structures demonstrate that there are often several equally valid ways to place the brackets in a repeating structure. There are official IUPAC rules on this, but, more commonly, one usually chooses the representation that emphasizes the monomer from which the polymer was made.
The quantity x represents the number of repeat units in the chain, and is called the degree of polymerization. Of course, there are end groups on the chain that are different from the repeat units, but these usually represent a negligible portion of the molecule, so they are seldom drawn.
The repeat units can all be identical, in which case the compound is a homopolymer.
If the repeat units are different, the result is a copolymer. For copolymers, the situation gets complicated quickly. At the limit of extreme complexity, there are biopolymers such as proteins and DNA that have exactly defined sequences of many repeat units to serve the purposes of the organism. Synthetic copolymers are seldom so elegant; however, there are still many possibilities for even simple systems. Consider a copolymer made from just two ingredients. There are infinite ways in which the two can be sequenced along the backbone. Here are some limiting cases:
There exist techniques for synthesizing these and many other related structures.
The chain itself can take many different overall structures, among them:
Most polymers consist of linear chains, but branched systems have important uses, and highly branched polymers with fractal structures are a current hot topic of research. Here are a few links:
Dendrimers and hyperbranched polymers at the University of Delaware
Dendrimers in the group of Prof. G. Newkome
Molecular modeling of dendrimers (Cal Tech)
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