From studying the page on polyesters we looked at how ester linkages could be formed in a condensation reaction between a dicarboxylic acid and a diol to produce polyesters. Polyamides are formed in a similar way to polyesters however this time a dicarboxylic acid is reacted with a diamine to form an amide bond. Amines are molecules containing the amino functional group (-NH2); so diamines are molecules containing two amino groups. The formation of a typical polyamide is outlined below:
One of the first synthetic polyamides to be produced was nylon. Nylon was first made in the 1930 by Wallace Carothers while he was working for the American pharmaceutical company Du Pont. One of the first uses for this new wonder material was to make ladies stockings. Stocking had previously been made of silk which unfortunately made them very expensive and not particularly hard wearing or long lasting. When the first nylon stocking went on sale in America they were so popular that millions of pairs sold in only a few hours and shops quickly ran out of stock.
There are many different types of nylon but one type; nylon-6,6 is often used in the manufacture of stockings and tights. Nylon-6,6 can be made from the reaction of the dicarboxylic acid 1,6-hexanedioic acid (or simply hexanedioic acid) and the diamine hexane-1,6-diamine (or you may see it written as 1,6-diaminohexane). The name nylon-6,6 comes from the fact that both of the monomers contain six carbon atoms. The formation of nylon-6,6 is outlined below:
Nylon is a thermoplastic polymer; this means it will soften and melt when heated. If molten nylon is forced through a device called a spinneret; this simply resembles a large shower head; then long filaments of nylon thread will emerge. If these filaments are then cooled by simply being exposed to warm air they will fully solidify and form long filaments of nylon. These are then stretched and drawn out to form long filaments or threads which can be spun onto bobbins and the filaments or threads can them be used to make fabrics or other items as necessary. Drawing or stretching the nylon filaments results in the formation of long parallel polymer chains which will pack closely together which enables them to form lots of intermolecular bonds to neighbouring polymer chains; this intermolecular bonding between the polymer chains dramatically increases the strength of the nylon filaments formed.
Amino acids contain the acidic carboxyl group (-COOH) and the basic amino group (-NH2) in one molecule. So instead of using two separate monomers; one with the acidic carboxyl group and one with the basic amino group to make a polyamide why not just use a single monomer with both these groups? Well in the case of amino acids that is exactly what happens when they form polypeptides and proteins; for example the amino acids alanine and glycine can undergo a condensation reaction to form a dipeptide molecule as shown below. This dipeptide molecule contains an amide or peptide bond. However the dipeptide molecule still has reactive amino (-NH2) and carboxyl groups (-COOH) on the ends of the molecule and can readily undergo more condensation reactions to form a polypeptide molecule or a protein.
It is not even necessary to start with different amino acids. Heating a single amino acid monomer will result in the formation of a polyamide. We can show this simply as follows where the amino acid glycine can polymerise to form the polymer polygylcine when heated.
