condensation polymerisation


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 a long chain polymer molecule. 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; this is outlined below:

amide formation using a diacid and a diamine


Ladies stockings are made from nylon

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 types, 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:

formation of nylon-6,6 from 1,6-diaminohexane and hexane-1,4-dioic acid

Nylon-6,6 is easy to prepare in the lab, however rather than using the reaction of a dicarboxylic acid and a diamine which is both slow and forms an equilibrium mixture it is much easier and more efficient if a diacyl dichloride is used instead of the dicarboxylic acid. Simply swap the hexanedioic acid for hexane-1,6-dioyl chloride. The dicarboxylic acid is readily converted in the diacyl chloride using thionyl chloride as a chlorinating agent:

hexanedioic acid + thionyl chloride hexane-1,6-dioyl chloride + sulfur dioxide + hydrogen chloride

An equation for the reaction of the diamine and the diacyl dichloride is shown below; though it is very similar to the one above using the dicarboxylic acid, the main differences are that this reaction does not involve an equilibrium mixture and that hydrogen chloride gas and not water is produced.

reaction of a diacyl chloride with a diamine to form nylon-6,6

Preparation, uses and applications of nylon

 molten nylon polymer is forced through a spinneret then cooled to form a filament of nylon

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 will form. 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 hydrogen bonds to neighbouring polymer chains; this hydrogen bonding between the polymer chains dramatically increases the strength of the nylon filaments formed.

Uses and applications of nylon

Nylon fibres have high tensile strength and they have high chemical and electrical resistance. They have excellent abrasion resistance which makes them ideal for use in sewing threads, ropes and belts and in all sorts of machinery. Nylon fibres are also easy to dye which means they are available in a wide range of colours, they are also lightweight, water resistant and hard wearing which makes them ideal for use in many types of clothing from tracksuits, swimming trunks, sportswear, socks and other items of hosiery, to jackets and raincoats. Nylon is also commonly used in the manufacture of umbrellas, tents and even parachutes.

Nylon being a thermoplastic means that it can be melted to form a viscous fluid which flows well and can be shaped and moulded into various objects such as pipes, tubes, films, screws as well as nuts and bolts. The fact that it is easily shaped gives it many industrial and commercial uses in such items as moulded machine parts to simple washer or gears and bearings in machinery, appliances and even children's toys. The bristles in your toothbrush are also likely to contain nylon fibres. Nylon is also mixed with other natural fibres to increase their ware resistance and usability e.g. nylon is mixed with wool to make carpets.

The montage below shows just a few of the many uses of nylon from guitar strings, to ropes, raw plugs and wind socks.

A quick search on Google will no doubt turn up many more uses of this versatile montage of items made from nylon

Amino acids and polyamides

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. amino acid molecules can react to form a dipeptide or a polypeptide which contains amide bonds

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. formation of polygylcine from glycine

Aramids- aromatic polyamides

Police officiers wearing body armour made of Kevlar

A group of polyamides formed from aromatic diamines and aromatic dicarboxylic acids are often referred to as aramids. Perhaps the two best known aramids are Kevlar and Nomex. These aramids have some very desirable properties, for example some of the useful properties of Kevlar include:

The polyamide Kevlar is made from the monomers benzene-1,4-dicarboxylic acid and benzene-1,4-diamine; as shown below. There is very strong hydrogen bonding between the polymer chains in Kevlar; the hydrogen bonding is between the carbonyl oxygen and the amine hydrogen atom on adjacent polymer chains. In fact the polymer chains in Kevlar are so tightly bonded together it is very very difficult to separate them.

Uses of Kevlar

Ropes made from Kevlar are lighter and stronger than steel ropes and they have the advantage that they will not corrode. Kevlar ropes secured the parachutes for the Mars Pathfinder robot on its long journey to Mars and Kevlar reinforced cushions helped it land safely on the Martian surface. Kevlar ropes are also used in some smaller suspension bridges where they replace traditional steel cables. Kevlar cables are lighter and stronger than steel cables and they will not corrode. Kevlar can also be spun in a fabric to make gloves and protective clothes; for example Kevlar gloves will offer protection from cuts and stabbing injuries many sharp objects such as knives, saws, drills, grinders, and other abrasive tools.

Perhaps the most famous use of Kevlar is in body armour; many police officers, soldiers and even traffic wardens and night club door supervisors may wear vests made of Kevlar that offer protection from stabbing from knife attacks and other sharp or pointed objects. Kevlar vests can also be made that are bulletproof. Its high temperature stability and electrical resistance is also useful for anyone working in hot environments such as fire fighters or welders. Kevlar hoses and tubes are also used extensively in the automotive industries. This list is only a few of the uses of this polyamide; an internet search will give you more information as needed.


Firemen fighting a fire while wearing Nomex fire resistant clothing

Nomex is another example of an aromatic polyamide or aramid that shares many of the properties of Kevlar. Its structure is also similar to that of Kevlar, this is simply because the monomers used to make it are similar to those use to make Kevlar. Nomex is made from the two monomers benzene-1,3-dicarboxylic acid and benzene-1,3-diamine; as shown below.

The shape of the repeating unit in Nomex means that the long polymer chains are not able to pack together or as closely as those in Kevlar, this means the intermolecular bonding will not be as strong as in Kevlar which will result in a more flexible polymer, that like Kevlar can be woven into fabrics. One of the main uses of Nomex is in flame retardant materials such as those worn by fire fighters. Nomex flame retardant clothing is also worn by military pilots, racing drivers as well as electrical workers and anyone at risk from fires or electrocution. When Nomex is exposed to intense heat it will not burn, melt or drip which makes it an ideal material for anyone working in an environment where there is a risk of fire or being exposed to high temperatures.
Nomex like Kevlar is also resistant to chemical attack by acids, alkalis and oils.

The formation of Nomex from its monomers

monomers and structure of repeat unit for Nomex

Key Points