Synthetic Polyamides (Nylons) Chemistry Tutorial

Key Concepts

• Polyamides are polymers in which the monomers come together to form amide links (also known as peptide bonds):

  			H |
  -	C	-	N	-
  	|| O

• In general, polyamides are formed during condensation polymerisation reactions in which the polymer is formed and a small molecule, such as water, is eliminated.
• Polyamides include naturally occurring proteins and synthetic polyamides which are also known as nylons.
• Nylons are described by a numbering system which indicates the number of carbon atoms in the monomer chains.

  Polyamides, nylons, prepared from amino acid monomers are designated by a single number.
  Polyamides, nylons, produced from diamines and dicarboxylic acids are designated by two numbers:
  (i) the first number is the number of C atoms in the diamine chain
  (ii) the second number is the number of C atoms in the dicarboxylic acid chain

• The amide groups (-CO-NH-) are polar:

  			Hδ+ |
  -	C	-	N	-
  	||   Oδ-

• Hydrogen bonds can form between one nylon polymer chain and another:

  Hδ+ | ---- C - N ----- ||   Oδ-

  . . . .

  Hδ+ | ---- C - N ----- ||   Oδ-

  
  In general, the more hydrogen bonds that can form between the polymer chains the higher the melting point will be and the greater the strength of the polymer.
• Polyamides (nylons) are hydrolysed in strongly acidic conditions.
• Nylons are used as textiles for clothing and carpet. They are also used to make rope, and nylons can be cast into different shapes to make things like cogs and bearings.

  Kevlar, known commonly as the material of which bullet-proof vests are made, is the brand name for a very strong, light polyamide which was first produced in 1965.
  Kevlar is also used in boat construction, in light-weight mountaineering ropes, and in many other things where it is important to combine light weight with great strength.

Nylon-6,6

Carothers reacted hexandioic acid (also known as adipic acid, a 6-carbon dicarboxylic acid) with 1,6-diaminohexane (also known as hexan-1,6-diamine or hexamethylenediamine, a 6-carbon diamine) in a condensation polymerisation reaction producing the polymer, nylon-6,6, and water. Water is the small molecule eliminated during the condensation reaction.

The first step in this process in which 1 molecule of hexandioic acid reacts with 1 molecule of 1,6-diaminohexane is shown below:

The organic product molecule, "a dimer", has a carboxylic acid functional group at one end, and an amine functional group at the other.
Its carboxylic acid functional group could react with a molecule of 1,6-diaminohexane:

And, the amine functional group can react with a molecule of hexandioic acid:

And so on, building up the polymer nylon-6,6 (or poly(hexamethylene adipamide) if you prefer).

Nylon-6,10

A solution of decanedioyl dichloride in cylcohexane is floated on an aqueous solution of 1,6-diaminohexane by gently pouring it down the side of the beaker to minimize mixing.

A greyish film of nylon-6,10 forms at the interface and can be pulled out as fast as it is produced forming a long thread (or nylon rope).
For this reason, the demonstration is often referred to as the "nylon rope trick".

The reaction is a condensation polymerisation reaction in which amide links are formed between the monomers and HCl is the small molecule eliminated during the reaction as shown below:

Kevlar

Amide links are formed between the monomers during the polymerisation and water is eliminated as shown below:

benzene-1,4-dicarboxylic acid (terephthalic acid)		1,4-diaminobenzene		kevlar	water
nHOOCCOOH	+	nH2NNH2	→	[-OCCO-HNNH-]n	+	(2n-1)H2O

Nylon-6

For this reason, nylon-6 is also referred to as polycaprolactam.

First, caprolactam undergoes ring opening with water. The ring opens up between the CO and NH groups:

caprolactam		aminocaproic acid monomer
	H2O →	H | O || H- N - CH2-CH2-CH2-CH2-CH2 -C -OH

The aminocaproic acid monomer can then polymerise by forming amide links.
The first step in this polymerisation reaction is shown below:

The overall polymerization reaction can be represented as:

Notice that although water is eliminated during the polymerisation part of the process this synthesis is not generally considered to be a condensation polymerisation reaction, instead it is referred to as a ring-opening polymerisation.

Hydrolysis of Polyamides

Polyamides can be hydrolysed in strongly acidic conditions. The products of the reaction are a dicarboxylic acid and the salt of an amine.

Nylon-6,6 can be produced using the rope trick by substituting hexanedioyl dichloride (adipoyl chloride) for decanedioyl dichloride.