11B. More on Other Synthetic Polymers - macromolecules - structure and uses

SYNTHETIC FIBRES like NYLON and TERYLENE - condensation polymers

Condensation polymerisation involves linking lots of small monomer molecules together by eliminating a small molecule. This is often water from two different monomers, a H from one monomer, and an OH from the other, the 'spare bonds' then link up to form the polymer chain plus H₂O.

In addition polymerisation there is only one monomer molecule with a double bond and one product, the polymer, and the linking occurs via a reactive double bond.

Condensation polymerisation involves monomers with two functional groups (one at each end of the molecule). When these types of monomers react, they join together (polymerise), small molecules such as water are eliminated in the process, and so the reactions are called condensation reactions, hence the process is called condensation polymerisation.

The simplest polymers are made from two different monomers with two of the same functional groups on each monomer.

• Terylene (a polyester) and nylon are good for making 'artificial' or 'man-made' fibres used in the clothing and rope industries.
  • In the manufacturing process the polymer chains are made to line up.
  • This greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made.
• A polyester can be made from ethane diol (an alcohol with two hydroxy groups two -OH's) and hexanedioic acid (a carboxylic acid with two -COOH groups, a dicarboxylic acid).
  • These are the two starting monomers prior to polymerisation and both must have a reactive group at each end - and a different functional group that can react with the other.
    • The ethanediol monomer molecule   HO-CH₂-CH₂-OH
      • This type of molecule is called a diol, because it has two alcohol groups -OH.
    • The hexanedioic acid molecule monomer molecule   HOOCCH₂CH₂CH₂CH₂COOH
      • This type of molecule is called a dicarboxylic acid, because it has two carboxylic acid groups -COOH.
  • Alcohols react with carboxylic acid to form esters with the elimination of water.
  • In this case the ester linkage is formed at both ends of each molecule with the elimination of water molecules.
  • Polyesters are condensation polymers because of how they are formed - by this condensation reaction that eliminates a small molecule to form the ester bond between the monomers.

Diagram to explain how a diol (alcohol) and a dicarboxylic acid condense together to give an ester linkage.

Here just one of each monomer have condensed together to make a bigger molecule - the water is eliminated as the new linking covalent bond is formed - an ester bond.

BUT, at each end of this molecule, the functional group (alcohol -OH, on left) can link with the other functional group (carboxylic acid, -COOH, on right) to create an even bigger molecule - eventually, a long chain polyester polymer will form.

• Where n is the very large number of monomer molecules, the condensation polymerisation of ethane diol and hexanedioic acid can be represented as ...

n HO-CH₂-CH₂-OH + n HOOCCH₂CH₂CH₂CH₂COOH

===> -(-CH₂-CH₂-OOC-CH₂CH₂CH₂CH₂-COO-)_n-  +  2n H₂O

or more simply:

n HO-[][]-OH + n HOOC-[][][][]-COOH ==> -(-[][]-OOC-[][][][]-COO-)_n-  +  2n H₂O

[][] and [][][][] represent the rest of the molecules, and n is a very large number !

• Terylene (a polyester) is formed by condensation polymerisation and the simplified structure of Terylene can be represented as
•  
• 3 units as partially displayed formula

More advanced displayed formula representations of Terylene and its formation

• However, above is a more accurate representation of the polyester Terylene, where you can see the ester linkage (COOC) more clearly, but this level of molecular structure is probably not needed at GCSE/IGCSE chemistry level.
• This is the same kind of 'ester linkage' (-COOC-) found in fats which are combination of long chain fatty carboxylic acids and glycerol (alcohol with 3  -OH groups, a 'triol').

• The plastic PET is the same as Terylene!

  • The acronym PET stands for polyethylene terephthalate.

  • When used in clothing fibres, it is called Terylene, but when used in bottles, it is called PET

  • PET is a clear, strong and lightweight plastic belonging to the polyester family.

  • It is typically called "polyester" when used for fibres or fabrics, and "PET" or "PET Resin" when used for bottles, jars, containers and packaging applications.

  • PET is the world's packaging choice for many foods and beverages because it is hygienic, strong, lightweight, shatterproof, and retains freshness.

  • PET is most commonly used to package carbonated soft drinks and water.
     

• Nylon (a polyamide) is formed by condensation polymerisation, the structure of nylon represented below where the rectangles represent the rest of the carbon chains in each unit.
  • For advanced molecular representations see Organic Nitrogen Compounds (A level Notes)
• Nylon is made polymerising a dicarboxylic acid and a diamine with the elimination of water.
  • Both monomers have the same functional group at each end, hence di.... in their names.

Diagram showing the formation of the polyamide link as a water molecule is eliminated when the carboxylic acid group in one monomer, bonds with amine group of the other monomer.

In this case two amino acids have a formed the simplest possible polypeptide - a simple dipeptide.

Note * that at each end of the molecule, the amine group (-NH₂, on left) and the carboxylic acid group (-COOH, on right) can both form a bond with another diamine molecule by further elimination of water molecules.

Diagram to explain how a diol (alcohol) and a dicarboxylic acid condense together to give an ester linkage.

Here just one of each monomer have condensed together to make a bigger molecule - the water is eliminated as the new linking covalent bond is formed.

BUT, at each end of this molecule, the functional group (alcohol -OH, on left) can link with the other functional group (carboxylic acid, -COOH, on right) to create an even bigger molecule - eventually, a long chain polyester polymer will form.

• n HOOC-[][][][]-COOH + n H₂N-[][][][]-NH₂ ==> -(-OC[][][][]-CONH-[][][][]-NH-)_n-+  2n H₂O
• 
• 3 units as partially displayed formula.
• is the displayed formula of a diamine.
• is the displayed formula of a dicarboxylic acid.

• This is the same linkage (-CO-NH-) that is found in linked amino acids in naturally occurring macromolecules called polypeptides and proteins, where it is called the 'peptide' linkage.
  • Nylon-6,6
  • Making Nylon-6,6 in the laboratory
    • In the 1st beaker, make a solution of 1,6-diaminohexane in water.
    • In a 2nd beaker, dissolve 1,6-hexanedioyl dichloride in a suitable organic solvent - it must be one that does not mix with water - an immiscible liquid like the organic solvent 1,1,2-trichloroethane.
    • Pour one solution on top of the other, and nylon is formed at the interface of the two solutions.
    • With a glass rod, you can extract a blob of nylon on the end, lift it up carefully and twist the glass rod around and wind up a steady 'mushy' thread of nylon..
    • At GCSE level you don't need to write an equation, but you might be expected to recognise the condensation polymerisation reaction AND the small molecule, hydrogen chloride HCl, is eliminated. The equation is:
  • n HOOC-(CH₂)₄COOH + n H₂N-(CH₂)₆-NH₂ ==> -(-OC(CH₂)₄-CO-NH-(CH₂)₆-NH-)_n- +  2n H₂O
    • Unless you are studying chemistry at an advanced level, you don't have to no why its called Nylon-6,6, but it's because both monomers have a chain of 6 carbon atoms!

See also Extra advanced Level notes on Nylon structure and synthesis

• Although these are actually thermoplastic polymers, nylon and Terylene can be drawn out into thin strong fibres for use in clothing.

• Some important structure, strength and 1D, 2D and 3D dimension concepts are in the Chemical Bonding notes.
• Nylon and polyester are typical synthetic fibres which have, in many cases, replaced cotton, silk and wool fabrics in the clothing industry.
  • They are cheap to make on an industrial scale compared to cotton from fields, silk from silkworms and wool from sheep.
  • As well as being cheaper, the physical properties of synthetic fibres have several advantages compared to their natural predecessors like cotton, silk and wool.
  • Compared to natural fibres, synthetic fibres tend to be ....
    • lighter - outdoor or indoor clothing,
    • more durable - harder tougher wearing fibres,
    • water-resistant - better water-proofed fabrics,
    • However, there are some disadvantages e.g.
      • they are not very breathable and sweat builds up making you feel uncomfortable.
      • The use of 'breathable' fabrics like GORE-TEX are described and discussed on the smart materials page 6. Gore-Tex and thinsulate etc.
  • The most common use of polyester today is called PET (for short!) and is used to make the plastic bottles for storing liquids in like soft drinks. PET is very useful because it is transparent, shatterproof and cheap!
    • Fine polyester fibres can be made into a variety of articles of clothing which are lighter and cheaper than traditional materials like wool.
    • Plastic bottles made from polyester can recycled and turned into fibres again and reused in clothing, are you wearing a plastic bottle!

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