What are Condensation Polymers? (Edexcel IGCSE Chemistry (Modular))
Revision Note
Condensation polymers
Condensation polymers are formed when two different monomers are linked together with the removal of a small molecule, usually water
This is a key difference between condensation polymers and addition polymers:
Addition polymerisation forms the polymer molecule only
Condensation polymerisation forms the polymer molecule and one water molecule per linkage
The monomers have two functional groups present, one on each end
The functional groups at the ends of one monomer react with the functional group on the end of the other monomer, in so doing creating long chains of alternating monomers, forming the polymer
Polyesters are formed from two different monomers and produce water
For every ester linkage formed in condensation polymerisation, one molecule of water is formed from the combination of a -H and an -OH group
An example is terylene which is a polyester made from dicarboxylic acid monomers (a carboxylic with a -COOH group at either end) and diols (an alcohol with an -OH group at either end)
Each -COOH group reacts with another -OH group on another monomer
An ester linkage is formed with the subsequent loss of one water molecule per link
The condensation reaction in which the polyester terylene is produced
The structure of terylene can be represented by drawing out the polymer using boxes to represent the carbon chains
This can be done for all polyesters
Diagram showing a section of the polyester terylene
Examiner Tip
Notice that the sequence of bonding in the polyester is the mirror image at either end of the link, NOT the link repetition due to the monomers containing the same functional group at either end.
Deducing the monomers from a polyester
If you are given the structure of a polyester, you can deduce the diol and dicarboxylic acid used to form it
It helps to think of this as the reversal of the formation of a polyester
When a polyester is formed, small molecules of water are removed
The OH comes from the acid / -COOH groups of the dicarboxylic acid
The H comes from the alcohol / -OH groups of the diol
To reverse this, water is added to the polyester which allows it to break back down into its monomers - this process is known as hydrolysis
Take the polyester, polybutylene terephthalate (PBT), as an example:
Diagram showing a section of the polyester polybutylene terephthalate, PBT
Identify the ester / -COO- linkages in the polymer
Separate the polymer into its two component monomers by breaking the C-O single bond in the ester linkage
This leaves 2 different structures:
Water is used to complete the molecules
Remember: When the polyester was made, the dicarboxylic acid lost OH groups and the diol lost H atoms
OH groups are added to the C=O carbon on one structure forming the two carboxylic acid / -COOH groups of the dicarboxylic acid
H atoms are added to the oxygen atoms on the other structure forming the two alcohol / -OH groups of the diol
The resulting molecules are the two monomers that are used to form polybutylene terephthalate, PBT
A diagram to show the two monomers used to form the polyester polybutylene terephthalate, PBT
Biopolyesters
Biopolyesters are a specific type of polymers that are synthesised from sugars and plant oils using microorganisms
They are able to biodegrade naturally in the environment after their intended purpose
The polymers are synthetically made, consisting of ester, amide and ether functional groups which gives them the characteristic of being biodegradable
The biodegradation of a bottle made of biopolyesters
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