Condensation Polymers (1) Polyesters

Relevant Specification Extracts:

OCR

6.2.3 Polyesters and Polyamides

Condensation polymers

(a) condensation polymerisation to form: 

1. polyesters (ii) polyamides 

Formation from carboxylic acids/dicarboxylic acids(or respective di acyl chlorides) and from alcohols/diols or amines/diamines.

Learners will not be expected to recall the structures of synthetic polyesters and polyamides or their monomers. 

Edexcel 18B

14. know that the formation of a polyamide is a condensation polymerisation reaction 

15. be able to draw the structural formulae of the repeat units of condensation polymers formed by reactions between: 

i  dicarboxylic acids and diols 

ii  dicarboxylic acids and diamines 

iii amino acids 

AQA

3.3.12.1 Condensation polymers (A-level only) 

Condensation polymers are formed by reactions between: 

dicarboxylic acids and diols
dicarboxylic acids and diamines
amino acids.
The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.
Typical uses of these polymers.
Students should be able to:
draw the repeating unit from monomer structure(s)
draw the repeating unit from a section of the polymer chain
draw the structure(s) of the monomer(s) from a section of the polymer
explain the nature of the intermolecular forces between molecules of condensation polymers.

Polyesters

In this first post on condensation polymers, I’m looking at the formation of polyesters, defining the repeat units of polyesters (e.g Terylene), and the nature of the intermolecular forces that exist between these polymer molecules.

I’m going to discuss how dicarboxylic acids (or acyl chlorides ) combine with diols and how to spot the repeat unit and the linkage between repeat units of polyester.

Formation of polyesters

As their name suggests polyesters are part of the ester family of molecules and contain the –COO– functional group.

Like aliphatic and aromatic esters polyesters are formed from either a  carboxylic acid and an alcohol or a carbonyl chloride and an alcohol.

However there is a distinguishing feature of these carboxylic acids and carbonyl chlorides: they are di carboxylic acids and chloride I.e. they have two acid groups or two acid chloride groups.  Similarly the alcohols used are diols.

A typical dicarboxylic acid is 1,4-benzene dicarboxylic acid and a typical alcohol might be ethan–1,2–diol. See below for their structures: 

When these molecules combine a small molecules is eliminated in the case above that molecule is water H₂O.

You can that described in the reaction scheme below:  

You can see how the structures allow for a chain to develop of units of acid and alcohol linked by ester groups.

You should be able to draw the repeat unit of a typical polyester such as Terylene.

You should also be able to highlight the ester functional group in the structure as below.

And you can also see the repeat units outline in two different ways.

If the acid used is 1,4- benzene dicarboxylic acid and the alcohol is ethan–1,2–diol then the resultant polymer was originally called Terylene at ICI back in the day (thats the old Imperial Chemical Industries).

Here is a list of some of the properties both useful and not so helpful of typical polyesters:

Advantages and Disadvantages of Polyesters

Advantages:

Tough and rigid

Recycled into useful products as the basis for resins in such applications as shower units and floor tiles etc..

PET flakes from PET bottles are in great demand for fibrefill for pillows and sleeping bags, carpet fibre and sheet mouldings etc..

Easy to print or mass colour or dye

Disadvantages:

Subject to attack from acids and bases

Low thermal resistance

Poor solvent resistance

And here is a picture of the actual material Terylene before it is spun and stitched into a textile:

Such polyester molecular chains stick together through the use of polar bonds (between CO groups) and temporary dipole forces (between aromatic groups).