Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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6.10 The manufacture, molecular structure, properties and uses of polyesters - condensation polymers

Sub-index for this page

6.10.1 Condensation polymerisation compared to addition polymerisation and the structure of a polyester

6.10.2 The manufacture, molecular structure of Terylene/PET (Poly Ethylene Terephthalate) and their uses

6.10.3 Examples of homopolymeric polymers - structure and uses of polylactic acid and Biopol

6.10.4 Disposal and other issues involving using polyesters

6.10.5 The hydrolysis of polyesters

6.10.1 Comparing condensation polymerisation & addition polymerisation and the structure of a polyester

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 monomer, the 'spare bonds' then link up to form the polymer chain plus H₂O molecules, or sometimes HCl molecules if one of the monomers is an acid chloride derived from a dicarboxylic acid.

By contrast, in addition polymerisation (illustrated below) there is only one monomer molecule with a double bond (e.g. alkene) and one product, the polymer, and the linking occurs via the reactive double bond - illustrated below.

Condensation polymerisation

Polyesters are made from dicarboxylic acids and diols (though can use one monomer with the two different groups present in its molecular structure See homopolymeric polyesters).

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.

This involves monomers with two functional groups, one at each end of the molecule - they can be the same or different, as long as one functional group and join and condense with the functional group of another monomer.

When these types of monomers react, they join together (polymerise) and small molecules such as water are eliminated in the process, and so the reactions are called condensation reactions, since a polymer is formed, the process is called condensation polymerisation.

e.g. in these three simplified cases, there are two different monomers, but each has the same functional group (hydroxy, amine, carboxylic acid or acid chloride) ....

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

 n H₂N-[][]-NH₂ + n HOOC-[][][][]-COOH ==> -(-[][]-NHOC-[][][][]-CONH-)_n-  +  2n H₂O

 n H₂N-[][]-NH₂ + n ClOC-[][][][]-COCl ==> -(-[][]-NHOC-[][][][]-CONH-)_n-  +  2n HCl

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

A polyester can be made from a diol (an alcohol with two hydroxy groups two -OH's) and a dioic 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.

An example of polyester (which is a commercial product) is made from the two monomers ...

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.

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

The OH group of one diol monomer and a COOH group of the other monomer react to form an ester linkage COOC and a water molecule H₂O is eliminated.

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 consisting of hundreds and thousands of monomer units.

Where n is the very large number of monomer molecules, the condensation polymerisation of ethane-1,2-diol and hexanedioic acid can be represented, where n is a very big number, as ...

n HO-CH₂-CH₂-OH + n HOOCCH₂CH₂CH₂CH₂COOH

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

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6.10.2 The formation, molecular structure and uses of the polyester PET/Terylene

PET is the acronym for Poly Ethylene Terephthalate, and it has the same molecular structure as Terylene!

Terylene (a polyester) is good for making 'artificial' or 'man-made' fibres used in the clothing industry.

benzene-1,4-dicarboxylic acid, (terephthalic acid) a carboxylic acid functional group at each end of the molecule

Ethane-1,2-diol (ethylene glycol), two primary alcohol groups, one at each end of the molecule;

benzene-1,4-dicarboxylic acid forms a condensation polymer with ethane-1,2-diol, the polyester Terylene and the eliminated molecule is water H₂O, 1st diagram below, and the 4th diagram shows several of the linked ester units.

You can carry out the reaction with a different aromatic acid derivative to make PET/Terylene e.g. using ...

 the diacid chloride of benzene-1,4-dicarboxylic acid and the eliminated molecule is hydrogen chloride (HCl).

 the dimethyl ester of benzene-1,4-dicarboxylic acid and the eliminated molecule is methanol (CH₃OH)l.

(see the two middle diagrams below)

More advanced displayed formula representations of Terylene/PET and its formation.

One of the equation is repeated in structural formula style and with the repeating unit shown.

 You need to be able to work out the structural formula of the original monomers i.e. the original aromatic dicarboxylic acid and the aliphatic diol.

 

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" e.g. the brand Terylene when used for fibres or fabrics, and "PET" or "PET Resin" when used for bottles, jars, containers and packaging applications - but they have the same molecular structure - its just how its processed into a particular product.

PET is a thermoplastic, softens on heating, that can be extruded into different shapes, particularly for water and fizzy drinks. The bottle keeps the dissolved carbon dioxide from escaping and it is shatterproof if dropped on the floor - so its quite a tough plastic.

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.

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!
 

Terylene polyester is a typical synthetic fibres which have, in many cases, replaced cotton, silk and wool fabrics in the clothing industry.

Fine polyester fibres can be made into a variety of articles of clothing which are lighter and cheaper than traditional materials like wool.

Shirts, sheets, socks and trousers are manufactured from a mix of a polyester and a natural fibre such as cotton and wool.

The polyester gives fabric strength and creases resistance and the natural fibre gives the material a softer more natural feel and also allows the material to absorb some perspiration from the wearer - remember plastic materials are usually hydrophobic - water repellent.

    drawing out fibres makes them stronger

In the manufacturing process the polymer chains are made to line up by drawing out the polymer into a fine fibre.

Increasing the polymer chain alignment greatly increases the intermolecular forces between the 'aligned' polymer molecules and strong fibre strands of the plastic can be made - the process increases the surface - surface contact, so increasing the instantaneous dipole - induced dipole forces and the permanent dipole - permanent dipole forces.

Terylene fibres 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,

Polyester fibres are good thermal insulators - trap air e.g. the stuffing in duvets.

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.

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6.10.3 Homopolymeric polyesters

A homopolymer is a polymer produced from a single type of monomer e.g. the production of poly(ethene) from ethene.

To produce a condensation homopolymer, the single type of monomer must have both functional groups (one at each end) in its molecular structure to enable it to condense together with another monomer and eliminate a small molecule when the linking bond is formed.

Hydroxy-carboxylic acids have stable functional groups at (or near!) the ends of the molecule i.e. HO-R-COOH.

These can be obtained from renewable resources such as corn starch and sugar cane.

The polymer products are sometimes referred to as 'bioplastics'.

 

Polylactic acid - structure and uses

2-hydroxypropanoic acid (lactic acid) is a hydroxy acid that can be polymerised to make a useful biodegradable polymer.

Lactic acid has two functional groups:

Hydroxy - alcohol and carboxylic acid, these can condense to form an ester linkage.

2-hydroxypropanoic acid can be manufactured by the fermentation of cornstarch.

Lactic acid is naturally formed e.g. in milk, so microorganisms can 'recognise' the polymer's structure and break it down.

Polymerisation equation and structure of polylactic acid - condensation reaction with loss of water

Uses of polylactic acid

Polylactic acid, also biodegradable, can be used for cold drink cups and biodegrades in 180 days.

Polylactic acid has many uses. It can be processed into plastic films in food handling, bottles, and biodegradable medical implant devices (e.g. screws, pins, rods, and plates that are expected to biodegrade in the body within 6-12 months).

Polylactic acid constricts under heat and is suitable for use as a shrink wrap material. 

One disadvantage is its low softening temperature makes polylactic acid products unsuitable to hold hot liquids e.g. plastic cups.

Supermarkets are using biodegradable polymers ('bioplastics') for packaging and carrier bags which can decompose in less than a month on the compost heap.

 

Poly(glycolic acid)

Poly(2-hydroxyethanoic acid), poly(glycolic acid), is another biodegradable polymer.

Again, the monomer has the two functional groups to make an ester linkage - primary alcohol and carboxylic acid, with water eliminated in the condensation polymerisation.

Poly(glycolic acid) makes useful fibres, but is quite easily hydrolysed.

However, this makes it particularly useful in as a suture in surgery - the 'stitches' gradually dissolve away!

 

Biopol - structure and uses

Biopol is a brand name for PHB (Polyhydroxybutyrate). It is an environmentally friendly, quality biodegradable plastic

As an energy store, 3-hydroxybutanoate, CH₃CH(OH)CH₂COOCH₃, is produced by the microorganism Alcaligenes. From this you obtain 3-hydroxybutanoic acid .

Polymerisation equation and structure of Biopol

Uses of Biopol

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6.10.4 Disposal and other issues involving using polyesters

Plastic bottles made from polyester can recycled and turned into fibres again and reused in clothing, so!, are you wearing a plastic bottle!

The bottles must be collected, sorted, and washed and after purification they can be recycled and softened - extruded once again into a useful product.

However, polyesters are not biodegradable, despite the ester linkage, which is present in compounds of all organisms, microorganisms have not yet evolved to hydrolyse the ester linkage in synthetic fibres.

There is growing concern of the build-up of microplastic particles in the environment, much of which seems to end up as 'fibres' in the sea.

Also, unfortunately, when burned, toxic fumes are easily produced unless the material is incinerated at a very high temperature with excess air (oxygen supply) under very carefully controlled conditions.

For see plastic recycling - methods and issues

See More on the uses of plastics, issues with using plastics, solutions and recycling

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6.10.5 The hydrolysis of polyesters

Stability to hydrolysis - PET/Terylene

Since the monomer linkage is an ester, its not unreasonable to consider the possibility that PET/Terylene might degrade in the environment by hydrolysis.

Fortunately, the ester linkage in polyesters is quite stable and no such degradation takes place with water and very slowly in contact with acids.

(i) However, if you reflux fragmented bits of PET/Terylene (to increase the surface area) for long enough in aqueous sodium hydroxide solution, they will eventually dissolve to form a solution of ethane-1,2-diol (one of the monomers) and the disodium salt of benzene-1,4-dicarboxylic acid, disodium benzene-1,4-dicarboxylate (the salt of the other monomer material).

(ii) If you add excess dilute hydrochloric acid to the mixture, a white precipitate of benzene-1,4-dicarboxylic acid forms.

C₆H₄ represents the benzene ring structure in these simple structural formula equations.

State symbols are omitted for simplification: PET and the acid (s) and everything else is (aq).

(i) -(-CH₂-CH₂-OOC-C₆H₄-COO-)_n-  +  2n NaOH ===> n HO-CH₂-CH₂-OH + n Na^+-OOCC₄H₄COO^-Na⁺

-(-CH₂-CH₂-OOC-C₆H₄-COO-)_n-  +  2n OH^- ===> n HO-CH₂-CH₂-OH + n ^-OOCC₄H₄COO^-

(ii) Na^+-OOCC₄H₄COO^-Na⁺  +  2HCl  ===>  HOOCC₄H₄COOH  +  2NaCl

^-OOCC₄H₄COO^-  +  2H⁺  ===>  HOOCC₄H₄COOH

Therefore it is unwise to store acidic, and especially alkaline, chemicals in PET containers and Terylene clothing should not come into contact with acids or alkalis.

 

The alkaline hydrolysis of poly(2-hydroxypropanoic acid), poly(lactic acid)

If poly(lactic acid) is refluxed with aqueous sodium hydroxide the product is a solution of sodium 2-hydroxypropanoate (sodium lactate).

The acid can be freed by adding dil. hydrochloric acid:

CH₃CH(OH)COO^-_(aq)  +  H⁺_(aq)  ===> CH₃CH(OH)COOH_(aq)

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