F324 Condensation Polymers

F324 Condensation polymers

Condensation polymers

Condensation polymerizationWe have previously met addition polymerisation, where a double bond in a monomer is broken and used to connect one monomer to another, progressively forming very long chain molecules.

Condensation polymerization requires - a monomer with two different functional groups that can react by condensation

OR- two identical functional groups on one monomer, that react by condensation

with two of a different functional group on a second monomer.

Role of Chemists in developing degradable polymersIt takes in excess of 5000 years to break down a plastic bag made of polyethene, left in landfill. We need plastics which will break down much more rapidly.

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Condensation polymers describe condensation polymerisation to form:

(i) polyesters, eg Terylene from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, poly(lactic acid) from 2-hydroxypropanoic acid (lactic acid)

(ii) polyamides, eg nylon-6,6 from 1,6-diaminohexane and hexane-1,6- dicarboxylic acid, Kevlar from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid

You will not be expected to recall the structures of synthetic polyesters and polyamides or their monomers. compare condensation polymerisation with addition polymerisation suggest the type of polymerisation from:

(i) a given monomer or pair of monomers,(ii) a given section of a polymer molecule;

identify the monomer(s) required to form a given section of a polymer (and vice versa) state the use of polyesters and polyamides as fibres in clothing

Hydrolysis and degradable polymers

describe the acid and the base hydrolysis of polyesters and polyamides outline the role of chemists in minimising environmental waste by development of

degradable polymers, similar in structure to poly(lactic acid), which is used for waste sacks, packaging, disposable eating utensils and medical applications such as internal stitches.

explain that condensation polymers:(i) may be photodegradable as the C=O bond absorbs radiation(ii) may be hydrolysed at the ester or amide group

Condensation is a reaction in which two small molecules react together to form a larger molecule, with the elimination of a small molecule such as water.

Condensation polymerisation is therefore when many monomers join together to form a polymer, with the elimination of a smaller molecule such as water each time a monomer joins the chain.


There are two types of degradable plastics:i) Petroleum-based plastics with additives which when exposed to heat/moisture cause swelling and expand the structure allowing the plastic to be broken down mechanically.

ii) Bioplastic which are inherently degradable because they contain links such as esters or amides.

PolyestersRecall that an ester can be made from the reaction of a carboxylic acid with an alcohol. An ester linkage (-COO-) is formed and water is eliminated.

We can form a polyester from a monomer which carries both a –COOH group and an –OH group (a hydroxycarboxylic acid) by condensation polymerisation

a bond forms between the hydroxyl group on one monomer and the carboxylic acid group on a different monomer molecule

an ester linkage forms and water is eliminated

Example: poly(lactic acid), PLA, - formed from the monomer lactic acid (2-hydroxypropanoic acid).

There has been much interest in poly(lactic acid): the starting material, lactic acid, is derived from maize / corn starch so is

renewable, rather than depleting finite crude-oil resources

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Definitions: A degradable polymer is a polymer which breaks down into smaller fragments in the environment. The polymer can be broken down by several different means:

Biodegradable – broken down by micro-organisms into CO2, water, and inert biomass. This may be under either aerobic or anaerobic conditions.

Photodegradable – broken down by light. C=O (e.g. in polyesters) absorbs uv light which promotes degradation, especially when the polymer is made with additives which also absorb light.

Hydrolysed – broken down by acidic or alkaline hydrolysis, typically of an ester link or a peptide link in the polymer. Such polymers are not hydrolysed by water on an appreciable timescale.


this is the most widely used of the bioplastics used for waste sacks, packaging, disposable eating utensils and medical

applications such as internal stitches.

Another biodegradable polyester, poly(glycolic acid), is also used in surgery for stitches, and as an inner coating for drinks bottles. Synthetic biodegradable polyesters also play a role in thermoformed trays for fresh produce and meat, as well as disposable plates, bowls and cups. These plastics are made from cellulose and potato starch.

We can also form a polyester by reacting two different types of monomer units:

- one monomer is a dicarboxylic acid with two –COOH groups- the other is a diol with two –OH groups

Example: Terylene (when a fibre) also known as PET (when solid e.g. in bottles)- made from the monomers ethane-1,2-diol and benzene-1,4-dicarboxylic acid

Polyesters are used in making carpets, sports clothing and shirts. In clothing and bedding, polyesters are often blended with cotton. Almost all polyesters can be machine washed and machine-dried. Polyester is a strong fibre which is resistant to stretching and shrinking and chemical attack. Unfortunately it burns easily, and should not be exposed to naked flames.

Breaking down polyestersAlkaline hydrolysisPolyesters, like esters, are hydrolysed by hot aqueous alkalis such as sodium hydroxide solution, but this happens much more slowly in the polymer. Each ester linkage is hydrolysed to:

- the sodium salt of a carboxylic acid, -COO-Na+

- a hydroxyl group, -OH

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Or in the case of a polyester formed from a single monomer (e.g. polylactic acid):

This means that polyester clothing is easily damaged (dissolved) by spills of alkalis in the laboratory – careful !

Acid hydrolysisPolyesters can also be hydrolysed with hot aqueous acid, such as hydrochloric acid, although the reaction is much slower. The monomer units of the polyester are produced.

PolyamidesWe can also use peptide (the –CONH- group is called an amide or a peptide) linkages to create a polymer. Polyamides are condensation polymers, with the amide linkage being formed by reaction of an –NH2 group with a –COOH group and the elimination of water. Examples include Nylon-6,6; Kevlar and proteins. There are two types of polyamide:

Polyamides made from a single monomerThe monomer is an amino acid, and the polyamide is a polypeptide or protein. We have met this already in the previous topic – see F324 notes on Amino Acids.

Polyamides made from two types of monomer unitThe monomers are a diamine with two –NH2 groups, and a dicarboxylic acid with two –COOH groups.

During condensation polymerisation a bond forms between an amine group on the diamine and a carboxylic acid group

on the dicarboxylic acid an amide (peptide) linkage forms and water is eliminated

Example: Nylon-6,6

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- is made from 1,6-diaminohexane and hexanedioic acid:

Nylon-6,6 is widely used in clothing.

The '6,6' comes from the number of carbon atoms between the NH2 groups of the diamine, and between in the dicarboxylic acid (including those of the COOH).

Example: Kevlar - formed from benzene-1,4-diamine and benzene-1,4-dioic acid.

Kevlar has some remarkable properties – fire resistant, and a higher strength than steel. It is used to make protective clothing e.g. for firefighters, and in bullet-proof vests.

Why is Kevlar so strong ?In part because the polar C=O groups along one chain has a δ- oxygen atom and will align exactly with the polar N-H groups in another chain which has a δ+ H atom. Extensive hydrogen-bond cross linking of chains is therefore possible.

Hydrolysis of polyamidesLike polyesters, polyamides can be hydrolysed by hot aqueous alkali, e.g NaOH(aq). Each amide linkage is hydrolysed producing an –NH2 group and the sodium salt of the acid, -COO-Na+

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Alternatively polyamides can be hydrolysed using hot aqueous acid, such as HCl(aq). The dicarboxylic acid is produced, along with the ammonium salt of the diamine.

Comparing addition and condensation polymerisationWe need to be able to compare, and to identify the type of polymerisation from either the monomer(s) or a section of the polymer.

Comparing:Addition polymers are made from one monomer (alkene); condensation polymers are made from two types of monomer or one with two functional groups

Addition polymers form only one product; condensation polymers have an elimination product such as H2O or HCl.

Identifying types of polymerisationWe may have the monomer to examine, or may have a section of the polymer chain.For addition polymerisation, look out for:

the monomer has a C=C double bond the polymer backbone is continuous chain of C atoms

For condensation polymerisation, look out for two monomers, each with two functional groups one monomer with two different reacting functional groups the polymer contains ester or amide linkages

EnvironmentalThe main environmental benefits of degradable polymers such as these over hydrocarbon-based polymers such as poly(ethene) are:

They may be photodegradable as the C=O bond absorbs radiation, leading to breakdown of the polymer chain.

They may be easily hydrolysed, breaking down the polymer chain.

In either case this means that environmental pollution from waste polymer material can be minimized.

- for example, polyethene lasts in excess of 5000 years in landfill- nylon breaks down within 30 – 40 years in landfill

They may be produced from renewable resources, rather than depleting finite resources such as crude oil.

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F324 Condensation Polymers

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