Transcript polymers - wellswaysciences

Week 8
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Describe condensation polymerisation forming polyesters and polyamides,
such as Terylene and poly(lactic acid).
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State the use of polyamides as fibres in clothing.
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Polymers
• Long chain molecule made up of many small monomer
units joined together.
• Addition polymers:
• Monomer units alkenes
• e.g. ethene → poly(ethene)
• One type of monomer only.
• 100% atom economy.
• Designed with specific properties.
• Can identify monomer from repeat unit which has no
obvious functional groups in skeleton of chain.
• Repeat unit – specific arrangement of atoms that
occurs in the structure over and over again. Repeat
units are included in brackets, outside which is a
letter n.
Week 8
Addition polymerisation of an alkene
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Week 8
Propene monomers forming poly(propene)
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Polymers
• Condensation polymers:
• Made by reactions where small molecules e.g. water or
HCl are eliminated.
• Chemical reactions between monomer units with
different (reactive) functional groups.
• Either 1 monomer with 2 different functional groups (
i.e. bifunctional monomers) or 2 different monomers,
each with 2 identical functional groups but the
functional groups are different on each monomer.
• Condensation is a reaction in which 2 small molecules
react together to form a larger molecule, with the
elimination of a small molecule such as water.
Esters
• Made by reaction of a carboxylic acid and an
alcohol.
• E.g. propanoic acid + methanol → methyl propanoate + water
How must this be modified to make a
POLYester?
Polyesters
• Monomers are joined by ester linkages with the loss
of water.
• Monomers must be single bifunctional molecules or 2
monomers, one a carboxylic acid and the other an
alcohol.
• The repeat units reflect the monomers used to make
them.
• If bifunctional single monomer then there is ONE
REPEAT unit with a bridging oxygen at one end and a
carbonyl group at the other.
• If 2 monomers are used the repeat unit has 2
adjacent bridging oxygen atoms and 2 adjacent
carbonyl groups.
Week 8
Formation of polyesters from two types of monomer (left) and from one type of
monomer (right)
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Condensation Polymerisation
• Polyesters must have monomers containing at least one
carboxylic acid group and one alcohol group.
• Terylene has 2 monomer units,1 a dicarboxylic acid (terepthalic
acid – benzene-1,4-dicarboxylic acid).
• And a diol, ethan-1,2-diol.
• Used to manufacture fibres. E.g. carpets, sports clothing, shirts,
bedding. Fibres are elastic and good for clothing because
multiple dipole-dipole attractions can break and reform making
it good for clothing.
• Polylactic acid has one single monomer- 2-hydroxypropanoic acid.
• It is both photodegradable and biodegradable.
• It is used for waste sacks, food and drinks packaging. In each
case an ester linkage is formed between the OH and the
carboxyl groups and water is eliminated.
Week 8
Formation of Terylene, a polyester
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Week 8
Condensation polymerisation of lactic acid
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Week 8
•
Describe condensation polymerisation to form the polyamides nylon-6,6 and
Kevlar.
•
State the use of polyamides as fibres in clothing.
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Polyamides
Polyamides contain this functional group.
Found in protein (polypeptide) chains –
which are natural polyamides.
As before either 1 bifunctional monomer
(e.g. amino acids) or 2 monomers, each with
the same functional group at both ends but
the functional groups are different on
each monomer.
Structure of the Skeleton
• As before:
• Monomers must have at least one amino group
and one carboylic acid group which react
together with the loss of a water molecule.
• If bifunctional single monomer then there is
ONE REPEAT unit with an NH at one end and
a carbonyl group at the other.
• If 2 monomers are used the repeat unit has 2
adjacent N-H groups and 2 adjacent carbonyl
groups.
Week 8
Formation of polyamides from two types of monomer (left) and from one type of
monomer (right)
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Week 8
Formation of the polyamide nylon-6,6
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Week 8
Formation of the polyamide Kevlar
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Uses of Polyamides
• Fibres in clothing.
• Nylon:
• Strong, stretchy fibres used in underwear,
upholstery, carpets, tents, ropes etc.
• Nylon is ideal for making fibres because
• i) the amide links in the backbone are themselves
strong.
• ii) the chains are aligned along the length of the
fibre. Hydrogen bonds form between the chains and
when the fibre is stretched and then released these
bonds re-form to bring the nylon molecules back to
their original position, making the nylon elastic. Good
for tights!
Uses of Polyamides
• Kevlar:
• High tensile strength. Used as lightweight
replacement for steel (5 times stronger).
Body armour. Immense strength from Hbonding between amide groups and very flat
planar structure means close packing and
strong attractions between stacked rings.
Kevlar showing close packing
Week 8
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Compare condensation polymerisation with addition polymerisation.
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Suggest the type of polymerisation from a given monomer or pair of
monomers, or a given section of a polymer molecule.
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Identify the monomer(s) required to form a given section of a polymer (and
vice versa).
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Addition or Condensation Polymers?
• Work through examples on p. 58 in text book
– supplied in the short term.
POLYMERISATION OF ALKENES
SPOTTING THE MONOMER
POLYMERISATION OF ALKENES
SPOTTING THE MONOMER
Week 9
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Describe the acid and base hydrolysis of polyesters and polyamides.
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Outline the role of chemists in the development of degradable polymers.
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Explain that condensation polymers may be photodegradable and may be
hydrolysed.
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Break down of condensation polymers
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All readily hydrolysed.
All react with hot, aqueous acid or alkali.
Acid hydrolysis of polyester:
The parent monomer units are made.
Alkaline hydrolysis:
The sodium salt and the diol are made.
Acid hydrolysis of polyamide:
The dicarboxylic acid and the ammonium salt of the
amine.
• Alkaline hydrolysis:
• Sodium salt of dicarboxylic acid and diamine formed.
Week 9
Acid and base hydrolysis of the polyester Terylene
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Week 9
Acid and base hydrolysis of the polyamide nylon-6,6
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Degradable Polymers
• Degradable polymer – breaks down into
smaller fragments when exposed to light,
heat or moisture.
• Biodegradable polymer – breaks down
completely into carbon dioxide and water.
• E.g. Poly(lactic acid) derived from cornstarch
• Poly(glycolic acid) from sugar cane and unripe
grapes.
• These are polyesters.
Biodegradable Polymers
• PLA- compost bags, food packaging,
disposable table ware.
• PGA- surgery for stiches.
• Cellulose and potato starch used to make
biopolymers- used in thermoformed trays for
fresh veg and meat etc.
• Also in disposable crockery.
• Carrier bags from tapioca starch.
Photodegradable polymers
• Become weak and brittle when exposed to
sunlight.
• Made by blending the polymer with light
sensitive additives which catalyse the
breakdown of the polymer in uv light.
• OR can be made including C=O bonds which
can absorb light energy and break.
• A waxy substance forms first and then
eventually bacteria convert it to carbon
dioxide and water only.