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Polymers
Polymers are giant molecules that are made up of
many, many smaller molecules.
Building blocks for polymers are called monomers.
Examples: plastics, rubber etc.
Biopolymers: proteins, polysaccharides, nucleic acids
Polymers
Biopolymers:
Proteins
• Basic building blocks are amino acids
• Amino acids linked together into proteins by amide groups
• Peptide bonds formed by condensation reactions between two
amino acids.
e.g. Alanine + Glycine:
Gly
Ala
Gly-Ala
Polymers
Biopolymers:
H
*
O
N
1
R
H
H
2
R
N
H
H
N
*
O
R = any amino acid
Representative structure of a segment of protein
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Amino acids:
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Polysaccharides (Carbohydrates)
• Monosaccharides linked together by ether-bridges
Representative segment of cellulose: a tough fibre.
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Nucleic acids
Composed of:
H3PO4 molecule
Organic base
5-C sugar
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In order to get a polymer formed we need a
bifunctional molecule.
X
X
Y
Y
(-XY)
X
Y
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Addition Polymerization
Example: ethylene H2C=CH2, can polymerize by
opening the C–C  bond to form C–C  bonds with
adjacent ethylene molecules (with the help of
radicals).
The result: polyethylene.
This is called addition polymerization because
ethylene molecules are added to each other.
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Step 1 Initiation: generation of radicals from catalyst
Step 2 radical adds to ethylene and polymerisation starts
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Step 3 Propogation: repeated addition of carbon radical
to other ethylene molecules
In
.
CH2CH2 + H2C CH2
In
Repeat
In
many times
(CH2CH2)nCH2CH2
CH2CH2CH2CH2
.
Step 4 Termination: radicals consumed to stop the reaction
.
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Draw a segment of polystyrene that consists of four styrene
molecules added together.
Styrene monomer
Now what is the repeating unit?
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This is the repeat unit
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Poly(vinyl chloride) has the following structure
Cl
Cl
Cl
n
Simplify the above to show its repeat unit only.
What is the monomer of the above polymer?
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repeat unit
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Condensation Polymerization
Condensation Polymerization: molecules are joined by the
elimination of a small molecule (e.g. water):
O
H O
N H + H O C
N C
H
+ H O H
Example of condensation polymerization: formation of nylon.
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O
OH +
HO
H2N
O
Adipic acid
NH2
Hexamethylenediamine
Heat
O
H
N
+ 2n
N
H
O
Nylon 66
n
H
O
H
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Another example of a condensation polymerisation is
the formation of nylon 6.
Used for clothing and mountaineering ropes amongst
others
O
H
N
H2O
HO
NH2
O
Caprolactam
6-aminohexanoic acid
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Show the condensation product of this molecule that occurs
between itself.
HO
NH2
O
6-aminohexanoic acid
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Polymers
Terylene is a polymer used for the carbonated drinks market
as it has a low permeability towards CO2 and prevents the
drink from going 'flat'.
O
O
+
HO
HOCH2CH2OH
OH
Draw a section of the polymer containing just the repeat unit
Polymers
Polymers
Types of Polymers
Plastic
Materials that can be formed into shapes by application
of heat and pressure.
Thermoplastics:
• Hard at room temp.
• Become soft + viscous when heated.  can be
shaped more than once.
• Little or no cross-linking  individual chains can slip
past each other.
• E.g.’s include polyethylene, nylon, polystyrene….
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Thermosetting resins:
• Become highly cross-linked when heated.  solidify
into a hard, insoluble mass.
• Can only be shaped once, polymerisation irreversible.
• Can withstand high temperatures.
• For example, Bakelite; used for adhesives, moulded
parts and coatings:
Phenol
formaldehyde
Polymers
Elastomers:
• Have the ability to stretch out & spring back to their
original shapes.
• Have a modest amount of cross-linking.
• Polymer chains have irregular shapes.
• Most common example is natural rubber
Polymers
Structure and Physical Properties of Polymers
Polymer chains tend to be flexible and easily entangled or
folded due to free rotation around the C–C single bonds.
Some regions of the polymer, may however, display a more
ordered arrangement of chains than other regions:
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Structure and Physical Properties of Polymers
The degree of crystallinity is a measure of the extent of
such ordering.
More ordering in a polymer = denser, harder, less soluble
polymers that are more resistant to heat.
e.g. Properties of PE as a Function of Crystallinity.
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Structure and Physical Properties of Polymers
Stretching or extruding a polymer can increase
crystallinity.
Degree of crystallinity is also determined by average
molecular mass:
Low density polyethylene (LDPE) has an average
molecular mass of 104 amu (used in plastic wrap);
High density polyethylene (HDPE) has an average
molecular mass of 106 amu (used in milk cartons).
Polymers
Structure and Physical Properties of Polymers
LDPE:
• Obtained by polymerization of ethylene at high pressure
and high temperature.
• Polymer chains have irregular branches and cannot pack
together in an ordered way.
• Result: LDPE is an open polymer of low density and little
mechanical strength.
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Structure and Physical Properties of Polymers
HDPE:
• Polythene can also be prepared catalytically at lower
pressures and temperatures.
• Result: regular non-branched chain polymer which is highly
ordered or crystalline.
• HDPE is tough and strong and the ordered structure means
that it has higher density.
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Structure and Physical Properties of Polymers
Polyethylene is a very versatile material & it’s properties
can be “fine tuned” by varying:
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Cross-Linking Polymers
Bonds formed between polymer chains make the polymer
stiffer.
Natural rubber is too soft and chemically reactive to make a
useful material.
By vulcanizing the rubber (cross-linking the polymer chains)
useful materials are made.
Rubber is usually cross-linked with sulfur.
Cross-linked rubber is stiffer, more elastic and less susceptible
to chemical reaction.
Polymers
Cross-Linking Polymers