polymers are giant molecules that are made up of many, many smaller molecules. building blocks for...
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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
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Proteins
Biopolymers:
• 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
NN
N
O
O
* *
H
H
H
R1 H
H R2
R = any amino acid
Representative structure of a segment of protein
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Biopolymers:
PolymersPolymersAmino acids:
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Representative segment of cellulose: a tough fibre.
Polysaccharides (Carbohydrates)
• Monosaccharides linked together by ether-bridges
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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.
Y Y
X
X X
Y
(-XY)
PolymersPolymersAddition 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.
PolymersPolymersStep 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 CH2CH2CH2CH2.
+
In (CH2CH2)nCH2CH2.Repeat
many times
Step 4 Termination: radicals consumed to stop the reaction
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Styrene monomer
Draw a segment of polystyrene that consists of four styrene molecules added together.
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):
Example of condensation polymerization: formation of nylon.
N
H
H H O C
O
+ N
H
C
O
H O H+
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O
HO
O
OHH2N
NH2
Heat
O
O
NH
HN
n
HO
H2n
+
+
Adipic acid Hexamethylenediamine
Nylon 66
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N
OH
H2O
O
HONH2
Caprolactam 6-aminohexanoic acid
Another example of a condensation polymerisation is the formation of nylon 6.
Used for clothing and mountaineering ropes amongst others
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O
HONH2
6-aminohexanoic acid
Show the condensation product of this molecule that occurs between itself.
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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
OH
O
HO
HOCH2CH2OH+
Draw a section of the polymer containing just the repeat unit
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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….
Thermosetting resins:
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• 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
PolymersPolymersElastomers:
• 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
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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).
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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.
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Cross-Linking Polymers