Docstoc

Polymers

Document Sample
Polymers Powered By Docstoc
					                    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          2
                                 H
                      H    R
    *     N                      N   *

           1
                      N
         R        H
                      H    O

               R = any amino acid

Representative structure of a segment of protein
               Polymers
Amino acids:
                        Polymers
Polysaccharides (Carbohydrates)
• Monosaccharides linked together by ether-bridges




 Representative segment of cellulose: a tough fibre.
                Polymers
Nucleic acids

                    Composed of:
                     H3PO4 molecule

                      Organic base
                      5-C sugar
                    Polymers

In order to get a polymer formed we need a
bifunctional molecule.

     X                 X      Y                  Y


                            (-XY)




          X                                  Y
                     Polymers
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.
                        Polymers
Step 1 Initiation: generation of radicals from catalyst




Step 2 radical adds to ethylene and polymerisation starts
                         Polymers

Step 3 Propogation: repeated addition of carbon radical
       to other ethylene molecules

              .                                                  .
In   CH2CH2 + H2C CH2                    In       CH2CH2CH2CH2


 Repeat
                                              .
                    In   (CH2CH2)nCH2CH2
 many times


Step 4 Termination: radicals consumed to stop the reaction
                      Polymers

Draw a segment of polystyrene that consists of four styrene
molecules added together.




                    Styrene monomer



 Now what is the repeating unit?
Polymers




       This is the repeat unit
                 Polymers

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?
              Polymers




repeat unit
                        Polymers
Condensation Polymerization

Condensation Polymerization: molecules are joined by the
elimination of a small molecule (e.g. water):


  H                 O                   H O
  N H + H O C                            N C      + H O H


Example of condensation polymerization: formation of nylon.
                           Polymers

     O
                               OH +                                  NH2
HO
                                          H2N
                           O
         Adipic acid                         Hexamethylenediamine
                                      Heat



 O
                           H                                         O
                           N                              + 2n
                                                  N              H       H
                                                  H
                       O                              n

                               Nylon 66
                     Polymers
Another example of a condensation polymerisation is
the formation of nylon 6.

Used for clothing and mountaineering ropes amongst
others
   O
           H
       N                  HO
               H2O                                 NH2
                               O
Caprolactam                 6-aminohexanoic acid
                     Polymers

Show the condensation product of this molecule that occurs
between itself.

  HO
                           NH2
       O
    6-aminohexanoic acid
Polymers
                       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
                             +      HOCH2CH2OH
HO                   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….
                            Polymers
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:
                        Polymers
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.
                     Polymers
  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.
                        Polymers
    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.
                       Polymers
    Structure and Physical Properties of Polymers

Polyethylene is a very versatile material & it’s properties
can be “fine tuned” by varying:
                         Polymers
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

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:5
posted:3/16/2012
language:
pages:32