POLYMERS

What is a polymer?

• Polymers are large molecules made up small repeating units.

• The repeating units in a polymer are called monomers.

• Monomers are linked together in a polymerization reaction.

• Polymers are widely abundant and may be natural or synthetic.

• Some examples of polymers are rubber, polyvinyl chloride (PVC), polyethylene, Teflon, Kevlar, carbohydrates, proteins

Addition Polymers

• Addition polymers result from addition reactions of monomers containing carbon-carbon double bonds.

• Some examples of addition polymers include polyethene, polypropene, polyvinyl chloride, polystyrene and Teflon

C C C C C C+ + C C C C C C

ethene etheneethenepolyethene or polyethylene

C C

n

OR

Example

Stages in an Addition Polymerization Reaction

• There are 3 stages in an addition polymerization reaction.

• Initiation an initiating molecule (such as a peroxide) with an unpaired electron forms a bond with one of the C’s in the double bond.

• Propagation the “growing” phase.• Termination the unpaired electron ends combine

forming a covalent bond ending the reaction.

Condensation Polymers

• Condensation polymers are formed when monomers are linked together through condensation reactions (removal of water).

• Examples of condensation polymers include polyester, proteins, carbohydrates and Kevlar

Example

O

CH2OH

OH

OH

OH

OHH

H

HH

H

galactose

O

CH2OH

O

OH

OH

OH

H

H

H

HH

glucose

H

O

CH2OH

OH

OH

OH

H

H

HH

H

O

CH2OH

OOH

OH

OH

H

H

H

HH

+ H2O

lactose

Types of condensation polymer linkages

a) Ester linkages (you have seen these in esterification reactions)

(a reaction between a carboxylic group and an alcohol (hydroxyl group))

- polymers with ester linkages have the general name of polyesters

Eg. Polyethylene, PET

Types of condensation polymer linkages

b) Amide linkages

- Occur between an amine group and a carboxyl group

- Called polyamides in general

- Examples include Kevlar, Dacron

The Effect of Substituted Groups on Polymer Properties

• The reactivity, solubility and strength of a polymer is due to the groups that are attached to the polymer chain.

• Teflon (polytetrafluoroethane) for example is very unreactive because the C-F is very strong.

• Monomers that contain 2 double bonds are able to add to other molecules in two ways which means a single monomer can be incorporated into 2 different polymer chains causing bridges between the polymer chains.

• These bridges are called “crosslinks” and may be formed intermittently along the polymer chains.

• These links between polymer chains are covalent bonds and are much stronger than the van Der Waals forces that would otherwise hold the chains together.

• As the degree of crosslinking increases so does the strength of the molecule.

• A high degree of crosslinking will also make the polymers heat resistant and more elastic.

• Crosslinks may also be formed with inorganic crosslinking agents such as sulfur.

Homework

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