Structure and properties of polymers

Addition polymerisation:

• If the monomers (represented by the letter A) contain a double bond, they can add together to make a polymer.
• A + A + A + A  ->  -A-A-A-A-
• No other products are formed.
• The polymer structure can be represented more simply by drawing the repeating unit.

Co-polymerisation:

• A co-polymer is made when two different monomers (represented by the letters A and B) beomce incorporated into the polymer chain.
• A + B + A + B  ->  -A-B-A-B-
• No other products are formed.
• The polymer structure can be represented more simply by drawing the repeating unit.

Chain length:

• the longer the chains, the stronger the polymer
• tensile strength increases because:
  • longer chains become more entangled
  • longer chains have stronger intermolecular bonds between them

Side groups on the polymer chain:

• the more polar the side groups, the stronger the bonds between polymer chains, so the polymer is stronger

Branching:

• straight, unbranched polymer chains can pack closer together, allowing stronger bonds between the chains, so the polymer is stronger

Chain flexibility:

• the more rigid the chain, the stronger the polymer
• hydrocarbon chains are very flexible, whereas incorporating benzene rings makes the polymer chains stiffer

Cross-linking:

• more extensive cross-linking makes the polymer harder to melt

Stereoregularity:

• the more regular the orientation of the side groups, the closer the packing, and the stronger the polymer 

Elastomers:

• soft and springy
• can be stretched, but they return their original shape

Fibres:

• can be made into strong, thing threads

• No cross-links between the chains
• Intermolecular bonds between the chains are much weaker than the covalent cross-links in a thermoset
• Attractive forces can be overcome by heating
• The chains can slide over one another and the polymer can be deformed
• After being deformed, on cooling, the weak bonds between the chains reform and the thermoplastic holds its new shape

• Extensive cross-linking between the polymer chains
• Bonds between the chains are much stronger than in thermoplastics
• The covalent bonds cannot be broken by heating
• Chains are unable to move relative to each other and the polymer cannot change shape
• If a thermoset is heated for an extensive period, the polymer chars and burns