Polymers Flashcards
Polymers
Large molecules built by repetitive linking of smaller units called monomers
Natural (e.g. carbohydrates and proteins) and synthetic (e.g. plastics) polymers exist
Polymer properties
Density, melting point, flexibility, stability (heat, light, chemicals), strength, weight
Addition polymerisation
Monomers are added together using the electrons from the double bond
All atoms present in the monomer are present in the polymer
Examples: polyethylene, polypropene
Chain length
The length of polymers allows them to have substantial dispersion forces and higher melting points
Extent of branching affects polymer melting point
If polymer forms long chains with very few, short branches they can pack closely together - high density. Leads to an orderly crystalline arrangement, making it tough with high melting point.
If polymer has a lot of branching they cannot arrange themselves well - low density. The structure is non-crystalline making it more soft and low melting point.
Arrangement of side groups
The way side groups arrange along a polymer chain can affect properties (isotactic, atactic, syntactic)
Isotactic
All side groups on one side of the chain
Atactic
Side groups randomly distributed
Syntactic
Side groups alternate
Condensation polymerisation
Monomers with functional groups react with each other
Small molecule (e.g. water)
Polyester
Monomers are carboxylic acids and alcohols
Ester link (C bonded to O, and double bond O)
Polypeptide (proteins)
Monomers are amino acids
Peptide link (C double bond O, to NH)
Polysaccharide
Monomers are glucose molecules
Glycosidic link (C - O - C )
HDPE branching
Low degree of branching allows polymer chains to stack closely together. Dispersion forces act over shorter distances, higher melting point.
LDPE branching
High degree of branching means polymer chains don’t stack well together. Dispersion forces act over longer distances, lower melting point.
