HSC Chemistry: Polymers

 

This is part of the HSC Chemistry course under the topic Polymers. There are two types of polymers: addition and condensation polymers.

HSC Chemistry Syllabus

• model and compare the structure, properties and uses of addition polymers of ethylene and related monomers, for example: – polyethylene (PE) – polyvinyl chloride (PVC) – polystyrene (PS) – polytetrafluoroethylene (PTFE) (ACSCH136)

• model and compare the structure, properties and uses of condensation polymers of ethylene and related monomers, for example: – polyesters – polyamides (nylon) (ACSCH136)

Addition Polymers: HDPE, LDPE, PVC, PS and PTFE

 

Addition Polymers

Addition Polymerisation

• Addition polymers are produced using monomers with double carbon-to-carbon bonds (C=C).

 

 

 

Polymerisation of ethylene produces polyethylene

 

• Reactive C=C of alkene molecules can ‘open up’ to form single carbon-carbon bonds with other alkene molecules. This results in the formation of a long, saturated polymer chain.

 

• Step 1: Initiation
  • An initiator molecule e.g. hydrogen peroxide (H₂O₂) produces free radical OH species with a highly reactive unpaired electron.
  • The unpaired electron forms a new covalent bond with one of the electrons in the C=C bond in the monomer. This produces another radical molecule with an unpaired electron that was formerly in the C=C bond.

 

• Step 2: Elongation
  • The radical molecule formed in initiation is able to react with another monomer via the same mechanism, that is the unpaired electron forms a covalent bond with one of the electrons in the C=C bond.
  • This reaction joins the two organic molecules to produce a larger molecule that also contains an unpaired electron.

 

• As the reaction between the radical molecule and monomers continues, the molecule grows in length

 

 

• Step 3: Termination
  • Elongation of the molecule stops when two radical molecules (with unpaired electrons) react to form a covalent bond. This terminates the polymerisation as the product no longer has an unpaired electron.
  • Termination can occur with hydroxyl radical molecules produced from hydrogen peroxide or another large polymer that has an unpaired electron. 

Two relatively large radical molecules react to produce a final product (polymer)

 

High-density Polyethylene (HDPE)

• Structure: HDPE consists of straight, linear chains of polyethylene with no or minimal branching.

 

Properties	Uses
The chains produced are linear and can pack closely together – high density.   Straight-chained HDPE have greater dispersion force than LDPE. Therefore, they have higher melting points, harder and are crystalline in	plastic utensils   Plastic bottles   Plastic toys

 

Low-density Polyethylene (LDPE)

• Structure: polyethylene chains with numerous branches.

 

Properties	Uses
Branched structure of LDPE causes it to have reduced area of contact and dispersion forces between polymer chains. This results in a lower melting point than HDPE.   Branched structure gives rise to LDPE’s amorphous nature i.e. the shape of LDPE can be easily changed.	Plastic bags   Films   Plastic packaging

Polyvinyl chloride (PVC)

Structure: PVC is produced from polymerisation of chloroethene (vinyl chloride).   Compared to ethylene, chloroethene has one hydrogen substituted by a chlorine.

 

Properties

Uses

Presence of Cl atom causes PVC to have much greater molecular mass than LDPE and HDPE. Cl’s electronegativity provides a permanent dipole-dipole force between polymer chains. This leads to stronger intermolecular force. PVC has higher melting points than HDPE and LDPE.   PVC can be manufactured to be either rigid or quite flexible, depending on its production method and other added chemicals.   PVC is durable, particularly resistant to weathering (suitable for outdoor uses).   PVC is resistant to acids.   Like most polymers, PVC is a poor conductor of electricity and so acts as a good electrical insulation material.

Rigid PVC Gas pipes and water drainage pipes because it is durable and rigid.   Window frames   Flexible PVC Cling wrap   Insulation for cables and wires   Inflatables

Polystyrene (PS)

Structure: PS is produced from polymerization of ethenylbenzene (styrene).   An ethyenylbenzene monomer is an ethene molecule with one of the hydrogen atoms replaced by a large and bulky benzene ring.   A benzene ring is a six-carbon membered ring with alternating C=C bonds.

 

 

Properties of PS Extensive dispersion forces are present between adjacent chains of polystyrene. However, the bulkiness of benzene rings causes PS to be brittle.   Transparent   Light weight and low density   Thermally insulating   Electrically insulating   Uses of PS In gas-expanded (aerated) form, polystyrene is used as Styrofoam in disposable containers and packaging   Transparency and light weight properties are useful in cassette cases.   Light weight and electrically insulating properties are useful in handles of screwdrivers.

 

 

 

 

Polytetrafluoroethylene (PTFE)

• Structure: PTFE is produced from polymerisation of tetrafluoroethene. Each monomer is essentially an ethene molecule with all 4 hydrogen atoms replaced by fluorine atoms.

 

Properties of PTFE Extensive dispersion and dipole-dipole forces between chains of PTFE gives rise to its high melting point. Thermally stable at temperatures up to 250 ºC.   Almost totally chemically inert.   Insoluble in most solvents, resistant to water   Flame resistant   Acid resistant   Very low coefficient of friction   Uses of PTFE   Non-stick cooking pans   Low frictional resistance warrants its use in many areas of engineering e.g. gaskets.

Condensation Polymers: Polyesters and Polyamides (Nylon)

 

Condensation Polymers

• In contrast to addition polymers:
  • Condensation polymers are formed through condensation reactions through which a small molecule e.g. H₂O is also formed.
  • Condensation polymerisation usually involves more than one monomer type.
  • Condensation polymers are usually thermoplastics, meaning they can be melted for reshaping and recycling.

 

Two types of condensation reactions used to produce polymers: Esterification: reaction between alcohol and carboxylic acid to form an ester. Amidation: reaction between amine and carboxylic acid to form an amide. (extension)   In general, the number of water molecules produced (eliminated) from a polymerisation reaction is always one less than the number of monomers in the final polymer. For example, if a polymer has 10 monomers, then 9 H2O molecules would have been produced.

Figure: Condensation reaction between methanol and butanoic acid.

 

Polyesters

• Structure: monomers are joined together via esterification i.e. reaction between an alcohol and a carboxylic acid functional group.
• A water molecule is eliminated every time two monomers are joined.
• A polyester can be produced by reacting dicarboxylic acids and dialcohols.

 

• A polyester can be produced by using a monomer that contains both carboxylic acid and alcohol functional groups.

 

• Polyethylene terephthalate (PET) is the specific polyester used in textiles. It is produced using two monomers: a dicarboxylic acid and a dialcohol.

 

Properties of Polyesters	Uses
High tensile strength   Can be drawn into fibres   Abrasion-resistant   Heat-resistant   Crease/wrinkle-resistant   UV-resistant   Thermoplastic   Hydrophobic; greater moisture resistance	Clothing   Carpets and other textiles   Fruit containers   Single-use plastic bottles   Toasters   Shower headers

 

• Naturally found polyesters are biodegradable but most synthetic polyesters are not. However, polyesters can be recycled and re-processed because their inter-chain linkages are easy to overcome without distorting the polymer backbone.

 

 

 

Polyamides

• Structure: monomers are joined together via amidation, that is a reaction between a carboxylic acid and an amine functional group. A water molecule is eliminated every time a dicarboxylic and a diamine molecule join together.
• In addition to dispersion and dipole-dipole forces, hydrogen bonds are also formed between polyamide chains. This is because polyamide molecules contain hydrogen atoms bound to nitrogen atoms, which allows them to donate hydrogen bonds to oxygen atoms in nearby polyamide chains.

 

Diagram shows hydrogen bonds formed between adjacent polyamide chains

 

• Nylon 6 is a common polyamide produced from 6-aminohexanoic acid (monomer)

  

• Nylon 66 is another common polyamide produced from two monomers: hexandioic acid (adipic acid) and hexan-1,6-diamine.

 

 

Properties of Nylon	Uses of Nylon
High tensile strength   Can be drawn into fibres   Abrasion-resistant   Elastic   Thermoplastic   Absorbs moisture	Clothing: Lingerie, tights, raincoats and swimwear   Carpets, drapes and bedding   Seat belts   Ropes, nets, sleeping bags, tents

 

 

 

Polyesters vs Polyamides

 

Property	Polyamide (nylon)	Polyester (PET)
Structure	Both are condensation polymers.   Both uses carboxylic acid functional group in monomer(s).   Production of polyamides and polyesters produce water as byproducts   Unlike polyesters, hydrogen bonds are formed between polyamide chains.
Properties	Similarities High tensile strength   Both can be drawn into fibres: good for textile use   Abrasion-resistant   Thermoplastic: can be re-shaped and recycled via melting   Differences   Polyesters are more heat resistant   Polyamides have greater tensile strength than polyesters    Nylon can absorb moisture whereas polyesters are more hydrophobic so they are less able to do so.
Uses	Similarities Both are useful in textiles e.g. clothing and carpets due to their high tensile strength, ability to be drawn into fibres and recyclable nature (thermoplastic)   Differences Polyesters are more heat resistant, so they are found in appliances that are exposed to high temperatures such as toasters.   Polyesters are found in plastic containers for fruits and plastic bottles.   Nylon is also found in fishing nets and ropes, applications that require greater tensile strength.

 

 

 

Addition Polymers vs Condensation Polymers

Similarities

• Both types of polymers are produced from organic monomers which consist of carbon atoms as the backbone.
• Relatively low cost compared to alternatives
• Most are thermoplastics
• Strong and light weight
• Non-biodegradable (environmental implications)

 

Differences

• Water is also produced during condensation polymerization.
• Different properties that determine polymers’ uses.
• Condensation polymers can be drawn into fibres (good for textile).
• Condensation polymers are more easily recycled.