Investigate the structural formulae, properties, formation and uses of addition polymers (polyethylene, polyvinyl chloride, polystyrene, polytetrafluoroethylene) and condensation polymers (nylon, polyester)

What this dot point is asking

NESA wants you to draw the structures of common polymers, write polymerisation equations using either an addition (one monomer with $C=C$, no byproduct) or condensation (two monomers with reactive groups at each end, releases water) mechanism, and explain how the chain architecture and intermolecular forces between chains determine real-world properties.

The answer

Two mechanisms, two polymer families

Addition polymers

The $\pi$ bond of an alkene opens; the carbons join in a long chain. The general scheme:

The square-bracketed unit is the repeat unit. Every atom of the monomer ends up in the polymer; nothing is lost.

Polyethylene (PE), from ethene $CH_2=CH_2$. The simplest polymer. Two grades:

• LDPE (low-density): branched chains, free-radical catalysis at high pressure. Flexible, used for plastic bags, squeeze bottles, cling film.
• HDPE (high-density): linear unbranched chains, Ziegler-Natta catalyst at low pressure. Rigid, used for milk bottles, piping, hard hats.

The same monomer produces dramatically different materials because chain architecture controls packing and intermolecular forces.

Polyvinyl chloride (PVC), from chloroethene $CH_2=CHCl$:

The chlorine atoms make PVC denser and stronger than PE, and the C-Cl dipole adds dipole-dipole forces on top of dispersion. PVC is rigid in pure form (pipes, window frames) and softened to flexible form with plasticisers (cables, hoses, vinyl flooring).

Polystyrene (PS), from styrene $CH_2=CHC_6H_5$ (phenylethene):

The pendant phenyl rings make the polymer rigid and brittle. PS is used for plastic cutlery, CD cases, and (blown with $CO_2$ or pentane) as expanded polystyrene foam (cups, packaging).

Polytetrafluoroethylene (PTFE, Teflon), from tetrafluoroethene $CF_2=CF_2$:

The C-F bonds are very strong and the fluorine shield is chemically inert. PTFE is heat-resistant up to 260 degrees C, has a very low coefficient of friction (used as non-stick coating), and resists almost all chemicals.

Condensation polymers

Two monomers, each with two reactive groups, react head-to-tail-to-head-to-tail. A small molecule (water) is expelled at each linkage.

Nylon 6,6 (polyamide), from hexane-1,6-diamine and hexanedioic acid:

The repeat unit contains two amide bonds. The "6,6" refers to the carbon count in each monomer (6 in the diamine, 6 in the diacid). The amide $N-H$ and $C=O$ groups hydrogen bond between adjacent chains, giving nylon high tensile strength, toughness and a high melting point (about 265 degrees C). Used for textiles (stockings, climbing ropes), engineering plastics (gears, bearings), and fishing line.

Polyester (PET, polyethylene terephthalate), from ethane-1,2-diol and benzene-1,4-dicarboxylic acid (terephthalic acid):

The repeat unit contains two ester linkages. The aromatic rings make PET rigid and dimensionally stable; the polymer can be drawn into strong fibres or blown into bottles. Used for soft-drink bottles, polyester clothing, packaging films.

Structure-property relationships

Chain length

Longer chains give greater dispersion forces overall, higher melting point and stronger material. Industrial polymers are typically 1000 to 10,000 monomer units long.

Branching

Linear chains pack closely (HDPE, drawn nylon fibre); branched chains pack loosely (LDPE). Closer packing means more dispersion force contact and higher density.

Functional groups in the chain

Hydrogen-bond-capable groups (amide, hydroxyl) raise melting point and tensile strength considerably. Halogen substituents add dipole-dipole forces. Aromatic rings add rigidity.

Crystallinity

Regular, regularly-spaced chains can crystallise (form ordered regions); irregular chains stay amorphous. Crystalline regions are stronger and more dense. HDPE is about 90% crystalline; LDPE only about 50%.

Crosslinking

Covalent bonds between adjacent chains turn a thermoplastic into a thermoset (vulcanised rubber, epoxy). Not usually examined at HSC but worth a mention.

Examples in context

Example 1. Polyethene at Qenos Botany. The Qenos plant at Botany Industrial Park produces both low-density and high-density polyethene from ethene feed cracked at the adjacent ethylene plant. LDPE forms in high-pressure (around 2000 atm) free-radical polymerisation, giving branched chains used in NSW supermarket plastic bags. HDPE forms with a Ziegler-Natta catalyst at low pressure, giving linear chains with higher crystallinity used in milk bottles and pipes. The chemistry is identical at the monomer level: $n CH_2=CH_2 \rightarrow [-CH_2-CH_2-]_n$, no byproduct. The HSC distinction between addition mechanism and structural outcome (branched vs linear) explains why one Qenos product is soft and the other rigid.

Example 2. Lycra and the NSW activewear supply chain. Activewear sold by Lorna Jane and 2XU outlets in NSW uses Lycra (spandex), a block copolymer containing both polyurea hard segments (amide-like) and polyether soft segments (ether). Each amide bond forms by condensation of a diisocyanate with a diamine with no net water loss but the same condensation principle. The HSC framework for condensation polymers (loss of small molecule, repeating amide linkage) generalises to this commercial example. Students predicting the monomer of a polymer from a fragment of its repeat unit are doing exactly the analysis a textile chemist does when reverse-engineering a competitor's fibre.

Try this

Q1. Distinguish between addition and condensation polymerisation, naming one example polymer of each. [3 marks]

• Cue. Addition: alkene monomer, no byproduct (PE); condensation: difunctional monomers, water lost per bond (nylon).

Q2. Calculate the average degree of polymerisation $n$ of a polyethene chain with average molar mass $5.6 \times 10^4$ g mol$^{-1}$. [2 marks]

• Cue. Monomer mass 28.05 g mol$^{-1}$; $n = 5.6 \times 10^4 / 28.05 = 2000$.

Q3. Polyester (PET) is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. (a) Draw the repeat unit. (b) Explain why PET fibre has high tensile strength. (c) State one environmental consequence of PET use in NSW. [2+2+1 marks]

• Cue. (a) $-OCH_2CH_2OOC{-}C_6H_4{-}CO-$. (b) Linear chains with strong intermolecular forces (hydrogen bonding to ester carbonyl, dispersion forces between aromatic rings). (c) Non-biodegradable; landfill burden or recycling load.