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.

Common traps

**Forgetting the brackets and the subscript $n$** in a polymer structure. The repeat unit must be in brackets with $n$ outside.

Writing water as a byproduct of addition polymerisation. Addition has no byproduct. Only condensation produces water (or sometimes HCl, etc.).

Naming the monomer of polyethylene as "ethylene". Ethylene is the older name; HSC prefers ethene. Same molecule.

Confusing PVC with polystyrene. PVC has C-Cl side groups, polystyrene has C-phenyl side groups.

Saying nylon has stronger IMFs because of the amide group, full stop. The mechanism is specifically hydrogen bonding between the $N-H$ of one chain and the $C=O$ of an adjacent chain. State that.

In one sentence

Addition polymers (PE, PVC, PS, PTFE) form when an alkene monomer's $\pi$ bond opens and links to give a saturated chain with no byproduct, while condensation polymers (nylon, PET) form when two difunctional monomers link with loss of water at each bond, producing amide or ester linkages that hydrogen bond between chains and give the polymer its strength.