Polymer Properties Differ from Monomer Properties

What students typically write

“Polyethene is a gas because ethene is a gas.” Or: “The polymer has the same boiling point as the monomer because they have the same chemical formula.” Or: “Poly(chloroethene) is toxic because chloroethene is toxic.”

Students reason that since the polymer is “made from” the monomer, it must share the monomer’s properties. This confuses chemical composition with physical behaviour.

Why it is wrong

Polymerisation joins hundreds or thousands of small monomer molecules into a single long chain. Although the repeating unit has the same atoms as the monomer, the resulting polymer molecule is enormous by comparison. This size change transforms the physical properties.

Ethene (C2H4) is a gas with a boiling point of -104 degrees C. Polyethene is a solid that does not melt until around 115-135 degrees C. The difference is not in the types of bonds but in the length of the molecules and the total intermolecular forces between them.

Short molecules have weak intermolecular forces and are easily separated (low boiling point). Long polymer chains have extensive intermolecular forces along their entire length, requiring much more energy to separate (high melting point, solid at room temperature).

Toxicity, flammability, solubility, and other properties also change upon polymerisation. PVC (poly(chloroethene)) is a stable, safe plastic used in water pipes, even though the monomer chloroethene (vinyl chloride) is a known carcinogen. The reactive C=C double bond in the monomer is consumed during polymerisation, making the polymer much less chemically reactive.

What the mark scheme actually wants

A typical question: “Ethene is a gas. Poly(ethene) is a solid. Explain why their physical states are different.” [2]

Marking points:

1. Poly(ethene) molecules are very long / have a much larger molecular mass than ethene (1 mark)
2. There are stronger intermolecular forces between the long polymer chains / more energy needed to overcome the forces (1 mark)

Worked example: wrong vs right

Question: Propene (C3H6) is a gas at room temperature. Predict the state of poly(propene) at room temperature and explain your answer. [2]

Wrong answer: “Poly(propene) is a gas because propene is a gas and they are made of the same thing.” This scores 0/2.

Right answer: “Poly(propene) is a solid at room temperature (1) because the polymer chains are very long with strong total intermolecular forces between them, requiring much more energy to separate than the small propene molecules (1).” This scores 2/2.

How to avoid this mistake

Remember that polymerisation is a transformation, not just a renaming. The monomer and polymer have the same repeating unit but vastly different molecule sizes. Size determines intermolecular forces, and intermolecular forces determine physical properties.

When comparing a monomer and its polymer, focus on molecule size and intermolecular forces, not on the composition of the repeating unit.

For the full treatment of addition polymerisation, see polymers. The properties of alkene monomers are covered at alkenes.