Quick questions on Addition polymerisation and polymer properties (QCE Chemistry Unit 4)

Each monomer is an alkene (one or more C=C). Under suitable conditions (free-radical initiator, heat, pressure; or a coordination catalyst like Ziegler-Natta), the pi bonds open and link successive monomers:

For each, the repeat unit retains every atom of the monomer; you should be able to draw both directions (monomer to repeat, repeat to monomer) on demand.

Three structural levers control polymer properties: substituent group, chain branching, and chain regularity (which sets crystallinity).

All five addition polymers in the QCAA list are thermoplastic: they soften on heating and can be reshaped repeatedly, because chains are not covalently cross-linked. Thermosetting polymers (Bakelite, epoxy resins) are condensation polymers with permanent cross-links and are discussed in the separate condensation-polymers dot point.

Given a target application, identify the property that matters and choose the polymer:

Repeat unit conventions. Bracketed unit, with bonds extending outside the bracket on both sides; subscript n outside the bracket; substituents drawn on the correct carbon. For PVC, the chlorine is on the same carbon as in the monomer; for polystyrene, the phenyl group is on the same carbon as in styrene.

Drawing the C=C in the repeat unit. The double bond is consumed in polymerisation; the repeat unit has only single bonds.

Made by high-pressure free-radical polymerisation. Chain transfer creates short alkyl branches every 20 to 50 carbons. Branches prevent close packing; density 0.91 to 0.93 g/cm^3; crystallinity around 40 to 50 percent.

Made by low-pressure Ziegler-Natta catalysis. Chains are essentially linear (few branches). Tight packing; density 0.94 to 0.97 g/cm^3; crystallinity around 60 to 80 percent.

Bracketed unit, with bonds extending outside the bracket on both sides; subscript n outside the bracket; substituents drawn on the correct carbon. For PVC, the chlorine is on the same carbon as in the monomer; for polystyrene, the phenyl group is on the same carbon as in styrene.

Replace the C=C with two C-C single bonds, one going to the previous unit and one to the next. The 2H on the CH2 end stay; the substituents on the CHR end stay.

Identify a single repeat unit (everything between consecutive identical patterns), insert a double bond where the chain crosses out of the unit, balance hydrogens. A single repeat unit contains exactly two carbons for the listed Unit 4 polymers.

The double bond is consumed in polymerisation; the repeat unit has only single bonds.

A repeat unit without brackets is just a fragment. QCAA marks for both.

LDPE is the branched, low-crystallinity one; HDPE is the linear, high-crystallinity one. The "L" and "H" refer to density, which tracks crystallinity.