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

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

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.

Draw the repeating unit of polypropene from propene ($\text{CH}_3 \text{-CH} = \text{CH}_2$). State whether the polymer is saturated. [3 marks]

Polystyrene and polyethene have similar molar masses, yet polystyrene has a higher glass-transition temperature ($100^{\circ}\text{C}$ vs $-110^{\circ}\text{C}$). Explain. [3 marks]

A petrochemical engineer compares HDPE and LDPE. (a) Describe the structural difference. (b) Predict which has higher density and tensile strength.