Question

Which of the following will have highest value of T_g?

• A. Polyethylene
• B. Polypropylene
• C. Cis polyisoprene
• D. Polystyrene

Hint:

Bulky side groups increase rigidity and raise the glass transition temperature.

Answer 1

The Correct Option is D

Step 1: Understanding glass transition temperature.
Glass transition temperature (T_g) depends on chain rigidity, bulky side groups, and intermolecular interactions. More rigid chains show higher T_g.
Step 2: Structural comparison.
Polystyrene contains bulky phenyl groups attached to the polymer backbone, which restrict molecular motion and increase chain stiffness compared to other polymers listed.
Step 3: Analysis of options.
(A) Polyethylene: Flexible chains with very low T_g.
(B) Polypropylene: Slightly higher T_g than polyethylene.
(C) Cis polyisoprene: Elastomer with low T_g.
(D) Polystyrene: Correct — Bulky phenyl groups lead to highest T_g.
Step 4: Conclusion.
Polystyrene has the highest glass transition temperature among the given polymers, so option (D) is correct.

Answer 2

Glass transition temperature rises when polymer chains have less room to wiggle past each other, so comparing the size and rigidity of the side groups attached to each backbone tells us which one resists segmental motion the most.

1. Polyethylene: Has the simplest possible backbone with only small hydrogen atoms as side groups, so chain segments can rotate and slide past each other very easily. This gives it an extremely low T_g, well below room temperature.
2. Polypropylene: Has a small methyl (-CH₃) group on alternate backbone carbons. This is bulkier than hydrogen and adds a modest steric hindrance to rotation, raising T_g somewhat above that of polyethylene, but the methyl group is still small enough to allow fairly free motion.
3. Cis polyisoprene: Has a double bond and methyl group along the chain, but the cis configuration keeps the chain kinked and loosely coiled with a lot of free volume between segments, which is exactly why natural rubber is so flexible at room temperature - a low T_g is expected.
4. Polystyrene: Has a large, rigid, flat phenyl ring attached to every other backbone carbon. This bulky aromatic ring severely restricts rotation around the backbone bonds (high steric hindrance) and packs chains less efficiently, both of which strongly suppress segmental mobility and push T_g well above room temperature.

Since the size of the side group scales directly with restricted chain mobility, and polystyrene's phenyl ring is by far the bulkiest among these four, it must have the highest T_g.

Therefore, the correct answer is Polystyrene.