Question

Which of the following additive is added to control molecular weight?

• A. Antioxidant
• B. Antiozonant
• C. Chain transfer agent
• D. Plasticizers

Hint:

Common chain transfer agents include thiols and halogenated compounds.

Answer 1

The Correct Option is C

Step 1: Understanding molecular weight control.
Controlling molecular weight during polymerization is important to obtain polymers with desired mechanical and processing properties. This is achieved by regulating chain length.
Step 2: Role of chain transfer agents.
Chain transfer agents terminate growing polymer chains and initiate new ones, thereby controlling the average molecular weight of the polymer formed.
Step 3: Analysis of options.
(A) Antioxidant: Prevents oxidation, not molecular weight control.
(B) Antiozonant: Protects rubber from ozone degradation.
(C) Chain transfer agent: Correct — Directly controls polymer chain length.
(D) Plasticizers: Improve flexibility but do not affect molecular weight.
Step 4: Conclusion.
Chain transfer agents are added to control molecular weight during polymerization, making option (C) correct.

Answer 2

The question is about which additive is used specifically to control the molecular weight of a polymer during polymerization. The key is to separate additives that act during polymerization (affecting chain length) from those that act after polymerization (affecting the finished material's properties).

1. Antioxidant: This is a post-polymerization stabilizer added to finished polymer to scavenge free radicals formed during service life (heat, light, oxygen exposure), preventing degradation. It plays no role in setting chain length during synthesis.
2. Antiozonant: Similar to an antioxidant but specifically protects rubber products from surface cracking caused by atmospheric ozone attack on double bonds. Again, this acts on the finished product, not during chain growth.
3. Chain transfer agent: This is added directly into the polymerization reaction mixture. It works by donating a hydrogen (or other) atom to a growing radical, terminating that chain while simultaneously generating a new radical on the transfer agent itself, which starts a fresh chain. By tuning the concentration of the transfer agent, the average number of monomer units added before each termination event - and hence the average molecular weight - can be precisely dialed in.
4. Plasticizers: These are small molecules blended into the finished polymer to increase chain mobility and flexibility (lowering T_g); they are a post-synthesis formulation additive, not a reaction-controlling one.

Only the chain transfer agent operates during the polymerization step itself to directly set chain length.

Therefore, the correct answer is Chain transfer agent.