Free-radical addition polymerisation is typically initiated by substances that decompose to form radicals. Which reagent class commonly serves as the initiator?

Introduction / Context:
Initiation of free-radical polymerisation requires a clean, controllable source of radicals. Selecting the correct initiator system is central to controlling molecular weight, rate, and polymer microstructure in industrial processes like PE (high-pressure), PS, PMMA, and many acrylics.

Given Data / Assumptions:

• We focus on addition polymerisation via radical mechanism.
• Common initiator families include peroxides and azo compounds.
• Strong mineral acids are not radical sources.

Concept / Approach:
Organic peroxides (e.g., benzoyl peroxide, dicumyl peroxide) thermally decompose to generate radicals capable of adding to vinyl monomers and propagating chains. Acids like sulphuric or hydrofluoric serve as catalysts for ionic mechanisms (e.g., esterification, alkylation) but are not radical initiators for vinyl polymerisation.

Step-by-Step Solution:
Identify the mechanism: free-radical → need radical donor.Peroxides homolyse the O–O bond to form radicals.Exclude mineral acids; they promote ionic rather than radical pathways.

Verification / Alternative check:
Polymer reaction engineering references list peroxides and azo initiators (e.g., AIBN) as standard radical sources.

Why Other Options Are Wrong:
Sulphuric acid and hydrofluoric acid are not radical initiators for vinyl addition; they catalyse other reactions.“None” is invalid since a correct class exists.

Common Pitfalls:
Confusing initiators (radical) with catalysts for ionic or condensation polymerisations.

Final Answer:
Organic peroxides