Step-growth (condensation) polymerisation generally involves one reaction type and mechanism; which of the following materials is NOT produced via step-growth routes?

Introduction / Context:
Industrial polymers are often classified by their synthesis mechanisms. Step-growth systems build molecular weight gradually by reactions of functional groups; chain-growth systems propagate through active centers adding unsaturated monomers. Recognising which materials belong to which category underpins processing and property expectations.

Given Data / Assumptions:

• Phenolic resins, polyesters, polyamides, and epoxies are classic step-growth polymers (often with small-molecule byproducts or stepwise network formation).
• Polystyrene (including crosslinked grades) is formed by chain-growth free-radical polymerisation of styrene; crosslinking is achieved by comonomers like divinylbenzene.

Concept / Approach:
Even when polystyrene is crosslinked, the base polymerisation remains a chain-growth addition process initiated by radicals (or anionic routes). By contrast, phenolics, polyesters, and polyamides form by condensations between functional groups (e.g., hydroxyl and carboxyl, amine and acid, phenol and aldehyde). Epoxy networks also form via step-growth reactions between epoxide groups and hardeners.

Step-by-Step Solution:
List typical step-growth polymers.Identify the outlier: polystyrene (addition polymer).Select crosslinked polystyrene as “not produced by step-growth.”

Verification / Alternative check:
Mechanistic schemes in polymer texts place vinyl/olefinic systems like styrene firmly in chain-growth categories.

Why Other Options Are Wrong:
Phenol–formaldehyde, polyesters, polyamides, epoxies: all step-growth systems.

Common Pitfalls:
Assuming crosslinking implies step-growth; crosslinking can occur in both mechanism families.

Final Answer:
Crosslinked polystyrene (e.g., with divinylbenzene)