Polyester formation in polymer chemistry:\nPolyesters are commonly manufactured by the polycondensation of which types of monomers?

Introduction / Context:
Polyesters are a cornerstone family of engineering plastics and fibres, produced industrially in massive volumes (e.g., PET for bottles and fibres). Knowing the typical monomer pair used in their synthesis helps students distinguish condensation (step-growth) systems from addition (chain-growth) systems and connects directly to properties such as melting point, crystallinity, and hydrolysis resistance.

Given Data / Assumptions:

• Commercial polyesters (e.g., PET) are formed by step-growth reactions.
• Polycondensation eliminates a small molecule such as water or methanol.
• We focus on the typical functional group pairing found in polyesters.

Concept / Approach:
A polyester repeats the ester linkage along the chain. To build that linkage via condensation, one needs a diacid (or diester) and a diol (dihydric alcohol). During polymerisation, ester bonds form repeatedly while releasing small molecules. This contrasts with vinyl addition polymers that arise from unsaturated monomers without elimination of small molecules. Therefore, the classic answer is the combination of a dibasic acid and a dihydric alcohol.

Step-by-Step Solution:

Verification / Alternative check:
Common production routes confirm this: PET is made from dimethyl terephthalate (or terephthalic acid) and ethylene glycol, producing methanol or water as by-product during polymer growth.

Why Other Options Are Wrong:

• A single monomer: possible only for special self-condensing systems, not the standard route taught for polyesters.
• Carboxylic acid with ethylene: ethylene is not a diol; no ester network forms.
• Inorganic acid with ethylene glycol: yields inorganic esters, not typical polyester chains.
• Dicarbamate with diamine: leads to polyurethanes or polyureas, not polyesters.

Common Pitfalls:
Confusing PET's “terephthalate” name with vinyl addition; it is a condensation product of diacid (or diester) and diol.

Final Answer:
Dibasic acid with dihydric alcohol