Natural vs. synthetic: which of the following is NOT a natural polymer?

Introduction / Context:
Polymers are broadly categorized as natural (biosynthesized) or synthetic (man-made). Correctly classifying examples is basic to polymer science and helps in predicting degradation, processing, and applications. The task is to pick the synthetic outlier from a list dominated by biopolymers.

Given Data / Assumptions:

• Nucleic acids, polysaccharides, and proteins are macromolecules produced by living organisms.
• Polyisoprene is the main constituent of natural rubber latex.
• Polyurethane is produced by reaction of diisocyanates with polyols.

Concept / Approach:
Natural polymers include DNA/RNA (nucleic acids), cellulose/starch (polysaccharides), proteins (polypeptides), and natural rubber (cis-1,4-polyisoprene). Polyurethanes are synthetic, engineered in countless formulations (foams, elastomers, coatings) by step-growth reactions between isocyanates and polyols; they do not occur as high-molecular-weight materials in nature.

Step-by-Step Solution:

Verification / Alternative check:
Standard textbooks list DNA/RNA, polysaccharides, proteins, and natural rubber among natural polymers; polyurethanes are unequivocally synthetic.

Why Other Options Are Wrong:

• Nucleic acids/polysaccharides/proteins: classic natural polymers.
• Polyisoprene: the principal chain in natural rubber latex.

Common Pitfalls:
Confusing “found in nature” derivatives (e.g., cellulose acetate) with truly natural polymers; derivatives are modified and count as semi-synthetic.

Final Answer:
Polyurethane