Polymers have a distribution of chain lengths described by average molar mass.
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Synthetic polymer samples are mixtures of chains with different lengths, so there is no single molecular weight. The number-average molar mass, Mn, is the simple mean over all chains, while the weight-average molar mass, Mw, weights each chain by its mass. The polydispersity index, Mw divided by Mn, equals 1 only for perfectly uniform chains and is greater than 1 for real polymers. Mechanical properties such as tensile strength, toughness and melt viscosity depend strongly on molecular weight, because longer chains entangle more. Gel permeation chromatography is commonly used to measure the distribution.
Key ideas: Polymers are mixtures with a distribution of chain lengths. Number average Mn equals total mass over number of chains, weight average Mw weights heavier chains more, so Mw is greater than or equal to Mn. Polydispersity index equals Mw over Mn (1 for uniform, about 2 for typical chain growth). Degree of polymerization n equals M of polymer over M of repeat unit.
Common mistakes and connections: Mistake: quoting a single molar mass as exact or confusing Mn with Mw. Higher molar mass increases strength and melt viscosity, so processing is harder. Methods: osmometry for Mn, light scattering for Mw, gel permeation chromatography for both. Links to colligative properties.
A real polymer sample is a distribution of chain lengths. Mw exceeds Mn, so the polydispersity index is above 1. The interactive lesson shows this as a live animation or chart.
A typical high-density polyethylene sample may have a weight-average molar mass near 100,000 g per mol.
Polymers have a distribution of chain lengths described by average molar mass.
Polymer Molecular Weight is a advanced topic in Biochemistry, Environment & Industry. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Biochemistry, Environment & Industry first: Bakelite, Polystyrene, Copolymers.
The next topics in Biochemistry, Environment & Industry are Plastic Recycling and Microplastics.
Yes. The interactive lesson includes a self-check question on Polymer Molecular Weight, and the Biochemistry, Environment & Industry quiz draws on every topic in this area.
Polymer Molecular Weight is topic 63 of 127 in Biochemistry, Environment & Industry.
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