In referring to the molecular mass of a polymer, we can...

Name: Problem 45, Chapter 27: General Chemistry: Principles and Modern Applications
Uploaded: 2020-06-24T21:52:34-08:00
Duration: 23 s
Channel: Catherine Lemar
Description: In referring to the molecular mass of a polymer, we can speak only of the average molecular mass. Explain why the molecular mass of a polymer is not a unique quantity, as it is for a substance like benzene.

In referring to the molecular mass of a polymer, we can speak only of the average molecular mass. Explain why the molecular mass of a polymer is not a unique quantity, as it is for a substance like benzene.

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Key Concepts

Average Molecular Mass and Molecular Weight Averages

Given the range of molecular masses in a polymer sample, a single unique molecular mass cannot be assigned to the polymer. Instead, average measures such as the number-average molecular weight (Mn) or the weight-average molecular weight (Mw) are used to describe the overall molecular mass characteristics of the sample, providing meaningful averages despite the inherent variability.

Polymer Structure and Chain Length Distribution

Polymers are composed of many repeating units that can combine in various ways during synthesis, resulting in chains of different lengths. Unlike small molecules, which have a discrete and well-defined molecular mass, polymer molecules exhibit a range of chain lengths and thus a distribution of molecular masses due to the statistical nature of polymerization reactions.

Polydispersity

Polydispersity quantifies the extent of the molecular weight distribution in a polymer sample. A higher polydispersity index indicates a broader distribution of chain lengths, emphasizing the fact that the polymer’s molecular mass is not uniform, and reinforcing the need to refer to average values rather than a single, unique molecular mass.

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