28. Polymers: Structure & Composition

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Session Overview

Modules Organic Materials
Concepts polymer architecture, polymer composition, backbone structure, polymer material properties, natural and synthetic polymers
Keywords macromolecule, -mer, monomer, polymerization index, ordered solid, disordered solid, Dalton (atomic mass unit), cooling curve, partial crystallization, homopolymer, copolymer, tacticity, stereoisomerism, isotactic, syndiotactic, atactic, conformality, covalent bridge, crosslinking, elastomer, disulfide bond
Chemical Substances polyethylene (PE), low density polyethelene (LDPE), high density polyethelene (LDPE), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), natural rubber
Applications Manufactured products of the past 80-100 years, vulcanized rubber


Before starting this session, you should be familiar with:

Looking Ahead

This session is the first of two devoted to polymers. The polymers sessions are prerequisites for Sessions 30-32 on biochemistry.

Learning Objectives

After completing this session, you should be able to:

  • Define key terms used in polymer chemistry.
  • Describe a polymer's structure in terms of –mer type ordering, tacticity, and backbone chain configuration.
  • Explain how polymer structure determines material properties, such as flexibility or transparency.
  • Explain how crosslinking produces the characteristic properties of an elastomer.
  • Appreciate the role of polymers in both natural and synthetic materials.


Book Chapters Topics
[Saylor] 12.8, "Polymeric Solids." Overview of biological and synthetic polymers

Lecture Video

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This resource may not render correctly in a screen reader.Lecture Slides (PDF)

Lecture Summary

Polymer chemistry is an example of applied organic chemistry. Polymers are long chain macromolecules built from repeated smaller chemical structures or '-mers.' They are found widely in the natural world (e.g. proteins, rubber, cellulose) and, increasingly the past century, in synthetic form (e.g. "plastics"). Polymers have been central to many technological innovations that shape the modern world, such as film that enabled the cinema, or the progression of vinyl LPs to polycarbonate CDs and DVDs that enabled recorded music and video distribution.

This lecture begins by describing some basic characteristics of polymers. A polymer's physical properties are influenced by the degree of ordering in the molecular chains. A comparison of low-density and high-density polyethylene (LDPE and HDPE) reveals that HDPE is molecularly more ordered, with more zones of partial crystallization, and thus more dense and rigid than LDPE. As a result, LDPE is quite flexible and transparent, suited for food wrap; whereas HDPE is optically translucent and more rigid, suited for a milk jug.

The lecture describes polymer molecular architecture in terms of:

  • Comparing pure (homopolymer) vs. mixed or "alloy" compositions (copolymer)
  • Ordering of -mer types (random, alternating, block, graft)
  • Forms of tacticity (isotactic, syndiotactic, atactic, eutactic)
  • Backbone chain configurations (conformality, linear vs. branched chains)
  • Crosslinking with disulfide covalent bridges, to create an elastomer with "rubbery" qualities

The class ends with a quick recounting of the discovery and impact of rubber vulcanization, in which difficult-to-manage raw rubber gum becomes a more useful material by heating and adding sulfide to create crosslinking.


Problems (PDF)

Solutions (PDF)

Textbook Problems

[saylor] Sections Conceptual Numerical
[Saylor] 24.2, "Isomers of Organic Compounds." 1, 3, 4, 5 none
[Saylor] 24.5, "Common Classes of Organic Compounds." 1, 4 none

For Further Study


Buy at Amazon The Beatles. "Polythene Pam." Abbey Road. Apple Records, 1969.

Other OCW and OER Content

Content Provider Level
Polymer Basics DoITPoMS Undergraduate


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