Lecture 28 - Polymerization and Plastics

For this lecture...

1) What is a "Polymer"?

2) How are polymers produced?

3) How does this processing affect the properties of this material?

Ok, we know that petroleum is used as a source of energy. How else does it impact our lives? While some is used for lubrication purposes, most of the petroleum and hydrocarbons not used as for energy are used to produce the materials that we commonly call "plastics".

A Plastic is a synthetic (artificial, "man-made") material consisting of long, chain-like molecules.

Polymerization is the process by which small molecules are joined together to form very long chains. Thus, how would you define the term "polymer"?

A Polymer is simply a very long molecule made of identical components joined together. How would you describe the properties of polymers? Different varieties of polymer can be light-weight, corrosion-resistant, stiff or flexible, strong or weak, clear or opaque, insulators or conductors, low friction, heat-resistant or sensitive, convert light or motion to electricity. Polymer simply means "many mers", where a mer is the term for the small molecules that are joined together. Are all polymers plastics? Are all plastics polymers? Can you think of any long, chain-like molecules that occur in nature?

There are three major polymerization mechanisms:

Addition: process by which individual mers are covalently bonded together to form very long chains of several thousand mers; addition of mers has a minimal effect on the properties of either molecule.

Condensation: process by which a covalent bond forms between two or more different types of molecules and a by-product, such as water, is released; significantly alters the properties of the components.

Cross-linking: process by which several chain-like molecules are joined together to form a three-dimensional network structure. Vulcanization links strands of rubber together.

There are three principal kinds of polymers:

Thermoplastic: Generally soft and ductile (plastic), especially when heated; consist of many chain-like molecules (formed by addition) loosely associated with each other. They can be heated, formed, cooled, re-heated, re-formed, etc. without permanently altering the physical structure of the material. Thus, it is easily recycled. What objects around you are made of thermoplastic polymers? Ex. polyethylene, polyvinylchloride, polyester, polystyrene.

Thermosetting: Usually hard and rigid network of cross-linked networks formed by condensation. While stronger than thermoplastic polymers, thermosetting polymers cannot be easily recycled; when heat is applied to the components, they condense or "set" in a one-way reaction. What objects around you are made of thermosetting polymers? Ex. Epoxy, Bakelite, Polycarbonate, Formica, etc.

Elastomer: Loosely cross-linked network of chain-like molecules that can deform enormous amounts without permanently changing shape; may or may not become plastic when heated. What object around you are elastomers? Do you think they become softer when heated? Ex. natural rubber (latex), tires (vulcanized rubber), Nylon.

Although polymers were first invented in the 1860’s, it was not mass-produced until the early 1900’s when Bakelite was used as a replacement for wood, ivory, tortoise-shell, etc. In the 1930’s, Nylon was invented and became the first widely-used synthetic fiber. The 1939 World’s Fair in New York City was heralded as the beginning of the "Age of Plastics". The industrial development of the Second World War led to the wide-scale use of polymers for many tasks where only natural materials had been used previously. This formed the foundation for our reliance on polymers that continues to this day.

Example of Polymerization: Polyethylene


Key terms…