Tacticity of Polymers: Definition, Types & Examples

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Instructor: Patricia Jankowski

Patricia has a BSChE. She's an experienced registered nurse who has worked in various acute care areas as well as in legal nurse consulting.

Polymers are molecules composed of carbon and hydrogen made up of repeating smaller units. They can have a variety of properties, depending upon their composition and structure. Here, we will explore the tacticity of polymers. Updated: 08/11/2022

What Is a Polymer?

You may have heard of polymers and thought that they sounded really boring, like some old plastic pipes inside of some old building. But they are truly a lot more than that! Naturally occurring polymers are found inside your body and can provide you with energy. They are also in products you use every day, from fabrics and paints to fiberglass and artificial heart valves. Some polymers are stronger than metal and take less energy to produce. So whether you think they're boring or not, they're still a huge part of your world!

A polymer is a large type of molecule that's composed of repeating chains of smaller molecules. The smaller molecules are called monomers. Polymer molecules can become so large that they connect thousands of monomers, and can have huge molecular weight. Polymers occur frequently in nature, but there are also many synthetic polymers. Rubber and silk are both natural polymers, while polyethylene and polystyrene are synthetic polymers.

Polymers can be classified according to the kinds of chains they form or according to what kind of polymerization reactions create them. Nearly all polymers have a backbone of hydrocarbon, which is a long chain of carbon and hydrogen atoms. They also have pendant groups, which are attached to the hydrocarbon backbone, hanging off of it like a pendant. These are usually a part of the repeating monomer group.

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  • 0:04 What Is a Polymer?
  • 1:30 Tacticity of a Polymer
  • 4:11 Polymerization Processes
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Tacticity of a Polymer

Polymers have a property that is called tacticity. Tacticity refers to the manner in which the pendant groups are arranged along the hydrocarbon chain. In reality, hydrocarbon chains are not long, flat chains lying across a surface, but are instead three-dimensional chains with some atoms projecting out of the plane.

However, they do follow a few different kinds of set patterns of chain and pendant; it is these patterns that define tacticity. Tacticity patterns are three-dimensional, which means that the pendant groups may not only be on the opposite side of a hydrocarbon chain, but they may also be in a different spatial orientation. Even so, flat images are sometimes used to depict tacticity to simplify the ideas.

The tacticity of a polymer is an important consideration because it affects its physical and chemical properties. There are three different types of tacticity in polymers, which are isotactic, syndiotactic, and atactic. However, one polymer may have more than one tacticity at a different location along the polymer chain. It all depends upon the local conditions to which the polymer is exposed, as well as the proximity to other reactive pendant chains.

  • An isotactic polymer is one in which all of the pendant groups are located on the same side of the hydrocarbon backbone chain. A popular form of polypropylene that is commonly marketed is an example of an isotactic polymer.
  • A syndiotactic polymer is one in which the pendant groups have a regular, alternating pattern along the hydrocarbon backbone chain. Gutta-percha, which is a type of permanent dental filling that is used in root canals because of its biological inertness, is an example of a syndiotactic polymer.
  • An atactic polymer is one in which the pendant groups are randomly arranged along the hydrocarbon backbone. Polystyrene is an example of an atactic polymer.

The mechanical, physical, and chemical properties of polymers depend upon their tacticity. In general, syndiotactic polymers form the most rigid, crystalline structures of the three tacticities. Isotactic polymers are semi-crystalline, and atactic polymers are amorphous in form, with no crystalline structure. If you picture in your mind a hydrocarbon backbone with pendant groups jutting out in any and every direction, you can imagine why it would be difficult to pack such molecules together into an organized crystal. That's why polymers with a more regular structure, like the isotactic and syndiotactic types, can have a crystalline structure.

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