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.
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.
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.
In today's world, polymers are commonly synthesized to meet all sorts of needs, and natural polymers are also sometimes altered to change their properties. The type of polymer you get depends upon what goes into it and how it's processed, so there are a variety of different processes by which polymers are either made or are somehow manipulated. Some common polymerization processes include free radical vinyl polymerization, metallocene catalysis vinyl polymerization, and Ziegler-Natta polymerization.
Free radical vinyl polymerization is used to make mostly atactic polymers from monomers that contain carbon-carbon double bonds. This process begins with one double-bonded molecule that falls apart, forming two free radicals, or molecules with unpaired electrons. The free radical vinyl process yields strong plastics like polystyrene.
Ziegler-Natta polymerization is a method of polymerization developed by two scientists named (you guessed it!) Ziegler and Natta. It is used to make isotactic polypropylene. The rather complex process involves a transition metal catalyst that forms a system to begin the polymerization. Isotactic polypropylene is an extremely useful, highly crystallized substance that has many important applications. From dishwasher-safe containers to gas pipelines, this polymer is seen almost everywhere.
The metallocene catalysis vinyl polymerization process can be adjusted to make either isotactic or syndiotactic polymers. This process is catalyzed by metallocenes, which are positively charged metal ions located between two negatively charged cyclopentadienyl anions. The final polymer product has some metal atoms in the backbone chain. Metallocene catalysis vinyl polymerization produces Kevlar, which is used to make bullet-proof vests. These huge, strong polymer molecules have molecular weights in the millions of grams per mole.
A polymer is a large molecule made up of repeating chains of smaller molecules, which are called monomers. Polymers occur naturally and may also be synthesized. Polymers have a backbone of hydrocarbon, which is a long chain of hydrogen and carbon atoms. They also have pendant groups, which are attached to the backbone chain.
Tacticity is a property of polymers that refers to the manner in which pendant groups are arranged along the hydrocarbon chain. Polymers are three-dimensional but are often depicted as flat on a page for simplicity.
The three different types of tacticity in polymers are:
- Isotactic: All pendant groups are located on one side of the hydrocarbon chain.
- Syndiotactic: Pendant groups have a regular, alternating pattern along the hydrocarbon chain.
- Atactic: Pendant groups are arranged at random along the hydrocarbon chain.
The type of tacticity of a polymer influences its physical, mechanical, and chemical properties. Atactic polymers are generally amorphous, while isotactic polymers are generally semi-crystalline and syndiotactic polymers are crystalline.
Polymers are produced by several different processes. Three common ones are:
- Free radical vinyl polymerization: Produces atactic polymers.
- Ziegler-Natta polymerization: Produces isotactic polymers like isotactic polypropylene.
- Metallocene catalysis vinyl polymerization: Produces isotactic or syndiotactic polymers.
Polymers are often huge molecules and can have molecular weights of millions of grams per mole.
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