Chemistry

What is a Polymer



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What is a Polymer?

You could start by breaking down the word: it's made of two pieces of course, "poly" and "mer". Most everyone should know that the prefix "poly" means "many", as in polygon (many angles), polymorph (an organism with many adult forms), polyunsaturated fats (fats that lack an attached hydrogen at many sites), and polywannacracker (many hungry parrots). "Mer", on the other hand, is likely unfamiliar to anyone who hasn't studied polymers. A "mer" is simply a single unit in a polymeric chain. If you want, you can think about a regular metal chain. A chain is made of many links, or rings. A polymer is a chemical chain, but instead of links, it is made from a series of many chemical units, each of which is called a "mer". More often, you can hear about a monomer - one single mer. The monomer is the mer before it is attached to other mers, making the chain that is a polymer.

How Are Polymers Made?

There are multiple ways of making polymers, but there are two primary pathways that we can discuss. The type of polymer you get depends on which method you use to make it. The names of the two major types reflect how they are made. There are "Step-Growth" or "Addition" polymers, and there are "Condensation" polymers.

Step-Growth

Step-growth, or addition polymers are seen very commonly in the plastics you use daily. You probably even know the names of many, or at least their abbreviations. High density polyethylene (HDPE) is your everyday milk jug. Polyethylene terephthalate (PET) is your standard two liter bottle of soda or pop. PVC (polyvinylchloride) is used to make cheap, chemical resistant pipes, among other things. Each of these is made in roughly the same way, just starting with a different monomer, which is the chemical name that follows the "poly" in each chemical.

The reason that these polymers are called step-growth, or addition, is because they are made by adding each new monomer onto the growing chain, one at a time, in sequence. The chemical reactions involved vary between different polymers, but the general steps are the same. First, the reaction is started in the Initiation Phase. This is often achieved by adding an acid, a base, or a radical (that's a chemical with an unpaired electron - not a surfer's cry or a revolutionary) to a mixture of the monomers. The initiator reacts with some of the monomers, converting them into a form that will react with another monomer. This starts a chain reaction, which continues one by one, adding monomer after monomer to the ever-growing chain in what is called the Propagation Phase. The propagation phase will continue as long as there are monomers to be had, if you allow it. Since a certain length of polymer chain is usually needed, the reaction is finally brought to an end in the Termination Phase, when another chemical is added to stop (or quench) the reaction. You can actually liken this process to a fire. The fire starts when little Billy lights a match and touches it to a stack of paper. The paper continues to burn as long as there is paper (and oxygen) to be had. Mom comes along and sees the fire, so she throws a bucket of water on it to quench it. Step-growth polymerization is usually a more controlled process, but the pattern is the same.

Condensation

When you step out of a steamy shower and look at the mirror, you can see that water has condensed on its cool surface. In that case, condensation refers to the removal of water from the air. In polymerization, condensation means that water is formed during the reaction, so we can think of it as being removed as well. Hopefully many of you will recall that when a strong acid and a strong base react, they make water and a salt. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) for example, make water (HOH) and table salt (NaCl). The same sort of reaction can be made to make a polymer. Each monomer is a molecule which has an acid or a base group at each end. There are different possible arrangements, of course - each could have an acid at one end and a base at the other, or there can be two chemicals, one with acids at both ends and the other with bases at both ends. Either way, making a polymer out of them is fairly simple. You mix them together, and the acid ends react with the base ends, giving off water, and linking the two chemicals together. The process continues, lengthening the chain until the reaction is brought to a halt by reacting with a chemical that only has an acid or base on one end. The only catch is that the reaction can sometimes be reversed, so the water has to be removed along the way. Common condensation polymers can be found in your everyday wardrobe. Nylon and polyester are good examples of condensation polymers.

Natural Polymers

Not all polymers are manmade. Really, we got the idea from nature in the first place. Humans have been using plant fibers to make threads far longer than we've had polyester, after all. The plants make a polymer called cellulose, a condensation polymer made up of sugar molecules. Those long chains are responsible for everyday materials like cotton or hemp. Rubber is also a natural polymer, from the sap of the rubber plant. You don't see the natural product often now, however, synthetic rubber is cheaper to come by, and can be made with the exact properties a consumer wants.

Even within your own body are millions of polymer chains. There are proteins, which are chains of amino acids. An amino acid has an amine (a base) on one end and a carboxylic acid on the other end. They join together as a condensation polymer to form the proteins in your body. If you've ever heard of "essential amino acids", they're essential because your body can't make them, and needs them to make certain proteins. Fat is another condensation polymer, made up of monomers called fatty acids. You shouldn't forget your DNA and RNA either. Those letters stand for Deoxyribonucleic acid and Ribonucleic acid, respectively. Each is a complex condensation polymer, named for the repeating sugar unit (deoxyribose and ribose, respectively) that makes up the framework of the chain.

More About Polymers

This has only been the barest introduction to polymers. If you want a more in depth set of lessons, you might choose to check out the text I learned from back in college - over a decade ago I fear. That book was Seymour and Carraher's "Polymer Chemistry - an Introduction", the fourth edition. It had an especially good focus on the industrial aspects of polymer chemistry. If you are more interested in natural polymers, especially those in your body, a biochemistry text would likely be best. As for me, I intend to write a few related articles in the near future, including the topics of dendrimers (polymeric webs) and silicones (think implants). I hope you'll share those with me as well.

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