Probably the first thing you're wondering is "what's the difference between a plastic and a polymer, or are they the same thing?" A plastic is a type of material, whereas a polymer is a type of molecule (a very, very large molecule!). The word plastic comes from the Greek, plastikos, meaning shapeable. The stuff we call plastics are (usually) easily shaped or formed by various processing methods, which helps make them cheap. They are found all around you in the things of everyday life. Some of these you may regard as useful or at least benign (from things that improve hygiene, like garbage bags, sewage and water pipes, to all sorts of medical equipment). Others you may regard as cheap, nasty and unnecessary (such as the diposable cups and plates that many abhor, but use, because they also make life easier!). Plastics are polymers, but so are elastomers (e.g., natural rubber), fibers, paints, adhesives, and so on.

In the video above we show you some examples, a montage of what some might call the good, the bad and the ugly. Obviously, most people would prefer real walnut dashboards and Corinthian leather in their cars to some sort of molded plastic part and vinyl seats. But how many people can afford these things and what would it mean for walnut trees if they could? Leaving apart economics, would we have society as we know it without elastomers? (Would you want to be sitting in a 747 that was going to land on wooden wheels?) How about printed circuit boards, or the seemingly mundane but essential bags, tubes, etc., that are a crucial part of medical devices? What about safety glass, light-weight, tough composites, and so on? Those who would like to make us into some version of a simple agrarian society might want to sweep these things away, but for most people the genie is out of the bottle and they like the things that make their lives easier.

So, one question we would like you to think about before we get into the heart of the subject is “have these materials, on balance, made life better”? We would like you to explore some of these issues on your own in order to get a feel for why these materials are so ubiquitous in modern life, but at the same time we want you to learn some basic materials science – aspects of the synthesis, structure, properties and processing of polymers. A lot of this we will put in the context of the historical development of both these materials and the way we think the world works. First, because we think it is easier to learn something when you have a feel for how ideas developed. Second, because this allows us to tell some great stories and you will hopefully see that like other human enterprises, scientific discoveries involve not only genius, but hard work, luck, perseverance against the odds, human frailties, tragedies and triumphs. Accordingly, in the rest of this introductory chapter we are first going to say a little more about the nature of polymer materials, then look at the early history of the subject and finish up with some of the controversies concerning the use of these materials. More of this latter stuff can be found in our "Fascinating Polymer" stuff, which you will find links to on the left. But, first things first, when we say polymers are big molecules, just how big do we mean?

How Big are Polymers?

It would be nice if we could assume that anyone with a high school diploma would be familiar with the molecular structure of say, water or benzene. But, to our jaundiced eye, the current staple of many high school science curriculums seems to be “Saving the Rainforest” and molecular science appears to be largely an afterthought. But that’s a different rant. The point we wish to make is that molecules like water, benzene and the like are generally called “low molecular weight” or “low molar mass” materials by polymer scientists. As a rule of thumb, molecules having molecular weights of, say, less than 500 g/mole are considered low molecular weight materials. High molecular weight polymers, on the other hand, are covalently bound, chain-like molecules that generally have molecular weights that exceed 10,000 g/mole and can be as high as 107 g/mole. Between these extremes of low and high molecular weights, there is a poorly defined region of moderately high molecular weight materials and such molecules are often referred to as oligomers.

To give you a feel for the difference between a low molecular weight material and a high molecular weight polymer, let’s next put them on a scale that we can all relate to. If we assume that a single methylene group, CH₂, may be represented by a single bead in the figure above, then the gas, ethylene, is simply two beads joined together. The chain of beads shown in the figure would represent not a polymer (it’s too short), but an oligomer made up of about 100 CH₂ units. Let’s further assume that the length of the ethylene molecule is 1 cm. Now, if we consider a polyethylene molecule that has a molecular weight of 700,000 g/mole, it would be made up of 700,000/14 or roughly 50,000 methylene groups. On our scale this would be equivalent to a chain roughly a quarter of a kilometer long. These are very big molecules indeed!
 

Many of the physical properties of polymers are simply a consequence of their large size. To also get a feel for this, let’s consider building a simple hydrocarbon chain one carbon atom at a time, filling up all the unsatisfied valences with hydrogen atoms, as shown in the figure below. If we have just one carbon atom (and hence four hydrogens), we have the gas methane, often referred to as “natural gas”, perhaps because it not only emanates from the ground, but also the rear end of cows. The next three in the series are ethane, propane and butane, which have, respectively, two, three and four carbons in their chains. These are also gases at ambient temperatures and pressures, the latter two being commonly used for heating and cooking. Liquids, commonly used as auto and jet fuels typically have carbon chain lengths of 6–12. As we increase the carbon chain lengths further, the viscosity increases and we go from liquid materials used for baby oils, to “semi-solid” materials used as soft and hard candle waxes. At even higher carbon chain lengths, typically exceeding 30,000, we encounter hard, solid polyethylenes.

Classification

Finally on this page, it is important to recognize that all polymers, be they natural or synthetic, are simply giant long chain molecules or macromolecules. It’s all chemistry, and there really is no basic difference between a natural or synthetic polymer, in that both obey the same physical laws. Cellulose, for example, the natural polymer that is found in cotton and numerous other plants (and the most abundant polymer on the face of the planet), is a long chain macromolecule composed of carbon, hydrogen and oxygen. So is the synthetic polyester fiber, poly(ethylene terephthalate) (PET). It’s just that the arrangement (architecture) of the atoms in the two polymer chains is different. However, it is common practice to distinguish between natural and synthetic polymers and also classify them in terms of plastics, fibers, elastomers, composites, paints, adhesives, etc., as shown in the table below. Not only are plastics made out of polymers, so are all the other things listed in this table.