Q. How is plastic made?

Northbrook, Ill.

A. There are all kinds of plastics. But they have one thing in common. They're all made of very, very long molecules, called polymers. Polymers are nothing more than long chains of identical repeating units, called monomers, which are just small molecules.

``Think of polymers as a string of pearls,'' says Michael Rubner, professor of polymer materials science and engineering at MIT. ``They're incredibly simple, but beautiful.'' Rubner says it's their amazing length, hundreds of thousands of units long, that gives plastics their unique properties of flexibility and strength.

The trick to making polymers is getting the individual monomers to link up. It's kind of like performing magic on a big box full of paper clips so they end up in one giant paper clip chain, all connected.

The monomers in plastic are molecules made up mostly of atoms of carbon, oxygen, hydrogen, and nitrogen. Atoms bond together in different ways, depending on how many electrons each has, and what kind of electrical charge it carries. Those connections are called chemical bonds. There are several kinds of them.

Chemists understand the natural laws that dictate which bonds form between which atoms under which conditions. By creating just the right conditions using temperature, or pressure, or other chemicals, they can break apart a molecule's natural bonds and cause new ones to form, making those monomer molecules to link up like our paper clips.

Here's an example of how they do that to make the plastic in garbage bags, polyethylene. Theys start with a monomer called ethylene. Think of a bunch of ethylenes in a vat as a crowd of people standing next to each other. You can try this with a bunch of friends.

The first rule, the most important rule, is that every hand has to be holding another hand. No free hands. The second rule, not quite as important, is that everybody has to pair up with somebody in the crowd, each person holding both hands of the other person. That's how ethylenes behave. They naturally pair up -- with a double covalent bond between their carbon atoms, written as C=C. (A covalent bond is where two atoms share their outmost electron.) Like two people, each holding the other's two hands.

Now along come the chemists, who change the rules for just one of the pairs in the crowd. Under the new rules, instead of standing as a pair, one person has to grab a balloon with one hand and hold just one hand of their partner. (In plastic manufacturing, the ``balloon'' is a chemical called an initiator. That's what starts changing the rules for the kinds of chemical bonds that form.) Now the ethylenes look like this. C-C.

But don't forget that there can be no free hands. The first person is all set. He has a balloon in one hand, and has the second person's hand in his other. But the second person has a free hand. A no-no. So, he pulls one hand away from a nearby pair. Now person two is happy, but one of the people in that second pair has a free hand. She grabs away a hand from a nearby pair, and just like that, you've got a chain reaction going, all started by that initiator. Soon, instead of pairs of people in your crowd, you've formed yourselves into a chain.

Sometimes the chemical reactions vary. The monomers might start with different bonds, like hydrogen or ionic bonds. The chemists might create new ones like single or triple covalent bonds. Sometimes atoms are freed up that don't connect into the chain. They produce a byproduct. Nylon production creates polymers and water. And sometimes, to give the polymer different properties, chemists combine several different monomers into the chain, like a string of pearls where one is white, the next one is black and the next one is blue. But the process is always basically the same: Start a chain reaction among individual units that end up bonding together into a long chain.

Those polymer chains are the raw material that, when rolled or spun into fibers or molded together into sheets or balls or other shapes, become the plastic products we're so familiar with.

Rubner says polymers grow at lightning speed. Millions of ethylenes floating around in pairs can become one long connected chain, C - C-C-C-C, in just thousandths of a second. The chain stops growing only when some chemical imperfection -- think of it as a molecule with no hands, or three hands -- fouls things up.

Crude oil is the most common raw material for plastic monomers, Rubner says, because ``it's the most abundant, cheapest, easiest-to-use supply of the organic chemicals you need to make monomers.'' About 4 percent of crude oil turns into plastics.

Rubner forecasts a revolution in plastics: ``We can now make polymer chains with bonds that conduct electricity and light. It may not be too far in the future that electronic components, even your entire computer screen or TV set, are exclusively plastic.''