The model on the right above is an image of the pdb model
you can view by clicking here or you can just click on the image itself.
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Poly(vinyl chloride) is the plastic known at the hardware store as PVC. This is the PVC from which pipes are made, and PVC pipe is everywhere. The plumbing in your house is probably PVC pipe, unless it's an older house. PVC pipe is what rural high schools with small budgets use to make goal posts for their football fields. But there's more to PVC than just pipe. The "vinyl" siding used on houses is made of poly(vinyl chloride). Inside the house, PVC is used to make linoleum for the floor. In the seventies, PVC was often used to make vinyl car tops.
PVC is useful because it resists two things that hate each other: fire and water. Because of its water resistance it's used to make raincoats and shower curtains, and of course, water pipes. It has flame resistance, too, because it contains chlorine. When you try to burn PVC, chlorine atoms are released, and chlorine atoms inhibit combustion.
Structurally, PVC is a vinyl polymer. (well, duh!) It's similar to polyethylene, but on every other carbon in the backbone chain, one of the hydrogen atoms is replaced with a chlorine atom. It's produced by the free radical polymerization of vinyl chloride.
And here, my friends, is that monomer, vinyl chloride:
The model above is an image of the pdb model
you can view by clicking here or you can just click on the image itself.
Close the new window that opens up after you finish.
PVC was one of those odd discoveries that actually had to be made twice. It seems around a hundred years ago, a few German entrepreneurs decided they were going to make loads of cash lighting people's homes with lamps fueled by acetylene gas. Wouldn't you know it, right about the time they had produced tons of acetylene
to sell to everyone who was going to buy their lamps, new efficient electric generators were developed which made the price of electric lighting drop so low that the acetylene lamp business was finished. That left a lot of acetylene laying around.
So in 1912 one German chemist, Fritz Klatte decided to try to do something with it, and reacted some acetylene with hydrochloric acid (HCl). Now, this reaction will produce vinyl chloride, but at that time no one knew what to do with it, so he put it on the shelf, where it polymerized over time. Not knowing what to do with the PVC he had just invented, he told his bosses at his company, Greisheim Electron, who had the material patented in Germany. They never figured out a use for PVC, and in 1925 their patent expired.
And the key development occurred in 1926, the very next year. An American chemist, Waldo Semon, was working at B.F. Goodrich when he independently invented PVC. But unlike the earlier chemists, it dawned on him that this new material would make a perfect shower curtain. He and his bosses at B.F. Goodrich patented PVC in the United States (Klatte's bosses apparently never filed for a patent outside Germany). Tons of new uses for this wonderful waterproof material followed, and PVC was a smash hit the second time around.
PVC has unique physical properties in addition to those described above. For one, it has semi-crystalline domains or regions that have a much higher softening point (Tm) than the other amorphous domains (Tg) that make up the solid polymer. The crystalline domains act as physical crosslinks to give the product you make toughness and strength. This means you can process PVC as a thermoplastic to make all those wonderful pipes and clear plastic seals around stuff you buy that you can't tear or cut easily.
So what kind of crystallinity does PVC have? Think about size: chlorine atoms have many more electrons than hydrogen and that makes it bigger. As a vinyl chloride monomer approaches the radical chain end during polymerization, the bigger chlorine wants to be further away from the chlorine already there. That leads to more syndiotactic placement than atactic or even isotactic. Those syndiotactic segments (3, 4, 5 or more repeat units) can get together with similar segments on other polymer chains and form small domains of crystalline material. Bingo! Good properties.
And here's an interesting fact: you can increase the syndiotactic content even more using a couple of tricks. Lower the temperature (say to -65 oC or so) and the steric effects during monomer addition increase. Or you can do the polymerization like normal but add a "complexing agent" that makes the chlorines look even bigger than they are. The simplest such agent is an aldehyde of some kind. Not sure why it works but it's reported to do a great job of making highly crystalline (syndiotactic) PVC. Look below for a procedure to try if you can be very, very careful: vinyl chloride is nasty, causing all kinds of health problems including cancer.
Tested Synthesis of PVC
To make highly crystalline PVC the way it's (not) made industrially, we have a procedure for you to follow. It uses an aldehyde to promote greater syndiotactic segments in the low molecular weight PVC formed: click here to view the method and here to download a copy.AND if you want to make PVC the way it actually IS made industrially (or at least, fairly close to that), click here to view the method and here to download a copy.
PVC NMR Spectra
So let's say you have a sample of what you're pretty sure is PVC. How do you confirm that hypothesis? Easy! Take an NMR spectrum or two. But wait! You need a standard to compare it to, right? Have we got a deal for you: both proton and carbon spectrum of commercial PVC.Go here to view a 1H spectrum and and here to see the 13C spectrum.
Other polymers used as plastics include:
Sources:
Burke, James; Connections, Little, Brown and Co., Boston, 1978.
Fenichell, Stephen; Plastic: The Making of a Synthetic Century,
HarperCollins, New York, 1996.
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