Introduction to
Inorganic Polymers
On most of these pages we've been talking about polymers whose backbone chains are made mostly of carbon atoms, if not entirely of carbon atoms. These we call organic polymers. But now we're going to leave convention behind and talk about some polymers that don't have any carbon atoms in the backbone chain. These are called, as if you couldn't guess, inorganic polymers. Here's a menu if the inorganic polymers on this page to help you navigate:
Silicones
You've certainly seen inorganic polymers before, especially the ones called silicones. Silicones are the most common of the inorganic polymers. They look like this:
They really should be called polysiloxanes. The bond between silicon and oxygen is very strong, but very flexible. So silicones can stand high temperatures without decomposing, but they have very low glass transition temperatures. You've probably seen rubber or caulking made of silicones somewhere before.
Polysilanes
Let's take a look at the element silicon for a moment. You can see that it's right beneath carbon in the periodic chart. As you may remember, elements in the same column or group on the periodic chart often have very similar properties. So, if carbon can form long polymer chains, then silicon should be able to as well.Right?
Right. It took a long time to make it happen, but silicon atoms have been made into long polymer chains. It was in the 1920's and 30's that chemists began to figure out that organic polymers were made of long carbon chains, but serious investigation of polysilanes wasn't carried out until the late seventies.
Earlier, in 1949, about the same time that novelist Kurt Vonnegut was working for the public relations department at General Electric, C.A. Burkhard was working in G.E.'s research and development department. He invented a polysilane called polydimethylsilane, but it wasn't much good for anything. It looked like this:
It formed crystals that were so strong that nothing could dissolve them. Burkhard tried to heat it, but it wouldn't melt below 250oC, when it decomposed, without melting. That made polydimethylsilane pretty much useless. He made it by reacting sodium metal with dichlorodimethylsilane like this:
This is important, because in the seventies, some scientists got the notion that they were going to make small rings of silicon atoms. So unwittingly did something similar to what Burkhard had done. They reacted sodium metal with dichlorodimethyl silane, but they also added some dichloromethylphenylsilane to the brew. And guess what happened! I'll give you a hint: they didn't get the rings they wanted. What they got was a copolymer, like this:
Maybe that polymer is more clearly drawn like this:
You see, those phenyl groups get in the way when the polymer tries to crystallize, so it isn't as crystalline as polydimethylsilane. This means it is soluble and can be processed and played with and studied.
So what are these good for? Polysilanes are interesting because they can conduct electricity. Not as well as copper, mind you, but a lot better than you'd expect for a polymer, and worth investigating. They're also very heat resistant, almost up to 300 oC, but if you heat them a lot higher you can make silicon carbide out of them, which is a useful abrasive material.
Polygermanes and Polystannanes
Okay, so if silicon can make long polymer chains, what about he other elements in Group IV? Can you make polymers out of germanium? You'd better believe you can! Not only can you make polymer chains out of germanium, but you can even make a polymer chain out of tin atoms. These polymers are called polygermanes and polystannanes, respectively.
Polystannanes are unique and nifty and wonderful and fabulous because they are the only known polymers with backbones made entirely from metal atoms. Like polysilanes, polygermanes and polystannanes are being studied for use as electrical conductors.
Polyphosphazenes I hate to break it to you folks, but we're out of Group IV elements. So the last inorganic polymer we're going to look at today is going to have to be made of something else. And that something else is phosphorus and nitrogen. Like the polysiloxanes, polyphosphazes are made of alternating atoms, in this case, the chain is made up of alternating phosphorus and nitrogen atoms, like this:
This backbone is very flexible, like the polysiloxane backbone chain, so polyphosphazenes make good elastomers. They're also very good electrical insulators. Polyphosphazenes are made in two steps:
First we take phosphorus pentachloride and react it with ammonium chloride to get a chlorinated polymer. Then we treat it with an alcohol sodium salt, and that gives us an ether-substituted polyphosphazene.
Well, that's enough of this fascinating subject for now. While there may be new and exciting inorganic polymers that have been made (research marches on, of course), we can't cover everything in these pages. And don't forget, there are interesting combinations of heteroatoms (non-carbon atoms) with carbon atoms in a variety of polymer backbones and pendent groups. These could be called "semi-inorganic," although that might be too artificial for some.
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