Radical Polymerization

Polymers are used to form plastics and other synthetic materials. These radical reactions use alkenes to link millions of repeating units together through an almost never-ending propagation step.

Concept: General features of Radical Polymerization. 

Video Transcript

When radicals are exposed to alkenes in excess, they can do something really weird. What that is that they can do something called polymerization. Polymerization is where you basically have a chain reaction that never stops. It never has a termination step because it keeps propagating forever. What you wind up getting is these hydrocarbon chains that could be 10,000, 20,000, hundreds of thousands of units long. That's actually what plastic is. A lot of synthetic materials come from polymerization.
What I wanted to go over right now was just the general mechanism for these polymerization reactions. And it is radical intermediated.
Basically, like I said, radical polymerization reactions use alkenes in excess to extend the propagation step. This is they way that industries use petroleum, petroleum just from underground. They use it and they convert it into plastic through this method. All it is is just a big chain reaction that just keeps on going and that's how you get stuff like tires and shopping bags or whatever. All these different things that are made out of weird plastics, all these synthetic materials, it's basically petroleum just linking up and cross-linking.
Basically, here's a really common byproduct of petroleum, propylene. Believe me, they mine tons of this stuff. They drill it and get tons of that stuff from underground. If you do a polymerization reaction using some kind of radical, I'm just going to put here a radical initiator like OR-, not negative, radical, what you wind up getting is a polymer. The general formula for a polymer is basically, n is just the number of units. And we don't really know how many units there are, so n is just going to stay like that.
What it means is that you have these repeating subunits that just keep going on forever. In this case, this repeating subunit would have basically two carbons and then another carbon here. The way that works is that the two carbons in between are always the ones that are made out of the double bond. In this case, that would represent these two right here.
This extra CH3 at the top, what's that got to do with? Well, that's the CH3 right here. All the other H's that are around are the other H's that are sticking out of this thing, so there's an H, H, and H. And these three H's are the three H's right there. This just keeps extending and extending. Actually, polypropylene is used to make AstroTurf and car tires and ropes and stuff. It's kind of cool how it actually has a real life application. 

Petroleum products are converted into various plastics through this mechanism:

Concept: The mechanism of Radical Polymerization. 


Example: Use the polymerization mechanism to draw the polymer formula for the following compound.