Free Radical Polymerization

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. 

Let's just get into the mechanism though because I know that's what you're interested in so the general mechanism is of course we need an initiation step so we're going to go ahead and in this case, we use peroxide usually that's the one that the industry uses so we would get two equivalents of my peroxide radical, OK? And that's going to react with my double bond, OK? Now in this case the radical is going to react directly with the double bond because the double bond is a good source of electrons so what I wind up getting is I'm just going to show you guys right here three arrows once again I'm going to get this going out into the middle of nowhere then I'm going to get the double bond attacking, now I just have to ask myself where would the radical, the extra radical be most stable on the primary carbon the red one or on the secondary carbon the blue one, the answer is secondary so my radical would go ahead and jump there what I would get for this first step is I would get now OR, OK? And I would get well there's no more double bond anymore it's just a single bond with a radical, OK? So that's my first step, well now how is that going to propagate? Well that's going to attach to another radical so I mean to another double bond so what winds up happening is that now this is going to do the same thing again I'm going to get 1, 2, 3 and this is the whole idea again about polymerization it's going to keep doing the same thing over and over so what I wind up getting here now is I'm going to draw the blue part just the way it was before but watch I'm going to twist it a little bit so now this goes here and that stage 3 that I had basically there that like not participating in blue I'm going to move that one up, OK? So that means that now what is that attached to? Well that's a new single bond I'm going to make that black that represents the new single bond that was made by the radical reaction and that's going to be attached to a new three carbon subunit that now has a radical there, OK? And this is going to keep on going and going and going so what you see is that now we have our general formula starting to prop up, this is going to be the repeating subunit is going to look like this, OK? Where basically I have this thing that just keeps going over and over and over again, OK?

So what of the termination step? Are you curious about that? We're just going to put here NA, why? Because that's the whole point of polymerization is that there is no terminations step it just doesn't terminate it just keeps on going forever until you run out of Alkenes, OK? And what you wind up getting is these really really really long polymers that have unusual properties like being elastic and never biodegrading and stuff like that, being terrible for the earth and killing animals so what I want you guys to do now.... That sounded like really jaded but I want you guys to do now is just figure out what the general subunit would look like if we were to polymerize this molecule, OK? And the best way to do that would just be to kind of draw the first parts of the mechanism and see if you guys can figure out what that repeating subunit is going to look like there, that's called the general formula of the polymer would be in those brackets so I'm going to give you guys some time to do it go ahead and use you don't have to know to draw the whole mechanism but use OR negative....Uhh I keep saying negative use the OR radical to start the reaction and then kind of just continue it and see if you can get that repeating subunit and then I'll give the answer.