Polyethylene

Polyethylene is probably the polymer you see most in daily life. It is one of the polymers called polyolefins, which is an odd name. Many names from the past have nothing to do with the actual chemical compositions of the molecules, but that's a story for another time.  Polyethylene is the most popular plastic in the world. This is the polymer that makes grocery bags, shampoo bottles, children's toys, and even bullet proof vests. For such a versatile material, it has a very simple structure, the simplest of all commercial polymers. A molecule of polyethylene is nothing more than a long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom. That's what the picture at the top of the page shows, but it might be easier to draw it like the picture below, only with the chain of carbon atoms being many thousands of atoms long:

Sometimes it's a little more complicated. Sometimes some of the carbons, instead of having hydrogens attached to them, will have long chains or branches of polyethylene attached to them. This is called branched, or low-density polyethylene, or LDPE. When there is no branching, it is called linear polyethylene, or HDPE. Linear polyethylene is much stronger than branched polyethylene, but branched polyethylene is cheaper and easier to make. It is also more flexible and works great for sandwich wrap.

Linear polyethylene is normally produced with molecular weights in the range of 200,000 to 500,000, but it can be made even higher. Polyethylene with molecular weights of three to six million is referred to as ultra-high molecular weight polyethylene, or UHMWPE. UHMWPE can be used to make fibers which are so strong they replaced Kevlar for use in bullet proof vests. Large sheets of it can be used instead of ice for skating rinks.

Polyethylene is vinyl polymer, made from the monomer ethylene. Here's a model of the ethylene monomer. It looks like some sort of four-legged headless animal if you ask me.

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. Either way, be sure to close the new window that opens up with the 3D model in it when you are ready to come back here. Branched polyethylene is often made by free radical vinyl polymerization. Linear polyethylene is made by a more complicated procedure calledZiegler-Natta polymerization. UHMWPE is made using metallocene catalysis polymerization.

But Ziegler-Natta polymerization can be used to make LDPE, too. By copolymerizing ethylene monomer with a alkyl-branched comonomer one gets a copolymer which has short hydrocarbon branches. Copolymers like this are called linear low-density polyethylene, or LLDPE. BP produces LLDPE using a comonomer with the catchy name 4-methyl-1-pentene, and sells it under the trade name Innovex¨. LLDPE is often used to make things like plastic films.

Other polymers used as plastics include:	Other polymers used as fibers include:
Polypropylene	Polypropylene
Polyesters	Polyesters
Polystyrene	Nylon
Polycarbonate	Kevlar and Nomex
PVC	Polyacrylonitrile
Nylon	Cellulose
Poly(methyl methacrylate)	Polyurethanes