March 20, 2019

The model above is an image of the isotactic polymer you can view
by clicking here or you can just click on the image itself.
The image below is of the atactic version with random backbone stereochemistry.
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.

Polypropylene is one of those rather versatile polymers out there. It serves double duty, both as a plastic and as a fiber. As a plastic it’s used to make things like dishwasher-safe food containers. It can do this because it doesn’t melt below 160oC, or 320oF. Polyethylene, a more common plastic, will anneal at around 100oC, which means that polyethylene dishes will warp in the dishwasher. As a fiber, polypropylene is used to make indoor-outdoor carpeting, the kind that you always find around swimming pools and miniature golf courses. It works well for outdoor carpet because it is easy to make colored polypropylene, and because polypropylene doesn’t absorb water, like nylon does.

Structurally, it’s a vinyl polymer, and is similar to polyethylene, only that on every other carbon atom in the backbone chain has a methyl group attached to it. Polypropylene can be made from the monomer propylene by Ziegler-Natta polymerization and by metallocene catalysis polymerization.

This is what the monomer propylene really looks like:

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.

Wanna know more?

Research is being conducted on using metallocene catalysis polymerization to synthesize polypropylene. Metallocene catalysis polymerization can do some pretty amazing things for polyolefins like polypropylene. Specifically, it allows polypropylene to be made with different tacticities. Most polypropylene we use is isotactic. This means that all the methyl groups are on the same side of the chain, like this:

Isotactic polypropylene has a high enough melting point that you can put it in your dishwasher and it won’t come out as a new form of plastic art. But sometimes we use atactic polypropylene. Atactic means that the methyl groups are placed randomly on both sides of the chain like the 3D image above or the figure below:

Atactic polypropylene has no commercial application because it’s pretty much a gooey, messy blob. However, by using special metallocene catalysts, it’s believed that we can make polymers that contain blocks of isotactic polypropylene and blocks of atactic polypropylene in the same polymer chain, as is shown in the picture:

This polymer is rubbery, and makes a good elastomer. This is because the isotactic blocks will form crystals by themselves, acting as physical crosslinkers. But because the isotactic blocks are joined to the atactic blocks, the little hard clumps of crystalline isotactic polypropylene are tied together by soft rubbery segments of atactic polypropylene, as you can see in the picture on the right.

To be honest, atactic polypropylene would be a very soft rubber without help from the isotactic blocks, but it wouldn’t be very strong. The hard isotactic blocks hold the rubbery isotactic material together, to give the material more strength. Most kinds of rubber have to be chemically crosslinked to give them strength, but not polypropylene elastomers.

Elastomeric polypropylene, as this copolymer is called, is a kind of thermoplastic elastomer. However, until the research is completed, this type of polypropylene will not be commercially available.

The polypropylene that you can buy off the shelf at the store today has about 50 – 60% crystallinity, but this is too much for it to behave as an elastomer. It does make it an excellent polymer for applications in plastic storage containers and bottles. Take a look on the bottom of what you use to store leftovers in the fridge. If it has a recycle number of “5,” it’s isotactic polypropylene.

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

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