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New type of catalyst makes hard plastic very elastic
STANFORD -- Polypropylene is a common plastic that in its hard form is found in containers, clothing, carpets and insulation. Its rarer soft form is used in roofing tar and melt adhesives.
Now, chemists at Stanford University have found a way to combine the two forms, resulting in a range of new plastics that vary considerably in elasticity. At one end of the scale, chemists can make polypropylene cords that stretch only slightly when pulled. At the other end, they can make cords that stretch easily and snap back as readily as rubber bands.
This new capability to precisely control the elasticity of polypropylene could open up whole new markets for the inexpensive plastic. For example, it might replace natural rubber and synthetic elastomers, which are considerably more expensive, in applications that range from automobile bumpers to clothes fibers to basketball shoes.
Polypropylene rubbers also have a significant environmental advantage. They can be readily recycled, and, unlike rubber bands, elastic polypropylenes can be melted and reused.
The successful production of elastic polypropylene was announced in the Jan. 13 issue of the journal Science by assistant professor of chemistry Robert M. Waymouth and doctoral student Geoffrey W. Coates.
As novel as the new plastics themselves is the method by which they are produced.
Using an analogy to pasta making, Waymouth explained how this is done: "If you want to make linguini, you use an attachment that extrudes linguini. To make spaghetti, you use the spaghetti attachment. Now imagine that you could change the shape of the attachment as you extruded the pasta. Then you would make individual strands of pasta that alternated: linguini-spaghetti-linguini. Our catalyst works something like that."
The catalyst involved is made from a material called metallocene. It consists of a metal atom sandwiched between two flat rings made out of five carbon atoms. To this basic compound, the chemists have attached an additional ring to the side of each of the two original rings. The metal atom is the site where the short propylene monomers are welded together to form the long polypropylene molecules. When the two additional rings are both on the same side of the catalyst, it makes the hard, or crystalline, form of the polymer. But when the two rings are on opposite sides, the soft, amorphous form is created.
There is one other complication. The two ring groups are rotating rapidly around an axis passing through the metal atom, a little like two lopsided bicycle wheels spinning around a common axis. Normally, they rotate much faster than the polymerization process, producing a random mixture of the two types of polypropylene.
To get control over the process, Waymouth and Coates added extra groups of atoms to the rotating rings. They did so in such a way that these groups slowed the rate of rotation until it was close to that at which the polypropylene chains were created. This resulted in a catalyst that oscillates between different configurations and therefore creates polymers that alternate in structure along the chain.
This enabled the scientists to control the lengths of the crystalline and amorphous sequences created on the polypropylene strands by varying the pressure and temperature of the reaction. In this fashion they have been able to vary the percentage of crystallites in the material from zero to 30 percent.
According to the scientists, this system represents a new strategy in constructing polymers - through design of a catalyst that changes its form while it works.
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