By Bob Chai
Most plastic items in use today are made from polypropylene, a series of propylene monomers joined together. Plastics are increasingly constituting a larger part of our household items, especially as new technology creates more uses for plastics. However, further applications of plastics are hindered by one major limitation: plastics are stiff.
In order to make plastics more flexible, chemists Geoffrey W. Coates of California Institute of Technology in Pasadena and Robert M. Waymouth of Stanford University have discovered what they call an "oscillating" catalyst. This oscillating catalyst works by adding stiff and flexible monomers to a growing polymer chain. "Regular polypropylene are isotactic, meaning that all the methyl groups of the polymer are projected outward in the same direction," explains Prof. Carl Gryte, a professor of chemical engineering at Columbia University. The methyl group's uniformity allows the polymer to crystallize, introducing rigidity and hardness into the polymer. On the other hand, random arrangement of the methyl groups, atacticity, does not allow for crystallization, leaving the polymer quite elastic and soft. In addition, Prof. Gryte warns that an atactic polymer has a high affinity of being oxidized, posing a big problem of keeping the polymer stable. However, by combining groups of hard isotactic polymers with soft atactic polymers, the oxidizing affinity of the soft polymer is reduced. Furthermore, the resulting "segmented elastome," as Prof. Gryte describes it, contains a combination of the rigid properties of the stiff polymer groups and the flexible property of elastic polymer groups. By monitoring the temperature and pressure of the polymer synthesis reaction, the composition of stiff monomers and elastic monomers in the resulting polymer can be predicted. As a result, plastics with varying degrees of flexibility and rigidity can be obtained.
The oscillating catalyst's ability to add two different monomer units during one reaction process depends on the molecular structure. Usually, only one binding site (a region where the reaction actually takes place) exists for a catalyst. However, the oscillating catalyst has freely moving parts rotating around a zirconium metal center. When the attachment of one monomer is taking place, the random movement of the free parts allows a different binding site to appear. Using this new binding site as a template for binding the next monomer and interchanging between the two binding sites, an alternating sequence of different monomers will be produced.
Applications of this new technology include plastic utensils that are much stronger (yet not as brittle), car wheels and sneakers made of softer and stronger plastics, plastic dishes that are less likely to break because of their greater elasticity. In addition, this rubber-like plastic polymer may be applied on a wider scale to rebuild the fallen structures in Kobe, Japan. Although currently inconceivable, it is certainly a thought for the future.