Rubbery plastics produced in Cornell lab

An entirely new class of rubbery plastics has been produced in the laboratory by a Cornell University researcher and two co-workers.

Because the material uses two common and inexpensive petroleum products, ethylene and polyethylene, for its feedstock, the research has the promise of greatly reduced production costs.

The development is the result of a chance discovery by Geoffrey Coates, assistant professor of chemistry and chemical biology at Cornell, of a long-sought catalyst that enables the new polymer to be 'grown' from molecules of ethylene and propylene. A catalyst - in this case, based on titanium - is a substance that increases the rate of chemical reaction without itself changing chemically. 'We didn't predict this. It was an example of serendipity,' says Coates.

Since ethylene and polyethylene, the two so-called monomers, or substrates, used in the process, are among the least expensive on the market, 'we anticipate that these new polyolefins will be dramatically cheaper,' says Coates.

Coates reported the research results at the national meeting of the American Chemical Society at the San Diego Marriott Hotel. Coates's paper was co-authored by Cornell graduate student Phillip Hustad and postdoctoral researcher Jun Tian.

The material produced in Coates's lab falls into a class of compounds called thermoplastic elastomers that, unlike most rubbers, have properties that allow them to be melted and recycled. The rubbers in this class most widely used today are polymers made from styrene and butadiene, two relatively expensive chemicals. Uses of materials in this class, which are made by Kraton Polymers, range from roofing to adhesives to shoe soles.

The hallmark of such rubbery polymers is that they are made by a 'living polymerization process,' a technique that literally grows molecules in connected monomer blocks, and can, in theory, endlessly enchain monomers to yield chains of equal length. In the case of the styrene-butadiene polymer, the material is made of three-block strands: the first block, the hard polystyrene; the second, the liquid polybutadiene; and the third the polystyrene. This combination of soft and hard molecules gives the material its elasticity. It can be injection molded, melted and recycled.

Coates likens this so-called 'sequential addition of monomers' process to a pasta machine turning out long strands of spaghetti, then linguine, then spaghetti again -- all connected. The trick, he says, is to use a catalyst that allows the pasta to continue these links without breaking.

Researchers have long reasoned that a lower-cost living polymerization process should be possible with ethylene and propylene if a catalyst could be found. For the past five years, in particular, the search for such a catalyst has been intense. But every new catalyst failed to enable the 'living' process to achieve the continuous growing of molecular blocks without breaking the chains.

Coates's titanium catalyst used in the creation of the Cornell rubbery polymer begins with propylene in a hard form called syndiotactic propylene. This block is then joined to a second, much softer, block of an ethylene-propylene copolymer. The third block is again the syndiotactic propylene. This 'very complicated polymer,' says Coates, 'has the properties of standard Kraton polymers, but has the advantage of being made from especially cheap materials - ethylene and propylene.'

One particular asset of the new polymer, he says, is that by 'tailoring' the size of the blocks in the strands, the plastic can be changed from a tough rubber to a gum elastomer, or soft rubber. 'To make the material more pliable, we can make the soft block bigger, and to make it harder, we would have more of the hard block,' he says.

Why hasn't this been achieved before? 'There is no way to rationally predict the action of compounds,' says Coates. 'We simply stumbled across this, luckily with our eyes open.'

The research was funded with grants from the Cornell Center for Materials Research and from Exxon Mobil.