In chemistry, just as in working life, a multi-tasker sometimes has the edge over a specialist. The California Institute of Technology has now unveiled a catalyst molecule capable of no fewer than three different jobs, each of which normally requires a tailor-made catalyst.
The versatile new molecule allows its creators, Robert Grubbs and co-workers, to make a complicated polymer at a single stroke. Polymers are chain-like assemblies of small molecules linked by chemical bonds, and are the components of virtually all plastics from rubber to the hard casing on computers.
Most are made up of just a single kind of small-molecule ('monomer') building block. But the polymers made by Grubbs and colleagues are composed of two different monomers.
Such 'hybrid' polymers are called co-polymers. A particularly useful variant of this sort is a 'block co-polymer', in which strings of many monomers of one type alternate with sections containing the other monomer. If the two monomers are denoted A and B, for instance, then the sequence might be 'AAAAABBBBBAAAAABBBBB' and so on.
Block co-polymers combine the properties of two types of material in a single one. For example, a co-polymer of hard polystyrene and rubbery polybutadiene is a rubbery material with chains that are pinned together by stiff clusters of the polystyrene blocks. It is a thermoplastic, which flows when hot but sets when cooled. It is used for the soles of sneakers.
The difficulty of making block co-polymers to order is that the reaction to link up one type of monomer (type A, say) might be quite different from that to link the other (type B). So two separate processes, with different catalysts to assist the reactions, must be used to build the chains. This makes the industrial process cumbersome and wasteful.
Grubbs and colleagues now show that a single catalyst can, with some clever planning, be designed to promote more than one kind of monomer-joining reaction.
They used a catalyst molecule whose reactive heart consists of an atom of the metal ruthenium. With various other molecular groups attached to the ruthenium, this molecule can activate the linking of methyl methacrylate molecules into poly(methyl methacrylate) (PMMA), which is the hard, clear plastic Plexiglas, or the gummy medium in acrylic paints.
A slightly different version of the ruthenium catalyst will break open ring-shaped hydrocarbon molecules to make polyolefins such as poly(cyclooctadiene) (PCOD), a tough, flexible material rather like polybutadiene.
Grubbs' team loaded the ruthenium catalyst with the attachments it needed to conduct both reactions, and then added it to a mixture of the two monomers (methyl methacrylate and cyclooctadiene). Within about a day, these monomers had been joined into block co-polymers: segments of PMMA linked to segments of PCOD.
Moreover, the researchers showed that their catalyst could facilitate a third reaction too. By exposing the polymerized mixture to hydrogen, they converted most of the PCOD segments to polyethylene, a less rubbery polymer. This ability to add hydrogen makes the range of materials available from the ruthenium catalyst even more diverse.