Rubbery reactions

Researchers from the RIKEN Advanced Science Institute in Wako, using a rare earth-based catalyst, have produced a novel polymer or chain of isoprene molecules with unique properties. In addition, much to their surprise, they found their polymerization system can be dramatically switched, simply by adding an aluminum-based compound, to produce a different isoprene polymer which is the main component of natural rubber.

The two polymers vary in which of the carbon atoms of the four-carbon isoprene molecules connect together to form the chain. The rubber chain (1,4-cis polyisoprene) is formed by joining the two end carbons of the isoprene molecules, whereas in the new polymer the molecules are joined at two adjacent carbons in a particular three-dimensional arrangement (isotactic 3,4-polyisoprene) (Fig. 1). It leaves the other two carbon atoms double-bonded on a side chain all along the same side of the main or polymer chain, a construction which allows a range of groups with different chemical functions to be added easily. The researchers are collaborating with a chemical company on employing this facility to transform 3,4-polyisoprene into further useful polymers.

The research group has been exploring how catalysts based on rare-earth metals—a group of elements which includes scandium, yttrium and the lanthanides—can be used to produce high-performance synthetic rubber¹. “We need a chemical way to produce rubber,” says project leader Zhaomin Hou. “Not only is the natural supply limited, but a big advantage of chemical synthesis is that it allows us to modify the properties of rubber in the laboratory.”

In their latest paper published in Angewandte Chemie International Edition ², the researchers discuss their work with a catalyst made up of an yttrium ion with bulky nitrogen-based groups called amidinates attached. The shape of the catalyst restricts the polymerization reaction to the two adjacent carbons at positions 3 and 4 in the isoprene molecule. The result is the new polymer 3,4-polyisoprene. But when they added trimethyl aluminum (AlMe₃), widely recognized as a co-catalyst which can potentially increase the activity of the catalyst system, they found the reaction began to change.

At levels below two AlMe₃ molecules to one catalyst molecule they were still producing 3,4-polyisoprene, but by five AlMe₃ molecules to one catalyst molecule, 98% of their product was rubber (1,4 cis-polyisoprene). “The finding that an aluminum alkyl compound can switch the selectivity of isoprene polymerization suggests that we should reconsider the role of aluminum compounds in such catalyst systems,” Hou says.