Treelike molecules branch out.

Hugging the Chippewa River some 130 miles southeast of Michigan's Sleeping Bear Dunes, a tree farm sprawls across the plains. Thousands of trees start life there as seedlings before branching into tall, elegant adults.

"Watching these trees with the eyes of a young chemist," says Donald A. Tomalia, now a researcher at the Michigan Molecular Institute in Midland, "I began to wonder whether one could make large molecules the same way."

In 1980, Tomalia synthesized the first dendrimer molecule, its name derived from the Greek dendron, or tree. At first, he could only produce small quantities of limited variety. Unlike crystals, which grow into ordered lattices, dendrimers accumulate additional material in a repeating, branched pattern. The structure and composition of the starting material determine the branching pattern.

Other chemists showed little interest in dendrimers until about 5 years ago, when powerful mass spectrometers and the need for specialized molecules produced an explosion of activity around the world. Today, "we're able to create macromolecules on demand with a wide range of sizes, shapes, and weights," Tomalia said at a meeting of the Council for the Advancement of Science Writing in Durham, N. C. "We're learning to produce these materials in kilogram quantities, in some cases up to thousands of pounds."

Just as a tree sprouts successive generations of branches, Tomalia says, these macromolecules branch in stages. Yet, unlike a tree, which produces its building materials internally, the growing molecular structure harnesses smaller molecules from solution at each juncture and fixes them in place according to a branching pattern. "In the end," he says, "you get a macromolecule that resembles a tree."

Unlike most polymers, which form randomly, dendrimers evolve into carefully constructed geometries with specified molecular masses and electronic structures. Such specificity distinguishes these polymers from other synthetic chain molecules and enables chemists to use dendrimers to construct even more complicated macromolecules.

"The dendrimers can grow two, three, four, five, or six limbs at each branching juncture, depending on how the molecule is designed," he says.

"Each time you change the dendritic architecture, you produce new materials with new properties and applications," he says. Because their complex forms are similar to those of many biological molecules, some dendrimers may prove useful in medicine, including genetic and immunological therapies. Others may lead to better microelectronics and plastics.