Dendrons, dendrimers and dendrigrafts.

The search for structural control at a nanoscopic level may well be over: tree-like dendritic polymers can be carefully orchestrated into a variety of tiny shapes

Three times this century, chemists have developed new architectural classes of polymers that have launched significant new industries.[1] The first involved linear, spaghetti-like polymers (for example poly(ethylene), poly(styrene) and nylon) known as 'thermoplastics'. The second class, referred to as 'thermosets', was based on bridging/crosslinking or vulcanising these spaghetti-type systems to give epoxies, urethanes and solvent resistant elastomers/rubbers. The third major class consists of randomly branched spaghetti-like polymers, which have emerged from recent breakthroughs in 'metallocene-type' catalysts to give new forms of commercially important amorphous, low density poly(ethylenes). These have novel and important performance properties that are currently being pursued by major chemical companies such as Dow, DuPont, Exxon and DSM [ILLUSTRATION FOR FIGURE 1 OMITTED].[2]

Based on proven architecture-property relationships observed for the first three classical polymer types, it is now accepted that new polymeric architectures are directly related to new properties, performances and materials.[3,4]

A new architecture

In the past decade, our group at the Michigan Molecular Institute and others have defined a fourth major class of macromolecular architecture [ILLUSTRATION FOR FIGURE 1 OMITTED].[5-7] This new architecture mimics the dendritic branching of trees and is referred to as dendrons, dendrimers or dendrigrafts. Other pioneers in this area include Fritz Vogtle at the University of Bonn who first conceptualised such dendritic branching in small molecules which he has referred to as 'cascade molecules'[8] and George Newkome at the University of South Florida who calls his dendritic molecules 'arborols'.[9]

During the 1980s, virtually all dendritic polymers were produced by starting construction from the root of the molecular trees. This involved assembling monomeric modules in a radial, branch upon branch motif according to certain dendritic rules and principles. This is called the divergent process [ILLUSTRATION FOR FIGURE 2 OMITTED] and is currently the preferred commercial route used by worldwide producers such as Dendritech (US), DSM (Netherlands) and Perstorp (Sweden).

In 1989, a convergent process [ILLUSTRATION FOR FIGURE 2 OMITTED] was introduced by Jean Frechet at the University of California, Berkeley (formerly of Cornell University) and others.[10] This starts by constructing the leaves of the molecular trees which are then attached to branched reagents to give mono-functionalised sub-dendrons. Stepwise reaction of these sub-dendrons according to dendritic rules and principles ultimately produces molecular trees by inward construction from the leaves to the branches to the roots. Although this approach is not amenable to large scale production, it offers the advantage of making laboratory quantities of ideally branched dendrimers that can …