Self-condensing vinyl polymerization was used to produce dendritic polymers with both highly branched structures and numerous reactive groups. A vinyl monomer will undergo self-polymerization if it contains a pendant group that can be transformed into an initiating moiety by the action of an external stimulus. The self-polymerization combines features of a classical vinyl polymerization process with those of a polycondensation because growth is accomplished by the coupling of reactive oligomers. Highly branched, irregular dendritic structures with a multiplicity of reactive functionalities are obtained by polymerization of 3-(1-chloroethyl)-ethenylbenzene.

Free radical polymerization remains one of the most widely used carbon-carbon bond-forming industrial processes, with millions of tons of polymers being produced annually. Unlike other vinyl polymerizations, it can be used with a broad variety of monomers and is also synthetically much less demanding than anionic or cationic processes. Despite these advantages and its tremendous commercial significance, free radical polymerization still lacks versatility in terms of its ability to control polymer architecture. Recent advances in "living" free radical polymerizations,1 notably the work of Georges et al., have allowed the preparation of narrow polydispersity linear polymers and some block copolymers, but dendritic structures such as hyperbranched polymers have remained elusive. This is unfortunate since 3-dimensional dendritic polymers,* 12 such as dendrimers and hyperbranched polymers, have been noted for their unusual properties.3 Their shape-and functionality-related properties4 can be exploited to design materials that have unique viscoelastic properties,5 can form unusual blends,6 or even act as sizeand shape-selective carrier molecules.7 In this report, we detail the first approach to hyperbranched and other complex macromolecular architectures based on living free radical polymerizations. This work is based on two recent developments in vinyl polymerizations. The first is the discovery by Hawker8 that well-defined unimolecular initiators can be used to initiate radical polymerizations with accurate control over molecular weights and chain ends.8•9 The second is the report by Fréchet et al.10 *of a novel "self-condensing" vinyl polymerization which, under cationic conditions, affords hyperbranched polymers with a broad polydispersity.

The ideal of living polymerization has defined research in polymer chemistry over the past 50 years. In this Perspective, we present the case that this concept has enabled the treatment of polymers as organic molecules, rather than impure mixtures of species, and allowed the translation of methods developed by synthetic organic chemists into ever more accessible living and/or controlled polymerization methods. The concurrent development of rapid analytical methods for screening new polymerization methods for living characteristics, chiefly size exclusion chromatography, has greatly aided in the expansion of living polymerization methods.

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