5924 Chem. Rev. 2009, 109, 5924–5973 Hyperbranched and Highly Branched Polymer ArchitecturessSynthetic Strategies and Major Characterization Aspects Brigitte I. Voit* and Albena Lederer Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany Received February 20, 2009 Contents 1. Introduction 1. Introduction 5924 “Life is branched” was the motto of a special issue of 1 2. Synthesis of Hyperbranched Polymers 5925 Macromolecular Chemistry and Physics on “Branched 2.1. General Aspects and Methodologies 5926 Polymers”, indicating that branching is of similar importance in the world of synthetic macromolecules as it is in nature. 2.1.1. Step-Growth Approaches 5926 The significance of branched macromolecules has evolved 2.1.2. Chain-Growth Approaches 5928 over the last 30 years from just being considered as a side 2.1.3. OtherSyntheticApproaches 5931 reaction in polymerization or as a precursor step in the 2.2. Examples of hb Polymers Classified by 5932 formation of networks. Important to this change in perception Reactions and Chemistry of branching was the concept of “polymer architectures”, 2.2.1. hb Polymers through Polycondensation 5932 which formed on new star- and graft-branched structures in 2.2.2. hb Polymers through Addition Step-Growth 5932 the 1980s and then in the early 1990s on dendrimers and Reactions dendritic polymers. Today, clearly, controlled branching is 2.2.3. hb Polymers through Cycloaddition 5934 considered to be a major aspect in the design of macromol- Reactions ecules and functional material. 2.2.4. hb Polymers through Self-Condensing Vinyl 5937 Hyperbranched (hb) polymers are a special type of Polymerization dendritic polymers and have as a common feature a very 2.2.5. hb Polymers through Ring-Opening 5938 high branching density with the potential of branching in Multibranching Polymerization each repeating unit. They are usually prepared in a one-pot 2.2.6. hb Polyesters 5938 synthesis, which limits the control on molar mass and 2.2.7. hb Polyurea and Polyurethanes 5940 branching accuracy and leads to “heterogeneous” products 2.2.8. Hyperbranched Glycopolymers and 5942 with a distribution in molar mass and branching. This Polypeptides distinguishes hyperbranched polymers from perfectly branched 2.2.9. Functional Materials with hb Architectures 5944 and monodisperse dendrimers. In the last 20 years, both 2.3. Architectures with Diluted Branching and 5945 classes of dendritic polymers, dendrimers as well as hb Linear-Hyperbranched Hybrids polymers, have attracted major attention because of their 2.3.1. Copolymers 5945 interesting properties resulting from the branched architecture 2.3.2. Dendrigraft and Arborescent Graft 5945 as well as the high number of functional groups.2 The Polymers, Hyper-Macs challenging synthesis of the dendrimers attracted especially 2.3.3. Linear-Hyperbranched Hybrid Structures 5948 scientists with a strong organic chemistry background and 2.3.4. Core-Shell Structures, Nanocapsules, 5950 led to beautifully designed macromolecules, which allowed Self-Assembly adeeperinsightintotheeffectofbranchingandfunctionality. 3. Characterization of Hyperbranched Polymers 5952 Dendrimers have been considered as perfect “nano-objects” 3.1. Structural Characterization 5954 where one can control perfectly size and functionality, which 3.1.1. Degree of Branching 5954 is of high interest in nanotechnology and biomedicine. 3.1.2. Intermolecular Hydrogen Bonding 5957 hb polymers, however, were considered from the beginning 3.2. Solution Characterization 5958 as products suitable for larger-scale application in typical 3.2.1. Molar Mass and Molar Mass Distributions 5958 polymer fields like coatings and resins, where a perfect 3.2.2. Molar Mass Dependencies of Structure 5960 structure is sacrificed for an easy and affordable synthetic Parameters route. Thus, the first structures that were reported paralleled 3.3. Bulk Characterization 5963 the chemistry used for linear polymers like typical polycon- densation for polyester synthesis. More recently, unconven- 3.3.1. Thermal Properties 5963 tional synthetic methods have been adopted also for hb 3.3.2. Melt Rheology 5964 polymers and related structures. Presently, a vast variety of 3.3.3. Thin-Film Properties 5965 highly branched structures have been realized and studied 4. Summary and Perspectives 5966 regarding their properties and potential application fields. 5. Acknowledgments 5967 Excellent reviews appeared covering synthesis strategies, 6. References 5967 properties, and applications, like the very recent tutorial by Carlmark et al.,3 the comprehensive book on hyperbranched * E-mail: [email protected]; [email protected]. polymers covering extensively synthesis and application 10.1021/cr900068q CCC: $71.50 © 2009 American Chemical Society Published on Web 09/28/2009 Hyperbranched and Highly Branched Polymer Architectures Chemical Reviews, 2009, Vol. 109, No. 11 5925 excellent solubility, compared to linear polymers, low solution viscosity, modified melt rheology, and high level of terminal end group functionality.7-10 In addition to “traditional” applications of hb polymers, the unique dendritic structure opens up opportunities for new “nanotechnology” applications based on specific confinement of functional units and the formation of pores and cavities, e.g., as in thin films in sensor devices and diagnostics, as porogens for nanofoams, and as carriers for special additives, catalytic species, and therapeutics.5,6,11-14 Polyesters dominate the field, with the Perstorp “Boltorn” products10 leading in product development and with hyper- branched polyesters based on the relatively easily available bis(4-hydroxyphenyl) pentanoic acid (BHPPA) often used in academic studies.15,16 However, the poly(esteramide)s Brigitte Voit received her Ph.D. in Macromolecular Chemistry in 1990 from commercialized by DSM under the trade name Hybrane,9 University Bayreuth, Germany. After postdoctoral work at Eastman Kodak in Rochester, U.S.A., she went in 1992 to Technische Universita¨t Mu¨nchen, poly(ethyleneimine)s from BASF SE under the trade name 17-19 20 receiving her habilitation degree in Macromolecular Chemistry there in Lupasol, and polyurethanes and polyesters developed 1996. In 1997 Brigitte Voit was appointed head of the Institute of as well at BASF SE are further examples for hyperbranched Macromolecular Chemistry at the Leibniz Institute of Polymer Research polymers specially suited for commercial coatings and resin (IPF) Dresden, as well as professor for “Organic Chemistry of Polymers” application.21 Hyperbranched polyglycerols, however, pre- at the University of Technology Dresden (TUD). Since 2002 she is also pared by HyperPolymers22 and more recently as self- heading the IPF Dresden as Managing Director and Chief Scientific Officer. 23 Her major research interest is in the synthesis of new functional polymer assembled nanocapsules by Nanotransport Systems have architectures covering topics such as dendritic polymers, functional block potential in biomedical applications, similar to the specially and graft copolymers, and thermo- and photolabile polymers. functionalized dendritic and hyperbranched polyesters based on bis(methylol)propionic acid as provided by Polymer Factory.24 This review will focus on the synthetic approaches used to prepare hyperbranched and highly branched polymers covering the classical lines but also giving special attention to the new trends to dilute the branching with linear polymer chains by creating increasingly complex linear-branched hybrid structures. Hyperbranched structures created on planar surfaces or on various (nano)particles and nanoscale objects and perfectly branched classical dendrimers and dendrons will be excluded from this review, since both topics are extensive and contain sufficient material for several indi- vidual reviews. Theoretical as well as application aspects will only be addressed peripherally to give background and perspectives for some of the presented synthetic aspects. The irregular branched structure in hb polymers and highly Albena Lederer obtained her Ph.D. in 1999 from the University of Mainz after her research in the field of physical chemistry of polymers at Max- branched derivatives leads also to significant and new Planck-Institute of Polymer Research Mainz. Since 2000 she is extensively challenges in the characterization of these materials. There- investigating the physicochemical properties of branched macromolecules. fore, the last part of this review will be devoted to the Since 2007 Albena Lederer is leader of the Polymer Separation group at structural characterization of hb polymers focusing on newly the Leibniz-Institute of Polymer Research Dresden. To her main research adapted and developed methods that help to define the special area belong the characterization of dendritic and multifunctional polymers features of these highly branched and functional materials. in solution and the development of new separation methods for branched polymers. 2. Synthesis of Hyperbranched Polymers 4 aspects that is about to be published, and the reviews by As already outlined, a vast variety of highly branched 5 6 Yan and Hayes. polymer structures has been reported since their theoretical Hyperbranched polymers have received much industrial treatment in the middle of the 20th century.25 Especially in attention and have been commercialized for several applica- the last 20 years,26,27 very strong synthetic activity emerged tions or are presently in the advanced development stage. on macromolecules