Hyperbranched polymers: from synthesis to applications

2.2.1. Polyisocyanates 6117 2.2.2. Polysilanes 6118 2.2.3. Polyacetylenes 6120 2.3. Foldamer-Based Helical Polymers 6124 2.3.1. Click Polymerization 6126 2.3.2. Ring-Closing Reaction 6126 2.4. Other Types of Helical Polymers 6127 2.4.1. π-Conjugated Helical Polymers 6127 2.4.2. Metallosupramolecular Helical Polymers 6128 2.5. Induced Helical Polymers 6130 2.5.1. Induced Helical Polyacetylenes 6130 2.5.2. Other Induced Helical Polymers and Oligomers 6132

Helical polymers: synthesis, structures, and functions

Synthetic helical polymers: conformation and function.

The legacy of Gilbert Newton Lewis (1875-1946) pervades the lexicon of chemical bonding and reactivity. The power of his concept of donor-acceptor bonding is evident in the eponymous foundations of electron-pair acceptors (Lewis acids) and donors (Lewis bases). Lewis recognized that acids are not restricted to those substances that contain hydrogen (Bronsted acids), and helped overthrow the "modern cult of the proton". His discovery ushered in the use of Lewis acids as reagents and catalysts for organic reactions. However, in recent years, the recognition that Lewis bases can also serve in this capacity has grown enormously. Most importantly, it has become increasingly apparent that the behavior of Lewis bases as agents for promoting chemical reactions is not merely as an electronic complement of the cognate Lewis acids: in fact Lewis bases are capable of enhancing both the electrophilic and nucleophilic character of molecules to which they are bound. This diversity of behavior leads to a remarkable versatility for the catalysis of reactions by Lewis bases.

Lewis Base Catalysis in Organic Synthesis

“Life is branched” was the motto of a special issue of Macromolecular Chemistry and Physics1 on “Branched Polymers”, indicating that branching is of similar importance in the world of synthetic macromolecules as it is in nature. The significance of branched macromolecules has evolved over the last 30 years from just being considered as a side reaction in polymerization or as a precursor step in the formation of networks. Important to this change in perception of branching was the concept of “polymer architectures”, which formed on new starand graft-branched structures in the 1980s and then in the early 1990s on dendrimers and dendritic polymers. Today, clearly, controlled branching is considered to be a major aspect in the design of macromolecules and functional material. Hyperbranched (hb) polymers are a special type of dendritic polymers and have as a common feature a very high branching density with the potential of branching in each repeating unit. They are usually prepared in a one-pot synthesis, which limits the control on molar mass and branching accuracy and leads to “heterogeneous” products with a distribution in molar mass and branching. This distinguishes hyperbranched polymers from perfectly branched and monodisperse dendrimers. In the last 20 years, both classes of dendritic polymers, dendrimers as well as hb polymers, have attracted major attention because of their interesting properties resulting from the branched architecture as well as the high number of functional groups.2 The challenging synthesis of the dendrimers attracted especially scientists with a strong organic chemistry background and led to beautifully designed macromolecules, which allowed a deeper insight into the effect of branching and functionality. Dendrimers have been considered as perfect “nano-objects” where one can control perfectly size and functionality, which is of high interest in nanotechnology and biomedicine. hb polymers, however, were considered from the beginning as products suitable for larger-scale application in typical polymer fields like coatings and resins, where a perfect structure is sacrificed for an easy and affordable synthetic route. Thus, the first structures that were reported paralleled the chemistry used for linear polymers like typical polycondensation for polyester synthesis. More recently, unconventional synthetic methods have been adopted also for hb polymers and related structures. Presently, a vast variety of highly branched structures have been realized and studied regarding their properties and potential application fields. Excellent reviews appeared covering synthesis strategies, properties, and applications, like the very recent tutorial by Carlmark et al.,3 the comprehensive book on hyperbranched polymers covering extensively synthesis and application * E-mail: voit@ipfdd.de; lederer@ipfdd.de. Chem. Rev. 2009, 109, 5924–5973 5924

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Hyperbranched and highly branched polymer architectures--synthetic strategies and major characterization aspects.

The First Isolable Dialkylsilylene

Carbon chemistry exhibits a rich variety in bonding patterns, with homo- or heteronuclear multiple bonds involving sp-hybridized carbon atoms as found in molecules such as acetylenes, nitriles, allenes and carbon dioxide. Carbon's heavier homologues in group 14 of the periodic table—including silicon, germanium and tin—were long thought incapable of forming multiple bonds because of the less effective pπ–pπ orbital overlap involved in the multiple bonds. However, bulky substituents can protect unsaturated bonds and stabilize compounds with formally sp-hybridized heavy group-14 atoms1,2: stable germanium2, tin3 and lead4 analogues of acetylene derivatives and a marginally stable tristannaallene5 have now been reported. However, no stable silicon compounds with formal sp-silicon atoms have been isolated. Evidence for the existence of a persistent disilaacetylene6 and trapping7 of transient 2-silaallenes and other X = Si = X′ type compounds (X, X′ = O, CR2, NR, and so on) are also known, but stable silicon compounds with formally sp-hybridized silicon atoms have not yet been isolated. Here we report the synthesis of a thermally stable, crystalline trisilaallene derivative containing a formally sp-hybridized silicon atom. We find that, in contrast to linear carbon allenes, the trisilaallene is significantly bent. The central silicon in the molecule is dynamically disordered, which we ascribe to ready rotation of the central silicon atom around the molecular axis.

A stable silicon-based allene analogue with a formally sp -hybridized silicon atom

Publisher Summary Remarkable progress of the chemistry of disilenes seems to encourage building up the modern general theory of bonding, structure, and reactions applied for the heavier main-group element chemistry in the near future; the general theory should include the theory for organic chemistry as a special case. This chapter discusses present advances in novel synthetic methods for various types of stable disilenes. Stable disilenes and other unsaturated compounds of heavier group-14 elements show unique spectroscopic properties that cannot be easily obtained from organic compounds. Bonding and structures of disilenes have also been discussed in the chapter. For a detailed discussion on the structure of disilenes, it is helpful to understand the intrinsic difference in the bonding between disilene and ethylene. The chapter highlights reactions and mechanisms of disilenes. During the past decade, many new types of reactions have been found for novel types of disilenes such as cyclic and bicyclic disilenes and conjugated tetrasiladienes. Mechanistic studies have recently been performed on several fundamental reactions of disilenes both theoretically and experimentally, and have greatly deepened the understanding of their reaction pathways, their potential energy surfaces, the factors determining the rates and stereochemistry, and so on.

Mitsuo Kira

Papers

The First Isolable Dialkylsilylene

A stable silicon-based allene analogue with a formally sp -hybridized silicon atom

Progress in the Chemistry of Stable Disilenes

An isolable dialkylsilylene and its derivatives. A step toward comprehension of heavy unsaturated bonds

Isolable silylene, disilenes, trisilaallene, and related compounds