Dendrimers limiting structures

If the polydispersity index (PDI) has a value of unity, the substance is designated as monodisperse. Monodispersity is considered to be a property of the casca-danes (defect-free dendritic molecules cf. Section 1.4) and almost perfect dendrimers. Since these compounds are synthesised via an iterative approach, monodispersity has so far generally been limited to lower generations. Should it prove possible to repeatedly remove all reactants and by-products of the individual synthetic steps during the construction of a dendrimer, then structurally perfect dendrimers will result. 

A way to narrow the MWD and to approach the structure of dendrimers is the addition of a small fraction of a/-functional initiator, to inimers [40,71]. In this process the obtainable degree of polymerization is limited by the ratio of inimer to initiator. It can be conducted in two ways (i) inimer molecules can be added so slowly to the initiator solution that they can only react with the initiator molecules or with the already formed macromolecules, but not with each other (semi-batch process). Thus, each macromolecule generated in such a process will contain one initiator core but no vinyl group. Then, the polydispersity index is quite low and decreases with / M /Mn l-i-l//. (ii) Alternatively, initiator and monomer molecules can be mixed instantaneously (batch process). Here, the normal SCVP process and the process shown above compete and both kinds of macromolecules will be formed. For this process the polydispersity index also decreases with/,but is higher than for the semi-batch process, M /Mn=Pn//. ... 

The recent development of structurally controlled dendrimers has led to the development of a wide range of new functional macromolecules. These dendrimers were first applied in the fields of chemistry, including catalysis, pharmacology, and materials science [23-26]. More recently there have been several reports of dendrimers having electro active, photoactive, and recognition elements [27-34]. Important applications in photonics have recently been exploited, though the number of reports is still limited. 

Initial efforts gave rise to well-characterized dendritic macromolecules, but applications remained limited because of the lack of specific functionalities. An exponential increase of publication volume observed for about 15 years testified the growing interest for dendrimers and has led to versatile and powerful iterative methodologies for systematically and expeditiously accessing complex dendritic structures. The perfect control of tridimensional parameters (size, shape, geometry) and the covalent introduction of functionalities in the core, the branches, or the high number extremities, or by physical encapsulation in the microenvironment created by cavities confer such desired properties as solubility, and hydrophilic/hydrophobic balance. Thus, creativity has allowed these structures to become integrated with nearly all contemporary scientific disciplines. 

Since they were first mentioned in 1978, the concept of cascade or dendritic structures has witnessed a meteoric rise. On the one hand, thanks to their aesthetic beauty, and on the other hand, because of their broad applicability, dendrimers are in considerable demand. As the concept is not limited to one class of substances and, furthermore, allows a simple access on several routes, there is -besides the researchers who started dendrimer chemistry - a growing number of research groups which are dendritically expanding their special areas of interest in different ways and describing new or varying already existing properties. 

Dendrimers with terminal functional groups represent mode compact precursors that are spherical and almost monodisperse, with reactive groups placed on their periphery. Their synthesis, structure and properties have been reviewed in monographs and review articles often together with hyperbranched polymers (cf., e.g. [15-20]), as well as in this book. Application of dendrimers as precursors for conventional materials is limited at this time by their relatively high cost. 

The second method referred to as the direct covalent bond formation method , produces semi-controlled, partial shell filled structures. It involves the reaction of a limited amount of nucleophilic dendrimer core reagent with an excess of... 

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