Monodisperse dendritic polymers

A useful toy theoretical model which captures the essential features of self-entangled dendritic polymers is the monodisperse Cayley tree in which each chain segment branches with a fixed functionality z at each of its ends, except those at the extremity of the molecule (see Eig. 13). Smaller versions of these structures, too low in molecular weight to be entangled, have been synthesised and are usually referred-to as dendrimers [47]. 

Dendrimers (from the Greek dendron tree) are highly branched, monodisperse (ideally) polymers, based around a central core moiety, from which multiple wedge-shaped dendritic fragments or dendrons spread out [163, 164] (see Fig. 15). 

CONTENTS Introduction to the Series An Editor s Foreword, Albert Padwa, Emory University. Preface, George R. Newkome, University of South Florida. A Review of Dendritic Macromolecules, George R. Newkome and Charles N. Moorefield, University of South Florida. Stiff Dendritic Macromolecules Based on Phenylacetylenes, Zhifu Xu, Benjamin Kyan, and Jeffery S. Moore, The University of Michigan. Preparation and Properties of Monodisperse Aromatic Dendritic Macromolecules, Thomas X. Neenan, Timothy M. Miller, Elizabeth W. Kwock, and Harvey E. Bair, AT T Bell Laboratories. High-Spin Polyarylmethyl Polyradicals, Andrzej Rajca, University of Nebraska. A Systematic Nomenclature for Cascade (Dendritic) Polymers, Gregory R. Baker and James K. Young, University of South Florida. Index. 

Since the production of perfect monodisperse dendrimers is too complicated and expensive, several researchers at DuPont Experimental Station working on dendritic polymers as rheology control agents and as spherical multifunctional initiators developed a route for a one-step synthesis of dendritic polymers. These... 

In recent years, the use of dendritic systems in different biomedical applications has grown exponentially. One reason for this can be found in their main characteristics, such as a well-defined size and structure, flexibility, monodispersity and a multivalent molecular surface. " Among the major possibilities, dendritic polymers have been studied as deliveiy carriers of drugs and for the development of synthetic vaccines, ... 

Physical properties of highly branched polymers are different from their linear analogs and have many potential applications in medicine and nanoengineering. Dendritic polymers with monodispersed composition and weU-defined tree-like structures are even attractive. Efficient reactions are required, and thiol-ene was used by Hawker and coworkers to prepare dendrimers starting from a tris-alkene core 2,4,6-triallyloxy-l,3,5-triazine. 1-Thioglycerol was used to introduce two hydroxy groups via UV-irradiated thiol-ene reaction, which was carried out at room temperature for 30 min. Due to the efficient nature of thiol-ene chemistry and less byproducts, hydroxy-terminated dendrimers... 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

The core first method has been applied to prepare four-arm star PMMA. In this case selective degradation of the core allowed unambiguous proof of the star structure. However, the MWD is a little too large to claim that only four-arm star polymers are present [81], Comb PMMAs with randomly placed branches have been prepared by anionic copolymerization of MMA and monodisperse PMMA macromonomers [82], A thorough dilute solution characterization revealed monodisperse samples with 2 to 13 branches. A certain polydispersity of the number of branches has to be expected. This was not detected because the branch length was very short relative to the length of the backbone [83]. Recently, PMMA stars (with 6 and 12 arms) have been prepared from dendritic... 

The near monodispersity typically associated with dendrimers prepared by both the divergent or convergent growth approaches presents a number of interesting characterization traits when compared to traditional narrow polydisper-sity linear polymers. These features are further enhanced by the high degree of symmetry normally associated with dendritic macromolecules and permit an unprecedented amount of structural information to be obtained using standard characterization techniques. 

Perhaps the most viable short-term use for dendritic macromolecules lies in their use as novel catalytic systems since it offers the possibility to combine the activity of small molecule catalysts with the isolation benefits of crosslinked polymeric systems. These potential advantages are intimately connected with the ability to control the number and nature of the surface functional groups. Unlike linear or crosslinked polymers where catalytic sites may be buried within the random coil structure, all the catalytic sites can be precisely located at the chain ends, or periphery, of the dendrimer. This maximizes the activity of each individual catalytic site and leads to activities approaching small molecule systems. However the well defined and monodisperse size of dendrimers permits their easy separation by ultrafiltration and leads to the recovery of catalyst-free products. The first examples of such dendrimer catalysts have recently been reported... 

In our opinion an adequate distinction between ideal, defect-free dendritic molecules, for which we propose the term cascadanef, and the more or less defect-free (monodisperse) dendrimers , and polydisperse, imperfect hyper-branched compounds has so far been lacking in the area between polymer chemistry and small-molecule chemistry. Other reasons for such a distinction are because highly pure compounds have al-... 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

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