Dendrimers initiator core

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//. ... 

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

The fact that the dendritic shell can produce localized microenvironments has been used by Diederich et al. who developed water-soluble dendritic cyclophanes (dendrophanes) as models for globular proteins (Figure 16.13) [5, 170, 171], These dendrimers contain well-defined cyclophane recognition sites as initiator cores for the complexation of small aromatic guests [172-174] and steroids [174-176], Enlargement of the cyclophane core could be used as a tool to complex larger steroid molecules. 

The number of surface groups (Z), branch cells (BC) and molecular weights for a dendrimer series can be calculated with the math expressions shown below. These parameters, as well as hydrodynamic dimensions for the series [EDA](G 0-10)de dn-PAMAM-(NH2)n are presented in Figure 25.1. The experimental procedures are general for a wide range of alkylenediamine initiator cores (e.g., NH2-(CH2)-nNH2). Characterization data for dendri-PAMAMs derived from these cores are included, where n = 2, 3,4, 5, 6. 

A commercially available hyper-branched polyester derived from bis-MPA was used as the multifunctional initiating core for the ROP of e-CL and this led to the synthesis of hybrid dendritic linear star polymers. The reactivities of the chain-end hydroxymethyl groups in the dendrimer were significantly greater than in the isomeric hyper-branched case. 

Star poly(methylmethacrylates) were synthesized via atom transfer polymerization using a small carbosilane dendrimer functionalized with a tertiary bromide moiety as an initiator core (Figure 12)100,101. a convergent approach to star polymers with a carbosilane dendrimer core was described in a report by Allgaier and coworkers102, in which living poly(butadienyl-lithium) arms were coupled with various SiCl-terminated carbosilane den-drimers. Utilizing smaller dendrimers with lower functionality was found to yield nearly ideal results in terms of substitution and polydispersity. 

Choose a reaction sequence so that each of the Nc reactive sites adds a reactant B, possessing new Nb (Nb > 1) reactive sites (thereby introducing multiplicity) to obtain dendrimer D0 of generation 0. With ammonia as initiator core, a p-alanine derivative (Nb = 2) might be chosen as reactant. 

Just as ancestry and lineage can be traced in higher organisms, a molecular-level parallel is observed when these transformations are considered in the context of (1) stored molecular information at each dendrimer stage and (2) molecular information transferred (transcribed) to the progeny dendrimer surface at each transformation. For example, if the initiator core is thought of as a primitive abiotic gene [6-8] molecular details are sequentially transcribed and stored to produce interior and ultimately exterior (surface) features which are characteristic of that dendrimer family (phenotype). 

Dendrimers possess three distinguishing architectural features (1) an initiator-core region, (2) interior zones containing cascading tiers of branch cells with radial... 

The initiator core (CD) may be as small as an atom or as large as a molecule. It may be homogeneous with the other dendrimer components or it may contain special design features (e.g., metal atoms, chromophores, etc.) that differentiate it from the dendrimer interior thus making it heterogeneous. 

The symmetry properties of the interior branch cells can dramatically affect the physical and perhaps chemical properties of a dendrimer. For example, symmetrical interior and surface branch cells, such as those in Figs. 11 and 15, exhibit identical connectivity paths from the initiator core to the termini. The equal radii place the terminal groups on a sphere or a segment of a sphere, thus making these dendrimers ideal models of functionalized nanoscopic spheres. 

Alternatively, dendrimers may be synthesized directly by our original II Divergent Core Proliferation method. This method may involve the exponential covalent assembly of monomer units around a multi-valent core to produce branch cells in situ or it may involve the direct use of pre-formed branch cell reagents. In either case the resulting covalent structure consists of precise numbers of dendrons organized around the initiator core. 

Structural verification is based on viscosity data (indicating Einstein spheroid characteristics), elemental analyses, light-scattering experiments, and direct observation of individual dendrimers by electron microscopy. The self-limiting molar-mass ranges correspond to approximately 38 branch-cell propagations (three tiers) around a germanium initiator core. Furthermore, analysis of molecular... 

Alternatively, initiator cores have been tethered to solid support (Merrifield resins [83]). Using protein synthesis procedures that avoid the workup difficulties involved in handling large excesses of reagents, one can readily control dendrimer growth. Unfortunately, only noncleavable linkers have been examined to date with these PAMAM dendrimers. The use of initiator cores possessing cleavable linkers should make dendrimer synthesis and isolation very facile. This was demonstrated for mono-dendrons derived from poly(lysines) as reported by Tam et al. [105-107] (Fig. 26). 

A variety of mono-, di-, tetra-, and polydendron PAMAMs have been synthesized from simple amines as well as linear polyamine cores [2, 83, 124]. Linear poly(ethylenimines) with core multiplicities (Nc of ca. 300-400) have been shown to produce high-aspect ratio, rod-like dendrimers at generation 3 or 4 [2]. Their length is determined by the degree of polymerization (n) of the initiator core and their diameter is derived from the number of generations (see Scheme 3). 

A unique class of structures referred to as iptycenes has recently been reported by Hart et al. [129-131]. These architectures represent the first examples of all-hydrocarbon dendrimer prototypes. Starting with either benzene or triptycene as the initiator core, branch junctures were constructed in situ by Diels-Alder adduct formation to give the heptaiptycene (A) and the nonaiptycene (tri-... 

Hall et al. [139] produced a low-molar-mass series of polyarylamine dendrimers by using a protection/deprotection scheme involving the protected, branch cell reagent 2,4-dinitrofluorobenzene. Using various anilines as initiator cores, the... 

Several dendrimer series in which ionic sites were incorporated at both the initiator core as well as the branch cell juncture have been reported by Engel et al. [140-142]. Dendrimers derived from the initiator cores 74, 75a-c and 76 have been synthesized by the reiterative sequence described in Scheme 27. These polycations possess some of the highest Nc and Nb multiples reported for any dendrimer series. 

These methods were used extensively for structure verification of dendrimers prepared by the divergent initiator core method such as Starburst PAMAM [124] poly(ether) [82], and poly(ethylenimine) dendrimers [2], as well as poly(siloxane), poly(phosphonium), poly (ary lalkyl)ether, poly(arylene) and poly(arylester) dendrimers. In many cases, small-molecule model systems were used for process optimization, defect identification, and stoichiometry studies. 

Presently, defects of the type A and B are the only serious side-reactions encountered in the synthesis of Starburst PAMAM dendrimers (generation = 0-10) by the divergent initiator core method . Defects of the types C, D and E have been minimized to under 3-5 mole% per generation. Polydispersities (Mw/Mn) of 1.0007 have been routinely observed for Starburst PAMAM dendrimers (generation = 4 NH3 core) which has an empirical formula of C465H933N187093 and a calculated molecular weight of 10632, as verified by electrospray mass spectroscopy [169]. 

Within this context and by using Figs. 5 and 46 as conceptual references, let us examine the degree of mimicry offered by the more simplistic dendrimers in the transfer of genealogical information. First, as one assembles poly(amidoamine) (PAMAM) dendrimer (see Sect. 5.2.3.1) around an ammonia initiator core, it becomes readily apparent that important molecular ( genetic ) information about the dimensions, shape and multiplicity of NH3 are being transferred from the initiator core to the surface of the PAMAM dendrimers as a function of generation. This may be visualized as shown in Fig. 47. In this context, the dendrimer initiator... 

Fig. 47. Molecular information (genealogy) transferred from the initiator core (NH3) to the surface of Starburst dendrimers as a function of generation, using a divergent synthesis protocol...

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