Core molecule

The molar mass distribution of hyperbranched polymers is, therefore, always larger than diat of titeir linear homologues and tends toward infinity when conversion becomes close to 1. The use of a B3, comonomer, acting as a chain limiter and core molecule, helps in reducing polydispersity and controlling the molar mass of the final polymer.197... 

This aliphatic hyperbranched polyester is prepared by the bulk polycondensation of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as AB2 monomer and 1,1,1-tris(hydroxymethyl)propane (TMP) as B3 core molecule, according to a procedure... 

The first true dendrimers were the polyamidoamines (PAMAMs). They are also known as starburst dendrimers, and the term starburst is a trademark of the Dow Chemical Company, who have commercialized these materials for a range of applications. These dendrimers use ammonia as the core molecule, and this is reacted with methyl acrylate in the presence of methanol, after which ethylenediamine is added. This is shown in Scheme 9.2. 

The generated quinone methide intermediates, during the disassembly, are highly reactive electrophiles and rapidly react with any available nucleophile (methanol or tetrabutylammonium hydroxide under organic solvent conditions). We could not isolate any significant amount of material that derived from the core molecule, probably due to generation of a mixture of compounds by the addition of different nucleophiles to the quinone methide. This molecule acts as an amplifier of a cleavage... 

The addition of ammonia to excess methyl acrylate (a linear monomer), followed by amidation with excess ethylenediamine afforded the resultant cascade molecule, and thus Tomalia [37] created the commercially available PAMAM starburst series of dendrimers (2, Fig. 2). Related core molecules such as ethylenediamine and aminoalcohols and other functionalizable groups such as thiol moieties were used to prepare similar dendrimers [38]. This methodology is applicable to most primary amines, resulting in a 1 —> 2 branching pattern. Recently, examples of related Si-, [39] P-, [40] and metallo systems [41], which follow this linear monomer protocol have been reported. 

The first example of a grafted dendrimer carrying non-racemic amino acid moieties at the surface has been reported in 1991 byNewkome et al. [20]. A four-directional core molecule, which they prepared from pentaerythrol [21], has been elongated with the branching units fra[carboxyethoxymethyl]amino-... 

Fig. 23. Formation of various 2nd-generation dendrimers or dendritic compounds using branch building blocks of (S)- or (.Reconfiguration at the benzylic centers [90], Three of the eight combinations tested by us led to dendrimer formation (products 63,65,66) while five halted at the stage of the doubly coupled products 64,70-73, cf. Fig. 22. Core molecule 69 has less hindered OH groups and is, of course, not an isomer of 54 and ent-54...

Almost 20 years later, Robert Zwanzig [4] followed a perturbative route to free energy calculations, showing how physical properties of a hard-core molecule change upon adding a rudimentary form of an attractive potential. The high-temperature... 

Concept A new approach to the rational design of enzyme inhibitors has emerged in the last ten to fifteen years that incorporates a substrate (or transition state) analog "core" molecule with additional binding determinants spanning beyond the immediate... 

Addition of a core molecule, or any other comonomers (chain extending AB, terminating A or B). 

In reality, the polydispersity of the hyperbranched polymer even in the absence of core is lower than that predicted for the ideal case. Cyclization and steric hindrance during polymerization can be the reasons. Polydispersity can also be lowered intentionally, for instance, by introduction of core molecules or by programmed addition of the monomers. 

Suzuki et al. [14] reported the Pd-catalyzed ring-opening polymerization of a cyclic carbamate in the presence of an initiator, which also acts as a core molecule, to afford a hyperbranched polyamine. The polymerization was proposed to be an in situ multibranching process, wherein the number of propagating chain ends increase with the progress of the polymerization. 

Its design versatility, as generic dendrons may be prepared to be used later as building blocks in conjunction with other reactive molecules, or coupled to a multifunctional core to afford functional dendrimers, dendritic-linear hybrids, dendronized polymers, etc. This may be a particularly significant advantage if the coupled reactive or core molecule is itself sensitive to the reaction conditions used in the multiple steps of the iterative synthesis of a dendrimer. 

Synthesis of GAGs requires the sequential attachment of an jV-acetyl amino sugar or uronic acid to the protein core molecule (Figure 9.2), each step being catalysed by a... 

The synthesis of hyperbranched polymers can often be simplified compared to that of dendrimers as it does not require the use of protection/deprotection steps. This is due to the fact that hyperbranched polymers are allowed to contain some linearly incorporated A B monomers. The most common synthesis route follows a one-pot procedure where A B monomers are condensed in the presence of a catalyst. Another method using a core molecule and an A B monomer has also been described. 

In agreement with Flory s predictions, hyperbranched polymers based on A,jB monomers reported in the literature exhibit a broad molecular weight distribution (typically 2-5 or more). The polydispersity of a hyperbranched polymer is due to the statistical growth process. A strategy to overcome this disadvantage is to add a By-functional core molecule, or a chain terminator, which Hmits the polydispersity and also provides a tool to control the molecular weight of the final polymer. The concept of copolymerizing an A2B monomer with a B3 functional core molecule was first introduced by Hult et al. [62] and more recently also utilized by Feast and Stainton [63] and Moore and Bharathi [64]. 

Feast and Stainton [63] reported on the synthesis of aromatic hyperbranched polyesters from 5-(2-hydroxyethoxy)isophthalate copolymerized with 1,3,5-benzenetricarboxylate (core molecule) as a moderator of the molecular weight. The degree of branching was found to be 0.60-0.67 as determined by C-NMR. Apparent molecular weights (M ) were found to be 5-36 kDa according to SEC characterization using linear polystyrene standards. 

As we conjectured in the introduction, the fundamental role of topology in this approach to entangled polymer dynamics would indicate that changes to the topology of the molecules themselves would radically affect the dynamic response of the melts. In fact rheological data on monodisperse star-branched polymers, in which a number of anionically-polymerised arms are coupled by a multifunctional core molecule, pre-dated the first application of tube theory in the presence of branching [22]. Just the addition of one branch point per molecule has a remarkable effect, as may be seen by comparing the dissipative moduli of comparable linear and star polymer melts in Fig. 5. 

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