Core monomers

Cores Monomers Branch Cells Dendrons Dendrimers... 

Hyperbranched polymers can be synthesized in several different ways, the most commonly used being classical condensation reactions. These reactions are made either in bulk or in solution where the A,jB monomers are condensed by themselves or in combination with a By core monomer. The use of a By core monomer improves the control over the molecular weight and dispersity of the hyperbranched polymer. Hyperbranched polymers can also be synthesized by self-condensing vinyl polymerization using vinyl-functional monomers. The introduction of this approach has greatly increased the number of possible monomers that can be used for this type of polymer. 

Numerical calculations inspired in the ZK method for stars have also been applied for the description of the dynamics of model dendrimers. La Ferla [232] used a freely-rotating model, including a topology-dependence stiffness parameter and preaveraged HI. With this model, he obtained a complex analytical expression for the mean size. Cai and Chen [233] used a Gaussian model without HI and performed a detailed analysis of the relaxation motions. They investigated the diffusion of the center of mass, the relaxation of the center of mass position relative to the core monomer, and also the rotational and internal modes. 

Figure 11.6 Snapshots of calculated structures of dendrimers. For clarity, internal and terminal monomers are shaded darker and lighter, respectively, and the size of the core monomer has been made larger than the other monomers. Reprinted from Giupponi and Buzza (2005). Copyright 2005 American Instimte of Physics.

Microcapsules containing polymer and pigment were prepared in [299] by dispersing a viscous suspension of pigment and oil-soluble shell monomer forming o/w emulsions. Subsequently, a water-soluble shell monomer was added to the emulsion droplets, encapsulating them via interfacial polycondensation. These microcapsules were then heated for free radical polymerisation of the core monomers. It has been shown that polyvinyl alcohol (PVOH) used as stabiliser reacts with the oil-soluble shell monomers. The decrease of PVOH concentration as result of this interaction leads to coalescence of the particles and to the increase of their equilibrium particle size, however, methods are proposed to prevent the depletion of PVOH. 

Vita et al. reported the synthesis and structural characterization of a main-chain hquid crystal polymer composed of a 1,2,4-oxadiazole-based bent-core repeating unit. This provided the first experimental evidence of cybotactic order in the N phase of an LC polymer based on a bent-core monomer (55 in Figure 15 2014AML91). 

Figure 15 Main chain of a liquid crystal polymer based on a bent-core monomer.

Example 15 StiUe Crosscoupling Polymerization of 4,4-Dilluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) Core Monomer (25) with Bis-EDOT and Bis-EDTT Derivatives (26, 27) to give P29a,b ... 

Crosslinking can be used to overcome the aforementioned obstacles, as in the case of the calamitic networks in which the orientation and polarity are preserved. First experiments were performed by swelling a calamitic LCE with a low-molar-mass BCLC [182, 183]. Up to 35 mol% of this material could be incorporated into the network. The resulting gel had a flexoelectric cmistant e-i of 20 nC/m. This is one-third of the value of the low-molar-mass BCLC, which corresponds to the volume fraction of the BCLC. Only recently has a pure bent-core elastomer (BCFLCE) been made [139, 169]. Following the synthetic route of Finkelmann, an oriented, transparent nematic elastomer sample could be obtained. As a polysiloxane backbone was used, the Tg was close to room temperature. The flexoelectric constant of 40 nC/m is somewhat smaller than for the pure bend-core monomer, but larger than the value obtained for the swollen elastomer. 

G1 preforms can also be fabricated by an interfacial-gel polymerization technique. The principle is basically the same as that of the photo-copolymerization method discussed above, except for the mechanism that forms the initial gel phase. In this method, the core solution (the monomer) is placed in a polymer tube rather than in a glass tube. The gel phase in the photo-copolymerization method is referred to as a prepolymer with a conversion of less than 100%, whereas in this method the gel phase comprises the polymer layer on the inner wall of the tube swollen by the core monomer. The reaction is carried out under UV irradiation or heating. 

Dendrimers are polymers forming a rigid star-like branched structure. Monomer units of this starshaped structure could vary widely from amino acids to polyesters, which also determines its characteristics. In addition breakthrough synthetic techniques such as lego (Brauge et al. 2001) and click (Wu et al. 2004) chemistries have advanced both the efficiency and innovation of possible dendrimer structures. Different combinations of the core, monomer units, and surface functionality have resulted in the emergence of more than hundreds of dendrimers. Table 85.1 lists major classes of dendrimers based on their chemical structure and also their use in drug delivery applications. 

The acetate method in combination with silylated carboxylic groups also proved to be a versatile approach to the preparation of hb copoly(ester amide)s [21-25]. As illustrated in Formula 11.2, ab2, abs, ab4, and abg monomers came into play. Several polycondensations were performed in the presence of acetylated a2 or a4 core monomers (see Formula 11.2). However, the acetate method is not restricted to fully aromatic monomers and polymers, and several semi-aromatic monomers were polymerized by this approach [26]. The polycondensation of monomer (e). Formula 11.2, is remarkable, because large amounts of cyclics were detected (see Sect. 11.7). Furthermore, several LC copolyesters were prepared from monomers (a)-(c). Formula 11.3 [27, 28]. 

However, side reactions, such as formation of ether groups or cychzation, were not investigated. Turner et al. [11] reported on the Bu2Sn(OAc)2-catalyzed polycondensation of 5-(2 -hydroxyethoxy) isophthalic acid in bulk at 190 °C. Dimethyl isophthalate served as core monomer in several experiments. Polycondensation promoted by bis(cyclohexyl)carbodiimide (DCC) were reported for the aromatic monomer (f), Formula 11.1 [35]. The mild reaction conditions prevented transesterification, but only low molar mass polyesters (Mw < 17 kDa) were obtained. Similar molar masses were achieved by Voit et al. [36] for carbodiimide promoted polycondensations of the triazene monomers (b) and (c), Formula 11.4. Somewhat more successful were DCC-activated polycondensations of trifunctional oligo(e-caprolactone)s such as (d), Formula 11.4, reported by Hedrick et al. [37, 38]. Syntheses of LC polyesters from monomers (e), Formula 11.4, and isomers were achieved by means of thionyl chloride and pyridine [39]. 

Consequently, the limitation of the chain growth (plateau of Mn) was attributed to the influence of ring formation consuming the a functions. Those authors also performed polycondensations in the presence of b2 or bs core monomers and observed that these core monomers did not prevent cyclization. 

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