Poly , hyperbranched structure

Kim and Webster [57] were the first to show that trifunctional benzene-based monomers can also be used to synthesize poly(phenylene)s, in this case hyperbranched structures 31 based on 1,3,5-trisubstituled benzene cores. They self-condensed l,3-dibromophenyl-5-boronic acid leading to the formation of soluble, hyperbranched PPP-type macromolecule 31. 

Well-defined complicated macromolecular structures require complex synthetic procedures/techniques and characterization methods. Recently, several approaches leading to hyperbranched structures have been developed and will be the focus of this section. The preparation of hyperbranched poly(siloxysilane) has been reported [198] and is based on methylvinyl-bis(dimethyl siloxysilane), an A2B type monomer, and a progressive hydrosi-lylation reaction with platinum catalysts. An appropriate hydrosilylation reaction on the peripheral - SiH groups led to the introduction of polymeric chain (PIB, PEO) or functional groups (epoxy, - NH2) [199]. 

Over the past decade, literally dozens of new AB2-type monomers have been reported leading to an enormously diverse array of hyperbranched structures. Some general types include poly(phenylenes) obtained by Suzuki-coupling [54, 55], poly(phenylacetylenes prepared by Heck-reaction [58], polycarbosilanes, polycarbosiloxanes [59], and polysiloxysilanes by hydrosilylation [60], poly(ether ketones) by nucleophilic aromatic substitution [61] and polyesters [62] or polyethers by polycondensations [63] or by ring opening [64]. 

Again, with the help of spectral analyses, the hyperbranched structures of the poly(aroylarylene)s were confirmed. Thanks to the 1,3,5-regioselectivity of the cyclotrimerization reaction, the NMR spectra of the hb-PAAs were... 

Because of the high functionality of glycerol it is not surprising that a significant amount of research has focused on new polymeric materials that incorporate glycerol (Fig. 8.4). This includes polyesters, poly ethers, and polycarbonates (Ray and Grinstaff, 2003 Fu et al., 2003). In addition to the linear and network polymers, dendritic or hyperbranched structures have been investigated extensively (Malmstrom et al., 1995 Hawker et al., 1997 Jayaraman and Frechet, 1998 Bosnian et al., 1999 Sunder et al., 1999, 2000 ... 

The self-condensing copper-catalyzed polymerization of macromonomer of poly(tBA) with a reactive C—Br bond (H-6) affords hyperbranched or highly branched poly(tBA).447 Copolymerization of H-1 and TV-cyclohexylmaleimide induced alternating and self-condensing vinyl polymerization.448 The residual C—Cl bond was further employed for the copper-catalyzed radical homopolymerization of styrene to give star polymers with hyperbranched structures. Hyperbranched polymers of H-1 further serve as a complex multifunctionalized macroinitiator for the copper-catalyzed polymerization of a functional monomer with polar chromophores to yield possible second-order nonlinear optical materials.325... 

A combination of other polymerization pathways also results in some hyperbranched structures. Multifunctional hydroxyl groups in polyglycerol with hyperbranched structure, prepared by anionic polymerization of glycidol, were esterified with 2-bro-moisobutyroyl bromide and then employed as a hyperbranched multifunctional macroinitiator for the copper-catalyzed radical polymerization of MA to give products with 45—55 poly(MA) arms.449... 

In a previous work, Cheng et al. [322] performed the same synthesis but without any ligand excess. The resulting macromonomer was similar to that described in Scheme 69 but with a chain-end bromine atom. This macromonomer was polymerized by the ATRP process, leading to a hyper-grafted polymer. The Mark-Houwink coefficient was 0.47, which characterized the hyperbranched structure. Hydrolysis of such a polymer led to the corresponding poly(acrylic acid). Similarly, Hua et al. [323] performed the synthesis of brushlike poly(acrylic acid). 

Photoswitchable supramolecular dendrimer-like structures were constructed starting from azobenzene-based and CD-based building blocks. For example a bis-azobenzene with a dipyridyl linker trans-Azo dimer) and a p-CD trimer formed an hyperbranched structure whose shape could be controlled by light (Fig. 11). AFM evidenced branched structures of several microns turning to disordered particles upon UV irradiation. Moreover a supramolecular dendrimer was recently built up in water starting from an hydrophilic hyperbranched polyglycerol with a-CD apex (CD-g-HPG) and an hydrophobic hyperbranched poly(3-ethyl-3-oxetane-methanol) with azobenzene apex (AZO-g-HPBO) (Fig. 12). The two components in 1 1 molar ratio selfassembled into a Janus-like dendrimer (JHBP, HPBO-6-HPG) of diameter ca. 5.1 nm, due to inclusion complexation of the azobenzene apex into the CD cavity of the partner... 

Other groups reported in a similar way the formation of metal-nanoparticles such as Ag, Au, Cu, Pt, and Pd stabilized through different kinds of hyperbranched polymers including poly(ethyleneimides) [67,68], poly(amidoamines) (structurally similar to PAMAM dentrimers) [69], poly(amine-esters) [70], and aromatic poly(amides) [71-73] and evaluated their activity towards various chemical reactions. 

Li Z, Li J. Control of hyperbranched structure of polycaprolactone/poly(ethylene glycol) polyurethane block copolymers by glycerol and their hydrogels for potential cell delivery. J Phys Chem B 2013 117(47) 14763-74. 

Scheme 24 Synthesis of hb poly(vinyl sulfide) (hb-PVS) with a sketch of the hyperbranched structure showing all potential units that may appear in the final polymer, depending on the molar ratio of A2 and B3. Reproduced with permission from [234]...

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). 

Experimental data on the solution properties and melt rheology of highly branched structures are scarcely found in the literature. This might be because of the structural nonuniformity of hyperbranched polymers, which makes it difficult to obtain reliable data. Because of the purely statistical nature of the poly-... 

A variation of the aromatic polyester structure was utilized by Hawker et al. when they described hyperbranched poly(ethylene glycol)s and investigated their use as polyelectrolyte media [76]. The highly branched structure implies that no crystallization can occur. Linear poly(ethylene) glycols usually crystallize, which has a detrimental effect on their use as polyelectrolyte media. 

The use of diisoprop anolamine in the synthesis of the hyperbranched poly-esteramide resins has led to defined molecules according to the predicted structures (Fig. 7) as confirmed by MALDI-TOF (Matrix Assisted Laser Desorption Ionization - Time Of Flight) and ESI (Electro Spray Ionization) mass spectra... 

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