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. 

Hyperbranched step-growth polymers, 14 Hyperbranched structures, polyimide, 284-287... 

Tomalia, D.A. Genealogically Directed Synthesis Starbust/Cascade Dendrimers and Hyperbranched Structures. 165, (1993). 

Reinhoudt et al. [174] reported the self-assembly of an AB2 type of monomer 83 with two Pd centers. Labile coordinating ligand, acetonitrile, when removed by heating from a solution of nitromethane in vacuo initiated an intramolecular coordination of benzylnitrile group with the tridentate Pd-centers leading to a hyperbranched structure. Coordination of the benzylnitrile group was moni-... 

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]. 

As described earlier in this book, the dendritic architecture is perhaps one of the most pervasive topologies observed at the macro and micro-dimensional length scales (i.e. jum-m). At the nanoscale (molecular) level there are relatively few natural examples of this architecture. Most notable are probably the glycogen and amylopectin hyperbranched structures that Nature uses for energy storage. 

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]. 

The main advantage for using hyperbranched structures may rest in their rheology (film formation from high-solids systems) and in some cases added value in properties (hardness), but deeper knowledge should be accumulated before a more comprehensive evaluation can be made. 

Tomalia, D. A. and Durst, H. D. Geneologically directed synthesis Starburst/ Cascade dendrimers and hyperbranched structures , Topics Curr. Chem., 165, 193— 313(1993). 

Aminopolysaccharides 9 and 10 can be expected to have unusual properties because of the hyperbranched structure. The thermal properties of 9 and 10 were examined (Figure 2) by thermogravimetric analysis (TGA). The TGA curve of 9 shows that decomposition of the tosyl group started at 166°C and showed a 45% weight loss up to 337°C. The residual material exhibited thermal resistance above 337°C, but a second weight loss occurred at... 

To the best of our knowledge, only one other example of a carboxylic acid functionalized hyperbranched structure is known in the literature, and this concerns a polyamide [19]. The synthesis reported starts from A2 (aminofunctional) and B3 (carboxylic acid functional) units and leads to low molecular weight products due to low conversion in dilute solution. These conditions were mandatory to prevent gelation [20]. Two different approaches to the synthesis of carboxylic acid functional hyperbranched polyesteramides are presented below [21]. 

Holter D, Burgath A, Frey H (1997) Acta Polym 48 30 Holter D, Frey H (1997) Acta Polym 48 298. Degree of branching is often used as a descriptor for hyperbranched structures see, e.g., Malmstrom E, Hult A (1997) JMS Rev Macromol Chem Phys 37 555 Dusek K (1997) TRIP 8 268... 

All films were touch-dried after one passage through the Mini-Cure (i.e. no oxygen inhibition was noticed). Raman spectra of the resins before and after curing showed no detectable amount of residual unsaturation (i.e. less than 5%). This showed that all acrylate groups are accessible to polymerization and not caged in the hyperbranched structure. 

Genealogically Directed Synthesis Starbust/Cascade Dendrimers and Hyperbranched Structures... 

Interesting comparisons have been made 17 between dendritic and the hyperbranched structures the thermal properties (glass transition temperature and thermogravimetric analysis) were independent of architecture and their solubilities were comparable, but greater than that shown for linear counterparts. 

Structural analysis of the homopolymers by spectroscopic methods confirmed that the diynes had undergone [2 + 2 + 2] polycyclotrimerizations by forming new benzene rings from their acetylenic triple bonds. The ratio of the 1,2,4- to 1,3,5-isomers of the trisubstituted benzene rings was estimated to be 2.2 1. Careful evaluation of the 111 NMR spectra unveiled that the number of terminal triple bonds in the final hb-PAs was much smaller than that in an ideal hyperbranched structure produced by the diyne polycyclotrimer-ization. This result suggests that intra-sphere ring formation might have been involved in the cyclotrimerization polymerization. 

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