Hyperbranched materials monomer synthesis

The polymerization using sugar oxazoline monomers is not limited to the synthesis of linear aminopolysaccharides as described above and can be extended to formation of a hyperbranched material. Synthesis of hy-perbranched aminopolysaccharide was achieved by acid-catalyzed polymerization of a sugar oxazoline monomer, 8, having two hydroxy groups at position 3 and 4, which can be considered as an AB2 monomer (Scheme 9) [12]. This is the first example of the synthesis of a hyper-... 

Hyperbranched polymers can be prepared by a variety of techniques, including the polycondensation of AB monomers as originally described by Flory [113], the reaction of A2 + B3 monomers, and self-condensing vinyl polymerization [139-141]. The first report [142] of using click chemistry in the synthesis of hyperbranched materials appeared at about the same time as the initial report for dendrimers prepared using CuAAC however, but much fewer examples have been reported that describe hyperbranched materials involving click chemistry. Nevertheless, these polymers represent an important class of materials, and both CuAAC [142-147] and thiol-ene [148] chemistry have found their way into the hyperbranched hterature. 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

Very recently, highly regular, highly controlled, dense branching has been developed. The resulting dendrimers often have a spherical shape with special interior and surface properties. The synthesis and properties of dendrimers has been reviewed (see e.g. G.R. Newkome et al. Dendritic Molecules , VCH, 1996). In this series, a chapter deals with the molecular dimensions of dendrimers and with dendrimer-polymer hybrids. One possible development of such materials may be in the fields of biochemistry and biomaterials. The less perfect hyper-branched polymers synthesized from A2B-type monomers offer a real hope for large scale commercialization. A review of the present status of research on hyperbranched polymers is included. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

Many methods have been reported to synthesize hyperbranched polymers. These materials were first reported in the late 1980s and early 1990s by Odian and Tomalia [9], Kim and Webster [10], and Hawker and Frechet [11]. As early as 1952, Hory actually developed a model for the polymerization of AB -type monomers and the branched structures that would result, identified as random AB polycondensates [46], Condensation step-growth polymerization is likely the most commonly used approach however, it is not the only method reported for the synthesis of statistically branched dendritic polymers chain growth and ringopening polymerization methods have also been applied. 

Scheme 22.13 Two examples of multifunctional monomers applied for the synthesis of a hyperbranched polymer 76 and insoluble microporous materials 77.

In 1997 Khauss et al. presented an elegant synthesis to star polymers with a hyperbranched core and (hyp>er)branched materials by slow addition of a suitable termimer (or a mixture of monomer and termination agent) to living, carbanionic polymer chains ( convergent anionic polymerization ).By this technique, a variety of different macromolecular architectures can be synthesized with moderate polydispersities. 

Bao, He and Li [14] also prepared the AB2 monomer illustrated in Scheme 6.4, and undertook a systematic study on the synthesis of hyperbranched polyesters using Ti(OBu)4, Sb203 and Zn(OAc)2 as transesterification catalysts. The final materials had Mn values of 11-60 kDa. 

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