Star-shaped architecture synthesis

Problem 12.13 The combination of RAFT chemistry and the FIDA cydoadditicm provides a simple and synthetically nondemanding pathway to well-de ned macromolecular star-shaped architectures. In support of this contention describe the synthesis of polystyrene (PSt) star polymers with up to 4 arms using the aforesaid two different RAFT end groups, namely, diethoxy-phosphoryldithioformate and pyridin-2-yldithioformate, and HDA coupling reactions. 

Another significant application of the ADMET polymerisation relates to the preparation of star-shaped polymers, which are branched macromolecules in which several linear polymer chains are attached to a unique branching point or core [48]. Montero de Espinosa, Winkler and Meier [49] described an ADMET approach to obtain those architectures (three- and four-arm) using small tri-acrylates and tetra-acrylates. More recently, Unverferth and Meier [50] reported the synthesis of well-defined star-shaped polymers via a head-to-tail ADMET polymerisation whereby di(trimethylolpropane)tetra-acrylate (four-arm) and dipentaerythritol hexa-acrylate (six-arm) served as core units, and fatty acid-derived 10-undecenyl acrylate as asymmetric a,(0-diene monomers. In this case, star-shaped polymers containing arms of 10 or 20 monomer units with an a,(0-unsaturated ester backbone and their subsequent post-polymerisation via a base-catalysed Thia-Michael addition were prepared. 

The versatility of the concept was also demonstrated for the synthesis of Frechet-type dendrimers by sequential etherification and CuAAC reactions starting from a triphenol core, using AB2 and CD2 types of monomers (Antoni et al., 2007). Because of the simplicity of this strategy, a wide variety of other macromolecular architectures, including star-shaped polymers and hyperbranched polymers, could be easily synthesized with unique characteristics that are tuned by the nature of the monomers. 

On the other hand, when aiming at the synthesis of star-shaped polymers via thermally and photochemically induced radical thiol-ene click chemistry, serious limitations of side reactions and incomplete conversions have been clearly demonstrated by the groups of Du Prez and Bamer-Kowollik. From this study, it was concluded that this type of thiol-ene reactions cannot be applied for the well-defined synthesis of complex polymer architectures in which polymer-polymer conjugation is needed (Koo et al, 2010). 

The so-called core-first method has been extensively used for the synthesis of various kind of star-shaped polymers, water-soluble or not. The functionalizable outer end of the branches offers an original access to a large scope of macromolecular architecture star-shaped polymers with copolymeric branches, functional star-shaped polymer networks, etc. The in-out method combines the advantage of the arm-first method and the core-first method allowing good control of the structure and the presence of functionalizable outer end of the branches. Different unsaturated compounds have been used to generate the core, such as DVB and DPE, the latter compound giving access to star-shaped terpolymers. 

Several others methods also exhibiting a living character are now being used for the construction of branched macromolecular architectures. One of them, group transfer polymerization, is briefly mentioned in the text. Special efforts are being made presently to apply living radical polymerization to the synthesis of star-shaped polymers. The performance of the different methods has been compared recently [94]. 

As discussed in Chapter 7, the absence of termination in living polymerization permits the synthesis of unusual and unique block polymers — star- and comb-shaped polymers. Living polymerization can also be employed to introduce a variety of desired functional groups at one or both ends of polymeric chains both in homo- and block polymers. In particular, living polymerization techniques provide the synthetic polymer chemist with a vital and versatile tool to control the architecture of a polymer complicated macromolecules can be synthesized to meet the rigid specification imposed by a scientific or technological demand. 

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