Synthesis of Star Polymers and Dendrimers

As we have noted earlier, TEC reactions compare favorably with CuAAC reactions, including the use of readily available starting materials and catalysts. However, unlike CuAAC, the TEC reaction is free of metals and can be performed under photochemical initiation. The TEC is also very fast (often quantitative reaction is obseved within a period of seconds at ambient temperature) compared to extended reaction times and elevated temperatures occasionally required for the CuAAC reaction (Binder and Sachsenhofer, 2007). The use of TEC reaction has therefore attracted attention as a means of preparing star polymers and more eomplex polymers, such as dendrimers and other sterieaUy hindered structures. 

Various commercially available thiols react photochemically with tetravinylsUane to give the corresponding tetrasubstituted thioether compounds  

Vinyl-terminated dendrimers up to the fth generation were thus prepared in excellent yields (78-94%). 

Since polymers synthesized by RAFT polymerization technique bear thiocarbonylthio end groups that can be reduced to thiol funetional species under facile conditions, RAFT-synthesized polymers can serve as masked macromoleeular terminal thiol-containing materials capable of undergoing TEC reactions. IMmary and secondary amines are convenient and ef cient reducing agents for the thiocarbonylthio end groups. Importantly, the thiol-ene reaction also proceeds 

Employing a similar approach as above, thiol-terminated RAFT polymers have also been reacted with a variety of low and high molecular weight acrylate species for ef cient end functionalization (Spruell et al., 2009) and immobilized onto ene-decorated microspheres of poly(divinyl benzene) (Goldmann et al., 2009). Considering the abundance of commercially available activated alkene substrates amenable to nucleophihc addition (e.g., acrylates and maleimides) and the simplicity-cum-robusrness of the aforesaid approach, the RAFT/thiol-ene combination can be expected to remain as a valuable tool for macromolecular synthesis (Sumerlin and Vogt, 2010). 

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