Diels-Alder Click Reaction for the Synthesis of Star-Shaped Polymers

3 Diels-Alder Click Reaction for the Synthesis of Star-Shaped Polymers 

In addition to the broad application of the CuAAC reaction, other types of highly efficient click reactions were used for the synthesis of star-shaped polymers. Tunca and coworkers used the Diels-Alder click reaction for the preparation of 3-arm star-shaped polymers using furan protected maleimide end-fiinctionalized PEO, PMMA or PtBuA and trianthracene functional linking agents (Dag et al., 2008). However, the resulting yields are comprised between 82 and 93% depending on the coupled polymer, the reaction conditions are more drastic than those used in CuAAC. 

The same group also reported on the synthesis of multiarm star block copolymers by using Diels-Alder cycloaddition reactions (Dag et al., 2009). First, an a-anthracene-end functionalized PS (PS-anthr) and furan-protected maleimide-end-functionaUzed polymers, including PMMA and PtBuA, were prepared via ATRP. The maleimide functionalities were protected as they can contribute to the copolymerization with MMA or tBuA. Moreover, the polymerization temperature was kept below 60 °C to prevent deprotection during the polymerization. In the next step, a 33-arm anthracene-end functionalized (PS) star polymer was obtained using PS-anthr as macroinitiator and divinyl benzene as crosslinker. These star polymers were then reacted with the unprotected maleimide end-functionalized PMMA or PtBuA to give multiarm star block copolymers via Diels-Alder click reaction. The efficiencies were foimd to be 96 and 88%, respectively. 

This result proves that well-defined structures with low degree of heterogeneity of the multiarm star-shaped polymers can be synthesized. Moreover, the method reported herein can also provide a synthetic pathway for the introduction of block copolymers synthesized via different polymerization routes (RAFT, ROP, etc.) onto the anthracene-end-functionalized multiarm star-shaped polymers. Although the Diels-Alder cycloaddition between anthracene and maleimide derivatives has proven to provide good results in the formation of complex architectures, the major drawback of this method remains the requirement of high temperature and relatively long reaction times. 

When combining the hetero-Diels-Alder strategy with the CuAAC reaction, star block copolymers have been obtained (SinnweU et al., 2008b). First, a-diene-co-alkyne 

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