(EDGP)s, and multiblock polymers with more than three blocks. In this methodology, two reaction steps, i.e., the polymer chain introduction and the regeneration of the same reaction site, are involved in each reaction sequence and iterated to construct the above-mentioned polymers. The following key building block is individually designed and used on the basis of their architectures: living chain-end-X-functionalized polymer (Scheme 1), living α-chain-end-((X)
2 or (X)4)-functionalized polymer (Schemes 4–6), living in-chain-X-functionalized diblock copolymer (Scheme 8), living in-chain-(X
1and X
2)-functionalized diblock copolymer (Scheme 9), living α-chain-end-(X
1and X
2)-functionalized polymer (Scheme 10), and living α-chain-end-X-functionalized diblock copolymer (Scheme 14). The
X, X1, and X
2functionalities are TBDMS ether, TMS ether, and TBDMS ether convertible to the BnBr or PA reaction site. In certain cases, the functional anions with TMS (X
1), TBDMS (X
2), and THP ethers (X
3) are employed (Schemes 2 and 3) instead of living functionalized polymers. One more important advantage of this methodology is that the use of complicated multistep selective reactions and reaction site with different reactivities generally required for macromolecular architecture synthesis are completely avoided because the polymer segment(s) are introduced one by one in each reaction step.
As often mentioned, all the iterative methodologies herein developed are basically the same in concept as the all-around iterative methodology mentioned in introduction. As the future synthetic potential, it is expected that different key building blocks can be employed together in the same methodology to result in the synthesis of more complex macromolecular architectures with mixed structures. Thus, synthetic limitation and difficulty of macromolecular architectures have been greatly surpassed with the progress of the iterative methodology.
Acknowledgments: This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society of the Promotion of Science (JP15K17907 for R. G.) and Mizuho Foundation for the Promotion of Sciences (2016−2018 for R. G.).
Author Contributions: Akira Hirao and Raita Goseki conceived the topic of the article; Raita Goseki, Shotaro Ito, Yuri Matsuo, and Tomoya Higashihara designed the overall structure of the article and reviewed the literature;
all authors co-wrote and edited the article.
Conflict of Interest: The authors declare no conflict of interest.
Scheme 15. Synthesis of multiblock copolymers by the iterative methodology using a living α-chain-end-X-functionalized polymer.
It is believed that the syntheses of triblock copolymers, triblock terpolymers except for the ABC type, and multiblock copolymers are difficult by the direct sequential block co- and terpolymerization using monomers with different reactivities. The methodology combining living sequential block copolymerization with a 1:1 addition reaction and the extended iterative methodology have demonstrated the useful means to successfully synthesize such (BP)s difficult by sequential block polymerization.
3. Concluding Remarks and Future Outlook
Throughout this article, we have demonstrated the successful development of a novel all-around iterative methodology for the syntheses of multicomponent (µ-SP)s, high generation (DHBP)s, (EDGP)s, and multiblock polymers with more than three blocks. In this methodology, two reaction steps, i.e., the polymer chain introduction and the regeneration of the same reaction site, are involved in each reaction sequence and iterated to construct the above-mentioned polymers. The following key building block is individually designed and used on the basis of their architectures: living chain-end-X-functionalized polymer (Scheme 1), living α-chain-end-((X)2 or (X)4)-functionalized polymer (Schemes 4–6), living in-chain-X-functionalized diblock copolymer (Scheme 8), living in-chain-(X1 and X2)-functionalized diblock copolymer (Scheme 9), living α-chain-end-(X1 and
X2)-functionalized polymer (Scheme10), and living α-chain-end-X-functionalized diblock copolymer (Scheme 14). The X, X1, and X2 functionalities are TBDMS ether, TMS ether, and TBDMS ether convertible to the BnBr or PA reaction site. In certain cases, the functional anions with TMS (X1), TBDMS (X2), and THP ethers (X3) are employed (Schemes2and3) instead of living functionalized polymers. One more important advantage of this methodology is that the use of complicated multistep selective reactions and reaction site with different reactivities generally required for macromolecular architecture synthesis are completely avoided because the polymer segment(s) are introduced one by one in each reaction step.
As often mentioned, all the iterative methodologies herein developed are basically the same in concept as the all-around iterative methodology mentioned in introduction. As the future synthetic potential, it is expected that different key building blocks can be employed together in the same methodology to result in the synthesis of more complex macromolecular architectures with mixed structures. Thus, synthetic limitation and difficulty of macromolecular architectures have been greatly surpassed with the progress of the iterative methodology.
Acknowledgments:This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society of the Promotion of Science (JP15K17907 for R. G.) and Mizuho Foundation for the Promotion of Sciences (2016−2018 for R. G.).
Author Contributions:Akira Hirao and Raita Goseki conceived the topic of the article; Raita Goseki, Shotaro Ito, Yuri Matsuo, and Tomoya Higashihara designed the overall structure of the article and reviewed the literature; all authors co-wrote and edited the article.
Conflicts of Interest:The authors declare no conflict of interest.
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