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Controlled/Living Radical Polymerization: Features, Developments, and Perspectives

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Controlled/Living Radical Polymerization: Features, Developments, and Perspectives www.aladdin-e.com Address:800 S Wineville Avenue, Ontario, CA 91761,USA Website:www.aladdin-e.com Email USA: [email protected] Email EU: [email protected] Email Asia Pacific: [email protected] Controlled/living radical polymerization: Features, developments, and perspectives Abstract Recent mechanistic developments in the field of controlled/living radical polymerization (CRP) are reviewed. Particular emphasis is placed on structure–reactivity correlations and ‘‘rules’’ for catalyst selection in atom transfer radical polymerization (ATRP), for chain transfer agent selection in reversible addition-fragmentation chain transfer (RAFT) polymerization, and for the selection of an appropriate mediating agent in stable free radical polymerization (SFRP), including organic and transition metal persistent radicals. Novel methods of fine tuning initiation, activation, and deactivation processes for all techniques are discussed, including activators regenerated by electron transfer (ARGET) and initiators for continuous activator regeneration (ICAR) ATRP, whereby Cu catalyst concentrations in ATRP can be lowered to just 10 ppm. Progress made in each technique related to the synthesis of both high and low molecular weight polymers, end functional polymers, block copolymers, expanding the range of polymerizable monomers, synthesis of hybrid materials, environmental issues, and polymerization in aqueous media is thoroughly discussed and compared. Contents 1. Introduction ............................................................................................................................................................... 94 2. Typical features of radical polymerization (RP) ...................................................................................................... 95 3. New controlled/living radical polymerization (CRP) ............................................................................................... 97 3.1. Fundamentals of CRP .................................................................................................................................... 97 3.2. Similarities and differences between RP and CRP ........................................................................................ 98 4. CRP by stable free radical polymerization (SFRP) ................................................................................................. 98 4.1. Basic mechanism ............................................................................................................................................. 98 4.2. Mediating species/initiation systems ............................................................................................................... 99 4.2.1. Nitroxides as persistent radicals ........................................................................................................ 99 94 www.aladdin-e.com 4.2.2. Alkoxyamine unimolecular initiators ............................................................................................... 100 4.2.3. Other organic mediators .................................................................................................................. 100 4.2.4. Metal mediated polymerization ....................................................................................................... 101 4.3. Re-evaluation of the persistent radical effect ............................................................................................... 101 4.4. Additional considerations ............................................................................................................................. 102 5. Principles of atom transfer radical polymerization (ATRP) .................................................................................. 103 5.1. Mechanism and components ........................................................................................................................ 103 5.2. Structure–reactivity relationships .................................................................................................................. 104 5.2.1. Understanding the ATRP equilibrium ............................................................................................ 104 5.2.2. Activation/deactivation structure–reactivity correlations ................................................................ 106 5.3. Initiation systems .......................................................................................................................................... 108 5.3.1. Normal/reverse/simultaneous reverse and normal initiation ........................................................... 110 5.3.2. Activators generated by electron transfer (AGET) ......................................................................... 110 5.3.3. Hybrid and bimetallic catalytic systems .......................................................................................... 111 5.3.4. Initiators for continuous activator regeneration (ICAR) ................................................................ 111 5.3.5. Activators regenerated by electron transfer (ARGET) ................................................................... 112 5.3.6. Inherent differences/advantages of each system .............................................................................. 112 5.4. Optimization of ATRP with respect to side reactions ................................................................................. 112 5.4.1. Avoiding side reactions .................................................................................................................... 113 5.4.2. Exploiting ‘‘side reactions’’ .............................................................................................................. 115 6. Degenerative transfer processes .............................................................................................................................. 116 6.1. Degenerative transfer by atom or group transfer ......................................................................................... 117 6.2. DT via addition–fragmentation with unsaturated polymethacrylates .......................................................... 118 6.3. DT with dithioesters and related compounds .............................................................................................. 118 6.3.1. Basic mechanism .............................................................................................................................. 118 6.3.2. Structure–reactivity relationships .................................................................................................... 119 6.3.3. Retardation and termination in RAFT ........................................................................................... 120 6.4. Additional considerations ............................................................................................................................. 121 7. Summary and comparison of SFRP, ATRP and DT processes ............................................................................ 122 7.1. SFRP. ............................................................................................................................................................ 123 7.2. ATRP ............................................................................................................................................................ 124 7.3. RAFT and other DT processes .................................................................................................................... 124 7.4. Recent progress in SFRP, ATRP and DT ................................................................................................... 125 8. Selected examples of controlled polymer architectures by CRP ............................................................................ 127 8.1. Topology ....................................................................................................................................................... 127 8.2. Composition .................................................................................................................................................. 129 8.3. Functionality ................................................................................................................................................. 130 9. Material applications .............................................................................................................................................. 130 10. Future perspectives .................................................................................................................................................. 132 Acknowledgements ................................................................................................................................................... 133 References ................................................................................................................................................................ 133 1. Introduction technology. Additionally, his quantitative descrip- tions of ion pairing phenomena and electron The discovery of living anionic polymerization by transfer processes greatly benefited physical organic Michael Szwarc had a tremendous effect on poly- chemistry [3,12]. mer science [1,2]. His work facilitated major The elimination of transfer and termination developments in both synthetic polymer chemistry reactions from chain growth polymerization formed
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