Abstract
Organic superbases reacted with alkyl iodides (R-I) to reversibly generate the corresponding alkyl radicals (R • ). Via this reaction, organic superbases were utilized as new and highly efficient organic catalysts in living radical polymerization. The superbase catalysts included guanidines, aminophosphines and phosphazenes. Low-polydispersity polymers (Mw/Mn = 1.1-1.4) were obtained up to high conversions (e.g., 80%) in reasonably short times (3-12 h) at mild temperatures (60-80 °C) for methyl methacrylate, styrene and several functional methacrylates. The high polymerization rate and good monomer versatility are attractive features of these superbase catalysts.
Highlights
Organic catalysts, in lieu of metal-based catalysts, have gained increasing attention, because many are environmentally benign, easy to handle and attractive alternatives in organic syntheses [1,2,3].Metal-free catalytic processes are often practical and serve to broaden synthetic applications
Living radical polymerization (LRP) has become increasingly important in polymer chemistry, because it allows for the synthesis of well-defined polymers with narrow molecular weight distributions [10,11,12,13,14,15,16,17,18,19,20]
We developed two mechanistically different systems, referred to as reversible chain transfer-catalyzed polymerization (RTCP) [25,26,27,28,29,30,31] and reversible coordination-mediated polymerization (RCMP) [31,32,33,34,35]
Summary
In lieu of metal-based catalysts, have gained increasing attention, because many are environmentally benign, easy to handle and attractive alternatives in organic syntheses [1,2,3]. Recent important studies in this field have involved the use of organic superbases, such as guanidines and phosphazenes, as catalysts. They exhibit high reactivity and high selectivity in many reactions, including fine organic transformations [4,5,6,7], such as asymmetric Michael addition, esterification and nitroaldol reactions, as well as fine polymer syntheses, such as ring-opening polymerization (ROP) [8]. RCMP involves reversible coordination of the catalyst to Polymer-I to generate Polymer and the catalyst-iodine complex (Scheme 1b). We demonstrate a unique reaction of alkyl iodides (R–I) with superbases to generate carbon-centered radicals (R) and the application of the superbases as highly active catalysts in RCMP. We studied the polymerizations of methyl methacrylate (MMA), styrene (St) and three functional methacrylates at 60–80 °C
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