Rapid And Well-Controlled Aqueous RAFT Polymerization Of Primary Amine Based Monomers Under Visible Light Radiation At Ambient Temperture

Rapid and well-controlled RAFT polymerization of the primary amine based monomers in aqueous solution under visible light radiation at 25℃was performed. To this end, N-(2-aminoethyl)methacrylamide hydrochloride (AEMA) and its corresponding long hydrophobic alkyl spacer monomer, N-(6-aminohexyl) methacrylamide hydrochloride (AHMA) were synthesized. The pH-dependent stability of 4-cyanopentanoic acid dithiobenzoate (CTP) in the presence of AEMA at 25℃was studied. The polymerization was performed in aqueous solution at pH 2.0 under visible light radiation at 25℃. The effect of ion pairing of these ionized monomers on the kinetics of RAFT polymerization was studied. A periodic light-on-off polymerization was performed under such conditions. The results demonstrated that CTP was stable in aqueous solution at pH 2.0, leading to the well-controlled RAFT polymerization under visible light radiation at 25℃. More importantly, this polymerization proceeded more rapidly in the presence of less polar alcohols due to the enhanced ion pairing of these monomers. For example,78% AEMA monomer was polymerized at in shortly 80 min and gave well-defined PAEMA at [AEMA]0:[CTP]0:[TPO]o=100:1:0.3 in 50 wt% butoxyethanol/water at pH 2.0 under visible light radiation at 25℃. As compared to AEMA, the longer hydrophobic alkyl spacer of AHMA led to shorter initialization period and slower propagating process. The more rapid polymerization upon increasing alcohol fraction or solution ion strength also confirmed the accelerating effect of this ion pairing. Moreover, this polymerization was immediately started or stopped upon periodically turning on or off this visible light. This demonstrated that the visible light exerted control over this aqueous RAFT polymerization not only through light-triggered initiating, but also through activating the fragmentation of "dormant" intermediate radicals. In addition, the CTP chain-ends were rapidly removed in alkali solution at pH 10 and 22℃, shortly in 30 min.