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Synthesis of Well-Defined Poly(acrylates) in Ionic Liquids via Copper(II)-Mediated Photoinduced Living Radical Polymerization

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journal contribution
posted on 11.08.2015, 00:00 by Athina Anastasaki, Vasiliki Nikolaou, Gabit Nurumbetov, Nghia P. Truong, George S. Pappas, Nikolaos G. Engelis, John F. Quinn, Michael R. Whittaker, Thomas P. Davis, David M. Haddleton
Herein we report the photoinduced living radical polymerization of acrylates in a variety of ionic liquids (ILs). 1-Ethyl-3-methylimidazolium ethyl sulfate [emim]­[EtSO4], 1-heptyl-3-methylimidazolium bromide [C7mim]­[Br], 1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim]­[BF4], 1-hexyl-3-methylimidazolium hexafluorophosphate [C6mim]­[PF6], and 1-octyl-3-methylimidazolium hexafluorophosphate [C8mim]­[PF6] were employed as solvents for the homopolymerization of a variety of acrylates including methyl acrylate (MA), n-butyl acrylate (n-BA), ethylene glycol methyl ether acrylate (EGA), and poly­(ethylene glycol) methyl ether acrylate (PEGA, Mn ≈ 480). Polymerization of MA, EGA, and PEGA in [C6mim]­[BF4], [C6mim]­[PF6], and [C8mim]­[PF6] proceeded in a controlled manner, as evidenced by kinetic studies, narrow molecular weight distributions (Đ ≈ 1.1), and quantitative conversions (>99%) within 30 min. MALDI-ToF-MS and 1H NMR confirmed very high end-group fidelity, which was further exemplified by in situ chain extensions and block copolymerizations, yielding well-defined block copolymers in a quantitative manner. While polymerization of n-BA in [C6mim]­[BF4] and [C6mim]­[PF6] yielded polymers with bimodal molecular weight distribution (potentially due to poor solubility), polymerization of the same monomer in [C8mim]­[PF6] was well-controlled yielding materials with a monomodal polymer peak distribution and low dispersity. Interestingly, all polymerizations in ILs experienced a significant acceleration on the rate of polymerization without compromising the end-group fidelity, as opposed to the slower rates observed when DMSO was used as the solvent. The versatility of the approach was also demonstrated by polymerization of MA to a number of chain lengths (Mn ≈ 4500–40 000 g mol–1) furnishing poly­(acrylates) with low dispersities in all cases (Đ ≈ 1.1). Importantly, extraction of the obtained polymer with toluene allowed the IL/catalyst solution to be reused as the solvent for further polymerizations without affecting the living nature of the polymerization. Moreover, the polymer extracted into the toluene (copper-free) can be used directly for post-polymerization modifications (e.g., click reactions).