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

Herein we report the photoinduced living radical polymerization of acrylates in a variety of ionic liquids (ILs). 1-Ethyl-3-methylimidazolium ethyl sulfate [emim][EtSO 4 ], 1-heptyl-3-methylimidazolium bromide [C 7 mim][Br], 1-hexyl-3-methylimidazolium tetrafluoroborate [C 6 mim][BF 4 ], 1-hexyl-3-methylimidazolium hexafluorophosphate [C 6 mim][PF 6 ], and 1-octyl-3-methylimidazolium hexafluorophosphate [C 8 mim][PF 6 ] 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, M n ≈ 480). Polymerization of MA, EGA, and PEGA in [C 6 mim][BF 4 ], [C 6 mim][PF 6 ], and [C 8 mim][PF 6 ] 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 1 H 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 [C 6 mim][BF 4 ] and [C 6 mim][PF 6 ] yielded polymers with bimodal molecular weight distribution (potentially due to poor solubility), polymerization of the same monomer in [C 8 mim][PF 6 ] 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 ( M n ≈ 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).