Synthesis of well-defined poly(acrylates) in ionic liquids via copper(II)-mediated photoinduced living radical polymerization
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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 (D ≈ 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 (Ð ≈ 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).