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] [EtSO4], 1-heptyl-3-methylimidazolium bromide [C(7)mim] [Br], 1-hexyl-3-methylimidazolium tetrafluoroborate [C(6)mim] [BF4], 1-hexyl-3-methylimidazolium hexafluorophosphate [C(6)mim] [PF6], and 1-octyl-3-methylimidazolium hexafluorophosphate [C(8)mini] [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, M-n approximate to 480). Polymerization of MA, EGA, and PEGA in [C(6)mim] [BF4], [C(6)mim] [PF6], and [C(8)mim] [PF6] proceeded in a controlled manner, as evidenced by kinetic studies, narrow molecular weight distributions (D approximate to 1.1), and quantitative conversions (>99%) within 30 min. MALDI-ToF-MS and H-1 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] [BF4] and [C(6)mim] [PF6] yielded polymers with bimodal molecular weight distribution (potentially due to poor solubility), polymerization of the same monomer in [C(8)mim] [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 (M-n approximate to 4500-40 000 g mol(-1)) furnishing poly(acrylates) with low dispersities in all cases (D approximate to 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).