Open-Vessel and Scalable Synthesis of Linear and Branched Poly(meth)acrylic Acid via Light-Mediated Atom Transfer Radical Polymerization in Water

Poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA) are synthesized on a large scale by conventional free radical polymerization (FRP). The access to architectural diversity by FRP is limited but can be addressed by reversible deactivation radical polymerization (RDRP), including atom transfer radical polymerization (ATRP). ATRP of methacrylic acid (MAA) and acrylic acid (AA) is challenging due to lactonization via the displacement of halide (X) end groups by penultimate carboxylate anions (CO₂^–) and the loss of chain-end functionality. Despite the successful polymerization of MAA or AA ((M)AA) using various RDRP methods, the oxygen-tolerant photo-ATRP of (M)AA has not yet been investigated. Herein, photo-ATRP of (M)AA in open vials was enabled by adding sodium pyruvate (SP) or pyruvic acid (PA) to the polymerization mixture. Photoirradiation of SP/PA generated radicals and enhanced the rate of polymerization at ambient temperature, which diminished lactonization reactions. This method allowed the synthesis of PMAA or PAA (P(M)AA) at low pH (1 to 3.2) with relatively low dispersity (Đ = 1.10–1.38 under optimized conditions) and good agreement between the theoretical molecular weight (M_n,theo) and the absolute molecular weight (M_n,abs). Photo-ATRP allowed the synthesis of PMAA in ≤1 h and also the synthesis of branched PAA by copolymerization with α-haloacrylic acids. Additionally, successful grafting of MAA from poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-co-CTFE) was achieved in dispersed aqueous media.