A Perspective on the History and Current Opportunities of Aqueous RAFT Polymerization

Abstract

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven itself as a powerful polymerization technique affording facile control of molecular weight, molecular weight distribution, architecture, and chain end groups - while maintaining a high level of tolerance for solvent and monomer functional groups. RAFT is highly suited to water as a polymerization solvent, with aqueous RAFT now utilized for applications such as controlled synthesis of ultra-high molecular weight polymers, polymerization induced self-assembly, and biocompatible polymerizations, among others. Water as a solvent represents a non-toxic, cheap, and environmentally friendly alternative to organic solvents traditionally utilized for polymerizations. This, coupled with the benefits of RAFT polymerization, makes for a powerful combination in polymer science. This perspective provides a historical account of the initial developments of aqueous RAFT polymerization at the University of Southern Mississippi from the McCormick Research Group, details practical considerations for conducting aqueous RAFT polymerizations, and highlights some of the recent advances aqueous RAFT polymerization can provide. Finally, some of the future opportunities that this versatile polymerization technique in an aqueous environment can offer are discussed, and it is anticipated that the aqueous RAFT polymerization field will continue to realize these, and other exciting opportunities into the future.

Keywords: aqueous RAFT; biocompatible polymerization; bioconjugation; controlled radical polymerization; photopolymerization; polymerization induced self-assembly; water.

Figure 1.

Common classes of CTAs that can be utilized to synthesize well-defined polymers and block copolymers using aqueous RAFT polymerization.

Figure 2.

a) Molecular structure of CTP and CTP-AMPS_n macro-CTAs and pseudo first-order rate plots for the hydrolysis of b) CTP, c) CTP-AMPS₃₈, and d) CTP-AMPS₉ at 70 °C. Reprinted with permission.^[6b] Copyright 2004, American Chemical Society.

Figure 3.

a) Fraction of CTA remaining as a function of time at pH 5.5. Reproduced with permission.^[6c] Copyright 2005, Wiley-VCH. b) Spectroscopic stability measurements of EDMAT in aqueous solutions of varying pH values. A₀ is the solution absorbance before irradiation, and A is the solution absorbance at predetermined time intervals. Reproduced with permission.^[7] Copyright 2009, American Chemical Society.

Figure 4.

a) Quantum dot photoinitiated aqueous RAFT polymerization, demonstrating light source control of b) monomer conversion reaction kinetics and c) control of polymer molecular weight as evidenced by representative gel permeation chromatography (GPC) traces. Reproduced with permission.^[14b] Copyright 2020, American Chemical Society.

Figure 5.

Photo-induced electron/energy transfer RAFT polymerization using a photocatalyst via either a) an oxidative catalyst pathway or, b) a reductive catalytic pathway (tertiary amine (NR₃) as reducing agent). Reproduced with permission.^[32] Copyright 2018, American Chemical Society.

Figure 6.

a) Schematic of the flow reactor setup when coupling two reactors to achieve block copolymers without intermittent purification. b) Representative TEM micrographs of pDMA-b-p(DAAm-co-DMA) nanoparticles in a one-step synthesis chain extending pDMA with DP200, DP400, and DP600 at 17.5 wt% solid content; b) w = branched worms, hbw = highly branched worms, v = vesicles. Reproduced with permission.^[37] Copyright 2019, American Chemical Society.

Figure 7.

a) Synthetic strategy toward aqueous RAFT of single stranded DNA functionalized polymers and nano-objects. b–e) Atomic force microscopy (AFM) images recorded by liquid AFM after aqueous RAFT dispersion polymerization from CTA modified DNA using a [DAAm]/[DMA] ratio of 80:20. Different degrees of polymerization were targeted: b) DP = 50, c) 100, d) 200, e) 250. The magnified images in b) and c) are 2.5 times magnified with respect to the original image. Reproduced with permission.^[43] Copyright 2020, Wiley-VCH.

Figure 8.

Schematic illustration of the one-pot aqueous photo-PISA approach to proapoptotic peptide brush polymer nanoparticles. Reproduced with permission.^[48] Copyright 2020, Wiley-VCH.

Figure 9.

Schematic of aqueous SI-PET-RAFT for the preparation of antifogging polymer brush films demonstrated here with superhydrophilic poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (pMETAC) polymer brushes, with fogging observed for the plain glass slide (left) and maintained transparency of the pMETAC polymer brush-modified slide (right). Reproduced with permission.^[51] Copyright 2021, American Chemical Society.