“Living” Polymer Beads in Supercritical CO₂

Abstract

Recently, controlled radical polymerization techniques have received considerable interest because of the ability to synthesize species exhibiting precise molecular architecture. 1 The (reversible addition fragmentation chain transfer) RAFT technique allows the formation of polymers with a very narrow molecular weight distribution from a wide range of monomers. 2,3 In addition, the resulting polymer is free from undesirable metal catalysts that are present following other controlled polymerization techniques (e.g., atom transfer radical polymerization). 4 Other authors have shown that RAFT-mediated polymerizations can occur in supercritical CO 2 , and product with low polydispersity (PDI), albeit low conversion, was generally observed. [5][6][7][8] Hydrophilic/CO 2 -philic stabilizers were synthesized by RAFT 9,10 and in one case used in the dispersion polymerization of poly-(hydroxyethyl methacrylate). 9 However, the RAFT-terminated stabilizer was not used to control the kinetics of the polymerization, and low PDIs were not observed. Our work takes a significant step further, yielding high conversion, high molecular weight polymer and living microparticles.Supercritical fluids have emerged as acceptable replacements for organic solvents. scCO 2 is nontoxic, nonflammable, and inert and has an easily accessible critical point (T c ) 31.0 °C, P c ) 7.38 MPa). 11 Since the first successful radical dispersion polymerization in scCO 2 by DeSimone and co-workers, 12 numerous authors have reported successful polymerizations of various vinyl monomers. [13][14][15][16][17] While careful control of the solvent density can lead to polymer particles with well-defined morphology, simultaneous control over the polymer molecular weight remains elusive.

Dispersion polymerization in non-polar solvent: Evolution toward emerging applications

“…Finally, due to the high mass transfer properties of scCO 2 (plasticization), controlled FRP can be easily run in scCO 2 . [116,117] Over the last two decades, because of its unique properties, two main challenges remained of interest for the dispersion polymerization in scCO 2 . The first one was to develop new stabilizers to replace the commonly used but extremely expensive, fluorinated and silicone-based stabilizers.…”

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

“…4,16 All CLRP systems developed to date operate on the same basic principle of propagating radicals alternating between active and dormant states. 7 NMP, [17][18][19][20][21][22][23] ATRP, [24][25][26][27] and RAFT 28,29 have all now been implemented as heterogeneous CLRP in scCO 2 .…”

“…28 Polymerizations were carried out at 65 C and 28 MPa using the RAFT agent a-cyanobenzyl dithionaphthalate (1 in Fig. 8), AIBN, and the colloidal stabilizer poly(dimethylsiloxane monomethacrylate) (PDMS-MMA).…”

“…The control/livingness was very good despite the polymeric RAFT agent being present from the outset of the polymerization. 28,29 Choe and coworkers 113 have reported that RAFT dispersion polymerization of S in ethanol at 70 C results in phase separation (a pink coagulant at the bottom of the reactor) when the RAFT agent is charged at the beginning of the polymerization. However, use of the two-stage method, [114][115][116] whereby the RAFT agent is added after the particle nucleation has been completed, results in good control/livingness without phase separation as well as a narrow particle size distribution.…”

“…However, use of the two-stage method, [114][115][116] whereby the RAFT agent is added after the particle nucleation has been completed, results in good control/livingness without phase separation as well as a narrow particle size distribution. It cannot be stated with certainty whether the scCO 2 /MMA RAFT dispersion polymerization 28,29 proceeds satisfactorily without application of the two-stage method due to the characteristic features of scCO 2 (e.g., low viscosity, high diffusivity, etc.). An alternative explanation may be that the value of J crit is markedly lower in the scCO 2 /MMA than the ethanol/S system [the lower the value of J crit , the smaller is the effect of the RAFT agent in delaying the nucleation stage (refer Morphology section)].…”