Nitroxide-mediated controlled/living dispersion polymerization of styrene in supercritical carbon dioxide (scCO2) has been performed successfully to high conversion using N-tert-N-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) and a poly(dimethylsiloxane-b-methyl methacrylate) stabilizer at 110 °C. The molecular weight distributions were narrow (M w/M n = 1.12−1.43), and the number-average molecular weight (M n) values agreed well with theory. A large excess of free SG1 was required to obtain satisfactory control in the dispersion polymerization, possibly due to SG1 partitioning. The critical degree of polymerization at which the polymer precipitates (J crit) was determined as 28 by visual observation using a novel approach. Polymerizations conducted in solution (toluene) proceeded at a similar rate (∼20% faster) to those in scCO2. The number of chains increased with conversion in both dispersion and solution, with the greater increase being observed in the dispersion system.

Methyl methacrylate (MMA) polymerizations have been conducted in the presence of large excesses of N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl) nitroxide] (SG1) at 1108C. It is demonstrated that such a protocol does not improve control/livingness in the nitroxide mediated polymerization (NMP) of this monomer, instead substantial levels of disproportionation between the nitroxide and propagating radical (PMMA l ) results. The extent of the disproportionation reaction increased with the SG1 concentration, eventually becoming the sole end forming event. Significant disproportionation between SG1 and PMMA l was also observed at substantially lower temperatures (458C) in the presence of large excesses of SG1. V V C 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45:

Supercritical carbon dioxide (scCO 2 ) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO 2 is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (J crit ). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well-defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well-defined colloidal particles being formed. In recent years, nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO 2 . This Highlight reviews this recent body of work, and describes the unique characteristics of scCO 2 that allows composite particle formation of unique morphology to be achieved.