Nitroxide-Mediated Radical Dispersion Polymerization of Styrene in Supercritical Carbon Dioxide Using a Poly(dimethylsiloxane-b-methyl methacrylate) Stabilizer

Abstract

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.

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 .…”

“…All our heterogeneous polymerizations in scCO 2 were performed at 110 C at [ 30 MPa, unless otherwise stated. [18][19][20][21][22][23] One of the fundamental differences between a CLRP and a conventional radical polymerization is the slow build up in MW due to the participation of dormant species and intermittent reversible activation. This has allowed us to use NMP to visually determine J crit , 19,22 where polymer chains become insoluble in the continuous homogeneous phase, and thus precipitate.…”

“…At 40% (w/v) S, 19 and using [TEMPO]/[AIBN] ¼ 1.82, J crit of 28 (M n ¼ 2900 g/mol) was obtained at $8% conversion, while at a higher monomer loading of 70% (w/v), 22 J crit of 114 (M n ¼ 11,850 g/mol) was obtained at $26% conversion, using a similar ratio of TIPNO to AIBN (110 C). It is important to note that J crit is a solubility parameter, and it is independent of both initiator and nitroxide, unless J crit is so low that end group effects cannot be neglected.…”

“…5). The increase in the number of low MW chains in S polymerizations in dispersed, solution, and bulk systems has been shown to be mainly due to chain transfer to monomer 19,21 (or presumably to the S-dimer 21 ). The utility of this technique was further illustrated by recovery of PS as a white powder at high conversion (92%) after 72 h by venting (depressurizing the reactor) of CO 2 .…”

“…[18][19][20] The first dispersion NMP in scCO 2 used PS-b-PDMS alkoxyamine generated in situ to achieve simultaneous initiation and stabilization (inistab 105 ). 18 This approach was earlier used successfully in dispersion ATRP of MMA in scCO 2 by Okubo and coworkers.…”