RAFT-Mediated Emulsion Polymerization of Styrene with Low Reactive Xanthate Agents: Microemulsion-like Behavior

Abstract

Xanthates ([1-(O-ethylxanthyl)ethyl]benzene (CTA1) and [1-(O-trifluoroethylxanthyl)ethyl]benzene (CTA2)) have the capacity to control the molecular weight distribution in emulsion polymerizations to produce very small nanoparticles below 20 nm. We form stable translucent polystyrene latexes using surfactant (sodium dodecyl sulfate, SDS) and a small amount of pentanol as cosurfactant. The high CTA concentration results in a greater retardation in rate until consumption of all the RAFT agent. With an increase in CTA1 the particle size decreases from 38 to 8 nm and the particle number concentration N c increases from 2 Â 10 18 to 2 Â 10 20 particles/L. Although an increase in N c should in principle lead to a faster rate of polymerization, we observe a greater retardation in rate with increasing CTA. The higher C tr,RAFT of CTA2 results in a greater initial retardation until consumption of all the RAFT agent and particle diameters lower than 5 nm and at high concentrations of CTA2 diameters that are not measurable. Kinetic simulations solving the Smith-Ewart equations explain the anomaly between R • (formed from the fragmentation of the R group from the RAFT agent) acting to nucleate micelles and terminate radicals within particles. The small and mobile R • radicals can exit particles, re-enter micelles or other particles, re-exit until they either nucleate micelles, or terminate with propagating polymeric chains. This process of exit and re-entry is similar to limit 3 in a conventional emulsion polymerization. The higher micelle nucleation rate through initiation within micelles by R • radicals results in smaller and a greater number of particles. Exit is the dominant mechanism for greater nucleation and retardation.

Living Radical Polymerization by the RAFT Process – A Third Update

“…In this context several groups have studied the kinetics of the RAFT emulsion polymerization of St [495,510] with xanthate RAFT agents. A detailed study of RAFT microemulsion of BA with xanthate RAFT agents has also been performed.…”

“…A* BA [125] 220 O S S F 3 C A* [495] (St) [495] 221 O S S A* (St) [495] (NES) [290] (BES) [290] (NVP) [103] NVC [496] (NVC-b-311) [496] NVC-b-314 [496] (Continued ) A NVP [103] 223 O S S G [103] (NVP) [103] …”

A detailed mechanistic study of bulk MADIX of styrene and its chain extension

“…1b) can be often assumed, strongly simplifying the overall kinetic description. 5,7,[30][31][32][33][34] A crucial design parameter, as for every RAFT polymerization, 15,16,35 is thus the selection of appropriate MADIX agent substituents as this determines C tr(,0) . For MADIX of MAMs, several kinetic studies have been performed with styrene as the monomer.…”

“…For MADIX of MAMs, several kinetic studies have been performed with styrene as the monomer. 7,17,19,25,32,34,36,37 For example, Adamy et al 7 investigated the influence of the chemical structure of the initial RAFT agent for MADIX of styrene in toluene and reported that with (O-ethyl xanthate)-2-ethyl propionate (OEXEP) as the initial RAFT agent, a Đ close to 2 (X m = 20%) can merely be obtained. In contrast, by increasing the electron-withdrawing capacity of the Z group by incorporation of fluorinated groups at β-position to the oxygen atom, as for instance in (O-2,2,2-trifluoro ethyl xanthate)-2-ethyl propionate (OtFOX), a lower but still high Đ of 1.6 can be achieved (X m > 20%), in agreement with an earlier experimental study by Destarac et al 36 The latter observation was attributed to an increased reactivity of the SvC bond due to the decreased availability of the oxygen lone pair to conjugate with the thiocarbonyl group.…”

Compartmentalization Effects on Bimolecular Termination in Atom Transfer Radical Polymerization in Nanoreactors

“…Over the last few years, theoretical work has shown unequivocally that compartmentalization effects are anticipated in nitroxide-mediated radical polymerization (NMP), [11,12,14,15,[18][19][20][21][22][23] atom transfer radical polymerization (ATRP) [13,16,18,21,24,25] and reversible addition-fragmentation chain transfer (RAFT) polymerization [20,21,[26][27][28][29][30] if the monomer droplets/particles are sufficiently small. In many cases, these theoretical predictions have now been verified experimentally.…”