Nitroxide-Mediated Radical Polymerization of Styrene in Aqueous Microemulsion: Initiator Efficiency, Compartmentalization, and Nitroxide Phase Transfer

Abstract

Nitroxide-mediated radical polymerizations (NMP) of styrene at 100 °C using the cationic emulsifier tetradecyltrimethylammonium bromide (TTAB) have been carried out employing 2,2′-azoisobutyronitrile (AIBN) and two nitroxides of different water solubilities. The polymerization rate was lower and the molecular weight distribution more narrow in microemulsion than bulk at low conversion. The results can be rationalized based on the lower initiator efficiency in microemulsion than bulk, presumably caused by the confined space effect (compartmentalization) on geminate termination of AIBN radicals, and possibly also the confined space effect causing an increase in deactivation rate. The extent of retardation relative to bulk was more severe for the less water-soluble 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy (TIPNO) than the more water-soluble N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) as expected based on more extensive nitroxide exit occurring in the case of SG1 than TIPNO. The results illustrate how effects of heterogeneity can profoundly influence the course of NMP in dispersed systems.

Synthesis of Nanosized (<20 nm) Polymer Particles by Radical Polymerization in Miniemulsion Employing in situ Surfactant Formation

“…Despite the fact that the present system is a miniemulsion (kinetically stable but thermodynamically unstable), it is interesting to compare the particle size data with previous microemulsion (thermodynamically stable system) NMP data, since the initial droplet/particle sizes are similar (approximately 25 nm). The d n versus conversion data in the present study are in sharp contrast to the styrene microemulsion NMP data previously reported by Zetterlund et al [23] (SG1/AIBN), which exhibited very small particle sizes at low conversion (d n ¼ 23 nm; translucent appearance), but the particle size gradually increased with conversion to reach 55 nm (opaque appearance) at 92% conversion. The conversion-time plots in the present work, as well as in the above-mentioned microemulsion NMP, are close to linear.…”

“…This value of f is somewhat lower than what is normally obtained in bulk/solution systems. [58] It has previously been reported that f is lower in microemulsion than in bulk for SG1-mediated polymerization of styrene at 100 8C, [23] attributed to increased levels of geminate termination of initiator radicals generated in pairs in small particles (the confined space effect), [51][52][53][54][55] and it would appear that this rationale also applies to the present data considering the very small particle size. The particle size decreased somewhat with increasing conversion (d n ¼ 25 and 13 nm at 0 and 92% conversion, respectively; Figure 2), which would appear to indicate secondary nucleation.…”

“…This is consistent with some fraction of monomer droplets (monomer-swollen micelles in the microemulsion work) acting as monomer reservoirs, supplying monomer to the actual polymerization loci (nucleated monomer droplets/monomer-swollen micelles). [23] Detailed analysis of the kinetics of polymerization of the present system, including discussion of compartmentalization effects, [55,59,60] will be presented in a forthcoming paper.…”

“…[7,8] Microemulsion CLRP, generating particle diameters well below 100 nm, have been conducted using nitroxide-mediated radical polymerization (NMP), [17,[19][20][21][22][23] atom transfer radical polymerization (ATRP), [24,25] and reversible addition-fragmentation chain transfer (RAFT) polymerization. [26][27][28] Miniemulsion polymerization [29] is an attractive method for implementation of CLRP [7,8] as well as for preparation of organic/inorganic hybrid and hollow polymeric nanoparticles [30][31][32] in the particle diameter range 50-1 000 nm.…”

Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems

“…The potential of microemulsion polymerisation has yet to be fully realised for solvophobic block copolymer syntheses, although the few published examples suggests that highly pure products can be obtained, 97,118 which may be a direct result of confinement effects, where termination and radical side reactions are suppressed. 154,155 Methods in which the monomer is initially soluble and the growing polymer precipitates can further simplify the polymerisation process i.e. dispersion and precipitation polymerisation.…”