Application of Stable Radicals as Mediators in Living-Radical Polymerization - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 421

provides the opportunity to produce block copolymers and polymers with complex architectures using a
process that enables the industrial production of large-scale materials and eliminates the high viscosity and
heat dissipation issues that exist in bulk polymerizations. Heterogeneous processes also offer the advantage
of being able to reach high molar mass polymers with high conversion at a faster rate of polymerization
as compared to bulk or solution systems.

11.5.1 Living-radical miniemulsion polymerization

Mechanistically, miniemulsion systems are much simpler than emulsion systems since they begin with
preformed, small, stabilized droplets. Nucleation and further polymerization of these droplets allow for
the formation of polymer particles. An issue with miniemulsion systems is the need for a small molecule
hydrophobe, such as hexadecane, to impart stability to the small monomer droplets against diffusional
degradation (Ostwald ripening). The hydrophobe is often regarded as an impurity in the final latex particle.
Furthermore, the use of a high shear device is required for the formation of the small droplets, adding
difficulty to large-scale syntheses. Nevertheless, colloidally stable latexes are readily achieved using living-
radical techniques under these conditions.
Initial SFR miniemulsion polymerizations 187,188 were performed using either BPO 189,190 or potassium
persulfate (KPS) 191 as the initiator, TEMPO as the stable radical mediator, and hexadecane as the costa-
bilizer. To avoid the use of aqueous phase initiation and remove the need for hexadecane, miniemul-
sion polymerizations were subsequently performed using long-chain alkoxyamines or TEMPO-terminated
polystyrene oligomer with good success. 192,193
While initial miniemulsion polymerizations were performed between 125 - 135 ◦ C in a pressurized
reaction vessel, the use of SG1 ( 6 ) allowed the temperature to be dropped to 90 ◦ C. 194 Miniemulsion
polymerizations using TEMPO have been accomplished at 100 ◦ C through the use of rate enhancing addi-
tives; however, the polymers tended to exhibit a broad molecular weight distributions (M n /M w ∼ 1 . 4 -1.6)

attributed to a low activation rate due to the lower temperature. 195
Various reaction parameters have been reported to affect miniemulsion polymerizations. Cunningham
et al . found an increase in the rate of polymerization with an increase in the concentration of the surfactant
sodium dodecyl benzenesulfonate (SDBS) at a constant particle size of

120 nm. No decrease in the
degree of livingness was observed. 196,197 Nakamura et al . 198 reported that particle sizes of greater than
170 nm give polymerization rates similar to bulk polymerizations and good control while smaller sizes
give faster reaction rates but less livingness. The results were explained by suggesting that a fraction of
the free TEMPO becomes deactivated by being absorbed at the interface between the organic and aqueous
phase resulting in a decrease in the deactivation rate. 199 The loss in livingness was further noted in the
preparation of block copolymers 200,201 Shifts to higher molecular weights of the initial homopolymer were
evident but the gel permeation chromatography plots showed a low molecular weight shoulder attributed
to dead homopolymer.
The concentration of a styrene-TEMPO macroinitiator has also been shown to affect the polymerization,
with concentrations below 0.02 M giving poor results due to the contribution of autoinitiation and the
suppression of deactivation due to the interfacial activity of TEMPO. 202,203 The latter argument was also
used to explain why bimolecular termination is higher in miniemulsion systems than in solution and
increases with decreasing particle size. 204 Also, in agreement with earlier studies at lower temperatures,
Alam et al . 205 reported that the spontaneous thermal rate of initiation is 3 - 15 times faster in miniemulsion
than in bulk at temperatures of 110 ◦ C and 125 ◦ C.
There has been a lot of work dealing with compartmentalization, with a conclusion that for a styrene
TEMPO system at 125 ◦ C and [PS - TEMPO]

∼

=

0.02 M, a particle size of less than 70 nm is required to see
slowing of the rate of polymerization due to compartmentalization of both the propagating chain and the