No abstract

In studying 2,2,6,6-tetramethy-1-piperidinyloxy (TEMPO)-mediated styrene miniemulsions, we have observed that the surfactant sodium dodecylbenzenesulfonate (SDBS) not only provides colloidal stability but also influences the rate of polymerization. Increasing the SDBS concentration results in higher polymerization rates, although the molecular weight distribution and particle size distribution are not significantly impacted. We have also examined another common sulfonate surfactant, DOWFAX 8390. In contrast to SDBS, DOWFAX 8390 does not affect the polymerization rate. Furthermore, DOWFAXstabilized polymerizations are slower than SDBS-stabilized polymerizations. TEMPOmediated bulk styrene polymerizations are also accelerated significantly in the presence of SDBS. Although the mechanism for the rate acceleration is unknown, the experimental evidence suggests that SDBS is participating in the generation of radicals capable of propagating, thereby reducing the TEMPO concentration within the particles. Figure 4. (a) Conversion versus time, (b) M n versus conversion, and (c) polydispersity versus conversion for bulk styrene polymerizations with and without added SDBS: (^) styrene/SDBS ¼ 0 and (n) styrene/SDBS ¼ 1:0.0041. 5984 LIN, HSU, AND CUNNINGHAM

The rate-accelerating effects of camphorsulfonic acid (CSA) on nitroxidemediated styrene miniemulsion polymerization were studied. Polymerizations were initiated with benzoyl peroxide (BPO) as an initiator and mediated with either 2,2,6,6tetramethylpiperidinyloxy (TEMPO) or 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (OH-TEMPO). Although CSA has been used to accelerate the rate in bulk nitroxidemediated polymerizations, it has not been well studied in emulsion/miniemulsion. With dispersed systems, the effectiveness of CSA is likely to be affected by partitioning between the aqueous and organic phases. In styrene miniemulsion experiments performed over a range of conditions, the effect of adding CSA varied from negligible to significantly increasing the final conversion and molecular weight, depending on the nitroxide:BPO ratio. At a ratio of nitroxide:BPO ϭ 1.7, the effect of CSA addition is small, whereas the final conversion and molecular weight are dramatically enhanced by CSA addition when the nitroxide:BPO ratio is 3.6. CSA is most effective in enhancing the rate and molecular weight when the initial free-nitroxide concentration is higher. The magnitude of the rate and molecular weight enhancement was similar for TEMPO and OH-TEMPO despite their differences in water solubility.

Living/controlled radical polymerizations provide significant advantages in the control of polymer resin microstructure compared to conventional radical polymerization. Advances in our ability to tailor polymer microstructure will enable improvements in coatings properties and possibly new applications of coating technologies. Adapting living radical polymerizations to heterogeneous media such as aqueous-based miniemulsion polymerization presents several challenges related to maintaining the livingness (the fraction of chains that are still "living" at the end of polymerization) of the polymer chains and also developing a commercially viable process. We have studied the nitroxide-mediated polymerization of styrene in miniemulsion, with the intent of maintaining a high degree of livingness by balancing the rates of biradical termination and disproportionation. We can now achieve >95% monomer conversion in less than three hours, while maintaining polydispersities ~1.3. Monomer conversion can be dramatically increased from about 60-95% by changing the concentration of sodium dodecylbenzenesulfonate (SDBS) surfactant. Conversions in Dowfax 8390 stabilized miniemulsions showed no comparable dependency. Reasons for this potentially commercially important effect are under investigation.

Nitroxide‐mediated living radical polymerizations of styrene were run in miniemulsion. Using a modified miniemulsion process that does not require the use of a volatile costabilizer, near complete conversions could be achieved in 2‐3 hours while preserving narrow molecular weight distributions and a high degree of chain livingness. Increased rates and final conversions were achieved by semi‐batch addition of the nitroxide scavenger ascorbic acid.