Stability and Controlled Polymerization of Trithiocarbonate Chain Transfer Agents Under Harsh Conditions

This study investigates the stability and application of trithiocarbonate-based chain transfer agents (CTAs) in reversible addition-fragmentation chain transfer (RAFT) radical polymerization under harsh conditions. We evaluated the stability of 4-cyano-4-(2-carboxyethylthiothioxomethylthio) pentanoic acid (Rtt-17) and 4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanylpentanoic acid (Rtt-05) at 60 °C under basic conditions using ¹H NMR and UV-vis absorption spectra, showing that Rtt-05 is more stable than Rtt-17. The greater stability of Rtt-05 is attributed to the hydrophobic dodecyl group, which allows it to form micelles in water, thereby protecting the trithiocarbonate group from the surrounding aqueous phase. In contrast, hydrophilic Rtt-17, without long alkyl chains, cannot form micelles in water. Following the stability assessment, Rtt-17 and Rtt-05 were employed for RAFT polymerization of hydrophilic monomers, such as N,N-dimethylacrylamide (DMA) and 2-(methacryloyloxy)ethyl phosphorylcholine (MPC). DMA can dissolve in both water and organic solvents, and MPC can dissolve in water and polar solvents. Both CTAs successfully controlled the polymerization of DMA, producing polymers with narrow molecular weight distributions (M_w/M_n) less than 1.2. Also, Rtt-17 demonstrated effective control of MPC polymerization, yielding M_w/M_n values of around 1.2. However, during the polymerization of MPC, Rtt-05 failed to maintain control, resulting in a broad M_w/M_n (≥1.9). The inability of Rtt-05 to control MPC polymerization is due to the formation of micelles, which disrupts the interaction between the hydrophilic MPC propagating radicals and the trithiocarbonate group in the hydrophobic core of Rtt-05 micelles. The findings provide critical insights into designing CTAs for specific applications, particularly for biomedical and industrial uses of hydrophilic polymers, highlighting the potential for precise molecular weight control and tailored polymer properties.