As we teach in our 2-day course “Industrial Phase-Transfer Catalysis”, phase-transfer catalysis has been used for the modification of polymers with reactive sites. One reference that we cite for nucleophilic substitutions on polychloromethylstyrene with KOAc, KOBz, KSAc, KSCN, NaN3, KNPht is Nishikubo, T. “Phase-Transfer Catalysis: Mechanism and Syntheses”, ACS Symposium Series 659, Halpern, M. editor, 1997, Chapter 17.
A recent patent (reaction shown in the diagram) described the use of PTC for modification of a variety of polyacrylamides functionalized with pendant mercapto alkyl ethers (mostly mercapto ethyl ether, mercapto propyl ether and mercaptohexyl ether). The mercaptan SH was deprotonated by a variety of trialkyl amines and reacted with several alkylating agents.
It is interesting to note that the inventors used a lot less phase-transfer catalyst with an iodide counterion than with chloride or bromide counterions. Perhaps that was due to iodide co-catalysis to activate the alkyl halide alkylating agents.
The inventors did not explain why they used quaternary ammonium phase-transfer catalysts for all 12 examples. We speculate that the reason may have been to create a loose ion pair between the mercaptide anion and the quat cation. The ammonium cation R3NH+ (before it turns into the amine hydrohalide) would likely not form a very loose ion pair and the relatively close pKa’s of the neutral mercaptan and the cation from the tertiary ammonium-H species doesn’t help in keeping the mercaptan completely deprotonated (which is likely why the inventors used an excess of trialkylamine).
By the way, this patent focused on temperature-responsive macromolecules that are effective in improving the shrinkage and crack resistance of concrete. It is always interesting to learn of the diverse applications upon which phase-transfer catalysis brings benefit.
