Tetraethylammonium bromide, TEAB, was surprisingly chosen for the PTC etherification described in Saint-Loup, R.; Vanbesien, T. (Roquette Freres) U.S. Patent 12,398,235, 26-Aug-2025.
The etherification was performed at the high temperature of 180 deg C and used 50% NaOH as base, an isosorbide-based diol prepolymer as the alcohol and epichlorohydrin to produce epoxy-functionalized oligomers. In this system, epichlorohydrin acts both as a chain-extension reagent and as a functionalization agent, introducing glycidyl ether end groups that determine the final “epoxide equivalent weight” (EEW) and reactivity of the prepolymer. EEW is highlighted in this patent as a key metric for the success of the reaction.
It’s the choice of TEAB as phase-transfer catalyst at this high temperature that is surprising in the presence of strong base.
First the good news. Table 2 in the patent clearly shows that the use of 1% TEAB increases reactivity, EEW and Mn relative to the absence of TEAB. TEAB is a low-cost phase-transfer catalyst on a weight basis and even more so on a molar basis.
However, the choice of a less-than-optimal phase-transfer catalyst will result in unreacted hydroxyls, broad EEW distribution and prolonged reaction times. From the standpoint of industrial phase-transfer catalysis, TEAB is a poor choice for several reasons.
First and foremost, tetraethylammonium is almost guaranteed to undergo Hofmann elimination under these reaction conditions and that will obviously destroy catalytic activity. The presence of three non-hindered β-hydrogens on each of the four ethyl groups, coupled with the strong basic environment of 50% NaOH, makes TEAB among the most thermally unstable PTC quats. Hofmann elimination of tetraethylammonium in the presence of 50% NaOH starts being severe at temperatures above 80 deg C. This reaction is performed at 180 deg C (!!!) and it’s extremely surprising to us that the quat survives.
Secondly, as we teach in the 2-day course “Industrial Phase-Transfer Catalysis,” TEAB has a high q-value (quaternary charge density) of 2.0 and a very low C# of 12. In an I-reaction system—as this prepolymer glycidylation is classified in the PTC course—the optimal PTC should have a low q-value, preferably 1.4 or lower, very preferably 1.0 or lower, and a high C#, preferably at least 16 or higher. TEAB simply doesn’t meet either of these two criteria.
There are many commercial processes for the production of glycidyl ethers and the phase-transfer catalysts chosen usually avoid having any ethyl groups, let alone four ethyl groups.
If your company needs to achieve the highest performance PTC-base processes at the lowest cost and complete development and optimization with minimum investment of R&D resources, now contact Marc Halpern of PTC Organics.
