Glycidyl methcrylate (GMA) is a valuable monomer for a variety of large scale commercial coatings. We are asking input from our readers about a patent that issued this month that describes the esterification of methacrylic acid with epichlorohydrin under neutral conditions that claims, with no evidence, that it is more selective than the reaction of the alkali metal salt of methacrylic acid with epichlorohydrin to form GMA.
The inventors use a 2-step process to form GMA from methacrylic acid and epichlorohydrin. The first step is shown in the diagram. The second step is ring closure to the epoxide using carbonate as base.
Even though this process uses two separate steps to form GMA, one of which uses base, they cite as an advantage the costly need to form the alkali salt of methacrylic acid. This appears strange since if one used phase-transfer catalysis, the alkali salt of methacrylic acid could be formed by adding the same base to what they consider the first step. A potential PTC process would consist of a single step esterification mixing methacrylic acid, the base (using exactly 1.00 equiv so no excess base would be present to cause side reactions), a phase-transfer catalyst and add epichlorohydrin (perhaps dropwise as these inventors did). This would avoid a separate step. With proper choice of base, we should be able to avoid hydrolysis of the epoxide in a 1-step PTC esterification.
The inventors used tetramethylammonium chloride (TMAC) as the only catalyst reported in the examples. They note that this catalyst enhances the rate but above 10 mole%, they imply that selectivity may be negatively affected and they further note that the reactive epoxide can lead to undesired byproducts.
One may speculate that the role of TMAC may be to hydrogen bond to the acid and transfer it to the epichlorohydrin phase for reaction, if the acid does not dissolve in epi by itself. It is hard to be confident that this is the mode of action. The chloride of TMAC, which is soluble in epichlorohydrin due to pairing with the quat can also open the epoxide of epichlorohydrin to 1,3-dichloro-2-propanol and possibly slow down the undesired side reactions of the epoxide. It is hard to have confidence in this speculation, as well, in being the source of benefit for using TMAC.
The inventors note that other quats including TMAB and TBAB can serve as suitable catalysts (no data). That is interesting since bromide could make epibromohydrin in situ or form 1-bromo-3-chloro-2-propanol. The in-situ formation of alkyl bromides would catalyze the nucleophilic substitutions more than having just chloride present, but it could also potentially exacerbate the selectivity issue.
It is interesting that none of the patent claims address potential structures for the catalyst, quat salt or otherwise.
We are seeking input from you about whether you see advantage in this patent or whether you feel that there the selectivity and reactivity of a 1-step PTC esterification of acrylic acid, base and epichlorohydrin is likely better. We are also seeking your input about the role of the tetramethylammonium chloride in the reactions conditions reported. We will share the input of our readers next month.
