How do you make polyethylene glycol ester?

Answer

Hi Jan,

PEG9000 is polyethylene glycol, with an average molecular weight of 9000. The structural formula can be found in figure 1, with n approximately equal to 200. You can now react the end -OH groups with stearic acid. This reaction is called the Fischer Esterification and proceeds with an acid catalyst such as sulfuric acid or para-toluene sulfonic acid. The global response is shown in Figure 2. I found a reference where one does something similar. (Al-Sabagh et al. J Chem Technol Biotechnol 74:1075 – 1081 (1999))

Because the concentration of end groups is quite low, the chance of reaction is small, and you have to take very long reaction times into account. It will also be necessary to force the equilibrium of the reaction to the right by removing the water. That can e.g. with a Dean-Stark setup (at least for a lab-scale synthesis).

You can also consider using a more reactive carboxylic acid derivative, such as the acid chloride, stearoyl chloride. This can be purchased from a chemical supplier. In this latter reaction, HCl is formed as a by-product. To avoid side reactions you have to eliminate this with, for example, pyridine.