Method for preparing dimeric acid methyl ester

The invention relates to a method for preparing dimeric acid methyl ester by using Brphinsted-Lewis double-acid ionic liquid to catalyze unsaturated fatty acid methyl ester dimer. The method is characterized in that the Brphinsted-Lewis double-acid ionic liquid consisting of alkyl sulfonic acid functionalized pyridine cations or alkyl sulfonic acid functionalized 1,3-dialkylimidazolinium cations or alkyl sulfonic acid functionalized quaternary ammonium cations and inorganic anions of Lewis acid is used as a catalyst; and under the protection of nitrogen, the Brphinsted-Lewis double-acid ionic liquid catalyzes a fatty acid methyl ester polymerization reaction for 2 to 8 hours at the reaction temperature of between 220 and 280 DEG C to prepare the dimeric acid methyl ester. Compared with the prior art, the method has the following advantages: 1, the method has higher polymerization capacity, does not need to pressurize, and has short reaction time; 2, a dimeric acid methyl ester product and the ionic liquid cat