To obtain the subject emulsifying agent for emulsion polymerization having good stability in emulsion polymerization, capable of improving water resistance, or the like, of coating films obtained from emulsion and further breaking a polymer emulsion readily in recovery of the polymer.

This emulsifying agent comprises a compound of formula I [R1 and R2 are each H, a 1-20C alkyl or the like, with the proviso that they are not simultaneously H; X is H or an ionic hydrophilic group such as anion; A is a 2-4C alkylene or the like; (n) is 0-100; the group (AO)n may be a homopolymer, a block polymer or a random polymer] (e.g. a compound of formula II). The compound of formula I is obtained by reacting, e.g. a compound of formula III (e.g. 2-methyl-2-undecyl-4-hydroxymethyl-1,3-dioxolane) with glycicylmethacrylate in the presence of a catalyst and, as necessary, a polymerization inhibitor and then subjecting the resultant compound to addition reaction with an alkylene oxide according to conventional method.