wherein R1 and R2 independently of each other are a member selected from the group consisting of alkyl having 1 to 24 carbon atoms, preferably 12 to 18 carbon atoms, and hydrogen, or R3 and R4 independently of each other are a member selected from the group consisting of alkyl having 2 to 22 carbon atoms, preferably 12 to 18 carbon atoms, and hydrogen, and x and y independently of each other are a