What is described is a compound of formula I wherein R1 is a linear alkyl, alkenyl, or alkynyl of 2 to 12 carbons, or a branched alkyl with 12 to 20 carbons, R2 is a branched alkyl with 12 to 20 carbons, L1 and L2 are independently a linear alkylene with 1-18 carbons or alkenylene with 2-18 carbons, X is S or O, L3 is a bond or an alkylene with 1 to 6 carbons, R3 is an alkylene with 1 to 6 carbons, and R4 and R5 are the same or different, each a linear or branched alkyl with 1 to 6 carbons.