A compound of formula: or a salt thereof, wherein R' is absent, hydrogen, or alkyl; with respect to R 1 and R 2, (i) R 1 and R 2 are each, independently, optionally substituted alkyl, alkenyl, alkynyl, cycloalkylalkyl, heterocycle, or R 10; (ii) R 1 and R 2, together with the nitrogen atom to which they are attached, form an optionally substituted heterocyclic ring; or (iii) one of R 1 and R 2 is an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 membered heterocyclic ring or heteroaryl with (a) the adjacent nitrogen atom and (b) the a group adjacent to the nitrogen atom; each occurrence of R is, independently, -(CR 3 R 4)-; each occurrence of R 3 and R 4 are, independently H or C 1 -C 4 alkyl; each occurrence of R 10 is independently selected from PEG and polymers based on poly(oxazoline), poly(ethylene oxide), poly(vinyl alcohol), poly(glycerol), poly(N-vinylpyrrolidone), poly[N-(2-hydroxypropyl)methacrylamide] and poly(amino acid)s, wherein (i) the PEG or polymer is linear or branched, (ii) the PEG or polymer is polymerized by n subunits, (iii) n is a number-averaged degree of polymerization between 10 and 200 units, and (iv) the compound has at most two R 10 groups; the dashed line to Q is absent; Q is absent or is -O-, -NH-, -N(R 5)-, -S-, -C -, -C O-, -OC -, -C N(R 4)-, -N(R 5)C -, -S-S-, -OC O-, -O-N=C(R 5)-, -C(R 5)=N-O-, -OC N(R 5)-, -N(R 5)C N(R 5)-, -N(R 5)C O-, -C S-, -C O- or -C(R 5)=N-O-C -; each occurrence of R 5 is, independently, H or C 1 -C 4 alkyl; X is -(CH 2) n - wherein n is 4, 5, 6, 7, 8, 9, or 10; M 1 is -OC - or -C O-; a is 1, 2, 3, 4, 5 or 6; b is 0, 1, 2, or 3; Z 1 is a C 6 -C 14 branched alkyl group; and Z 2 is C 4 -C 20 alkenyl, wherein the alkenyl group may optionally be substituted with one or two fluorine atoms at the alpha position to a double bond which is between the double bond and the terminus of Z 2. R' is absent, hydrogen, or alkyl; with respect to R 1 and R 2, (i) R 1 and R 2 are each, independently, optionally substituted alkyl, alkenyl, alkynyl, cycloalkylalkyl, heterocycle, or R 10; (ii) R 1 and R 2, together with the nitrogen atom to which they are attached, form an optionally substituted heterocyclic ring; or (iii) one of R 1 and R 2 is an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 membered heterocyclic ring or heteroaryl with (a) the adjacent nitrogen atom and (b) the a group adjacent to the nitrogen atom; (i) R 1 and R 2 are each, independently, optionally substituted alkyl, alkenyl, alkynyl, cycloalkylalkyl, heterocycle, or R 10; (ii) R 1 and R 2, together with the nitrogen atom to which they are attached, form an optionally substituted heterocyclic ring; or (iii) one of R 1 and R 2 is an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 membered heterocyclic ring or heteroaryl with (a) the adjacent nitrogen atom and (b) the a group adjacent to the nitrogen atom; each occurrence of R is, independently, -(CR 3 R 4)-; each occurrence of R 3 and R 4 are, independently H or C 1 -C 4 alkyl; each occurrence of R 10 is independently selected from PEG and polymers based on poly(oxazoline), poly(ethylene oxide), poly(vinyl alcohol), poly(glycerol), poly(N-vinylpyrrolidone), poly[N-(2-hydroxypropyl)methacrylamide] and poly(amino acid)s, wherein (i) the PEG or polymer is linear or branched, (ii) the PEG or polymer is polymerized by n subunits, (iii) n is a number-averaged degree of polymerization between 10 and 200 units, and (iv) the compound has at most two R 10 groups; the dashed line to Q is absent; Q is absent or is -O-, -NH-, -N(R 5)-, -S-, -C -, -C O-, -OC -, -C N(R 4)-, -N(R 5)C -, -S-S-, -OC O-, -O-N=C(R 5)-, -C(R 5)=N-O-, -OC N(R 5)-, -N(R 5)C N(R 5)-, -N(R 5)C O-, -C S-, -C O- or -C(R 5)=N-O-C -; each occurrence of R 5 is, independently, H or C 1 -C 4 alkyl; X is -(CH 2) n - wherein n is 4, 5, 6, 7, 8, 9, or 10; M 1 is -OC - or -C O-; a is 1, 2, 3, 4, 5 or 6; b is 0, 1, 2, or 3; Z 1 is a C 6 -C 14 branched alkyl group; and Z 2 is C 4 -C 20 alkenyl, wherein the alkenyl group may optionally be substituted with one or two fluorine atoms at the alpha position to a double bond which is between the double bond and the terminus of Z 2.