A compound of Formula: or a salt thereof, wherein R' is absent, hydrogen, or alkyl (e.g., C 1 -C 4 alkyl); with respect to R 1 and R 2, (i) R 1 and R 2 are each, independently, optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, 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 heterocylic ring; or (iii) one of R 1 and R 2 is optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 member heterocyclic ring or heteroaryl (e.g., a 6-member ring) with (a) the adjacent nitrogen atom and (b) the a group adjacent to the nitrogen atom; each occurrence of R is, independently, -(CH 2)-; 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) wherein the compound of formula has at most two R 10 groups; the dashed line to Q is absent; Q is -O- or -C O-; X and Y are each, independently, -(CH 2) c -; M 1 and M 2 are each, independently, -OC - or -C O-; a is 1, 2, 3, 4, 5 or 6; b is 0, 1, 2, or 3; each occurrence of c is, independently, 2-10; and Z 1 and Z 2 are each, independently, (i) C 3 -C 10 cycloalkyl, (ii) C 3 -C 10 cycloalkyl(C 1 -C 6 alkyl), or (iii) wherein each of R 8 and R 9 is a C 2 -C 8 alkyl. R' is absent, hydrogen, or alkyl (e.g., C 1 -C 4 alkyl); with respect to R 1 and R 2, (i) R 1 and R 2 are each, independently, optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, 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 heterocylic ring; or (iii) one of R 1 and R 2 is optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 member heterocyclic ring or heteroaryl (e.g., a 6-member ring) 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, cycloalkyl, 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 heterocylic ring; or (iii) one of R 1 and R 2 is optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, or heterocycle, and the other forms a 4-10 member heterocyclic ring or heteroaryl (e.g., a 6-member ring) with (a) the adjacent nitrogen atom and (b) the a group adjacent to the nitrogen atom; each occurrence of R is, independently, -(CH 2)-; 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) wherein the compound of formula has at most two R 10 groups; the dashed line to Q is absent; Q is -O- or -C O-; X and Y are each, independently, -(CH 2) c -; M 1 and M 2 are each, independently, -OC - or -C O-; a is 1, 2, 3, 4, 5 or 6; b is 0, 1, 2, or 3; each occurrence of c is, independently, 2-10; and Z 1 and Z 2 are each, independently, (i) C 3 -C 10 cycloalkyl, (ii) C 3 -C 10 cycloalkyl(C 1 -C 6 alkyl), or (iii) wherein each of R 8 and R 9 is a C 2 -C 8 alkyl.