A copolymer comprises at least two repeating units of formula I In this formula each R1 is independently an alkyl, haloalkyl, aralkyl, alkenyl, aryl, or aryl substituted with an alkyl, alkoxy, or halo; each Y is independently an alkylene, aralkylene, or a combination thereof; each G is independently a bond or a divalent residue equal to a diamine of formula R3HN-G-NHR3 minus the two —NHR3 groups; each R3 is independently hydrogen or alkyl or R3 taken together with G and to the nitrogen to which they are both attached form a heterocyclic group; each n is independently an integer of 0 to 1500; each p is independently an integer of 1 to 10; and each q is independently an integer of 1 or greater, and wherein at least 50% of the q's are the integer 2.