The amine functional group at the end of the Methoxy-PEG allows selective pegylation of molecules or surfaces with complementary functional groups, such as carboxylic acids or activated esters. This can be achieved through various coupling reactions, such as the use of N-hydroxysuccinimide (NHS) esters or carbodiimides. PEG amines also react readily with epoxides or aldehydes to form secondary amines (permanent-) or imines (reversible pegylation) .