Homobifunctional PEG Homobifunctional PEG HO-PEGn-OH (11) H2N-PEGn-NH2 (11) HOOC-PEGn-COOH (8) NHS-PEGn-NHS (9) Mal-PEGn-Mal (5) Mal-PEGn-CH2CH2Mal (3)