H2N-PEG8-tBu alpha-aMine-oMega-propionic acid hexaethylene glycol Boc-1-amino-3,6-dioxa-8-octanediamine Z-8-aMino-3,6-dioxaoctanoic acid Octaethylene glycol 2,5,8,11,14,17-Hexaoxanonadecan-19-amine NH2-PEG8-COOH Pentaethylene glycol monomethyl ether Tetraethyleneglycol monomethyl ether PEG7-t-butly ester