active esters of peg acids and related polymers are provided that have a single propionic or butanoic acid moiety and no other ester linkages . these polymer acids have a half life in water of from about 10 to 25 minutes . for example , alpha - methoxy , omega - propionic acid succinimidyl ester of peg has a nearly ideal reactivity with amino groups on proteins and other biologically active substances . the half life of methoxy - peg - spa is about 16 . 5 minutes in water . the invention also provides conjugates with proteins , enzymes , polypeptides , drugs , dyes , nucleosides , oligonucleotides , lipids , phospholipids , liposomes , and surfaces of solid materials that are compatible with living organisms , tissue , or fluid .