Multi-armed, monofunctional, and hydrolytically stable polymers are described having the structure ##STR00001## wherein Z is a moiety that can be activated for attachment to biologically active molecules such as proteins and wherein P and Q represent linkage fragments that join polymer arms poly.sub.a and poly.sub.b, respectively, to central carbon atom, C, by hydrolytically stable linkages in the absence of aromatic rings in the linkage fragments. R typically is hydrogen or methyl, but can be a linkage fragment that includes another polymer arm. A specific example is an mPEG disubstituted lysine having the structure ##STR00002## where mPEG.sub.a and mPEG.sub.b have the structure CH.sub.3O--(CH.sub.2CH.sub.2O).sub.nCH.sub.2CH.sub.2-- wherein n may be the same or different for poly.sub.a- and poly.sub.b- and can be from 1 to about 1,150 to provide molecular weights of from about 100 to 100,000.