Multi-armed, monofunctional, and hydrolytically stable polymers are described having the structure 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 polya and polyb, 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 where mPEGa and mPEGb have the structure CH3O—(CH2CH2O)nCH2CH2— wherein n may be the same or different for polya- and polyb- and can be from 1 to about 1,150 to provide molecular weights of from about 100 to 100,000.