A water soluble polymer is described, the water soluble polymer comprising multiple hydrolytically degradable carbonate linkages in a backbone and, for each carbonate linkage in the backbone, an oligomer linked thereto by the carbonate linkage, wherein the oligomer is branched and, prior to being linked by the carbonate linkage, has the formula: R.sub.3[O--(--CHR.sub.1CH.sub.2--O--).sub.n--].sub.mH where R.sub.3 is a core branching moiety, R.sub.1 is H, (n) can range from 2 to 2000, and m is at least 3.