All chemicals and solvents (analytical grade) were obtained from Sigma-Aldrich and used as received unless differently mentioned. Hydroxy-terminated tetra-arm polyethylene glycol (tetra-PEG-OH) was obtained from JenKem Technology USA and purified by dialysis in water (ZelluTrans, Roth, 1000 g mol -1 molecular weight cutoff) and precipitated twice from THF in cold diethyl ether. The number-average molar mass (Mn) was determine by SEC-MALLS in water (Mn = 10 kg mol -1 , Ð = 1.02). ɛ-Caprolactone (ɛ-CL) was dried under reduced pressure over CaH2 for at least 24 hours, then purified by vacuum distillation and stored under nitrogen atmosphere. Tin(II)-2-ethylhexanoate (Sn(oct)2) was purified by vacuum distillation and stored under nitrogen