Eight-armed poly(ethylene glycol)-poly(trimethylene carbonate) star block copolymers (PEG-(PTMC)8) linked by a carbamate group between the PEG core and the PTMC blocks were synthesized by the metal-free, HCl-catalyzed ring-opening polymerization of trimethylene carbonate using an amine-terminated eight-armed star PEG in dichloromethane. Although dye solubilization experiments, nuclear magnetic resonance spectroscopy, and dynamic light scattering clearly indicated the presence of aggregates in aqueous dispersions of the copolymers, no physical gelation was observed up to high concentrations. PEG-(PTMC9)8 was end-group-functionalized using acryloyl chloride and photopolymerized in the presence of Irgacure 2959. When dilute aqueous dispersions of PEG-(PTMC9)8-Acr were UV irradiated, chemically cross-linked PEG-PTMC nanoparticles were obtained, whereas irradiation of more concentrated PEG-(PTMC9)8-Acr dispersions resulted in the formation of photo-cross-linked hydrogels. Their good mechanical properties and high stability against hydrolytic degradation make photo-cross-linked PEG-PTMC hydrogels interesting for biomedical applications such as matrices for tissue engineering and controlled drug delivery systems.