Focusing on material properties and application, but without addressing the network formation process and its influence on network defects, synthesis of highly biocompatible and biodegradable ACNs based on hydrophobic PCL and hydrophilic PEG have been reported several times in the literature. ACNs with these components are also subject of the present publication, but, with special focus on the process of network formation. With the aim of obtaining networks with highly defined structures, the synthesis was carried out following the tetra-arm approach of Sakai et al. In contrast to Sakai, in our method presented here, hydrophobic PCL and hydrophilic PEG tetraarm stars were linked by the much slower 2-(4-nitrophenyl)-benzoxazinone/amine reaction in organic solvents similar as described earlier for the synthesis of A2-B4 hybrid networks. In our approach, hydroxy-terminated star polymer tetra-PCL (1) was first reacted with 2-(4-nitrophenyl)-4-oxo-4H-benzo[d] [1,3]oxazine-7-carboxylic acid chloride (2), resulting in 2-(4-nitrophenyl)benzoxazinone-terminated tetra-PCL (3), which was cross-linked in different organic solvents with amino-terminated star polymer tetra-PEG (5) in a second step to give ACN gels (see Scheme 1). The moderate rate of the cross-linking reaction allows the precursor star polymers to mix and diffuse and, moreover, to monitor this process by NMR spectroscopy and oscillatory shear rheology.