Hydroxy-PEG4-CH2COOH is a PEG-based PROTAC linker that can be used in the synthesis of PROTACs[1].
References
[1] An S, et al. Small-molecule PROTACs: An emerging and promising approach for the development of targeted therapy drugs. EBioMedicine. 2018 Oct;36:553-562. DOI:10.1016/j.ebiom.2018.09.005