In the search for biocompatible amphiphilic polymers, carborane-substituted poly(ethylene glycol)s (PEGs) have been prepared in either a linear mono- and disubstituted or starlike tetrasubstituted shape. The reaction was performed by UV-initiated radical thiol/ene coupling of para-carborane-1-thiol to the corresponding allyl-terminated PEG, diallyl-terminated PEG, and tetraallyl-terminated starlike PEG. After the optimization of reaction conditions, the products were characterized by standard spectroscopic and chromatographic techniques. It was also demonstrated that it is possible to separate individual species differing in the number of attached carborane moieties by means of HPLC on an (R,S)-hydroxypropyl-modified β-cyclodextrin column. To understand the mechanism in atomistic and energetic detail, this reaction was examined by means of quantum mechanics (QM) calculations. Further, because the carborane cluster is hypothesized to behave as a radical trap, QM calculations of the stability of various radicals derived from all the stable carboranethiol isomers with thiol group attached either to carbon or boron atoms were performed. The newly prepared carborane PEGs have an amphiphilic structure and might therefore be utilized in bio-oriented applications.