Today's olefin metathesis catalysts show high reactivity, selectivity, and functional group tolerance and allow the design of new syntheses of precisely functionalized polymers. Here the synthesis of a new end-capping reagent is investigated allowing the introduction of a highly reactive activated ester end-group at the polymer chain end as well as its prefunctionalization to directly introduce functional moieties. The versatility of this new end-capping reagent is demonstrated by utilizing it to synthesize a so-called termimer (a monomer with termination capabilities). Copolymerization of a norbornene derivative with the termimer leads to hyperbranched ring-opening metathesis polymerization polymers as proven by gel permeation chromatography and MALDI-ToF-(matrix-assisted laser desorption/ionization time of flight) mass spectrometry.