Using 2-hydroxyethyl a-bromoisobutyrate as initiator, atom transfer radical polymerization (ATRP) of tert-butyl acrylate leads to poly(tert-butyl acrylate) (PtBA) with a hydroxyl group at one and a bromine atom at the other end. Esterification of the hydroxyl group of these heterotelechelic polymers with acryloyl chloride yields PtBA ((M) over bar(n) = 3060) with a polymerizable double bond at one end and a bromine atom at the other end which can act as an initiator in ATRP ("macroinimer"). Self-condensing ATRP of such a macroinimer leads to hyperbranched or highly branched PtBA. The polymer was characterized by GPC viscosity measurements. Even at (M) over bar(w) = 78 800, a rather low polydispersity index of (M) over bar(w)/(M) over bar(n) = 2.6 was obtained. A significantly lower value for the Mark-Houwink exponent (a = 0.47 compared to a = 0.80 for linear PtBA) indicates the compact nature of the branched macromolecules.