A new two-step synthesis of polyisobutylene (PIB) with precisely one thymine functionality per chain (PIB-T) is reported. The primary hydroxyl-functionalized PIB (PIB-OH) precursor was prepared by direct functionalization via living carbocationic polymerization of isobutylene initiated by the alpha-methylstyrene epoxide/TiCl4 system. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS) of a low molecular weight PIB-OH precursor demonstrated the effectiveness of direct functionalization by this method. A PIB-acrylate precursor (PIB-Ac) was obtained from such a PIB-OH, and the PIB-T was subsequently prepared by Michael addition of thymine across the acrylate double bond. MALDI-ToF MS of the products verified that all polymer chains carried precisely one thymine group. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3501-3506, 2010