The RAlCl2 x (OPr2)-Pr-i-co-initiated (R = Bu-i or Et) cationic polymerization of isobutylene in the presence of externally added water (0.016-0.1 mM) in nonpolar n-hexane at 10 degrees C and high monomer concentration ([IB] 55.8 M) has been investigated. It was shown that the sequence of H2O introduction into the system had the crucial effect on the polymerization rate, saturated monomer conversion, and, to a lesser extent, the content of exo-olefin end groups. Particularly, the highest polymerization rate (>70% of monomer conversion in 10 min) and acceptable exo-olefin end groups content (similar to 83%) were observed when (BuAlCl2)-Bu-i x 0.8O(i)Pr(2) reacted with suspended in n-hexane H2O before the monomer addition. Better functionality can be obtained when H2O is introduced into the system in the course of the polymerization (after 3-10 min since the initiation of reaction). Under these conditions, highly reactive polyisobutylenes (exo-olefin content is 86-89%) with desired low molecular weight (M-n = 1000-2000 g mol(-1)) in a high yield (75-90% of monomer conversion in 20 min) were readily synthesized. (c) 2014 Wiley Periodicals, Inc.