To get deep insights into the structure–reactivity relationship for ring-opening oligomerization reactions toward targeted design of novel main-chain boron-containing materials, detailed DFT B97D/TZVP calculations are carried out to compare the ring-opening oligomerization of both unsubstituted and tert-butyl (tBu)-substituted 9H−9-borafluorenes. In contrast to substituent exchange between normal boranes, such reactions are initiated by substituent exchanges involving double BCB bridged intermediates. On tBu-substitution, the BCB, and BHB bridged dimer intermediate is stabilized mainly due to enhanced barrier of 18.1 kcal/mol toward further trimerization channel and higher isomerization barrier of 22.5 kcal/mol toward the double BHB bridged dimer. In good agreement with available experiments, it is clearly shown that various product channels can be efficiently controlled by bulky substitution and by reaction temperatures, pointing out the way toward desired higher oligomers with improved thermal stability. © 2014 Wiley Periodicals, Inc.