Ammonia borane (AB) is an outstanding hydrogen-storage material, which has been gained many attentions. It can be regenerated from polyborazylene based on the experimental study, but the uncontrollable B-N dehydro-coupling in the polymerization from AB to borazine is still a problem for the continuous application of AB. Frustrated Lewis Pairs (FLPs) is a landmark discovery to dissociate small molecules by activating strong nonpolar bonds reversibly, which is recently applied to directly polymerize three molecular AB to a cyclotriborazylene (CTB, B3N3H12, which transforms into borazine after releasing 3 equiv. H-2). Here we analyse how a dimethylxanthene-derived FLPs catalyse AB to CTB. The cyclization experiences three steps by our calculation: the connection between FLPs and AB (1), the growth of B-N chain (2) and an intermolecular dehydrogenation between terminal N-H delta+ and delta-HB (3), which gives us a clue of a possible reversibility of the dehydrogenation of AB and broadens the applicability of this concept in the relevant catalyst design. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.