Thermoreversible gelation of polymer chains bearing hydrogen-bonding functional groups is studied by off-lattice Monte Carlo simulation with semiflexible bead-and-spring model chains. To see the formation of zipper-like sequential crosslink junctions (domino effect), we introduce stabilization energy -Delta epsilon between the nearest neighboring hydrogen-bonded beads along a chain in addition to the ordinary pair-wise hydrogen-bond energy -epsilon. It is found that the condition n theta/(xi) over bar = 2 is fulfilled at the sol/gel transition point, where (xi) over bar is the average zipper length, theta the zipper content per chain, and n the total number of beads on a chain. It is also shown that, at low temperature, zipper growth dominates the nucleation of new zippers, and as a result, there is another transition from a three-dimensional network to a pairwisely bound state (network/pair transition). (c) 2005 Wiley Periodicals, Inc.