Simulations were performed on solutions of associating polymers, where the
pairing energy between "stickers" is comparable to thermal energy, to gain
insights into the nature of reversible gelation. At high temperatures, inte
rchain associations relax over microscopic time scales, so that geometrical
percolation is unrelated to macroscopic gelation. However, with decreasing
temperature, the stickers undergo a clustering "transition," resulting in
a transient localization of stickers and hence the chains. These findings i
mply a close similarity between the dynamics of associating polymer solutio
ns and vitrification of glass-forming liquids, in agreement with many exper
imental results.