Ligand-induced polymerization.

Three models of ligand-induced polymerization are considered, encompassing dimerization of acceptor-ligand complex and cross-linking of monomer units via a ligand bridge to form either dimer or linear chains. For each model a binding equation is developed and examined in terms of limits, form, and intersection within a family of curves, each member constructed with fixed but different total acceptor concentration. The characteristics of the binding curves that emerge are correlated with those found by examining the dependence on ligand concentration of either the weight-average molecular weight of the acceptor constituent or a function of it. In the latter plots, a maximum exists for the cross-linking models at the same ligand concentration that the corresponding binding curves intersect: the existence of the maximum implies that at saturating concentrations of ligand only monomeric acceptor constituent remains. The practical implication of these findings is discussed in terms of experimental systems reported previously in the literature. In addition, particular comment is made on the forms of binding curves for the three models, which assume shapes generally associated with positive and negative cooperativity. Finally, comparison is made with those systems in which a shift toward polymeric constituent is effected by ligand perturbation of a preexisting association equilibrium.