Folding-driven, nucleation-elongation polymerization of phenylene ethynylene imines

Bis-imino functionalized oligo(m-phenylene ethynylene)s were equilibrated in a closed system under conditions that promote reversible imine metathesis. Equilibration in polar solvents gave high molecular weight polymers, while only low molecular weight oligomers were produced in chloroform. This polymerization is hypothesized to be driven by the free energy gained from the folding of the long polymer chains favored by the noncovalent, intramolecular aromatic stacking and solvophobic interactions. Polymerizations conducted in a series of solvents revealed a good correlation of the product molecular weight with the folding stability of the m-phenylene ethynylene oligomers. Consistent with the notion that the polymerization is a consequence of the intramolecular solvophobic association, higher degrees of polymerization followed from starter sequences with a backbone of enhanced solvophobicity. The equilibrium state of the metathesis reaction was also dependent on the chain length of the starter sequences. With a pair of trimeric precursors, hexameric macrocycle instead of polymer emerged, stabilized by intermolecular aggregation into columnar structures.; As a result of the helical structure of the polymeric product, the polymerization of oligo(m-phenylene ethynylene) imines in solution should inherently show nucleation-elongation in chain growth. The evidence for this behavior was obtained from polymerizations conducted under conditions of imbalanced stoichiometry. Since the polymerization proceeds via metathesis between a pair of bifunctional monomers of type A-A and B-B, the molar ratio of the polymerizing functional groups can be arbitrarily varied. Alternatively, stoichiometry can be controlled by the addition of a monofunctional oligomer. Similar results were obtained in both cases whereby high molecular weight polymers were observed to coexist with the monomer in excess. Thermodynamic equilibrium was established by showing that the same distribution was reached starting either from a monomer mixture or from high polymer to which one monomer was added. These results are in great contrast to the low molecular weight oligomers produced from a reaction that proceeds without the nucleation event. Equilibrium models that capture the features of nucleation-elongation under imbalanced stoichiometry have been developed and qualitatively support the experimental observation by showing the monomer-polymer coexisting state to be the thermodynamic equilibrium distribution.