Supramolecular polymerization.

ion of the proton from the amino group of the monomer (phenyl 4-(alkylamino)benzoate) by the base, the reactivity of the phenyl ester moiety is deactivated, which prevents monomers from reacting with each other. The anion produced by proton abstraction from the monomer will only react with initiator, leading to an activated monomer that possesses a more reactive phenyl ester moiety compared to the anionic monomer. As a result, only activated monomer will react, resulting in a chain-growth polycondensation291,292 instead of a step-growth polycondensation. In the case of structural cooperativity, initial polymerization is thermodynamically less favorable than elongation and polymerization becomes favorable only when the growing polymer has reached a critical length at which, due to a conformational or structural change in the growing polymer, growth becomes more favorable. The critical oligomer length at which polymer elongation becomes more favorable than dissociation is called the nucleus. In the case of cooperativity arising from structural changes, the nucleus is the smallest possible species at which an unstructured, disordered oligomer is converted into an ordered conformation. In classical covalent polymerizations structural cooperativity has been observed for the acid-initiated polymerization of isocyanides to form a helical polymer. For this polymerization, a mechanism was proposed in which an initial helical oligomer needs to be formed, which then acts as a template for the incorporation of subsequent monomeric units.293 Okamoto and co-workers reported the asymmetric, anionic polymerization of triphenyl methyl methacrylate initiated by 9-fluorenyllithium, in the presence of chiral ligands, to form a one-handed helical polymer.294 They observed that the reactivity of each oligomer anion depended on the degree of polymerization, which was correlated to the specific conformation of the oligomer anions. Only when a stable helical conformation of the oligomer was formed, which occurred for a DP of 7-9 units, did further monomer addition occur more readily. Oya and co-workers proposed a similar cooperative growth mechanism for the heterogeneous polymerization of amino acid anhydrides into synthetic polypeptides.295,296 In the early stages of the polymerization, only antiparallel -sheet type oligopeptides were observed due to the fact that the chains are too short to give the R-helical structure. When the degree of polymerization reaches a value of approximately 8, a conformational change to an R-helical conformation occurs and chain growth proceeds more favoarbly via addition of monomers to the active chain end into the R-helical conformation. Figure 25. Schematic energy diagram of a cooperative downhill supramolecular polymerization. The nucleus was chosen to be a tetramer. The abscissa in this plot represents the size of the oligomer (i), whereas the ordinate measures the free energy ∆G° in arbitrary units. 5716 Chemical Reviews, 2009, Vol. 109, No. 11 De Greef et al.