Well-Defined Condensation Polymers with Narrow Polydispersity via Unsymmetrical Ladderphanes by Sequential Polymerization

Sequential polymerizations for the synthesis of three different kinds of functional polymers with well-defined degree of polymerization and narrow polydispersity are described. Both conjugated and nonconjugated functional polymers are obtained selectively. Monomers are designed to have an ester function connecting the first polymerizable norbornene moiety and the second polymerizable module. ROMPs of the norbornene moiety with the first generation Grubbs catalyst yield the template polymers where the molecular weights and their distributions are well-controlled. The presence of N-arylpyrrolidene ring fused at 5,6-endo positions of the norbornene moiety is essential to direct the stereoselectivity of the polymerization. The second polymerizable groups attached to such aryl pendants via an ester linkage then proceed further polymerization leading to unsymmetrical double stranded ladderphanes. Hydrolytic cleavages of these ester-linked ladderphanes yield the corresponding daughter polymers that can be easily separated from the polynorbornene template containing carboxylic acid functions. Thus, cross metathesis of divinylarene pendants provides a convenient route for the synthesis of m-arylene-vinylene copolymers with narrow polydispersity and well-defined chain lengths. Moreover, the hydroxyl groups in each of the monomeric units in the daughter polymer would offer possibility for further modifications. Accordingly, highly fluorescent conjugated benzofuranylene-ethynylene copolymers are obtained. Unsymmetrical ladderphanes containing polynorbornene as one strand and butadiynylene-para-phenylene as the other are isolated. Hydrolytic cleavage of these ladderphanes yields ortho-hydroxy-substituted phenylene-butadiynylene intermediate, which would undergo intramolecular annulation to afford benzofuranylene-ethynylene copolymers. Its STM image offers a direct evidence for the formation of unsymmetrical double stranded ladderphanes. In addition, Claisen condensation reaction of arylene-bisacetate has been used to construct nonconjugated alt-poly(acetonylene-m-arylene)s in a controlled manner.