Selective Chain-End Postpolymerization Reactions and Property Tuning of a Highly Conjugated and All-Thiophene Polyazomethine

Abstract

A highly conjugated polyazomethine (3) consisting uniquely of thiophenes was investigated for sustaining postpolymerization property tuning. This was courtesy of the ever-present and reactive amino and aldehyde chain ends. Subjecting the polymer to polymerization conditions in the absence of monomers resulted in a DPn increase from 15 to 70. The aldehyde chain end was selectively reacted with an aliphatic amine by undergoing reductive amination with sodium triacetoxyborohydride (NaBH(OAc)3) while the imines along the conjugated polymer framework remained intact. This illustrates the robustness of the azomethine bond. Reductive amination of 3 with a monofunctional amine resulted in selective chain-end capping and prevented repolymerization. 3 was resistant to dynamic component exchange and monomer shuffling as a result of its high degree of conjugation. Conversely, chain extension was possible both by reductive amination with an α−ω-diamine and by reheating the polymer. Covalent attachment of two separate segments of 3 was possible by reductive amination with a 2:1 polymer/α−ω-diamine stoichiometry. This resulted in molecular weight doubling and serves to illustrate the possibility of multiblock tethering via the ever-active chain ends. Meanwhile, a 1:1 polymer/α−ω-diamine stoichiometry gave rise to a polymer with an aliphatic and aromatic amine functionalized at either chain end. As a result of the noncomplementary of the two chain ends, the polymer could not be repolymerized. However, it exhibited consistent molecular weight under various repolymerization reaction conditions indicating the robustness of the heteroconjugated bonds and their tolerance toward dynamic component exchange. The color of 3 was also changed from its inherent blue color to green via the covalent attachment of a yellow dansyl derivative by reductive amination at the aldehyde chain end. The collective property modification including repolymerization, coblock tethering, color modification, and increased degree of polymerization presenting unequivocal evidence that polyazomethines are capable of undergoing postpolymerization property tailoring.