Silylium cation initiated sergeants-and-soldiers type chiral amplification of helical aryl isocyanide copolymers

Abstract

A series of silylium cations [(R₁R₂R₃Si)₂H]⁺[B(C₆F₅)₄]⁻ (1–6) (1: R₁ = R₂ = R₃ = Et; 2: R₁ = R₃ = H, R₂ = Ph; 3: R₁ = R₃ = Me, R₂ = Ph; 4: R₁ = R₂ = R₃ = Ph; 5: R₁ = R₂ = R₃ = ⁱPr; 6: R₁ = R₂ = R₃ = OEt) serve as highly efficient metal-free single-component cationic initiators for the cationic polymerization and copolymerization of chiral and achiral aryl isocyanides R–NC (a–k) (a: R = 2-C₁₀H₇ (NI); b: R = 4-(COOEt)–C₆H₄ (EPI); c: R = 3-[COOⁱPr]–C₆H₄ (IPI); d: R = 4-{COO-[(1S,2R,5S)-(2-ⁱPr–5-Me–C₆H₆)]}–C₆H₄ (D-IMCI); e: R = 4-{COO-[(1R,2S,5R)-(2-ⁱPr–5-Me–C₆H₆)]}–C₆H₄ (L-IMCI); f: R = 4-[4-(Ph)CC(Ph)₂–C₆H₄]–C₆H₄ (ITPB); g: R = 4-{COO–(CH₂)₉–O–[4-(4-Ph–2-C₉H₅N)–C₆H₄]}–C₆H₄ (BNB); h: R = 4-{COO-[4-(4-Ph–2-C₉H₅N)–C₆H₄]}–C₆H₄ (PQPI); i: R = 4-(4-Ph–2-C₉H₅N)–C₆H₄ (IPQ); j: R = 4-{COO–(CH₂)₉–O–[4-(Ph)CC(4-N(C₂H₅)₂–C₆H₄)₂–C₆H₄]}–C₆H₄ (PPNI); k: R = 4-(NN–Ph)–C₆H₄ (IPD)). Nine homopoly(aryl isocyanide)s are prepared, in which optically active poly(d) and poly(e) exhibit obvious negative and positive Cotton effects at 364 nm and adopt one-handed helical conformations, the poly(f) containing tetraphenylethylene (TPE) substituent shows an aggregation-induced emission (AIE) nature, and the poly(g) containing 2,4-diphenylquinoline (DPQ) substituent serves as a new fluorescent probe for the detection of Fe³⁺ or Fe²⁺ ions. By fine-tuning the types of chiral and achiral aryl isocyanides, different degrees of chiral amplification of copolymer helicity can be achieved in the copolymerization of chiral aryl isocyanides d and e with achiral aryl isocyanides b, c, f, and k. Such chiral amplifications of the helical poly(e-co-b)s, poly(d/e-co-c)s, poly(d/e-co-f)s, and poly(d/e-co-k)s with the same helical sense as the corresponding poly(d) and poly(e) obey the “sergeants-and-soldiers” rule. Determination of reactivity ratios of these copolymerizations demonstrates that the different nonlinear relationship between the intensity of CD signals at 364 nm and the D/L-IMCI contents of these copolymers depends on the sequence distribution of chiral aryl isocyanide monomers in these copolymers. Based on the in situ¹H NMR, FT-IR, and high resolution ESI-MS spectra, a possible cationic (co)polymerization mechanism is proposed.