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

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.