Macromolecules, Vol.50, No.7, 2637-2644, 2017
Induction of Chirality in beta-Turn Mimetic Polymer Conjugates via Postpolymerization "Click" Coupling
Chiral and achiral beta-turn mimetic polymer conjugates were synthesized, and their chiroptical properties were investigated. Chiral helical poly(n-hexyl isocyanate)s (PHICs) with alkyne functional end groups (4.0-5.4 kDa) were synthesized via coordination polymerization using either a chiral or an achiral alkyne-functional organotitanium catalyst. The obtained helical polymers were coupled with a chiral beta-turn mimetic structure using the copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction, in turn allowing to tune the distance between the PHIC-helix and the chiral center from 7 to 14 angstrom. The successful linkage of the PHIC-helix to the beta-turn mimetic was confirmed by size exclusion chromatography (SEC), high performance liquid chromatography (HPLC), H-1 NMR spectroscopy, and MALDI-ToF mass spectrometry, proving the attachment of either one or two PHIChelices to the central beta-turn. Circular dichroism (CD) spectra confirmed chirality induction from the chiral beta-turn mimetic structure to the polymer backbone of the achiral PHIC over five chemical bonds via postpolymerization modification, and revealed a distance-dependent effect for the transmission of stereochemical information. In contrast, a linkage effect leading to reduced chirality was observed when an additional linker unit was incorporated in-between the beta-turn mimetic structure and the polymer backbone. The reported data do show that transfer of chirality is possible after polymerization over a relatively short distance of atoms.