Exploring Versatile Sulfhydryl Chemistry in the Chain End of a Synthetic Polylactide

Abstract

Synthesis of an end-functionalized polylactide by ring-opening polymerization of lactide monomer using a functional initiator containing pyridyl disulfide group is reported. Molecular weight of the polymer determined by GPC matched very well with that determined by end-group analysis using the UV/vis method, suggesting survival of the end-group functionality during polymerization. DTT-induced reduction of the pyridyl disulfide group produced free sulfhydryl group quantitatively which was utilized for versatile chain-end modifications using various thiol-mediated high-yielding chemical transformations including thiol–ene, thiol–maleimide, and thiol–acrylate “click” reactions. This strategy was further extended to link two macromolecules by reaction of sulfhydryl-functionalized polylactide and acrylate-terminated poly(ethylene oxide) (PEO) which produced a block copolymer with an acid-labile β-thiopropionate linker between the two constituent blocks. This functional group could be cleaved under mild acidic condition to produce the individual parent polymers. Further as-synthesized pyridyl disulfide-terminated polylactide was treated with thiol-functionalized sugar moiety and n-type semiconducting naphthalene diimide (NDI) chromophore which also generated quantitative chain-end functionalization by thiol–disulfide exchange reaction. NDI-functionalized polylactide showed white light emission due to mixed emission from monomeric and excimer-type species. Further atomic force microscopic (AFM) studies revealed NDI-functionalized polymer formed uniform spherical aggregates upon drying of a drop-casted film on silicon surface possibly due to solvent-evaporation-induced defined organization of the polymer chain dictated by strong π-stacking interaction among the NDI chromophores.