Chiroptical Switching Polyguanidine Synthesized by Helix-Sense-Selective Polymerization Using [(R)-3,3-Dibromo-2,2-binaphthoxy](di-tert-butoxy)titanium(IV) Catalyst

摘要

A series of chiral binaphthyl titanium alkoxide complexes were synthesized.Among them,chiral titanium complex [(R)-3,3'-dibromo-2,2'-binaphthoxy](di-tert-butoxy)titanium(IV) (R-3) exists as a crystal-lographic C_2 dimer in the solid state but a monomer in solution at room temperature.Application of R-3 in the helix-sense-selective polymerization of achiral carbodiimide,N-(1-anthryl)-N-octadecylcarbodiimide (1),yielded a well-defined regioregular,stereoregular poly[N-(1-anthryl)-N-octadecylguanidine] (poly-1b) with a relatively narrow polymer dispersity index of 2.7.Full racemization of poly-1b at +80degC in toluene requires more than 100 h.Interestingly,poly-1b was found to undergo fast reversible chiroptical switching at +38.5 degC in toluene.Furthermore,at room temperature,poly-1b shows a positively signed Cotton effect in toluene,but negative ones in THF and chloroform,respectively.The chiroptical switching takes place around the toluene content of 90% (vol) in the mixed toluene/THF solvents.This is the first example of chiroptical switching phenomenon occurring in a helical polymer possessing no chiral moieties in the polymer chains.We believe this reversible chiroptical switching phenomenon occurs by reorientation of anthracene rings relative to the chain director.