"Helix-sense-selective Induction" Strategy For Constructing Chirally Helical Substituted Polyacetylene Micro-nano Materials

Chirality and helicity are fundamental characteristics of nature and responsible for the functioning of living systems,thus attracting enormous attention from researchers engaged in multi-disciplines.They also show great fascination to polymer scientists,especially when they are combined in one polymer(i.e.,chirally helical polymer).In recent years,chirally helical polymers have witnessed explosive progress.Their types and synthetic methods have been largely diversified,and many of them have found important applications in chiral-related fields,such as enantioselective recognition/separation,asymmetric catalysis,etc.However,in most cases,such advanced polymers still cannot be prepared without chiral monomers.This situation has seriously hindered the further progress of the relevant research topics due to the high cost and limited designability of chiral monomers.Fortunately,helix-sense-selective polymerization and helical chirality induction techniques were established for preparing chirally helical polymers from achiral monomers and the corresponding polymers,respectively.Nevertheless,to date,these two techniques are still basically limited to solution systems and simply provided optically active solutions or irregular bulk materials.In order to overcome the limitations,we in this thesis developed helix-sense-selective precipitation polymerization,helically twining polymerization,and helix-sense-selective co-precipitation,by which we obtained optically active micro-nano materials with regular and controllable morphology.The major innovations and achievements of this thesis are as follows.1.We realized the first helix-sense-selective polymerization in precipitation polymerization systems,by which optically active helical polymer particles were constructed from achiral monomer.Achiral acetylenic monomer M1 with bulky adamantyl group smoothly underwent helix-sense-selective precipitation polymerization(HSSPP)with(nbd)Rh+B-(C6,H5)4 as catalyst in the presence of Boc-L-or Boc-D-alanine as chiral additive.Scanning electron microscopy(SEM)images show that the polymer particles with regular spherical morphology can be prepared in certain solvent mixture(THF/n-heptane = 1/10(mL/mL))in high yield.Circular dichroism(CD)and UV-vis spectra clearly demonstrated that helix-sense-selective polymerization took place in the course of precipitation polymerizations.Moreover,the induced helical structures with preferential screw sense exhibited relatively high thermo-stability.According to the FT-IR spectra,we proposed a possible mechanism for the occurrence of HSSPPs and the formation of stable helical structures:chiral additive molecules inserted in the helical grooves by forming hydrogen bonds with the neighboring two amide structures in the polymer pendant groups,thus controlling and stabilizing the screw sense of the helical polymer chains.Based on this mechanism,a "chiral-lock effect" was further put forward.2.We designed and synthesized a novel kind of achiral acetylenic monomer M2 with triphenyl group.This interesting monomer can form physical gels when polymerized in non-aromatic solvents with low polarity(such as CHCl3)due to the strong ?-? interaction among the polymer chains.Inspired by this phenomenon,we achieved the first HSSPP for directly constructing optically active physical gels(OAPGs)by using M2 as monomer and R-or S-1-phenylethylamine as chiral additive.The one-handed helical conformations of the resulting polymers and the optical activity of the gels thereof were verified by CD and UV-vis absorption spectra.SEM images show that the OAPGs were constructed by helical nanofibers with remarkable one-handed screw sense.After complete removal of the chiral additive,the purified OAPGs and the corresponding polymers still showed intense CD signals,demonstrating their excellent chiral memory ability.This significant advantage and the simple preparative methodology endow the OAPGs with great potentials as chiral functional materials.3.The synthesis of chiral crosslinked microspheres(CCMs)of substituted polyacetylenes with diameter of 1-50 ?m remains as an intractable task.In this work,such CCMs(with diameters of 1.3-2.3 ?m)were successfully prepared for the first time through precipitation polymerization of M3 using 1,4-Diethynylbenzene(DEB)as crosslinker.Based on macroscopic and microscopic observations,a mechanism for the precipitation polymerization of acetylenic monomers in the presence of crosslinker is proposed.In previous studies,the chiral additives cannot be reused in helix-sense-selective polymerization of substituted acetylene monomers,which leads to the waste of chiral resources.Excitingly,the CCMs prepared above were successfully applied as reusable chiral additive for the helix-sense-selective polymerization of M1.Moreover,the CCMs have the potential to be applied as chiral stationary phase for high-performance liquid chromatography.4.Inspired by the above“chiral-lock effect" and the helical chirality induction in literature,we in this study developed a novel 3-step methodology for preparing double helices consisting of helical substituted polyacetylenes(DHSPs):(1)an optically inactive helical polymer is synthesized using an achiral monomer;(2)the as-obtained polymer is induced to form one-handed helix by“locking" a chiral monomer in the helical grooves;(3)the "locked"chiral monomer undergo helically twining polymerization along the pre-formed one-handed helical polymer chains,thereby constructing the DHSPs.We used achiral acetylenic monomer M4 and chiral acetylenic monomer M5 to verify the above methodology.CD and UV-vis absorption spectra in combination with high resolution transmission electron microscopy(TEM)images demonstrate the optical activity,double helical structure,and helix sense of the thus-obtained DHSPs.The convenient synthetic strategy is expected to provide various kinds of double helical polymers.5.We prepared a new class of optically active helical polymer nanoparticles/graphene oxide(OAHPNs/GO)hybrid nanocomposites through an unprecedented strategy—the co-precipitation of optically inactive helical polymers(PI,P4,P6)and chiral ly modified GO in tetrahydrofuran/water system.This approach is named as helix-sense-selective co-precipitation(HSSCP),in which the chirally modified GO acted as chiral source for inducing and further stabilizing the predominantly one-handed helicity in the polymers.SEM and TEM images show the quite similar morphologies of all the obtained OAHPNs/GO nanocomposites;specifically,the chirally modified GO sheets were uniformly decorated with spherical polymer nanoparticles.CD and UV-vis absorption spectra confirmed the preferentially induced helicity in the helical polymers and the optical activity of the nanocomposites.The established HSSCP strategy is thus proven widely applicable and is expected to produce numerous functional OAHPNs/GO nanocomposites.