Synthesis And Characterization Of Optically Active Amphiphilic Diblock Copolymers

Chirality plays an important role in many fundamental interactions of living systems,since biological activity is strictly correlated with stereochemistry.Chiral micelles,aggregated by amphiphiles with a segment of optical activity,have attracted more and more attention in the past decades because of their fascinating properties such as molecular recognition,asymmetric catalysis,and chiroptical switch in aqueous milieu.In the present work,two novel types of optically active amphiphilic block copolymers with poly(ethylene glycol)as the hydrophilic segment,PEG-b-poly[N-(2-oxazolinylphenyl) maleimide](PEG-b-PPhOPMI)and PEG-b-poly[N-(2-oxazolinyl phenyl)methylacrylamides/acrylamides(PEG-b-POPMAM,PEG-b-POPAM),were synthesized via oxyanion-initiated polymerization and coupling reaction,respectively. The copolymers.have well-defined structure as ascertained by ¹H NMR,elemental analysis,gel permeation chromatography(GPC),and Fourier transform infrared (FT-IR).Also,the self-assembly behaviors of these amphiphilic copolymers were examined in detail.The amphiphilic copolymer PEG-b-PPhOPMI could be synthesized by using potassium alkoxide of poly(ethylene glycol)monomethyl ether(MeOPEGO^-K⁺)as the macroinitiator for the oxyanion-initiated living polymerization of the chiral monomer,N-{o-(4-pheny1-4,5-dihydro-1,3-oxazol-2-yl)phenyl}maleimide [(R)-PhOPMI].The length of the PPhOPMI segment and the specific rotation of the obtained copolymers could be controlled by the molar ratio of the chiral monomer to the macroinitiator.The micellization was examined for the amphiphilic block copolymers in aqueous milieu by fluorescence spectroscopy(FL),dynamic light scattering(DLS), and circular dichroism(CD).The results indicate that the copolymers could form regular spherical micelles with core-shell structure when the hydrophilic component was long enough;in contrast,the copolymers containing shorter PEG segments formed aggregates in large dimension due to the considerable interaction between hydrophobic PPhOPMI components.Also,it was found that the aggregated structure and chiraoptical properties of the polymeric micelles are strongly dependent on the medium nature and the polymer concentration,which means by studying the chiraoptical properties of the copolymer in water we can obtain the information of micellization such as critical micelle concentration(cmc).The radical polymerization of N-(2-oxazolinylphenyl)acrylamides in the absence and presence of yttrium trifluoromethanesulfonate(triflate)[Y(OTf)₃]in n-butanol has been investigated by CD and TEM.The results suggest that Y(OTf)₃-assisted radical polymerization afforded isotactic-rich polymers,which could form helical structure in solution.It was found that acrylamide-derived optically active polymers (POPAM and POMPAM)and their monomers exhibit fluorescence properties.The excimer emission of polymers is larger than that of monomers,with the fluorescence quantum yield(Φ_f)about 10%.Moreover,the configuration of polymer main chain affects the fluorescence properties of the polymer to some extent.Coupling reaction was applied to the synthesis of novel optically active amphiphilic block copolymers,PEG-b-POPAM)and PEG-b-POMPAM,which possessing photoluminescence(PL)properties.The investigation on micellization behaviors,chiroptical and fluorescence properties of copolymer in aqueous milieu indicated that the variation in chiroptical and fluorescence properties is evidently correlative with the aggregated state of the block copolymers,that is,the cmc value of this type of copolymers could be determined by CD or fluorescence technique.The solvent nature affected the self-assembly behavior,chiroptical and fluorescence properties of PEG-b-POPAM and PEG-b-POPMAM significantly.Especially,their fluorescence effect was enhanced greatly by aggregation in highly hydrophilic solvent system.This class of organic fluorophores,showing aggregation-enhanced emission, is currently at the very initial state of investigation,with some demonstrations of promising uses for electroluminescence devices,fluorescent switching or sensing,and optical storage.The chiral recognition ability of homopolymer(PMeOPAM and PMeOMPAM) and chiral micelles formed from PEG-b-PMeOPAM or PEG-b-PMeOPMAM was investigated by steady-state fluorescence technique using 1,1'-bb2-naphthol [(R)/(S)-BINOL],(D)/(L)-malic acid,and(R)/(S)-mandelic acid as analytes.The results indicated that the homopolymers showed considerable chiral recognition ability to above enantiomeric substrates,and highly isotactic polymers exhibiting better recognition performance.The chiral discrimination of chiral micelles to R/S-BINOL in water was more powerful than corresponding homopolymer in THF. But to the water-soluble analytes,i.e.,(D)/(L)-malic acid and(R)/(S)-mandelic acid, no chiral discrimination was observed in aqueous media.The fluorescence of PEG-b-PMeOPAM micelle in the presence of BINOL with various enantiomeric compositions was studied,showing a linear relationship betweenâ… /â… ₀ and the percent of the S component of BINOL.The enantioselectivity in fluorescence response was found to be due to the difference in enantioselective solubilization of hydrophobic micelle core to BINOL,which resulted to the time-dependenece of chiral discrimination.That is,the discrimination process possessed a dynamic character,the prolongation will led to the disappearance of chiral recognition ultimately.It is expected that the optically active amphiphilic block copolymers obtained in the present research could be used in the asymmetric catalysis chiral recognition and separation of enantiomers.