Synthesis Of Novel Optically Active Polymethacrylamides And Study Of Their Properties

Recently,optically active polymers have attracted ever-increasing attentions owing to their unique structures and properties such as chiral recognition,enantiomer separation,asymmetric catalysis,chiral memory,and so on.Various kinds of optically active polymers had been synthesized in the past several decades and widely applied in the fields of chiral separation materials,liquid-crystal materials,photoelectric sensors,biomedical chemistry,etc.In this work,a series of optically active methacrylamide derivatives bearing chiral pendants were synthesized,respectively.Then their chiral recognition abilities as chiral stationary phases(CSPs)for HPLC and anion recognition abilities as chemosensor were investigated.Three novel optically active methacrylamide derivatives,N-[(R)-((R)-1-phenylethylaminocarbonyl)-benzyl]methacrylamide(RR-PEBM),N-[(R)-((S)-1-ph-enylethylaminocarbonyl)-benzyl]methacrylamide(RS-PEBM),and N-[(R)-phenylme-thylaminocarbonylbenzyl]methacrylamide(R-PMBM),were separately synthesized and radically polymerized to obtain the corresponding polymers(P(RR-PEBM)s,P(RS-PEBM)s,and P(R-PMBM)s)in the present study.An interesting effect of the chiral carbon atom,which was away from the backbone,on the formations of the hydrogen bonds between the amide groups and chiroptical properties of polymers was observed.The specific optical rotation of P(RS-PEBM)and P(R-PMBM)showed a slight dependence on the solvent polarity,but in contrast,the polarity of the polymerization solvents clearly affected the values of the specific rotation of the P(RR-PEBM)s.In addition,the opposite sign of P(RR-PEBM)s compared to the monomer indicated that the optical activity of the P(RR-PEBM)s did not solely arise from the chirality of the side groups but also from the chiral secondary structure,most likely with a helical conformation,when the polymer main chain was generated.The polymer prepared in toluene displayed a larger optical rotation and stronger cotton effects than the one prepared in methanol.These results indicated that the hydrogen bonds formed between the amide groups in the monomers played an important role in the formation of the partial helical structure of the polymer.The secondary structure of P(RR-PEBM)was relatively stable upon heating but unstable in THF by adding MeOH.The novel methacrylamide monomer RR-PEBM and RS-PEBM were polymerized at 60? initiated by AIBN to obtain corresponding polymers in good yields.The effects of the solvents,Lewis acids and the concentration of Lewis acids on polymerization of the monomer were examined.The results showed that the rare earth metal trifluoromethanesulfonates affected the stereoregularity of polymers.When the Lewis acids were used in polymerization,the optical active polymers possessed isotactic-rich structure.The results of optical rotation and CD spectra showed that the optical activity of the polymers were relative to the stereoregularity of polymers.In addition,optically active copolymers were prepared via free radical polymerization of RR-PEBM with MMA initiated by AIBN at 60?.GPC?polarimeter and DSC were employmed to characterize the properties of the products.A remarkable change in optical activity was observed during the transformation from monomers to the corresponding copolymers.The opposite sign of copolymers compared to the chiral monomer from which it was formed indicates a chiral secondary structure of the copolymer main chain was generated.On the other hand,the composition of monomers had great effects on the properties of copolymers.Due to the formations of hydrogen bonds of monomer RR-PEBM with methanol,the copolymer prepared in toluene displayed a great difference on the chiroptical properties than the one prepared in methanol,these results indicated that hydrogen bonds played an important role in properties of copolymers.One chiral monomer N-[(R)-?-tert-butoxycarbonylbenzyl]methacrylamide(R-BCBMAM)was synthesized via amidation reaction between methacryloyl chloride and R-phenylglycine tert-butyl ester.Optically active polymers were prepared via free radical polymerization of(R-BCBMAM)initiated by AIBN at 60?.In the presence of catalytic amount of trifluoroacetic acid,the tert-butyl of P(R-BCBMAM)were removed to obtain poly {N-[(R)-a-hydroxylcarbonylbenzyl]methacrylamide} P(R-CB-MAM).1H-NMR.IR?CD and GPC were employmed to characterize the chemical structure of the products.The results indicated that the polymerization condition had slight effects on the chiroptical properties of P(R-BCBMAM).The results from specific optical rotation and circular dichroism spectra indicated that the P(R-CBMAM)and P(R-BCBMAM)had great difference on the chiroptical properties.On the basis of the results from circular dichroism(CD),the optical activity of P(R-BCBMAM)was attributed not only to the chirality of amino acid side groups in monomeric units but also to the steric repulsion of tert-butyl groups.P(R-BCBMAM)and P(R-CBMAM)were coated onto silica gel as the chiral stationary phase(CSP)for high performance liquid chromatography(HPLC).The CSPs exhibit superior chiral recognition ability for some racemates.On the other hand,1H-NMR measurements of mixture of polymers and racemic 1,1'-bi-2-naphthol(BINOL)were conducted in chloroform-d.The characteristic hydroxyl proton signal of BINOL was split into two peaks ascribed respectively to the levo-and dextro-isomer.The novel optically active monomers N-[(S)-((S)-1-phenylethylaminocarbonyl)-benzyl]methacrylamide(SS-PEBM)and RR-PEBM were polymerized to obtain the corresponding polymers bearing chiral pendants via RAFT method.The effects of the concentration of the monomers on polymerization were examined.As evidenced by the nearly linear relationship between the SS-PEBM/CDB ratio and number-average molecular masses(Mn),a good control of polymerization was achieved with this system.The average molecular weights determined by GPC analysis were consistent with the theoretical values for these polymers.This control of the polymerization was also supported by the relatively low polydispersity,with Mw/Mn values ranging from 1.15 to 1.23.The resulting polymers showed high selectivity for F-relative to other anions including,Cl-,Br-,HSO4-,AcO-,NO3-and H2PO4-in DMSO solution as judged from UV-vis,fluorescence,and circular dichroism spectrophotometric titrations.Compared with its monomer,the polymer had proven to be more attractive for detection of F-in terms of sensitivity and reproducibility.The results showed that the polymer could be used as a new type of chemosensor to detect the F-.