Optically Active Polymers Of N-Phenylacrylamides Bearing A Bulky Oxazoline Pendant: Synthesis And Characterization

Natural occurring polymers, such as proteins, polysaccharides and nucleic acids, are optically active and have a helical structure, which plays an important role for the polymers to exhibit their potential functions, such as molecular recognition, chiral resolution and asymmetric catalysis. Just like chiral small compounds, their optical activity is induced by their asymmetric structure in configuration or/and conformation. Therefore, the design and synthesis of optically active polymers and the structure-chiroptical property relationship have become one of the most attractive challenges in polymer science.It was found that some polymers exist stable one-handed helical structure in the presence of bulk side chains. Here, a novel type of optically active N-substituted acrylamides bearing an oxazoline group (OPAMs) has been synthesized: N-[o-(4-methyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]acrylamide ((S)-MeOPAM), N-[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]acrylamide ((5)-P_rⁱOPAM), and N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl)-phenyl]acrylamide ((S)-MOPAM). The present synthesis gives the monomers in moderate yields and is more economical with improved reaction conditions. The monomers have been structurally confirmed by FT-IR, ¹H NMR, and elemental analysis.The radical polymerization of these chiral N-substituted acrylamides has been investigated in detail in the absence and presence of various rare earth metal trifluoromethanesulfonates (triflates) (Ln(OTf)₃) in different solvents, especially alcohol solvents. We found that rare earth metal triflates, as a Lewis acid, catalytically changed the stereoregularity of polymers during the radical polymerization. The optical rotation [α]²⁵_D of poly(MeOPAM) in the presence of Y(OTf)₃ obtained in n-butanol changed from +10.1° to-33.4°. This reversion of optical rotation also happened to poly(MeOPMAM)s in the same condition. The nature of solvents strongly influences the effect of the rare earth metal triflates, and the polymerization at 60° in n-butanol, afforded the polymer having higher isotacticity, while syndiotactic-rich polymers were obtained in methanol.The ¹H NMR study on interaction of polymers with racemic 1, 1'-bi-2-naphthol (BINOL) was carried out and it is noteworthy that highly isotactic polymers have stronger interactions with BINOL, indicating the optically active polymers showed considerable chiral recognition ability. The reason can presumably be ascribled to the helical structure formed in the polymer to some extend. Higher asymmetric structure always endows the polymer better chiral recognition ability.In the presence of Y(OTf)₃, a sharp change of molecular weight was observed for poly(MeOPMAM)s and poly(P_rⁱOPAM) obtained in n-butanol solution. The number-average molecular weight of poly(MeOPMAM) changed from 4.1×10³ to 27.8×10³. These results were not found for poly(MeOPAM)s and poly(PhOPAM)s though. Noteworthy is also the polymers obtained in THF only give a relatively low molecular weight compared with those obtained in alcohol solvents.In conclusion, the effects of the polymerization conditions were strongly depended on the reaction conditions, such as rare earth metal triflates, solvents as well as the monomer structures. The polymers of different molecular weight and tacticities can be obtained under moderate conditions. Highly isotactic poly(OPAM)s and poly(OPMAM)s will give great potential functions while immobilized on silica gel and used as chiral stationary phase (CSP) for HPLC resolution of various racemic compounds.