Optically Active Polymers Of N-Phenylmaleimides Bearing Bulky Oxazoline Substituents And Chiral Supramolecular Aggregatation

Chirality is one of intrinsic properties for the nature. All the reactions relative to the living activity carry out in highly asymmetric environment. Biomacromolecules, including protein, amylose, and nucleic acid, are of chiral. As same as chiral small compounds, the optically active polymers have an asymmetry in configuration or/and conformation. Studies on both the synthesis for optically active polymers and the structure-chiroptical property relationship have become a challenging topic in the polymer science, which allow the development of noval materials with chiral functions.N-Substituted maleimide derivatives can polymerize in a trans-addition manner due to their cyclic structure. In the present work, a novel type of chiral monomers has been synthesized by introducing an oxazoline group into the parent compound N-phenylmaleimide, i.e., N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]maleim-ide ((R)-PhOPMI), N-[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]maleimide ((S)-PrOPMI), N-[o-(4-ethyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]maleimide ((R)-EtOPMI). and N-[o-(4-methyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]maleimide ((S)-MeOPMI). The present synthesis is more economical and effective than the reported procedure. The structure of the monomers and their precursors were confirmed based on FT-IR, ¹H NMR, and elemental analyses.The polymerization characteristics have been examined in detail for the oxazoline-substituted N-phenylmaleimides (OPMI) with initiators including n-BuLi, Et₂Zn, Et₂Zn/(-)-Sparteine, and Et₂Zn/n-BuLi. The polymerization initiated by n-BuLi produced optically active poly(OPMI)s with number-average molecular weights of 2×10³ - 6×10³. Among them, the biggest value of specific rotation [α]²⁵_d is-60.0°. It is noteworthy that the linear relationship of specific rotations versus molecular weights was observed for poly(PhOPMI)s; also, the directional reversion in the optical rotation presents at M_n=2700. ¹³C NMR analysis revealed that the polymer main-chain possess threo-diisotactic structure predominantly, which due to asymmetric inductive effect of chiral oxazoline pendants.The polymers obtained from Et2Zn-initiating system exhibit a quite strong optical activity, and [α]²⁵_D reached the value of -169.3°. For poly(PrⁱOPMI) and polyCMeOPMI), the optical rotation is opposite to their monomer's in direction. To explain the observation, a model of chiral supramolecular aggregate/complex containing Zn(II) was proposed based on ¹³C NMR, FT-IR, and CD spectral analysis. A similar initiator, Et₂Zn/(-)-Sparteine, yielded poly(OPMI)s with threo-diisotactic -rich structure in the main chain, indicative of strongly asymmetric inducation in this system. In this case, no polymeric complexes in-site formed because of the intensive coordination between sparteine and Zn(II) ions.The binary initiating system composed of Et₂Zn and n-BuLi combined the advantages of both metal alkyls, which gave the polymers with higher molecular weights and moderate optical activity ([α]²⁵_D = -97.1°～+24.3°). It was found that the content of Zn bound in polymeric matrix is all-important to the formation and optical activity for supramolecular aggregates. Also, the optical nature of the supermolecular aggregates depends on the (R)- or (S)-configuration of lateral groups.In the presence of copper(II) salt, a unique supramolecular aggregation was observed in tetrahydrofuran solution for the polymers obtained by Et₂Zn or Et₂Zn/n-BuLi initiators. The induced supramolecular chirality was found to be dependent on the absolute configuration of lateral residues, that is, the polymers with (R)-configurational oxazolinyl chromophores tended to form chiral aggregates with levorotatory handedness, while polymers obtained from the (S)-monomer offered dextrogyrate one upon the addition of Cu(II) salt. A model, in which side groups would arrange in a helix fashion, is employed for describing the unusual chirality.Copolymerization of OPMI and methyl methacrylate (MMA) was conducted by free radical mechanism. The monomer reactivity ratio and Alfrey-Price Q-e values have been determined. The resulting copolymer with higher molecular weight (M_n = 2600-27000) and optical activity ([α]²⁵_D = -7.8°～—36.6°) showed an excellent heat-resistance and moderate film-forming ability. Structural analysis indicates that OPMI monomer has a distinct tendency to copolymerization.It is expected that the optically active polymers and their chiral supramolecular aggregates obtained in the present research have some potential applications in the chiral recognition, separation of enantiomers, and asymmetric catalysis.