乙烯基二联苯单体的螺旋选择性自由基聚合

为了深入理解乙烯基二联苯单体自由基聚合过程中的手性传递,进行了手性单体(+)-2-(S)-异丁氧羰基-5-(4′-己氧基苯基)苯乙烯、非手性单体2-丁氧羰基-5-(4′-己氧基苯基)苯乙烯的均聚反应及它们二者的共聚反应,探讨了聚合温度和溶剂性质对手性单体均聚物旋光活性、手性单体含量对共聚物旋光活性以及聚合反应溶剂的超分子手性对共聚物旋光活性的影响.研究发现,降低聚合温度、采用液晶性反应介质有利于得到旋光度大的聚合物;少量手性单体的引入即可诱导共聚物形成某一方向占优的螺旋构象,比旋光度随手性单体的含量增加呈线性增长;在胆甾相液晶中制备的非手性单体聚合物不具有光学活性.这些结果表明,该类乙烯基二联苯聚合物具有动态螺旋构象,其光学活性主要依赖于主链的立构规整度和侧基不对称原子的手性.

Helix-sense-selective Radical Polymerization of Vinyl Biphenyl Monomers

In order to understand in depth the chirality transfer during the radical polymerization of vinylbiphenyl monomers,the homopolymerizations of chiral monomer,(+)-2-(S)-sec-butyloxycarbonyl]-5-(4'-hexyloxyphenyl)styrene,and achiral monomer,2-(n-butyloxycarbonyl)-5-(4'-hexyloxyphenyl)styrene,as well as their copolymerization were carried out.The effects of the polymerization temperature and solvent on the chiroptical properties of chiral homopolymers,the chiral monomer content on the chiroptical properties of copolymers,and the supramolecular chirality of polymerization media on the chiroptical properties of the homopolymers derived from achiral monomer were systematically investigated.All the resultant homopolymers and copolymers containing chiral units took helical conformations with an excess of screw sense and showed much more intensive optical activities than the monomer.It was found that decreasing polymerization temperature and using liquid crystalline reaction media favored obtaining polymers with large optical rotations.The effect of using ordered reaction media on the optical activity of polymer was larger than that of lowering reaction temperature.This was probably due to the increased stereoregularity of polymer main chain yielded in liquid crystalline phase,which diminished the number of helical reversal.The incorporation of chiral monomer could induce an excess of helix sense of copolymer main chain.Contrary to "sergeants-and-soldiers" effect,where a tiny amount of chiral monomer can endow the copolymer with the optical activity equal to that of the homopolymer consisting of chiral monomer,the optical rotations of copolymers scaled up linearly with the chiral component.It was considered that the length of helical block between two helical reversals was too short for the copolymer units to effectively cooperate and amplify the chirality of stereocenters in the pendant groups.The polymer obtained from the polymerization of achiral monomer in cholesteric phase showed no discernible optical rotation.These results suggested that the vinylbiphenyl polymers studied in this work possessed dynamic helical conformations and their optical rotations were mainly relied on the stereoregularity of polymer main chain and the chiral atoms of the pedant groups.