光学活性(甲基)丙烯酰胺的立体定向自由基聚合——立体控制机理探讨

利用Lewis酸三氟甲磺酸镱(Yb(OTf)3)调控(S)-N-(2-羟基-1-苯乙基)甲基丙烯酰胺((S)-HPEMA)和(S)-N-(2-羟基-1-苯乙基)丙烯酰胺((S)-HPEA)的自由基聚合,得到相应的光学活性聚合物;研究了反应条件对聚合物立体结构的影响,发现以甲醇和正丁醇为溶剂时,Yb(OTf)3的存在可显著提高聚合物的mm三元组的含量,而以DMSO为溶剂时,Yb(OTf)3对聚合物的立构规整性没有明显改变;通过1H-NMR研究了Yb(OTf)3与单体之间的相互作用,结果表明单体的酰胺基团与Lewis酸的稀土金属离子间的配位以及单体的羟基与Lewis酸的三氟甲磺酸根间的氢键影响了单体的加成方向和聚合物的立体结构.

STEREOSPECIFIC RADICAL POLYMERIZATION OF OPTICALLY ACTIVE (METH) ACRYLAMIDE——STUDY ON THE MECHANISM OF STEREOCONTROL

Optically active poly(meth)acrylamides were synthesized by the stereospecific radical polymerization of(S)-N-(2-hydroxy-1-phenylethyl)methacrylamide((S)-HPEMA) and(S)-N-(2-hydroxy-1-phenylethyl)acrylamide((S)-HPEA) in the absence and presence of Lewis acid ytterbium trifluoromethanesulfonate(Yb(OTf)3).The effect of the Lewis acid on the radical polymerization of(S)-HPEMA and(S)-HPEA was investigated.In the presence of a catalytic amount of the Lewis acid,the isotactic specificity of the polymers remarkably increased when the radical polymerization proceeded in methanol and n-butanol.However,the isotactic selectivity disappeared when the radical polymerization proceeded in DMSO.The interaction between the Lewis acid and monomers(S)-HPEMA and(S)-HPEA was investigated by 1H-NMR.It was found that the monomers strongly interacted with the Lewis acid in methanol-d4,which included the coordination of the amide group to the rare earth metal ion and the hydrogen bond between the hydroxyl group of the monomers and the trifluoromethanesulfonate ion of the Lewis acid.The plausible mechanism of stereocontrol in the radical polymerization of(S)-HPEMA and(S)-HPEA was analyzed.The monomers are activated by the interaction with the Lewis acid,and the activated monomer is preferentially polymerized.Therefore,the Lewis acid is incorporated into the propagating chain end.The Lewis acid could interact with two or more structural units,because of its high coordination numbers and more than one triflate ions,and control the stereochemistry of the polymerization to an isotactic-selective manner.The Lewis acid might weakly interact with the polymer chain to be readily transferred to another monomer and activate it.