几种手性原子转移自由基聚合引发剂的合成及其在螺旋选择性聚合中的应用

Synthesis of Several Chiral Atom Transfer Radical Polymerization Initiators and Its Helix-sense-selective Initiating Function for ATRP

合成了(S)-2,2'-二(溴甲基)-1,1'-联萘,(S)-2'-甲基-2-溴甲基-1,1'-联萘,α-溴代苯乙酸薄荷酯,N-薄荷基-α-溴代苯乙酰胺和α-溴代苯乙酸胆甾烷醇酯5种旋光的溴代烷并将其用作非手性单体甲基丙烯酸-1-苯基二苯并环庚醇酯(PDBSMA)的原子转移自由基聚合(ATRP)的手性引发剂.为了使这些手性引发剂在引发一步生成的初级自由基不发生消旋化,引发剂中的手性中心都不直接与溴原子相连.用这5种手性溴代烷做引发剂引发PDBSMA的ATRP所得聚合物可分成四氢呋喃(THF)可溶部分和THF不溶部分.THF可溶部分具有较大的比旋光度.对THF可溶部分的手性光学性质研究以及比较该部分聚合物和在同样引发条件下得到的甲基丙烯酸甲酯聚合物的比旋光度,我们得出聚合物大的比旋光度是由聚合物单手性螺旋过量引起的结论,即合成的手性引发剂对PDBSMA的ATRP均有一定的螺旋选择性,其中(S)-2,2'-二(溴甲基)-1,1'-联萘、(S)-2'-甲基-2-溴甲基-1,1'-联萘螺旋选择性最好.引发剂的螺旋诱导能力跟聚合反应的温度有很大关系,聚合温度上限为70℃,在0～70℃之间,随着温度的升高引发剂的螺旋选择性逐渐增强.

Five optically active alkyl bromides were synthesized and were used as chiral atom transfer radical polymeriza- tion （ATRP） initiators for the helically selective polymerization of 1-phenyldibenzosuberyl methacrylate （PDBSMA）. In order to avoid the racemization of the primary free radical formed in initiating step, the optically active alkyl bromide initia- tors were designed as such a structure in which the bromine atom did not connect to the chiral center. The chiral ATRP ini- tiators tested here are （S）-2,2＇-di（bromomethyl）-l,l＇-binaphthyl, （S）-2-bromomethyl-2-methyl-l,l＇-binaphthyl, menthyl a-bromophenylacetate, N-menthyl a-bromophenylacetamide and （3fl,5a）-cholestan-3-yl a-bromo phenylacetate. （S）-2,2＇- Di（bromomethyl）-l,l＇-binaphthyl, （S）-2-bromomethyl-2-methyl-l,l＇-binaphthyl and menthyl a-bromophenylacetate were synthesized following the method reported by literatures. N-menthyl a-bromophenylacetamide was synthesized by adding dropwise the solution of α-bromophenylacetyl chloride in tetrahydrofuran （THF） into the mixture of menthylamine, triethyl- amine and THF with stirring at room temperature within 3 h and stirring at room temperature for another 8 h after the addi- tion. （3β,5α）-Cholestan-3-yl α-bromo phenylacetate was synthesized using the method for the synthesis of menthyl a-bromophenylacetate except using （3β,5α）-cholestanol instead of menthol. The poly-PDBSMA obtained from the ATRP initiated by the optically active alkyl bromides was fractionated into THF-soluble part and THF-insoluble part. THF-soluble part was optically active. By studying the chiroptical properties of the THF-soluble part of the poly-PDBSMA and by com- paring the optical rotation of the THF-soluble part of the poly-PDBSMA with that of the poly（methyl methacrylate） （poly-MMA） obtained under the same polymerization condition, we concluded that the ATRP initiated by the optically active alkyl bromides was helix-sense selective. The NMR study of poly-MMA derived from the THF-insoluble part showed that the polymer produced from the polymerization initiated by （S）-2,2＇-di（bromomethyl）-l,l＇-binaphthyl was isotactic, which proved further that the ATRP initiated by the optically alkyl bromines was helix-sense selective. Among the five optically active alkyl bromides, （S）-2,2＇-di（bromomethyl）-l,l＇-binaphthyl and （S）-2-bromomethyl-2-methyl-l,l＇-binaphthyl had the best helical selectivity. The helix-sense selectivity depended on the polymerization temperature. The ceiling temperature is 70 ℃. In the range of 0--70 ℃, the helix-sense selectivity increased with the increase of the polymerization temperature.