环状温敏性双亲性嵌段共聚物的合成及性能研究

Synthesis and Properties of Cyclic Thermo-responsive Double Hydrophilic Block Copolymers

首先基于端基为叠氮基团的可逆加成断裂转移试剂CPADB-N3,通过连续两步可逆加成断裂转移(RAFT)聚合制备了线性温敏性双亲性嵌段共聚物聚(N-异丙基丙烯酰胺)-b-聚(寡聚乙二醇单甲醚甲基丙烯酸酯)(PNIPAAm-b-POEGMA);进一步通过一锅法胺解-迈克尔加成反应将二硫酯修饰为炔键,得到了线性前体l-(NIPAAm-b-POEGMA),最后通过极稀条件下,一价铜催化的分子内点击化学的偶联制备得到了环状温敏性双亲性嵌段共聚物c-(PNIPAAm-b-POEGMA).通过核磁共振氢谱(1H-NMR)、碳谱(13C-NMR)、红外光谱(FTIR)、尺寸排除色谱与多角度激光光散射联用(SEC-MALLS)等方法表征了聚合物的分子结构并证明了环状聚合物的成功合成;通过紫外可见分光光度计、动态光散射和透射电镜对比研究了环状与线性聚合物的温敏性与自组装行为.研究发现环状聚合物具有比线性聚合物更高的较低临界溶解温度(LCST),同时其在水相中自组装形成的胶束具有更小的粒径和更窄的粒径分布.

Cyclic topology exerts significant effects on the properties and potential applications of polymers;however, the precisely controlled systhesis of cyclic diblock copolymers remains challenging. In this study, we reported a novel and versatile synthetic strategy toward cyclic diblock copolymers by using the linear polymer precursors generated from reversible addition-fragmentation transfer (RAFT) polymerization. Key innovation of the technique proposed lies in the facile and complete conversion of terminal RAFT groups on linear polymers into clickable alkyne groups via a one-pot aminolysis/Michael addition reaction, which laid a foundation for subsequent intrachain Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAc) on the linear α-alkyne-ω-azide preursors. Full decoration of the RAFT termini was confirmed by Ellman study. Specifically, a cyclic double hydrophilic block copolymer (DHBC), poly(N-isopropylacrylamide)-b-poly(oligo(ethylene glycol) monomethyl ether methacrylate)(c-(PNIPAAm-b-POEGMA)), with thermo-responsiveness was synthesized following this approach successfully as confirmed by 1H-NMR, 13C-NMR, FTIR, and SEC-MALLS analyses. Thermo-induced phase transitions and self-assembly behaviors of the resulting cyclic DHBCs were then investigated by the combined analytical techniques of UV-Vis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM), and further compared with those of the linear analogues. Intriguingly, the cyclic thermo-sensitive DHBCs exhibited a lower critical solution temparature (LCST, 41.5 ℃) significantly higher than the linear counterparts (39 ℃), for POEGMA moiety with two block juntions in the cyclic copolymers could raise the LCST of PNIPAAm segment. More importantly, the spherical micelles self-assembled from cyclic DHBCs above LCST were smaller in size and narrower in size distribution compared with the ones derived from linear analogues, which resulted most likely from a more restricted cyclic topology. This study therefore developed an efficient alternative synthetic method for cyclic diblock copolymers and meanwhile provided new insights into the structure-property relationships of cyclic DHBCs.