Pinaud Julien, Placet Emeline, Lacroix-Desmazes Patrick, Trinh Thi Kim Hoang, Malval Jean Pierre, Chemtob Abraham, Pichavant Loïc, Héroguez Valérie
ICGM, Université de Montpellier, CNRS, ENSCM;
ICGM, Université de Montpellier, CNRS, ENSCM.
J Vis Exp. 2018 Nov 29(141). doi: 10.3791/58539.
We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesHBPh4) as the protected form of NHC is employed. IMesHBPh4 can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesHBPh4, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.
我们报道了一种在365 nm紫外光照射下生成N-杂环卡宾（NHC）1,3-二甲基咪唑-2-亚基（IMes）的方法，用于表征IMes并确定相应的光化学机理。然后，我们描述了一种使用该NHC光生成系统在溶液和微乳液中进行开环易位聚合（ROMP）的方案。为了光生成IMes，采用了一种以2-异丙基噻吨酮（ITX）为敏化剂、1,3-二甲基咪唑四苯基硼酸盐（IMesHBPh4）为NHC保护形式的体系。IMesHBPh4可通过1,3-二甲基咪唑氯化物与四苯基硼酸钠之间的阴离子交换一步获得。描述了一种实时稳态光解装置，这表明光化学反应分两个连续步骤进行：1）ITX三线态被硼酸根阴离子光还原；2）随后咪唑阳离子发生质子转移以生成预期的NHC IMes。实施了两种单独的表征方案。首先，将CS2添加到反应介质中，通过形成IMes-CS2加合物来证明NHC的光生成。其次，使用酸碱滴定法定量原位释放的NHC的量。还讨论了该NHC光生成系统用于降冰片烯的ROMP。在溶液中，通过将ITX、IMesHBPh4、[RuCl2（对异丙基苯）]2和降冰片烯在CH2Cl2中混合，然后在紫外反应器中照射该溶液来进行光聚合实验。在分散介质中，首先形成单体微乳液，然后在环形反应器内照射以产生稳定的聚（降冰片烯）胶乳。
