Single-Electron-Transfer Nitroxide-Radical-Coupling Reaction at Ambient Temperature: Application in the Synthesis of Block Copolymers
Single-Electron-Transfer Nitroxide-Radical-Coupling Reaction at Ambient Temperature: Application in the Synthesis of Block Copolymers
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DOI：
10.1021/ma900156y
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发表时间：
2009-06
期刊：
影响因子：
5.5
通讯作者：
Q. Fu;Zhongning Zhang;Wencheng Lin;Junlian Huang
中科院分区：
文献类型：
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作者：
Q. Fu;Zhongning Zhang;Wencheng Lin;Junlian Huang
“Sharpless型点击化学”在过去十年中引起了相当大的关注，因为它提供了一种简单的方法来获得复杂的大分子结构，例如线性、星形、环状、接枝聚合物和作为树枝状聚合物的共聚物。然而，“Sharpless型点击化学”中使用的带有叠氮基的聚合物，由于其光敏性和热不稳定性，难以保存，需要特别小心。另一种类型的点击化学，狄尔斯-阿尔德反应 (DA) [4+2] 系统，提供了一种通过分子内或分子间反应使用二烯和亲二烯体的偶联策略。这显示出基于高分子化学的巨大潜力，特别是提供新材料。然而，马来酰亚胺或蒽末端官能化聚合物通常需要多步合成和纯化。 Monteiro 还报道了一种自由基偶联方法，通过外球电子转移机制从双官能 PS 制备高分子量多嵌段共聚物。最近，我们的研究小组发现，在金属催化剂和缀合物存在下产生的大分子自由基可以通过形成烷氧基胺键而被另一个聚合物链中的2,2,6,6-四甲基哌啶基-1-氧基（TEMPO）基团高效捕获。这种反应被称为“原子转移氮氧自由基偶联”（ATNRC）反应。一般以CuBr和N,N,N0,N0 0,N0 0-五甲基二亚乙基三胺(PMDETA)为催化剂，在较高温度下在ATNRC中生成大分子自由基，引起交联、链转移等副反应。一些由活性单体（例如甲基丙烯酸酯）获得的聚合物不能在 ATNRC 中进行，因为大分子自由基会发生大量的 β-氢转移。为了克服这些缺点，需要优化ATNRC的条件。 Percec报道了在环境温度下通过单电子转移活性自由基聚合（SET-LRP）制备的各种功能单体超快速合成超高分子量聚合物，并用Cu取代Cu产生自由基，这为我们在ATNRC系统中生成大自由基提供了一种新策略。本文将SET的反应条件应用于硝基氧自由基偶联反应，在环境温度下SET机制产生的大分子自由基被硝基氧自由基捕获，称为单电子转移硝基氧自由基偶联（SET-NRC）反应（方案1）。通过 SET 机制，Pn-X（含卤素聚合物）将 Cu 氧化为 Cu，生成活性大自由基（Pn ），并被 TEMPO-Pm（含 TEMPO 聚合物）有效捕获。同时，在N配体存在下，Cu在极性溶剂（甲醇）中歧化成Cu和Cu，从而再生Cu。然而，在 THF 中，未发现 Cu 歧化（参见支持信息）。在Cu/N,N,N0,N00, N00-五甲基二乙烯三胺存在下，溴端聚合物如聚苯乙烯（PS）、聚（丙烯酸叔丁酯）（PtBA）、聚（丙烯酸甲酯）（PMA）、聚（甲基丙烯酸甲酯）（PMMA）和TEMPO端聚合物、聚（环氧乙烷）（PEO）、聚（己内酯）（PCL）之间进行偶联反应。 (PMDETA)在环境温度(25(5℃)下。通过EO在四氢呋喃(THF)中的开环聚合(ROP)制备含TEMPO的PEO(TEMPO-PEO，Mn，NMR(来自1H NMR的分子量)：3700g/mol，Mn，GPC(来自GPC的分子量)：3500g/mol，Mw/Mn：1.08)，使用使用标准 ATRP 技术制备二苯基甲基钾 (DPMK) 和 4-羟基 2,2,6,6-四甲基哌啶基-1-氧基 (HTEMPO) 聚苯乙烯（PS-Br, Mn,NMR: 4100 g/mol; Mn,GPC: 4000 Mw/Mn: 1.07）前体（参见支持信息）。程序如下：将装有 TEMPOPEO (0.37 g, 0.1 mmol)、Cu(0) (6.4 mg, 0.1 mmol)、PMDETA (0.02 mL, 0.1 mmol) 和 THF (3 mL) 的安瓿通过三个冷冻-泵-解冻循环脱气，然后将溶解在 THF (2 mL) 中的 PS-Br (0.41 g, 0.1 mmol) 引入到反应釜中。室温下磁力搅拌3次后，将粗品浸入液氮中，通过中性氧化铝蒸发除去铜络合物，将粗品再溶解于CH3OH中，将保留的CH3OH溶液浓缩至恒重，得到纯品。在 SET 中，低解离能的 C-X 键通过外层单电子转移过程形成自由基阴离子中间体而裂解，这表明在我们的体系中，当 PS-Br 在 Cu(0)/PMDETA 存在下混合时，C-Br 键通过自由基阴离子中间体的形成和分解而发生解离。室温下，形成的大分子自由基(PS)立即被TEMPO基团捕获，在此过程中，PS-Br和TEMPO-PEO的摩尔比控制在1:1，TEMPO-PEO基团的存在可能会增加溶剂的极性，有利于偶联反应，但在偶联产物的紫外光谱中，仅在262 nm处检测到了Cu，而在720 nm处没有检测到Cu。当THF作为SET中的溶剂时，不进行Cu的配比（详细信息参见支持信息），通过TEMPO-PEO的膜分离和PS-Br的环己烷萃取，可以轻松去除剩余的TEMPO-PEO和PS-Br残留物（详细信息参见支持信息） 带有溴端基的聚苯乙烯（PS-Br）、带有TEMPO端基的聚环氧乙烷的凝胶渗透色谱（GPC）痕量。 (TEMPO-PEO) 和 PS-b-PEO 如图 1 所示，具有高斯分布和低多分散性 (Mw/Mn< 1.10)，通过引入 HTEMPO (0.017 g，0.1 mmol) 代替 TEMPO-PEO 进行偶联反应，以便*应通过 C A U L C O N SO R T 找到对应关系。 IA 澳大利亚 7 月 1 0, 2 00 9
The “Sharpless-type click chemistry” has attracted considerable attention during the past decade since it provides an easy way to obtain complex macromolecular architectures such as linear, star, cyclic, graft polymers, and copolymers as dendrimers. However, the polymers with azide group used in “Sharpless-type click chemistry” are difficult to be preserved due to their photosensitivity and thermal instability, whichmeans special care should be taken. Another type of click chemistry, the Diels-Alder reaction (DA) [4+ 2] system, provides a coupling strategy using a diene and dienophile by intraor intermolecular reaction. This shows great potential based on the macromolecular chemistry particularly providing new materials. However, the maleimide or anthracene end-functionalized polymers generally require multistep synthesis and purification. Monteiro also reports a radical coupling method to make highmolecular-weightmultiblock copolymers from a difunctional PS, by an outer-sphere electron transfer mechanism. Recently, our group found macroradicals, generated in the presence of metal catalyst and conjugates, could be instantly captured by the 2,2,6, 6-tetramethylpiperidinyl-1-oxy (TEMPO) group in another polymer chain by formation of alkoxyamine linkage with high effiency. This kind of reaction is named as “atom transfer nitroxide radical coupling” (ATNRC) reaction. Generally, CuBr and N,N,N0,N0 0,N0 0-pentamethyldiethylenetriamine (PMDETA) were used as the catalyst to generate macroradicals in ATNRC under a relatively high temperature, which could cause side reactions such as cross-link and chain transfer. Some polymers obtained from active monomers, such as methacrylic esters, cannot be conducted in ATNRC due to the significant β-hydrogen transfer from the macroradicals. To overcome these disadvantages, the condition ofATNRC should be optimized. Percec reported that an ultrafast synthesis of ultrahigh-molecular-weight polymers from various functional monomers prepared by single-electron-transfer living radical polymerization (SET-LRP) at ambient temperature and Cu was used to substitute the Cu to generate the radicals, which provides us a new strategy to generatemacroradicals in ourATNRCsystem. In this paper, the reaction conditions for SET are applied in the nitroxide radical coupling reaction and the macroradicals generated by SET mechanism at ambient temperature are trapped by nitroxide radicals, named as single-electron-transfer nitroxideradical-coupling (SET-NRC) reaction (Scheme 1). A living macroradical (Pn ) is generated from Pn-X (halogen-containing polymers) by SETmechanism by the oxidation of Cu toCu, which is efficiently trapped by TEMPO-Pm (TEMPO-containing polymers). Meanwhile, Cu is disproportionated into Cu and Cu to regenerate the Cu in polar solvent (methanol) in the presence of N ligands. However, in THF, no disproportionation of Cu was found (see Supporting Information). The coupling reactionswere conducted between bromine end polymers, such as polystyrene (PS), poly(tert-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), and TEMPO end polymers, poly(ethylene oxide) (PEO), poly(ecaprolectone) (PCL), in the presence of Cu/ N,N,N0,N00, N00-pentamethyldiethylenetriamine (PMDETA) under ambient temperature (25 ( 5 C). The TEMPO-containing PEO (TEMPO-PEO, Mn,NMR (molecular weight from H NMR): 3700 g/mol, Mn,GPC (molecular weight from GPC): 3500 g/mol, Mw/Mn: 1.08) was prepared by ring-opening polymerization (ROP) of EO in tetrahydrofuran (THF) using diphenylmethylpotassium (DPMK) and 4-hydroxyl2,2,6,6-tetramethylpiperidinyl-1-oxy (HTEMPO) as initiator. Polystyrene (PS-Br, Mn,NMR: 4100 g/mol; Mn,GPC: 4000 Mw/Mn: 1.07) precursor was prepared using the standard ATRP techniques (see Supporting Information). The coupling reaction was carried out as the followingprocedure:Anampule chargedwithTEMPOPEO (0.37 g, 0.1 mmol), Cu(0) (6.4 mg, 0.1 mmol), PMDETA (0.02 mL, 0.1 mmol), and THF (3 mL) was degassed by three freeze-pump-thaw cycles. Then PS-Br (0.41 g, 0.1 mmol) dissolved in THF (2 mL) was introduced into the ampule with additional three freeze-pump-thaw cycles. After 24 h magnetic stirring at ambient temperature, the ampulewas immersed in liquid nitrogen. Crude products were diluted with THF, and the solution was passed through neutral alumina to remove copper complex. THF was removed by evaporation, and the crude products were redissolved in CH3OH. The latter was passed through an ultrafiltration membrane to get rid of the impurities. The pure products were obtained by the concentration of reservedCH3OH solution to constant weight and dried at 35 C in vacuo. In SET, the C-X bond with low dissociation energy is cleaved by the formation of radical anion intermediates via an outersphere single-electron-transfer process, which does not show obvious dependency on the nature of the halide group. This means in our system C-Br bond dissociation occurred through the formation and decomposition of the radical anion intermediates. When PS-Br was mixed with TEMPO-PEO in the presence of Cu(0)/PMDETA at room temperature, the formed macroradicals (PS) were trapped by TEMPO groups immediately by the formation of alkoxyamines. In this process, the feed molar ratio of PS-Br and TEMPO-PEO was controlled at 1:1, and the presence of TEMPO-PEO groupmay increase the polarity of the solvent to benefit the coupling reaction. However, in the UV spectra of coupling product, only Cu at 262 nmwas detected; no Cu at 720 nm was found. It means that when THF was used as solvent in SET, no proportionation of Cu was conducted (see Supporting Information for details). After reaction, the remaining TEMPO-PEOand PS-Br residues were easily removed by membrane separation for TEMPO-PEO and extraction with cyclohexane for PS-Br (see Supporting Information for details) The gel permeation chromatography (GPC) traces of polystyrene with bromine end group (PS-Br), poly(ethylene oxide) with TEMPO end group (TEMPO-PEO), and PS-b-PEO are shown in Figure 1 with Gaussian distribution and low polydispersity (Mw/Mn< 1.10). A comparable experiment was carried out by introducing HTEMPO (0.017 g, 0.1 mmol) instead of TEMPO-PEO for the coupling reaction, in order to *To whom correspondence should be addressed. D ow nl oa de d by C A U L C O N SO R T IA A U ST R A L IA o n Ju ly 1 0, 2 00 9