Citation: Li Huimei, Wang Jie, Ni Yunzhou, Zhou Yongfeng, Yan Deyue. Synthesis of a Linear-Hyperbranched Supramolecular Polymer and Its Light-Responsive Self-Assembly Behavior[J]. Acta Chimica Sinica, ;2016, 74(5): 415-421. doi: 10.6023/A16020076 shu

Synthesis of a Linear-Hyperbranched Supramolecular Polymer and Its Light-Responsive Self-Assembly Behavior

1.
College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240

Corresponding author: Zhou Yongfeng, yfzhou@sjtu.edu.cn
Received Date: 1 February 2016

Fund Project: the National Natural Science Foundation of China 21474062the National Basic Research Program 2013CB834506the National Natural Science Foundation of China 91527304the China National Funds for Distinguished Young Scholar 21225420

Figures(9)

Herein we reported the synthesis, self-assembly and light-responsive disassembly of a "linear-hyperbranched" supramolecular polymer. Firstly, a hyperbranched polymer CD-g-HPG composed of hyperbranched polyglycerol with a β-cyclodextrin group in the center was synthesized by ring-opening multibranching polymerization (ROMBP). Secondly, a linear polymer AZO-PS composed of polystyrene with an azobenzene group at the end was synthesized via atom transfer radical polymerization (ATRP). Then, the linear AZO-PS and hyperbranched CD-g-HPG were conjugated together through the specific CD/AZO host-guest interactions, leading to the formation of the "linear-hyperbranched" supramolecular polymer PS-b-HPG. This supramolecular polymer was amphiphilic and could self-assemble into vesicles in water. The host-guest complexation ability was characterized by UV-Vis titration. In the case of keeping the concentration of AZO-PSs unchanged, the absorption peak at 330 nm increased gradually with the addition of CD-g-HPGs, which supported the occurrence of complexation between β-CD groups in CD-g-HPGs and AZO groups in AZO-PSs. The host-guest CD/AZO complexation constant of 4.14×10⁴ M^-¹ was calculated by the Benesi-Hildebrand plot. A Job plot was generated, from which it was determined that the binding stoichiometry between AZO-PS and CD-g-HPG is 1:1. The self-assemblies of the amphiphlic linear-hyperbranched supramolecular polymers were characterized by SEM and TEM. The SEM images showed that the self-assemblies were spherical particles, and the holes directly be seen in some particles indicated that they were vesicles or hollow spheres with a very thin wall thickness. The TEM images of self-assemblies stained with ruthenium tetroxide (RuO₄) indicated that the spherical particles were vesicles according to a clear contrast difference between the inner pool and the outer thin wall. At last, we showed that the vesicles could disassemble under UV light due to the trans-to-cis isomerisation of the AZO groups. With the continuous UV irradiation on vesicles, the absorption peak of trans-AZO diminished gradually and almost completely disappeared after 900 seconds. Meanwhile, the solution was transformed from turbid to transparent followed with the appearance of yellow precipitates in the bottom of the bottle.
- supramolecular polymers,
- linear-hyperbranched,
- host-guest interactions,
- self-assembly,
- light responsive
1    引言

图图式1 “线性-超支化”超分子聚合物的制备、自组装及解组装

Figure 图式1. Preparation, self-assembly and disassembly process of "linear-hyperbranced" supramolecular polymer

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目前, 超分子聚合物的研究主要集中在结构和性能调控, 以及在生物化学、医学及材料科学的应用等方面^[12～14].如今已经报道了多种具有不同拓扑结构的超分子聚合物, 如线性、交联、树形等^[15～18].其中, “线性-超支化”超分子聚合物是指线性聚合物和超支化聚合物通过非共价键相互作用连接起来的超分子聚合物.这种“线性-超支化”结构和自然界中大树的结构类似, 自然界的大树具有线性的树干和超支化的树冠, 因此也具有“线性-超支化”结构.然而由于超支化聚合物在反应过程中难以保留初始单元, 因此直接合成“线性-超支化”聚合物有很大困难.目前, “线性-超支化”聚合物的合成方法分为“先线性链法”(chain-first), “先超支化法”(hyperbranched-first)和“超支化接枝法”(hypergrafting)三种. 1998年, Kricheldorf等^[19]利用端基修饰为三甲基硅氧醚的遥爪低聚醚酮为大分子核与支化单体3, 5-二乙酰氧基苯甲酸三甲基硅烷酯共聚首次获得了以聚醚酮为线性部分, 超支化聚3, 5-二羟基苯甲酸酯为支化部分的“线性-超支化”超分子聚合物.这属于“先线性链法”.其后, Frey等^[20～22]提出了比较成熟的“hypergrafting”法来合成“线性-超支化”超分子聚合物.该方法是首先合成一条线性的嵌段聚合物, 聚合物的一端带有大量的超支化聚合单体的引发基团, 用这些引发基团来引发AB2单体, 从而将超支化聚合物接枝到线性链的末端, 得到“线性-超支化”聚合物.

和共价连接的聚合物相比, “线性-超支化”超分子聚合物展示出了易于合成和可控的特点, 尤其表现出独特的自组装行为, 很好地诠释了超分子聚合物的动态特性, 因此一经发现就引起了大量关注.但是, 需要指出的是, 目前“线性-超支化”超分子聚合物的种类还十分有限.本文通过CD-g-HPG和偶氮苯基团修饰的线性聚苯乙烯(AZO-PS)通过主客体识别作用制备了“线性-超支化”超分子聚合物PS-b-HPG.这是一类新的“线性-超支化”超分子聚合物.该两亲性超分子聚合物可在水中发生自组装形成囊泡结构(图式1).一般而言, 常用TEM来表征纳米囊泡的结构, 但由于高分子的衬度低, 很难用TEM来区分.在本文中, 采用了四氧化钌对囊泡中的聚苯乙烯片段进行染色和固定, 因此很容易的表征了囊泡结构, 得到清晰的囊泡照片^[24].而且由于偶氮苯在紫外光照射下发生顺反异构化, 使得所制备的囊泡在紫外光的照射下可以发生光控的解组装现象.

然而在我们之前“线性-超支化”聚合物主要是通过基于共价键的化学合成来实现的, 得到的都是共价高分子.近几年, 我们课题组成功制备了基于非共价键的“线性-超支化”超分子聚合物^{[15, 23]}.首先以环糊精为引发剂, 通过可控阴离子开环聚合(ROMBP)在环糊精表面接枝超支化聚缩水甘油醚(CD-g-HPG), 从而制备出一种含一个CD官能团的超支化聚合物.随后又制备了末端带有金刚烷基团的十八烷分子(AD-C₁₈).两者通过环糊精/金刚烷之间的主客体络合作用偶联在一起形成“线性-超支化”超分子聚合物C₁₈-b-HPG.该聚合物在水中可自组装成纳米级的囊泡, 在加入竞争性的主体分子β-环糊精后, 可实现囊泡的解离.而且该类囊泡展示出了独特的力学自适应性能, 可以在外力作用下发生大形变.此外, 我们还合成了一种通过末端含偶氮苯基团的线性聚甲基丙烯酸甲酯(AZO-PMMA), 并与CD-g-HPG通过主客体相互识别作用得到“线性-超支化”超分子聚合物PMMA-b-HPG^[23].使用该超分子聚合物对金表面进行修饰, 可以发现金表面呈现出光控的可逆亲疏水性变化.

超分子聚合物是超分子化学的一个比较新兴的分支, 最初是指单体单元通过非共价键(如:氢键、范德华力、金属配位键、主客体相互作用等)连接而成的大分子.后来该概念有了进一步的拓展, 大分子片段通过非共价连起来也称为超分子聚合物^[1～4].与传统的共价键聚合物相比, 非共价键赋予超分子聚合物许多优异的性能.比如, 超分子聚合物的结构动态可调, 对外界环境具有刺激响应性, 具有自修复性能及优异的加工性能等, 因而越来越受到了研究者的关注^[5～11].

2    结果与讨论

2.1    CD-g-HPG的表征

图1 (a) CD-g-HPG的¹H NMR谱图和(b) CD-g-HPG的GPC曲线

Figure 1. (a) ¹H NMR spectrum of CD-g-HPG and (b) GPC curve of CD-g-HPG

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CD-g-HPG的合成示意图见支持材料的图S1.对其进行了¹H NMR和GPC表征. 图 1(a)为CD-g-HPG的¹H NMR谱图.从图中可以看出, 4.9～5.1的化学位移为环糊精1号位质子的信号峰, 3.0～4.0为HPG中的亚甲基和次甲基及环糊精2～6位的质子信号峰.对峰面积进行积分, 根据公式DP＝[(S_B－6S_A)/5]/(S_A/7)(其中S_A为环糊精1号位质子信号峰的积分面积, S_B为3.0～4.0区间内的质子信号峰积分面积), 计算得到CD-g-HPG中HPG的聚合度约为57.结合缩水甘油单体的分子量(74 g/mol)和β-CD的分子量(1135 g/mol), 得到CD-g-HPG的分子量约为5300 g/mol. 图 1(b)为CD-g-HPG的GPC曲线.用线性聚乙二醇为标样, 测得CD-g-HPG的相对分子量为3400, 多分散指数(PDI)为1.29.聚合物的GPC曲线为比较窄的单峰, 说明产物纯净. GPC结果和核磁结果不符合, 主要是因为我们合成的CD-g-HPG为超支化结构, 其流体力学半径小于同分子量的线性聚合物, 而且超支化聚合物的大量末端基团可能会被GPC柱子吸附, 所以其保留时间比线性聚合物要长.实验中用线性聚乙二醇为标样, 结果会比实际偏小.所以CD-g-HPG的分子量以核磁结果为准.

2.2    AZO-PS的表征

图2 AZO-PS的合成

Figure 2. Synthesis of AZO-PS

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如图 2所示, AZO-PS的合成分为两步.首先是ATRP引发剂2-溴-2-甲基丙酸偶氮苯4-苯酚酯(AZO-Br)的合成, 然后是一端带有偶氮苯基团的聚苯乙烯(AZO-PS)的合成. ATRP引发剂AZO-Br的表征参见文献^[25].

图3 (a) AZO-PS的¹H NMR谱图和(b) AZO-PS的GPC曲线

Figure 3. (a) ¹H NMR spectrum of AZO-PS and (b) GPC curve of AZO-PS

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图 3为AZO-PS的¹H NMR和GPC表征结果.从¹H NMR图谱可以看到, 所有信号峰均和AZO-PS上的质子对应, 峰面积积分比例也与大分子中相应的质子数量比相同. GPC结果测得AZO-PS的分子量为5800, PDI为1.20.以上结果表明成功制备了AZO-PS.

2.3    CD-g-HPG与AZO-PS的复合能力表征

使用紫外滴定法对亲水性CD-g-HPG和疏水性AZO-PS之间的主客体复合能力进行了表征.如图 4(a)所示, 以DMF/H₂O (1/1, V/V)作为溶剂, 随着CD-g-HPG的不断加入, AZO-PS在330 nm处的紫外吸收峰(归属于反式AZO基团)不断增加.反式AZO吸光度的增加是因为CD与AZO复合后摩尔消光系数的增加^[26], 这一结果证明了聚合物CD-g-HPG中CD基团与聚合物AZO-PS中AZO基团发生了复合.通过定量计算AZO浓度相同的情况下, 在加入CD-g-HPG及未加入CD-g-HPG时吸光度的差值ΔA, 利用改进的Hildebrand-Benesi方程: 1/ΔA＝1/(K_aΔε[G₀])*1/[CD]＋1/(Δε[G₀]) (K_a是超分子聚合物中AZO与CD的复合常数; Δε是AZO与AZO-CD络合物在相同波长处的摩尔消光数的差值; [G₀]是AZO基团的浓度, 实验中为1.0×10^－5 mol/L; [CD]是加入CD-g-HPG后溶液中CD基团的浓度), 得到1/ΔA随1/[CD]变化的Benesi-Hildebrand曲线[图 4(b)], 曲线的拟合方程为y＝1.3550×10^－4x＋5.6071, R²＝0.994.通过拟合曲线的斜率和截距计算得到“线性-超支化”超分子聚合物中CD与AZO的复合常数约为K_a＝4.14×10⁴ M^－1, 该值与报道的CD与AZO的复合常数比较类似^[27]. CD-g-HPG与AZO-PS的络合比例可以通过Job曲线来测定. 图 5是固定CD与AZO的总浓度不变, 通过改变两者的配比, 测定AZO基团在加入与不加CD时的吸光度差值(ΔA)与AZO-PS在溶液中的摩尔比χ_AZO-PS作图, 得到AZO-PS与CD-g-HPG结合的Job曲线.从曲线中可以看到最高点对应的横坐标在0.5, 由此可以得到AZO-PS与CD-g-HPG以1:1进行识别结合, 形成超分子两嵌段共聚物PS-b-HPG.

图5 AZO-PS和CD-g-HPG在DMF-H₂O (1/1, V/V)溶液中的Job曲线

Figure 5. A Job plot of AZO-PS and CD-g-HPG in DMF-H₂O (1/1, V/V) solvent

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图4 (a) AZO-PS在DMF-H₂O (1/1, V/V)溶液中, 不同CD-g-HPG浓度下的紫外-可见光吸收曲线和(b) AZO基团在最大吸收波长(λ＝330 nm)下的Benesi-Hildebrand曲线

Figure 4. (a) The UV-Vis absorption spectra of AZO-PS at varied content of CD-g-HPGs in DMF-H₂O (1/1, V/V) and (b) Benesi-Hildebrand plot at the maximum absorption wavelength (λ＝330 nm) of AZO groups

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2.4    组装体形貌的表征

超分子聚合物的形成和组装是一步实现的.先将CD-g-HPG和AZO-PS溶于共溶剂DMF中, 然后往里缓慢滴加去离子水, 溶液逐渐出现乳光, 说明聚合物发生了自组装.随后将该乳浊液装入分子量为3500 Da的透析袋中, 用去离子水透析3 d, 除去DMF, 最后得到带有强烈乳光的组装体水溶液, 最终组装液中超分子聚合物的浓度为6.9×10^－5 mol/L.使用SEM和TEM表征了所得组装体的形貌.如图 6(a)所示, SEM照片显示组装体具有球形结构, 尺寸分布不均匀.插图展示出具有破口的粒子, 说明组装体是中空的. 图 6(b)展示了组装体的TEM照片, 为了提高衬度和分辨率, 在制样后进一步用四氧化钌对组装体中的PS片段进行染色.从TEM图片可以清晰的看出组装体具有空的腔和薄的壁层, 证明形成了囊泡.根据TEM照片测量出的囊泡壁厚大约为25 nm, 结合聚合物的结构, 推测组装体囊泡具有双分子层结构.

图6 (a)组装体的SEM图片和(b)组装体经RuO₄染色后的TEM图片

Figure 6. (a) The SEM image of the self-assembies and (b) The TEM image of the self-assemblies stained with RuO₄

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2.5    组装体的解组装研究

众所周知, 偶氮苯是一种经典的具有顺反异构体的光响应性分子, 在紫外光照射下会发生异构化, 由反式结构转变为顺式结构; 在可见光下, 又可以由顺式结构恢复为反式结构.而且由于尺寸效应, 只有反式偶氮苯才可以与环糊精发生主客体识别而形成复合物, 而顺式偶氮苯却不能^[28].因此, 紫外光照会引起环糊精/偶氮苯之间的主客体识别作用被破坏, 进而引起PS-b-HPG超分子嵌段共聚物的解离及组装体结构的破坏. 图 7a是在紫外光照射下PS-b-HPG囊泡溶液的UV-vis曲线, 可见其在330 nm处对应于反式偶氮苯的吸收峰在逐渐降低. 图 7b展示了相应峰的吸光度对紫外光照时间的变化规律.可以看出, 在紫外光照500 s后, 反式偶氮苯的吸收峰几乎消失.证明了异构化的发生.

图 8为囊泡溶液在紫外光照射前和照射2 h后的数码照片.可见光下的组装体溶液为乳浊液, 当用紫外光照射2 h后, 乳浊液变澄清, 并且瓶底出现许多沉淀, 这些下层沉淀为不溶于水的AZO-PS.取上层清液进行DLS表征, 数据显示溶液中存在粒径为2.7 nm左右的粒子, 对应于解组装后的亲水的CD-g-HPG.以上结果表明, 组装体囊泡经紫外光照射后发生解组装而成为溶于水的CD-g-HPG和不溶于水的AZO-PS两部分.然而需要指出的是, 由于AZO-PS在水中形成沉淀, 因此即使在可见光长时间放置, AZO-PS和CD-g-HPG仍难以可逆形成超分子聚合物并形成囊泡.但进一步研究表明, 加热溶液可促使可逆过程的发生.将解组装并形成沉淀的透明水溶液在80 ℃下加热40 h, 可以看到沉淀部分溶解, 溶液逐渐变得不透明, 并伴随着囊泡的重新形成.证明加热有利于解组装体重新通过主客体复合可逆形成超分子聚合物, 并进一步组装形成囊泡.但这个过程十分缓慢.

图7 (a) AZO-PS和CD-g-HPG组装体水溶液在不同紫外光照射时间下的紫外-可见光吸收图谱和(b) 330 nm波长下, 以照射时间为横坐标的吸光度函数曲线

Figure 7. (a) UV-Vis absorption spectra of the aqueous solution of AZO-PS and CD-g-HPG assemblies at different UV irradiation time and (b) Absorbance at λ＝330 nm as a function of the irradiation time

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图8 组装体囊泡在紫外光照射前和照射2 h后的数码照片.放大部分中, 上层清液为解组装后的CD-g-HPG.下层为不溶于水的AZO-PS沉淀

Figure 8. Digital photos of self-assembly vesicles before and after UV irradiation for 2 h. The amplified part of the upper clear liquid shows the residual CD-g-HPG. The bottom shows the AZO-PS precipitates

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3    结论

分别通过可控阴离子开环聚合(ROMBP)和原子转移自由基聚合(ATRP)的方法, 制备了以β-环糊精为中心的超支化聚缩水甘油醚(CD-g-HPG)和一端带有偶氮苯基团的聚苯乙烯(AZO-PS).两者通过β-环糊精和偶氮苯基团之间的主客体识别作用形成“线性-超支化”超分子聚合物PS-b-HPG, 该聚合物可以在水中发生自组装形成囊泡结构.使用Benesi-Hildebrand曲线计算出超分子聚合物中CD与AZO的复合常数K_a＝4.14×10⁴ M^－1. Job曲线结果说明AZO-PS与CD-g-HPG以1:1进行识别结合.通过SEM和TEM表征了组装体的形貌, 证明组装体为中空的囊泡结构, 该囊泡可以在紫外光照射下实现解组装.

4    实验部分

4.1    材料和表征

具体的原材料和表征过程参见支持材料.

4.2    CD-g-HPG的合成

参考文献^{[15, 23]}如图S1所示.以b-CD为引发剂, 采用可控阴离子开环聚合(ROMBP)的方法, 引发缩水甘油醚的聚合.具体合成方法如下:在干净的100 mL茄型瓶中, 放入磁子, 在明火烘烤条件下, 反复抽真空通氮气, 除尽瓶中的氧气和水分.将瓶放入手套箱中, 依次加入30 mL无水DMF, β-CD (0.3 g, 0.264 mmol)和18-冠-6 (0.495 g, 1.875 mmol), 充分溶解后, 加入KH (0.084 g, 2.1 mmol), 室温条件下剧烈搅拌2 h, 使KH与b-CD充分反应后升温至80 ℃, 用微量注射泵将缩水甘油单体(2.4 g, 30.9 mmol)缓慢滴加到反应液中, 滴加结束后继续反应24 h, 停止反应.将茄型瓶从手套箱中取出, 加入少量的水终止反应, 并将反应液用去离子水透析1周以除去未反应的单体, 环糊精及冠醚等物质, 冷冻干燥后获得微透明略带黄色的粘稠液体.该反应的产率约为53%.

4.3    AZO-PS的合成

参考文献^[29], 首先合成ATRP引发剂2-溴-2-甲基丙酸偶氮苯4-苯酚酯(AZO-Br), 合成步骤如下:在干净的250 mL圆底烧瓶中, 放入磁子, 依次加入4-羟基偶氮苯(1.98 g, 10 mmol)和80 mL三氯甲烷, 搅拌使其溶解后加入三乙胺(2.8 mL, 20 mmol), 放入冰水浴中.然后向溶液中逐滴加入溶于30 mL三氯甲烷的2-溴-2-甲基丙酰溴(4.6 g, 20 mmol)溶液.室温下搅拌12 h.反应完成后, 过滤, 收集滤液, 向其中加入30 mL去离子水进行洗涤, 重复三次.然后加入无水硫酸镁, 静置2 h.过滤收集滤液, 旋掉溶剂, 再加入50 mL无水乙醇, 重结晶, 重复三次.最后得到的产物放入真空烘箱于50 ℃下干燥24 h, 得到淡黄色有粘性的粉末, 即AZO-Br (3.16 g, 产率约74%).

一端带有偶氮苯基团的聚苯乙烯(AZO-PS)的合成步骤如下:首先在干净的50 mL梨形圆底烧瓶中, 放入磁子, 依次加入AZO-Br (34.7 mg, 0.1 mmol)和CuBr (14 mg, 0.1 mmol), 装置密封后, 抽真空-充氮气, 重复三次.用注射器加入PMDETA (20 μL, 0.1 mmol)和苯乙烯(4.0 g, 38 mmol)的混合溶液, 冷冻-抽真空-充氮气-解冻, 重复三次.然后将烧瓶放入80 ℃油浴中, 反应2 h.停止反应后, 加入少量四氢呋喃溶解产物, 将烧瓶中的混合液逐滴滴入500 mL甲醇中进行沉淀, 重复三次.最后将沉淀产物放入50 ℃真空烘箱中干燥24 h, 得到浅黄色固体粉末, 即AZO-PS (1.35 g, 产率约34%).

4.4    “线性-超支化”超分子聚合物的制备及自组装

CD-g-HPG (5.3 mg, 0.001 mmol CD)和AZO-PS (5.8 mg, 0.001 mmol AZO)在超声条件下共溶于2 mL DMF中.然后在超声条件下缓慢滴加去离子水, 溶液逐渐出现乳光, 预示超分子聚合物发生了自组装.最后共加入5 mL去离子水, 得到白色乳浊液.将该乳浊液装入分子量为3500 Da的透析袋中, 去离子水透析三天以除去DMF.最后得到带有乳光的浑浊的组装体水溶液.

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