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1、MacromoleculesCommunications to the EditorVolume 38, Number 26 December 27, 2005 Copyright 2005 by the American Chemical SocietyComb-Coil Polymer Brushes on the Surface of Silica Nan particlesHanding Zhao,* Xiao Kang, and Li Leakey Laboratory of Functional Polymer Materials, Ministry of Education, D
2、epartment of Chemistry, Nankai University, Tianjin 300071, P. R. China Received August 17, 2005Revised Manuscript Received November 4, 2005During the past decade, surface modification of inorganic Nan particles by attachment of polymer brushes has attracted much interest due to the improvement of th
3、e properties of the Nan particles, especially the dispersion and stability of the particles in various solvents.Recently, many groups reported preparation of polymer brushes on the surface of gold nanoparticles,1magneticnanoparticles,2and silica nanoparticles.3There are two principal techniques to g
4、raft polymer brushes on the surface of the inorganic particles: (1) the “grafting to” method, where the end-functionalized polymers react with the functional groups on the inorganic particle surface, and (2) the “grafting from” method, where the polymer chains grow from the initiator-modified inorga
5、nic particle surface. Because of the satiric hindrance imposed by the grafted polymer chains, it is difficult to prepare polymer brushes with high graft density on the particle surface using the “grafting to” method. How-ever, in the “grafting from” method polymer chains grow from the initiators tha
6、t have been initially anchored to the particle surface, and the grafted chains will not hinder the diffusion of the small molecular monomers to the reaction sites, so the polymer brushes with higher graft density can be obtained. In recent years, there have been increasing research activities in the
7、 use of various polymerization methods to grow polymer chains on the silica particle surface. These methods include anionic polymerization,3bcat-Ni ionic polymerization,3dring-opening polymerization,3cradical polymerization,3eand atom transfer radical polymerization (ATRP).4-8Because ATRP allows bet
8、ter control over the molecular weight and distribution of the target polymer, much attention has been paid to these of this polymerization method in the synthesis ofpolymer brushes from a surface. Recently, Matyjasze-wskis group reported an improved ATRP method, activators generated by electron tran
9、sfer (AGET) ATRP.9In a typical AGET ATRP system, an alkyl halide is used as initiator, and a transition metal complex in its ox datively stable state (e.g., Cu2+/ligand) is used as catalyst. The activator is generated by using an electron transfer to reduce the higher oxidation state transition meta
10、l. In their experiments tin(II) 2-ethylhexanoate and ascorbic acid were used as the reducing agents. The AGET ATRP method has all benefits of normal ATRP and remains tolerant to air during sanitation, so it will be very useful in the preparation of polymer brushes on the silica nanoparticles surface
11、 .In this paper we report the first synthesis of comb -coil polymer brushes on the surface of silica nanoparticles. In this approach two steps are involved. At the first step, poly(2-hydroxyethyl methacrylate) (PHEMA)brushes on the surface of ATRP-initiator-anchored silicananoparticles were prepared
12、 using ATRP. At the second step using a combination of ring-opening polymerization and AGET ATRP, poly(DL-lactate) -poly(n-butyl acryl-late) (PLA -PBA) comb-coil polymer brushes were synthesized from the backbone and the terminal site morpheme brushes, respectively. This process is illus-traded in S
13、cheme 1. To synthesize PHEMA polymer brushes, ATRP-initiator-anchored silica nanoparticleswere prepared by a reaction of original silica particles with 3-(triethoxysilyl)polyamine and followed by an-other reaction with 2-bromoisobutyryl bromide. PHEMAbrushes were synthesized by ATRP. PLA comb and Pa
14、colet were synthesized by a combination of ring-opening polymerization and AGET ATRP (Scheme 1). The hydroxyl groups on PHEMA chains can be used in the ring-opening polymerization of LA, and the bromide Scheme 1. Schematic Representation for the Preparation of Comb-Coil Polymer Brushes on the Surfac
15、e of Silica Nanoparticles groups at the end of PHEMA chains can be used as initiators in AGET ATRP of BA. In this polymerization system, the Cu2+/bipyridine complex was used as catalyst for AGET ATRP, and tin(II) 2-ethylhexanoate was used as a reducing agent. Meanwhile, tin(II) 2-ethylhexanoate was
16、also used as catalyst for the ring-opening polymerization of DL-lactate (LA). So the ring-opening polymerization of LA at the backbone of PHEMA chains and AGET ATRP of BA at the end of PHEMA chains were taken place in the same polymerization system (Scheme 1). In this case each monomer should indepe
17、ndently propagate via two different mechanisms (ring opening and radical) and form different structures (comb structure at the backbone of PHEMA and coil structure at the end of PHEMA chain).The primary amino groups were introduced onto particle surface by a reaction with 3-(triethoxysilyl)-polyamin
18、e (Scheme 1). The elemental analysis result shows that the nitrogen content is about 0.93%, which means the concentration of amino groups on the surface is about 0.66 mol/g. This result keeps consistent with the literature report.3a ATRP-initiator-anchored silica particles were prepared after a reac
19、tion of laminated silica particles with 2-bromoisobutyryl bromide. The elemental analysis result shows that the bromide content on the silica particle surface is about 6%. This result indicates that on the particle surface the amount of ATRP initiator is roughly the same as that of amino group.Therm
20、o gravimetric analysis (TGA) result indicated that the weight retention of ATRP-initiator-anchored silica nanoparticles at 800 C was about 90%. But it has to be pointed out that part of the weight loss of the modified silica particles is due to the continued condensation reaction and associated wate
21、r loss.8,10 If the weight retention of the residue at 800 C is used as the reference, the weight retention of PHEMA brushes on the surface is about 76%, and that of the comb-coil polymer brushes is about 50%.Comb-coil polymer/silica composite was etched by HF acid, and the polymer was measured by 1H
22、 NMR. Figure 2 shows the 1H NMR spectrum of the comb-coil polymer. In Figure 2 the peak at 3.8 ppm indicates the successful ring-opening reaction of LA by -OH group on PHEMA backbone. In the spectrum the peaks at 5.16 ppm (d) and at 2.28 pap (e) represent the signed to ethylene protons adjacent to t
23、he -OH group in PHEMA basically disappears, which methane protons in PLA and PBA backbone, respectively.11 Using these two peaks it can be calculated that the molar ratio.Figure 1. Thermo gravimetric analysis of (a) ATRP-initiator anchored silica nanoparticles, (b) PHEMA brushes on the surface of si
24、lica nanoparticles, and (c) PLA-PBA comb-coil polymer brushes on the surface of silica nanoparticles.Figure 2. 1H NMR spectrum of PLA-PBA comb-coil polymer of LA to BA is about 1:0.36. The peak at 4.36 ppm (b) is attributed to the ethylene group of PHEMA. Using this peak and the peak at 5.16 ppm (d)
25、, the molar ratio of PHEMA repeating units to PLA repeating units can be obtained. Assuming all the -OH groups take part in the ring-opening polymerization, there are 20 LA repeating units on each PHEMA repeating unit, or the average molecular weight of PLA comb chain is about 1400.The molecular wei
26、ght of the etched polymer was also measured by GPC. The GPC result shows that the molecular weight (Mn) of the comb-coil polymer is about 44 000 and the polydispersity is about 1.55.Transmission electron microscopy (TEM) results confirmed the formation of comb-coil polymer brushes on the surface of
27、silica particles. The specimens for TEM measurement were prepared by dipping the copper grids into the diluted solution of grafted (or bare) silica nanoparticles in THF and evaporating in air. To increase the contrast, the comb-coil polymer chains were stained by exposing in hydrazine vapor for 6 h
28、and OsO4 vapor for 3 h.12 Figure 3a shows a TEM image of bare. Figure 3. TEM images of bare silica particles (a) and comb coil polymer grafted silica particles (b, c). Part c is a magnified TEM image of comb-coil polymer grafted silica particles.silica particles. The bare silica particles were obser
29、ved as aggregates with an average size of several hundred nanometers. In Figure 3a no isolated bare silica particles can be found. In contrast, silica particles with comb-coil polymer brushes were observed to have fine dispersion, and the particle size ranges from 20 to 30 nm. To observe the particl
30、es clearly, a magnified TEM image was shown as Figure 3c. On the novel method to synthesize comb-coil polymer brushes on the surface of silica nanoparticles. For the synthesis of comb-coil polymer on the particle surface, PHEMA polymer brushes were prepared by ATRP method. On the basis of a combinat
31、ion of AGET ATRP and ring opening polymerization, PLA-PBA comb-coil polymer brushes were prepared from the backbone and the end of PHEMA chains, respectively. The comb-coil polymer brushes form wormlike structure on the silica particle surface. It is obvious that our approach can be extended to othe
32、r systems, such as Au nanoparticles. This type of polymer modified nanoparticles may find their applications in the area of biomaterials, chemical mechanical polishing (CMP), etc.Acknowledgment. This project was supported by National Natural Science Foundation of China under Contract No. 20544001 an
33、d start-up funds from Nankeen University. Supporting Information Available: Experiments details and two control experiments. This material is available free of charge via the Internet at http:/pubs.acs.org.References and Notes(a) Shan, J.; Nupponen, M.; Jiang, H.; Viitala, T.; Kauppinen, E.; Konttur
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41、rt B: Polym. Chem. 2004, 42, 76-82. (b) You, Y.; Hong, C.; Wang, W.; Lu, W.; Pan, C.高分子38卷，26期 2005年 12月27 .2005年被美国化学社会授予版权与作者的交流梳状形状的高聚物梳在二氧化硅纳米颗粒的表面上。赵含因、康小丽、刘丽。功能高分子材料重点实验室，教育部，化学系，南开大学，中国P.R天津3000712005年8月17日被接收，2005年11月4日，修订手稿后被接收。过去的10年间，在无机纳米颗粒表面上梳附上一层高分子来对它的表面进行改性从而吸引了很多的研究者的兴趣，因为从而提高了纳米颗粒的
42、性能，特别是在不同的溶剂中粒子的分散和稳定性方面。许多研究团体报告出在金色和有磁性的纳米颗粒表面上附上一层高分子梳的准备。以下有两种基本的技术使聚合物梳附在无机颗粒上：一种是通过“接枝到”的方法，把没有功能的高分子与高能性的高分子在无机颗粒表面上发生反应。第二种是“接枝向”的方法，在引发剂改性了的无机颗粒表面上进行高分子的链增长。因为被接枝的高分子链产生的位阻，很难准备用高接枝密度的聚合物梳在无机颗粒上进行“接枝到”的方法。然而，在“接枝向”的方法中高分子从原始状态开始链增长就已经被固定在了颗粒的表面上。并且接枝的链不会阻碍小分子单体扩散到反应的部位，所以具有高接枝浓度的聚合物梳将被生成。在最
43、近的几年中，产生了越来越多关于利用各种聚合方法在二氧化硅颗粒的表面上生成高分子链的研究活动。这些方法中包括阴离子聚合、阳离子聚合、开环聚合、原子团聚合、原子转化为原子团聚合。因为原子转化为原子团聚合更好的控制分子的重量以及靶子聚合物的分配，所以很多人把关注放在了利用聚合的方法在表面上合成聚合物梳。最近,Matyjaszews团队报告了一个改善原子转化为原子团聚合的方法，通过电子转化成电子团体聚合生成了活性剂。在一个典型的原子转移自由基聚合体系中，一个烷基卤代物被用来做引发剂，一个转化成金属络合物在它稳定的氧化状态被用来做催化剂，生成的催化剂是通过电子转移来降低高氧化态跃迁的金属。在他们的实验二
44、中乙基乙基铅和抗坏血酸用来做还原剂，AGET ATRP 有标准的原子转移自由基聚合所有的好处，而且在声波降解法中可以保留空气，所以它是一种在二氧化硅纳米粒子表面生成聚合物梳很有用的方法。在这篇论文中我们报表了在二氧化硅纳米粒子表面生成聚合物梳的第一个合成物。这个方法包含了两个步骤，聚(甲基丙烯酸羟乙基亚胺)(水凝胶) 在二氧化硅硅纳米粒子表面梳引发剂是准备使用ATRP（原子转化自由基聚合）在第二个步骤结合使用开环聚合和AGET ATRP,聚dl丙交酯)聚(正丁基acryl-late)(PLA pay)梳状线圈聚合物梳的是分别合成的骨干和终端站点的水凝胶梳子，这个过程在图表一种有说明。为了合成水
45、凝胶聚合物梳，ATRP催化剂固定在二氧化硅纳米粒子表面是为带有丙胺的纯的二氧化硅粒子与溴异丁酰溴反应做准备的。聚甲基丙烯酸羟乙酯梳是有原子转移自由基聚合合成而得的，聚乳酸梳和丙烯酸丁酯梳是由开环聚合和自由基聚合共同合成而得的。聚甲基丙烯酸羟乙酯链上的羟基团可以作用乳酸的开环聚合，聚甲基丙烯酸羟乙酯的端链中的溴化物团可以在丙烯酸丁酯的自由基聚合中作为引发剂。在这个聚合体系中，Cu2+和二吡啶的复合体被作为催化剂在AGET自由基聚合中，锡(II)2乙基己基铅被用作还原剂，同时锡(II)2乙基己基铅还可以用作催化剂在乳酸的开环聚合中，所以乳酸的开环聚合在水凝胶的支柱链和水凝胶端链的AGET自由基聚合
46、是发生在相同的聚合体系中，在这种情况下每个单体应该独立地传播通过两种不同的机制(开环和激进的)和形成不同的结构(梳状结构水凝胶的支柱和线圈结构最后的水凝胶链)。方案1。示意图表示的二氧化硅纳米粒子表面上制备聚合物刷梳线圈原始的氨基基团被引到了颗粒表面通过反应与3 -丙胺的反应，元素分析的结果表明含氮量约为0.93%，这意味着在粒子表面氨基基团的浓度为0.66mol/g。这个结果使符合文献报告。自由基引发剂固定在硅粒子表面是为胺化了硅与溴异丁酰粒子的反应做准备，这个元素分析结果表明,溴化物在二氧化硅粒子表面上的含量大约是6%。这结果表明,在粒子表面的ATRP引发剂数量氨基酸团的数量是大致相同的。
47、 热重分析(TGA)结果表明这个重量保留自由基引发剂固定在二氧化硅纳米粒子在800C是大约90%，但是它有被指出改性二氧化硅粒子的部分重量减少是由于持续冷凝反应和相关的水损失。如果保持在800的残渣重量用作参考,保留在表面的水凝胶梳重量约76%,梳子线圈聚合物梳的是大约50%。聚合物/二氧化硅复合材料梳理线圈是蚀刻的氟化氢酸，测量聚合物是通过核磁共振，图2显示了核磁共振谱的梳子线圈聚合物，在图2的峰值为3.8 ppm分配亚甲基质子毗邻- -OH基团在水凝胶基本上消失。这表明成功的开环反应,由乳酸基团的反应通过-OH基团在水凝胶骨干中。在频谱峰值为5.16 ppm(d)和2.28 ppm(e)分
48、别代表次甲基质子聚乳酸军和聚丙烯酸丁酯骨干，使用这两个峰，它可以计算出乳酸与丙烯酸丁酯摩尔比大约为 1:0.36,峰值为4.36 ppm(b)由于亚甲基群水凝胶,使用这个峰值和峰值为5.16 ppm(d),用水凝胶的重复单位与聚乳酸的重复单位的摩尔比可以获得。假设所以的-OH团参与开环聚合，有20个乳酸在每一个重复单位水凝胶重复单元，或者聚乳酸梳链的平均摩尔质量为1400。图1。热重分析(一)自由基引发剂-锚定硅纳米颗粒,(b)水凝胶刷的表面的二氧化硅纳米粒子,和(c)聚乳酸梳线圈聚合物刷的二氧化硅纳米粒子的表面上。图2。聚乳酸与水凝胶梳线圈的核磁共振谱被风化了的聚合物分子量通过GPC测定。结
49、果表明,凝胶渗透色谱分子量(Mn)的聚合物是梳线圈44 000和多分散性是1.55。 图3。透射电镜的裸硅粒子(a)和梳状线圈聚合物接枝硅粒子(b、c)。部分c是一个放大的TEM图像的梳子线圈聚合物接枝二氧化硅粒子。透射电子显微镜(TEM)结果形成梳线圈聚合物刷上二氧化硅粒子的表面的结构，这些标本为TEM 测量是通过浸铜网格做好准备到稀释溶液接枝(或裸)二氧化硅纳米粒子在四氢呋喃和蒸发在空气中。进一步对比，聚合物链的梳状线圈渍暴露在肼蒸气 6h和OsO蒸汽3 h，图3展示了光秃秃的二氧化硅纳米粒子横向电磁场的图像，裸露的硅粒子被观察到作为聚合物平均大小几百纳米。在图片3（a）中没有发现孤立裸露的硅粒子。相反，梳状的聚合物的硅粒子可以看到有很好的散布，而且粒子的大小在20-30nm之间。为了更清楚的观察粒子，3（c)是放大了的横向电磁场图片，在图片中我们可以看到在粒子的表面有像虫状的梳状