Room temperature growth of surface-confined poly(acrylamide) from self-assembled monolayers using atom transfer radical polymerization
Room temperature growth of surface-confined poly(acrylamide) from self-assembled monolayers using atom transfer radical polymerization
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DOI：
10.1021/ma012254q
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发表时间：
2002-05-07
期刊：
影响因子：
5.5
通讯作者：
Husson, SM
中科院分区：
文献类型：
--
作者：
Gopireddy, D;Husson, SM
先前的研究人员1-3已经讨论了使用所谓的“接枝”方法来生产聚合物改性表面的优势。该方法采用表面限制的聚合引发剂，聚合物链由表面限制的聚合引发剂生长。该文献描述了多种类型聚合4-6 的表面限制聚合方案，主要在二氧化硅2、3、5、7、8 基材上进行，还有一些在金上的例子。 1, 4, 6, 9 使用金上的 SAM 作为基础对于研究表面限制聚合具有优势：这些系统具有明确、均匀的结构； 10 它们提供了控制面积表面引发剂密度以及聚合物链密度的机会，并且它们有助于通过常用于研究薄膜的分析技术进行表征。历史上使用这些系统的限制是它们在聚合所需温度下的热不稳定性。 10, 11 近年来，通过使用交联聚（硅氧烷）粘合层来连接引发剂 9 以及开发 12 和使用 4, 12, 13 促进室温聚合的催化剂，这一限制已被克服。我们已经采用这样的催化剂系统在镀金硅片上从 SAM 中生长出表面限制的聚（丙烯酰胺）。在表面限域聚合中使用平面基材本质上对受控链增长提出了挑战：ATRP 反应的受控性质依赖于足够浓度（∼ 10-3 M8）的失活物质来抑制增长的链自由基。通常，这些持久的失活分子在聚合的初始阶段通过终止反应形成，并用于控制链增长。 14 对于平面上的表面限域聚合，在典型的反应条件下，引发剂分子太少，无法产生足够浓度的钝化剂分子。（我们的晶圆表面积为 1 cm2；即使所有 SAM 链都用引发剂封端，根据已知的面积 SAM 链密度，引发剂的摩尔量将为∼ 7.9× 10-10 mol。15）在 Matyjaszewski 的工作之前等人进行了 8 项从平坦表面生长受控聚合物的实验，需要不受束缚的“牺牲”引发剂来产生钝化剂。虽然成功，但该策略导致了表面限制聚合物和溶液相聚合物的形成。为了克服这一限制，Matyjaszewski 等人。 8设计了一种策略，在反应开始之前将足够浓度的钝化剂物质添加到反应混合物中。然而，在基于丙烯酰胺的聚合中，ATRP 的转化率普遍较低；这一事实可以通过增长的链自由基的缓慢活化以及快速失活来解释，表明催化剂的氧化还原电位不足。 13 添加钝化剂会增加钝化速率，从而导致聚合率非常低或不聚合。初步研究证实了这一结果；因此，这项初步研究没有采用“牺牲”引发剂或预先添加的钝化剂物种。将提供的数据支持与所研究的系统的单体浓度有关的一级聚合速率。这些数据还表明聚合物的生长不受控制。
Previous researchers1-3 have discussed the advantages that exist for using the so-called “grafting from” method to produce polymer-modified surfaces. This method employs surface-confined polymerization initiators off of which are grown the polymer chains. The literature describes surface-confined polymerization schemes for many types of polymerization4-6 primarily on silica2, 3, 5, 7, 8 substrates and a few examples on gold. 1, 4, 6, 9 Using SAMs on gold as a foundation has advantages for studying surface-confined polymerization: These systems have well-defined, uniform structures; 10 they provide opportunities to control areal surface initiator densities and hence polymer chain densities, and they facilitate characterization by analytical techniques commonly used for studying thin films. A limitation to using these systems has historically been their thermal instabilities at the temperatures needed for polymerization. 10, 11 This limitation has been overcome in recent years by using cross-linked poly (siloxane) adhesion layers for initiator attachment9 and also with the development12 and use4, 12, 13 of catalysts that promote room temperature polymerization. We have employed such a catalyst system to grow surface-confined poly-(acrylamide) from SAMs on gold-coated silicon wafers. The use of flat substrates in surface-confined polymerization inherently presents a challenge for controlled chain growth: The controlled nature of the ATRP reactions relies on a sufficient concentration (∼ 10-3 M8) of deactivating species for the growing chain radicals. Typically, these persistent deactivating molecules form during the initial stages of polymerization via termination reactions and serve to control chain growth. 14 For surface-confined polymerization on flat surfaces, too few initiator molecules are available to generate a sufficient concentration of deactivator molecules under typical reaction conditions.(Our wafers have 1 cm2 surface area; even if all of the SAM chains were capped with initiator, the molar amount of initiator would be∼ 7.9× 10-10 mol based on known areal SAM chain densities. 15) Prior to the work of Matyjaszewski et al., 8 experiments to grow controlled polymers from flat surfaces required untethered “sacrificial” initiator to generate the deactivator. While successful, this strategy resulted in the formation of both surface-confined and solution-phase polymers. To overcome this limitation, Matyjaszewski et al. 8 devised a strategy that adds the deactivator species in sufficient concentration to the reaction mixture prior to the start of the reaction. However, in the case of acrylamide-based polymerization, low conversions are common for ATRP; this fact has been explained by a slow activation of the growing chain radicals in conjunction with fast deactivation, indicating an inadequate redox potential of the catalyst. 13 Addition of deactivator species increases the deactivation rate, resulting in very low or no polymerization. Preliminary studies confirmed this result; therefore, this initial study did not employ “sacrificial” initiator or preadded deactivator species. Data will be presented that support a first-order polymerization rate with respect to monomer concentration for the system studied. These data also indicate that polymer growth is not controlled.