Stimuli-responsive polymers based on L-phenylalanine residues: Protonation thermodynamics of free polymers and cross-linked hydrogels
Stimuli-responsive polymers based on L-phenylalanine residues: Protonation thermodynamics of free polymers and cross-linked hydrogels
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DOI：
10.1021/ma047652i
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发表时间：
2005-03-22
期刊：
影响因子：
5.5
通讯作者：
Ito, Y
中科院分区：
文献类型：
--
作者：
Casolaro, M;Paccagnini, E;Ito, Y
合成了带有L-苯丙氨酸残基的乙烯基聚合物，它们以游离形式和交联水凝胶形式存在，与N-异丙基丙烯酰胺（NIPAAm）形成均聚物和共聚物。采用电位法和溶液量热法研究了COO-基团在水介质中、不同温度和增加氯化钠浓度下的质子化反应热力学。可溶性聚合物聚（N-丙烯酰基-L-苯丙氨酸）及其低NIPAAm含量的共聚物显示出特有的特征。他们的碱度常数（log K），以及相对于（质子化程度）的焓（-Δ H度）的变化显示在α = 0.5时突然下降。这归因于质子化和电离的相邻COO-基团之间形成氢键。该过程是由侧链芳环驱动的，其改善了疏水相互作用。由于大分子构象自由度的增加和亲水基团周围水分子的释放，熵变（AS '）急剧增加。相应的交联聚合物形成响应pH、温度和离子强度的水凝胶。这两种水凝胶，P9（均聚物与9摩尔%的交联）和CP 2（共聚物与90摩尔%的NIPAAm和2摩尔%的交联），其特征在于其pH值和温度响应行为的平衡和振荡溶胀研究。它们表现出强烈的pH依赖性体积相变和不寻常的氯化钠相变现象。此外，水凝胶CP 2在水溶液中表现出温度依赖性体积相变（LCST，低临界溶液温度）行为，其中LCST通过降低pH值而降低。它对RAW 264细胞系是无毒的。
Vinyl polymers carrying L-phenylalanine residues were synthesized in the free and in the cross-linked hydrogel form, as homo- and copolymers with N-isopropylacrylamide (NIPAAm). The protonation reaction thermodynamics of the COO- group were studied in aqueous media, at different temperatures and at increased concentrations of sodium chloride, mainly by potentiometry and solution calorimetry. The soluble polymer, namely poly(N-acryloyl-L-phenylalanine), and its copolymers with low NIPAAm content displayed characteristic features. Their basicity constants (log K), as well as the enthalpy (-Delta H degrees) changes in relation to a (degree of protonation) showed an abrupt drop at alpha = 0.5. This was ascribed to the formation of hydrogen bonds between the protonated and the ionized neighboring COO- groups. The process was driven by the side-chain aromatic rings that improved hydrophobic interactions. The entropy (AS') changes sharply increased as a result of the increased macromolecular conformational freedom and the release of water molecules surrounding the hydrophilic groups of the polymer. The corresponding cross-linked polymers formed hydrogels that were responsive to pH, temperature, and ionic strength. The two hydrogels, P9 (homopolymer with 9 mol% cross-links) and CP2 (copolymer with 90 mol% of NIPAAm and 2 mol% cross-links), were characterized for their pH- and temperature-responsive behavior by equilibrium and oscillatory swelling studies. They demonstrated a strong pH-dependent volume phase-transition and an unusual sodium chloride phase-transition phenomenon. Moreover, the hydrogel CP2 exhibited a temperature-dependent volume phase-transition (LCST, lower critical solution temperature) behavior in aqueous solution, where the LCST decreased by lowering the pH. It was nontoxic against the RAW264 cell line.