Department of Chemistry, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam-Golm, Germany.
Langmuir. 2011 Apr 19;27(8):4465-73. doi: 10.1021/la200115p. Epub 2011 Mar 10.
A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was synthesized by sequential reversible addition-fragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20-25 °C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40-55 and 70-80 °C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 °C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.
一系列由聚 N-丙基丙烯酰胺（PNPAM，A）、聚甲氧基二乙二醇丙烯酸酯（PMDEGA，B）和聚 N-乙基丙烯酰胺（PNEAM，C）组成的三温响应嵌段共聚物通过顺序可逆加成-断裂链转移聚合合成。制备了具有不同嵌段序列 ABC、BAC 和 ACB 的聚合物，其相转变温度按 A<B<C 的顺序升高。使用动态光散射、浊度法和 NMR 光谱法研究了它们在稀水溶液中的聚集行为。发现这些聚合物的自组织强烈依赖于嵌段序列。虽然具有末端低 LCST（低临界溶液温度）嵌段的聚合物在 20-25°C 时在第一个相转变温度以上发生聚集，但具有中间低 LCST 嵌段的三嵌段共聚物仅在第二个相转变温度以上发生聚集。中间嵌段的坍塌不足以诱导聚集，而是产生具有坍塌中间嵌段的稳定单分子胶束，这得到了 NMR 和荧光探针数据的支持。所有共聚物的进一步加热导致在 40-55 和 70-80°C 时出现另外两个热转变，可以分别与 B 和 C 嵌段的相转变相关联。一旦诱导聚集，所有聚合物都表现出高度的团聚倾向。使用溶剂化变色荧光染料尼罗红研究了三响应嵌段共聚物对疏水性试剂的载体能力。通过经过第一个热转变，嵌段共聚物能够溶解尼罗红。在具有一个末端低 LCST 嵌段的 ABC 或 ACB 嵌段共聚物的情况下，在该阶段已经可以溶解大量染料。相比之下，BAC 嵌段共聚物形成单分子胶束，疏水性探针的掺入效率要低得多。只有当 B 嵌段坍塌，达到约 45°C 的第二个相转变时，才会发生聚集，并且溶解变得有效。在 ABC 和 ACB 聚合物的情况下，疏水性探针似乎在原始坍塌的 A 链和在疏水性较低的 B 嵌段坍塌后形成的额外疏水链之间分配。
