Stereocomplexation of Helical Polycarbodiimides Synthesized from Achiral Monomers Bearing Isopropyl Pendants,Macromolecules

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Stereocomplexation of Helical Polycarbodiimides Synthesized from Achiral Monomers Bearing Isopropyl Pendants
Macromolecules ( IF 5.1 ) Pub Date : 2017-11-22 00:00:00 , DOI: 10.1021/acs.macromol.7b01633
Dumindika A. Siriwardane ₁ , Oleg Kulikov ₁ , Yekaterina Rokhlenko ₂ , Sahila Perananthan ₁ , Bruce M. Novak ₁

Affiliation

A high level of the permanent asymmetry was built into the poly(N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide) system by introducing a bulky, substituted phenyl group which revealed a very interesting phenomenological behavior upon heating. This polymer undergoes P/M racemization upon thermal annealing, thus leading to the formation of a stereocomplexed structure. Predominantly P and M helices have been obtained through helix sense selective polymerization by using chiral BINOL-Ti(IV) diisopropoxide initiator with achiral N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide monomer. Upon thermal annealing, the specific optical rotation (SOR) of the single-handed polymer begins to decrease but never reaches zero. The SOR plateaus at a large value (−286° for M helices or +283° for P helices), and shortly thereafter the polymer forms a precipitate. The process that polymer undergoes is attributed to stereocomplexation between two complementary strands via racemization. Inspired by the phenomena analogous to classical leucine zippers with isobutyl termini (interlocking motifs), a unique polycarbodiimide scaffold bearing isopropyl pendant groups was designed to play a vital role in the aggregation process with a calculated energy barrier of around 19 ± 0.4 kcal/mol. To investigate the effect of regioregularity in isopropyl groups, a series of isomeric polymers bearing isopropyl segments at the ortho, meta, and para positions have been synthesized, and their self-assembly behavior has been studied by using AFM, SEM, p-XRD, and TEM analytical techniques. To take advantage of both isopropyl zipping motif and increased solubility in organic solvents imparted by octadecyl lateral chains, a new block copolymer, poly(N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide)-b-poly(N-phenyl-N′-octadecylcarbodiimide) (P-1,2), was designed. The first block, containing the substituted aryl functional group, contributes to the stereocomplexation phenomena, while the second block copolymer, composed of the octadecyl group, imparts solubility and morphological attributes. This unique polymeric scaffold exhibits interesting morphologies such as spherical particles, capsules, wrinkled surface patterns, and fiber-like motifs, which may be associated with supramolecular aggregation. Detailed stereocomplex formation studies will bestow new possibilities in diverse areas, including drug delivery applications, catalysis, and chiral separations.

中文翻译：

含异丙基侧链的非手性单体合成的螺旋聚碳二亚胺的立体络合物

通过引入庞大的取代苯基，在加热后显示出非常有趣的现象学行为，从而在聚（N-甲基-N '-（2-异丙基-6-甲基苯基）碳二亚胺）体系中建立了高度永久的不对称性。该聚合物在热退火后经历P / M外消旋作用，从而导致形成立体复合结构。通过使用手性BINOL-Ti（IV）二异丙氧基引发剂和非手性N-甲基-N进行螺旋有义选择性聚合，主要获得了P和M螺旋。'-（2-异丙基-6-甲基苯基）碳二亚胺单体。热退火后，单手聚合物的比旋光度（SOR）开始降低，但从未达到零。SOR平稳值较大（M螺旋为-286 °，P为+ 283°螺旋），然后不久，聚合物形成沉淀。聚合物经历的过程归因于外消旋作用在两条互补链之间的立体复合。受到类似于具有异丁基末端的经典亮氨酸拉链（互锁图案）现象的启发，带有异丙基侧基的独特的聚碳二亚胺支架被设计为在聚集过程中起着至关重要的作用，其能量屏障的计算值约为19±0.4 kcal / mol。为了研究异丙基中区域规则性的影响，合成了一系列在邻位，间位和对位带有异丙基链段的异构体聚合物，并通过原子力显微镜，扫描电子显微镜，p-XRD和TEM分析技术。为了同时利用异丙基拉链基序和十八烷基侧链赋予的在有机溶剂中增加的溶解度，一种新的嵌段共聚物，聚（N-甲基-N '-（2-异丙基-6-甲基苯基）碳二亚胺）-b-聚（N-苯基-N'-十八烷基碳二亚胺）（P-1,2），被设计。包含取代的芳基官能团的第一个嵌段有助于立体络合现象，而由十八烷基组成的第二个嵌段共聚物则赋予了溶解性和形态学特性。这种独特的聚合物支架表现出有趣的形态，例如球形颗粒，胶囊，起皱的表面图案和类似纤维的基序，这可能与超分子聚集有关。详细的立体复合物形成研究将在不同领域赋予新的可能性，包括药物递送应用，催化作用和手性分离。

更新日期：2017-11-22

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