Stereocomplexation of Helical Polycarbodiimides Synthesized from Achiral Monomers Bearing Isopropyl Pendants

A high level of the permanent asymmetry was built into the poly­(<i>N</i>-methyl-<i>N</i>′-(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 <i>P</i>/<i>M</i> racemization upon thermal annealing, thus leading to the formation of a stereocomplexed structure. Predominantly <i>P</i> and <i>M</i> helices have been obtained through helix sense selective polymerization by using chiral BINOL-Ti­(IV) diisopropoxide initiator with achiral <i>N</i>-methyl-<i>N</i>′-(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 <i>M</i> helices or +283° for <i>P</i> 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 <i>ortho</i>, <i>meta</i>, and <i>para</i> positions have been synthesized, and their self-assembly behavior has been studied by using AFM, SEM, <i>p</i>-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(<i>N</i>-methyl-<i>N</i>′-(2-isopropyl-6-methylphenyl)­carbodiimide)-<i>b</i>-poly­(<i>N</i>-phenyl-<i>N</i>′-octadecyl­carbodiimide) (<b>P-1,2</b>), 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