Impact of Helical Chain Shape in Sequence-Defined Polymers on Polypeptoid Block Copolymer Self-Assembly

Abstract

Controlling the self-assembly of block copolymers with variable chain shape and stiffness is important for driving the self-assembly of functional materials containing nonideal chains as well as for developing materials with new mesostructures and unique thermodynamic interactions. The polymer helix is a particularly important functional motif. In the helical chain, the traditional scaling relationships between local chain stiffness and space-filling properties are not applicable; this in turn impacts the scaling relationships critical for governing self-assembly. Polypeptoids, a class of sequence-defined peptidomimetic polymers with controlled helical secondary structure, were used to systematically investigate the impact of helical chain shape on block copolymer self-assembly in a series of poly(n-butyl acrylate)-b-polypeptoid block copolymers. Small-angle X-ray scattering (SAXS) of the bulk materials shows that block copolymers form hexagonally packed cylinder domains. By leveraging sequence control, the polypeptoid block was controlled to form a helix only at the part either adjacent to or distant from the block junction. Differences in domain size from SAXS reveal that chain stretching of the helix near the block junction is disfavored, while helical segments at the center of cylindrical domains contribute to unfavorable packing interactions, increasing domain size. Finally, temperature-dependent SAXS shows that helix-containing diblock copolymers disorder at lower temperatures than the equivalent unstructured diblock copolymers; we attribute this to the smaller effective N of the helical structure resulting in a larger entropic gain upon disordering. These results emphasize how current descriptions of rod/coil interactions and conformational asymmetry for coil polymers do not adequately address the behavior of chain secondary structures, where the scalings of space-filling and stiff−elastic properties relative to chain stiffness deviate from those of typical coil, semiflexible, and rodlike polymers.

Properties and applications of precision oligomer materials; where organic and polymer chemistry join forces

“…Peptoids have also been used to examine fundamental questions in polymer science, such as the impact of chain stiffness on self‐assembly. Using a diblock platform composed of disperse poly( n ‐butyl acrylate) (pBA) and discrete peptoid oligomers, Segalman and Zuckermann were able to tune the rigidity of the peptoid block through control of the secondary structure 130 . Utilization of a homochiral or racemic α‐methyl benzyl side chain in the peptoid block generated sequences with either a stiff helical structure or an unstructured chain, respectively, with both types of diblocks self‐assembling into hexagonal cylinders.…”

“…Using a diblock platform composed of disperse poly(n-butyl acrylate) (pBA) and discrete peptoid oligomers, Segalman and Zuckermann were able to tune the rigidity of the peptoid block through control of the secondary structure. 130 Utilization of a homochiral or racemic α-methyl benzyl side chain in the peptoid block generated sequences with either a stiff helical structure or an unstructured chain, respectively, with both types of diblocks self-assembling into hexagonal cylinders. Despite the helical block filling less space than the unstructured analog, larger domain spacings were observed for the chiral diblocks.…”

Recent advances in crystallization and self‐assembly of polypeptoid polymers

“…Segalman and coworkers studied self‐assembly of a series of poly( n ‐butyl acrylate) block copolymers containing chiral polypeptoids . By tuning the chirality of side chains, that is, racemic or enantiomeric, the stiffness of polypeptoids can be controlled along the whole chains or part of chains.…”

“…B, TEM of polypeptoid block copolymers shows well‐ordered cylinder nanostructures. The sample is in a thin film configuration on a silicon nitride grid; the polypeptoid block is stained with RuO 4 . Adapted with permission from ref.…”

Linking two worlds in polymer chemistry: The influence of block uniformity and dispersity in amphiphilic block copolypeptoids on their self‐assembly

“…These hierarchical structures can assemble due to a rather complex and strictly defined sequence and/or exact chain‐length. The uniformity in chain length and sequence of the molecules is believed essential, because the interactions between each monomer unit is crucial for the formation of higher orders as demonstrated by integrating disruptive units into the sequence, designing a protein‐like sequence or simply varying the chirality of the side chain . Further, this uniformity facilitates the design of desired configurations or functions via computational modeling because no variations in chain length or chemically composition have to be considered …”