Abstract:
We describe a novel class of periodically grafted amphiphilic copolymers (PGACs) that could serve as nonionic
functional mimics of ionenes, the primary difference being that the periodically occurring charged units along the backbone of
ionenes are replaced by hydrophilic oligoethylene glycol segments. The synthesis and properties of this new class of segmented
polymers that carry a hydrophobic alkylene polyester backone with periodically placed hydrophilic oligo(oxyethylene) pendant
units are presented. When the length of the intervening alkylene segment is adequately long, 22-carbons in this case, and the
pendant unit is a hexaethylene glycol monomethyl ether (HEG) segment, the polymer chain appears to adopt a folded zigzag
conformation, reminiscent of the accordion-type structure formed by cationic ionenes. This transformation is driven by the
intrinsic immiscibility of the alkylene and HEG segments and is reinforced by the strong tendency for long chain alkylene
segments to crystallize in a paraffinic lattice. Evidence of the formation of such structures comes from the AFM images, which
reveal the formation of remarkably flat pancake-like aggregates that are formed by the lateral aggregation of individually collapsed
polymer chains; importantly, the heights of these structures match well with the lamellar layer-spacing obtained from SAXS
studies of bulk samples. DSC studies further confirm the crystallization of the intervening alkylene segments, especially when
they are long (C22), suggesting the formation of the folded zigzag structures. In a suitably designed PGAC that carries
diacetylene units symmetrically placed within the alkylene segment, attempts were made to cross-polymerize the diacetylene
units and generate PEGylated nanoparticles. However, these attempts were unsuccessful demonstrating the very stringent
geometric requirements for the topotactic polymerization of diacetylenes.
