Macromolecular co-assembly

Up to now, studies on IPECs have mostly been focused on the macromolecular co-assemblies formed by the oppositely charged linear polyions, both synthetic and natural (e.g., DNA). However, in recent years there have also been reports of IPECs based on nonlinear polyionic species, such as slightly cross-linked PE... 

The domain of water-soluble IPECs was thoroughly and systematically investigated by Kabanov and Zezin and their coworkers, who suggested that such macromolecular co-assemblies can be considered as peculiar amphiphilic block copolymers whose solubility in aqueous media results from charged fragments of the HPE that do not form interpolymer salt bonds with their polymeric counterparts [33], The structure of water-soluble nonstoichiometric IPECs with ladder-like sequences of interpolymer salt bonds proposed by Zezin and Kabanov is schematically depicted in Fig. 1. 

This review deals with recently obtained experimental results on IPECs based on branched PE species, specifically including PE stars, star-like micelles generated in aqueous solutions of ionic amphiphilic block co- and terpolymers, and cylindrical PE brushes. In addition, we will also present the results of molecular dynamics (MD) simulations performed for some of these systems, which enable the possible structural organization of the formed macromolecular co-assemblies to be revealed. 

The advances in controlled polymerization achieved in recent years have opened up the possibility of synthesizing well-defined star-shaped PEs, particularly those containing a large number of arms. To the best of our knowledge, no publications on the interaction of PE stars with oppositely charged macromolecules or investigations on the properties of the resulting macromolecular co-assemblies can be found in the literature to date, except for a few papers [79-81] that are discussed below. 

It is remarkable that an aqueous solution of the reference linear PAA becomes turbid after adding the first portion of the aqueous solution of P4VPQ (Fig. 2, inset). This clearly indicates that the reference linear PAA, in contrast to the (PAA) y stars, cannot form water-soluble IPECs via this method under the same conditions and also clearly manifests a pronounced effect of the topology of the polymeric component involved in interpolyelectrolyte complexation on the possibility of preparing water-soluble macromolecular co-assemblies. 

A detailed examination of the homogeneous mixtures of (PAA)x stars and P4VPQ [79] by dynamic light scattering (DLS) provided evidence of a distinct coexistence of the two populations of macromolecular co-assemblies (1) A dominant fraction comprising particles of water-soluble IPECs that result from interactions between oppositely charged polymeric components their hydrodynamic size was... 

Fig. 4 Proposed inhomogeneous core-corona structure of the macromolecular co-assemblies formed in aqueous mixtures of (PAA)2i stars (in the form of sodium salt) and P2VPQ. In structure /, some of the arms of the star-shaped HPE are embedded in the core and others are extended. In structure II, all of the arms contribute segments of equal length to the core. Reprinted from [80] with permission from Springer...

An interesting example of macromolecular co-assemblies derived from starshaped polyionic species was reported by Ge et al. [81]. The authors found that a star-shaped double hydrophilic poly(methacrylic acid)-poly(ethylene oxide) heteroarm copolymer [(PMAA)x-PDVB-(PEO)x, with PDVB being poly(divinylbenzene) and X denoting the number of PMAA and PEG arms] can interact in alkaline media with a double hydrophilic poly(ethylene oxide)-block-quaternized poly[2-(dimethylamino)ethyl methacrylate] (PEO- -PDMAEMAQ) diblock copolymer. At Z = [PDMAEMAQ]/[PMAA] = 1, well-defined water-soluble onion-like (core-shell-corona) macromolecular co-assemblies are formed, with a hydrophobic core consisting of a PDVB microgel. The interaction of the PMA arms of the hybrid coronas of such copolymer stars with the PDMAEMAQ+ blocks of the diblock copolymer generates an insoluble inner layer (shell) around a PDVB core. Meanwhile, PEG blocks from both PEG- -PDMAEMAQ and (PMAA)x-PDVB-(PEG)x build up a hydrophilic nonionic corona that stabilizes the whole complex in aqueous media. 

Using atomic force microscopy (AFM), Xu et al. [85] showed that similar macromolecular co-assemblies derived from the cylindrical PE brush based on quat-ernized poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMAQ) complexed with short poly(sodium styrene sulfonate) (PSSNa) have distinct longitudinal undulations (Fig. 8), thereby apparently providing experimental proof of a peculiar pearl-necklace structure of IPECs based on such branched PEs. 

Further investigations carried out by Chelushkin et al. [73] for various combinations of micelles formed in aqueous media by ionic amphiphilic diblock copolymers [PS-fc-P4VPQ, poly(styrene)-fetocA -poly(sodium acrylate) (PS- -PANa), and PS- -PMANa)] with oppositely charged linear PEs (polycarboxylates, polysulfonates, polyphosphates, and aromatic, aliphatic, and alicyclic quaternized polyamines) have demonstrated that the solubility of IPECs is decisively determined by (1) the aggregation state of the excess polymeric component (micelles versus individual polymeric coils) and (2) the procedure or method employed for the preparation of such macromolecular co-assemblies. 

The results of experimental and theoretical research on water-soluble (nonstoichio-metric) IPECs based on nonlinear (branched) polyionic species (HPE) complexed with oppositely charged linear PEs (GPE) demonstrated that the main feature of such macromolecular co-assemblies is their pronounced compartmentalized structure, which results from a distinctly nonuniform distribution of the linear GPE chains within the intramolecular volume of the branched HPE. In the case of star-shaped PEs or star-like micelles of ionic amphiphilic block copolymers, this com-partmentalization leads to the formation of water-soluble IPECs with core-corona (complex coacervate core) or core-shell-corona (complex coacervate shell) structures, respectively. Water-soluble IPECs based on cylindrical PE brushes appear to exhibit longitudinally undulating structures (necklace) of complex coacervate pearls decorated by the cylindrical PE corona. 

Einally, the enormous number of possible combinations of oppositely charged polymeric components that can be involved in interpolyelectrolyte complexation offers attractive perspectives for the preparation of water-soluble multicompartment nanosized macromolecular co-assemblies with desired properties. We believe that such novel IPECs are very promising and will be in demand for their future applications in nanomedicine (e.g., gene and drug delivery, and diagnostic systems), biotechnology, and nanotechnology as nanocontainers, nanoreactors, and molecular templates for nanoelectronic devices. 

Abstract This review considers interpolyelectrolyte complexes, with a particular emphasis on advanced macromolecular co-assemblies based on polyionic species with nonlinear topology and on polymer-inorganic hybrids formed by inteipolye-lectrolyte complexes containing metal ions and/or metal nanoparticles. 

Keywords Interpolyelectrolyte complexes Interpolyelectrolyte reactions Macromolecular co-assembly Metal nanoparticles MetaUo-containing interpolyelectrolyte complexes Nanostructures Polyelectrolytes Polymer-inorganic hybrids... 

Nowadays, IPECs are typically considered to be self-organizing and ordered macromolecular co-assemblies. This concept is based not only on semi-intuitive understanding of such systems as products of the cooperative interpolyelectrolyte interaction but also on some supporting experimental findings. 

Self-organization processes proceeding in IPECs are not timited by the nanometer scale, pronouncedly manifesting themselves on the scale corresponding to mesostructures and in bulk. The self-organization can lead to the selective formation of highly ordered structures. An example of such a stracture is stoichiometric IPEC formed by PA anions and protonated linear poly(ethylene imine) (PEI) cations. The study of chemical transformatirais in such macromolecular co-assemblies provided evidence that more than about 80% (mol) of interpolymer salt bonds can be converted to covalent amide braids, as illustrated by Scheme (11) ... 

To broaden the window of Z-values corresponding to the formation of water-soluble IPECs, star-shaped polyions can be complexed with double hydrophilic (bis-hydrophUic) diblock copolymers comprising an ionic block and a nonionic hydrophilic block. In this case, the formed macromolecular co-assemblies remain water-soluble even if oppositely charged groups of the polymeric counterparts are taken in 1 1 ratio (Z = 1) [49], provided that the length of hydrophilic nonionic block is sufficiently long. Apart from the enhanced water-solubUity of the formed complex species, one can additionally impart new properties and desired functionalities, such as biocompatibUity, thermosensitivity, etc., through the hydrophilic nonionic block of the copolymer. 

Similar micelle-like macromolecular co-assemblies were reported [50] for an anionic star-shaped bis-hydrophilic heteroarm copolymer PMAA-PEO coupled with a cationic bis-hydrophilic diblock copolymer, PDMAEMAQ-Wock-PEO (PDMAEMAQ-b-PEO), in alkaline media. In this case, however, the PEO blocks of both polymeric counterparts form a hydrophilic corona of each of the complex species formed. 

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