Synthesis and Assembly of Designer Styrenic Diblock Polyelectrolytes

The Role of Chemically Innocent Polyanions in Active, Chemically Fueled Complex Coacervate Droplets

Complex coacervation describes the liquid-liquid phase separation of oppositely charged polymers. Active coacervates are droplets in which one of the electrolyte's affinity is regulated by chemical reactions. These droplets are particularly interesting because they are tightly regulated by reaction kinetics. For example, they serve as a model for membraneless organelles that are also often regulated by biochemical transformations such as post-translational modifications. They are also a great protocell model or could be used to synthesize life-they spontaneously emerge in response to reagents, compete, and decay when all nutrients have been consumed. However, the role of the unreactive building blocks, e.g., the polymeric compounds, is poorly understood. Here, we show the important role of the chemically innocent, unreactive polyanion of our chemically fueled coacervation droplets. We show that the polyanion drastically influences the resulting droplets' life cycle without influencing the chemical reaction cycle-either they are very dynamic or have a delayed dissolution. Additionally, we derive a mechanistic understanding of our observations and show how additives and rational polymer design help to create the desired coacervate emulsion life cycles.

Effect of Charged Block Length Mismatch on Double Diblock Polyelectrolyte Complex Micelle Cores

Polyelectrolyte complex micelles are hydrophilic nanoparticles that self-assemble in aqueous environments due to associative microphase separation between oppositely charged blocky polyelectrolytes. In this work, we employ a suite of physical characterization tools to examine the effect of charged block length mismatch on the equilibrium structure of double diblock polyelectrolyte complex micelles (D-PCMs) by mixing a diverse library of peptide and synthetic charged-neutral block polyelectrolytes with a wide range of charged block lengths (25-200 units) and chemistries. Early work on D-PCMs suggested that this class of micelles can only be formed from blocky polyelectrolytes with identical charged block lengths, a phenomenon referred to as chain length recognition. Here, we use salt annealing to create PCMs at equilibrium, which shows that chain length recognition, a longstanding hurdle to repeatable self-assembly from mismatched polyelectrolytes, can be overcome. Interestingly, D-PCM structure-property relationships display a range of values that vary systematically with the charged block lengths and chemical identity of constituent polyelectrolyte pairings and cannot be described by generalizable scaling laws. We discuss the interdependent growth behavior of the radius, ionic pair aggregation number, and density in the micelle core for three chemically distinct diblock pairings and suggest a potential physical mechanism that leads to this unique behavior. By comparing the results of these D-PCMs to the scaling laws recently developed for single diblock polyelectrolyte complex micelles (S-PCMs: diblock + homopolymer), we observe that D-PCM design schemes reduce the size and aggregation number and restrict their growth to a function of charged block length relative to S-PCMs. Understanding these favorable attributes enables more predictive use of a wider array of charged molecular building blocks to anticipate and control macroscopic properties of micelles spanning countless storage and delivery applications.

Translocation Behaviors of Synthetic Polyelectrolytes through Alpha-Hemolysin (α-HL) and Mycobacterium smegmatis Porin A (MspA) Nanopores

DNAs have been used as probes for nanopore sensing of noncharged biomacromolecules due to its negative phosphate backbone. Inspired by this, we explored the potential of diblock synthetic polyelectrolytes as more flexible and inexpensive nanopore sensing probes by investigating translocation behaviors of PEO-b-PSS and PEO-b-PVBTMA through commonly used alpha-hemolysin (α-HL) and Mycobacterium smegmatis porin A (MspA) nanopores. Translocation recordings in different configurations of pore orientation and testing voltage indicated efficient PEO-b-PSS translocations through α-HL and PEO-b-PVBTMA translocations through MspA. This work provides insight into synthetic polyelectrolyte-based probes to expand probe selection and flexibility for nanopore sensing.

Advances in the Structural Design of Polyelectrolyte Complex Micelles

Polyelectrolyte complex micelles (PCMs) are a unique class of self-assembled nanoparticles that form with a core of associated polycations and polyanions, microphase-separated from neutral, hydrophilic coronas in aqueous solution. The hydrated nature and structural and chemical versatility make PCMs an attractive system for delivery and for fundamental polymer physics research. By leveraging block copolymer design with controlled self-assembly, fundamental structure-property relationships can be established to tune the size, morphology, and stability of PCMs precisely in pursuit of tailored nanocarriers, ultimately offering storage, protection, transport, and delivery of active ingredients. This perspective highlights recent advances in predictive PCM design, focusing on (i) structure-property relationships to target specific nanoscale dimensions and shapes and (ii) characterization of PCM dynamics primarily using time-resolved scattering techniques. We present several vignettes from these two emerging areas of PCM research and discuss key opportunities for PCM design to advance precision medicine.

Weakly Ionically Bound Thermosensitive Hyperbranched Polymers

We synthesized novel amphiphilic hyperbranched polymers (HBPs) with variable contents of weakly ionically tethered thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) macrocations in contrast to traditional covalent linking. Their assembling behavior was studied below and above the lower critical solution temperature (LCST). The HBPs underwent a morphological transition under changing temperature and ionic strength due to the LCST transition of PNIPAM and the reduction in the ionization degree of terminal ionic groups, respectively. We suggest that, in contrast to traditional branched polymers, ionically linked PNIPAM macrocations can reversibly disassociate from the sulfonate groups and form mobile coronas, endowing the dynamic micellar morphologies. In addition, assembly at the air-water interface confined PNIPAM macrocations and resulted in the formation of heterogeneous Langmuir-Blodgett (LB) monolayers with diverse surface morphologies for different peripheral compositions with circular domains formed in the condensed state. The HBPs with 25% PNIPAM showed larger and more stable circular domains that were partially preserved at high compression than those of HBPs with 50% PNIPAM. Moreover, the LB monolayers showed variable surface mechanical and surface charge distribution, which can be attributed to net dipole redistribution caused by the behavior of mobile PNIPAM macrocations and core sulfonate groups.

Spatiotemporal Formation and Growth Kinetics of Polyelectrolyte Complex Micelles with Millisecond Resolution

We have directly observed the in situ self-assembly kinetics of polyelectrolyte complex (PEC) micelles by synchrotron time-resolved small-angle X-ray scattering, equipped with a stopped-flow device that provides millisecond temporal resolution. A synthesized neutral-charged diblock polycation and homopolyanion that we have previously investigated as a model charge-matched, core-shell micelle system were selected for this work. The initial micellization of the oppositely charged polyelectrolytes was completed within the dead time of mixing of 100 ms, followed by micelle growth and equilibration up to several seconds. By combining the structural evolution of the radius of gyration (R_g) with complementary molecular dynamics simulations, we show how the self-assemblies evolve incrementally in size over time through a two-step kinetic process: first, oppositely charged polyelectrolyte chains pair to form nascent aggregates that immediately assemble into spherical micelles, and second, these PEC micelles grow into larger micellar entities. This work has determined one possible kinetic pathway for the initial formation of PEC micelles, which provides useful physical insights for increasing fundamental understanding self-assembly dynamics, driven by polyelectrolyte complexation that occurs on ultrafast time scales.

SAXS methods for investigating macromolecular and self-assembled polyelectrolyte complexes

Polyelectrolyte complexation is driven by associative interactions between oppositely charged polyelectrolytes, resulting in formation of a macroscopic polymer dense phase and a polymer dilute phase with applications in coatings, adhesives, and purification membranes. Beyond macroscale phase separation, precision polymer synthesis has enabled further development of polyelectrolyte complex (PEC)-based self-assembled micelles and hydrogels with applications in biotechnology. Interestingly, it has been suggested that mechanisms similar to polyelectrolyte complexation drive formation of biological condensates that play an indispensable role in cellular biogenesis. The formation pathways and functionality of these complex materials is dependent on the physical properties that are built into polymer structure and the resulting physical conformation in the dilute and dense phase. Scattering techniques have enabled in situ investigation of structure-function relationships in PEC materials that may address unresolved biophysical questions in cellular processes as well as catalyze the development of novel materials for diverse applications. We describe preparation of PEC materials with controlled polymer characteristics (length, blockiness, charge density), small-angle X-ray scattering (SAXS) techniques employed to probe appropriate length scales, and the data analysis routines from a practical standpoint for new users. This article deals with bulk complexes and not with the related, important and interesting area of non-equilibrium layer-by-layer assembly of polyelectrolytes.

Comparing Zwitterionic and PEG Exteriors of Polyelectrolyte Complex Micelles

A series of model polyelectrolyte complex micelles (PCMs) was prepared to investigate the consequences of neutral and zwitterionic chemistries and distinct charged cores on the size and stability of nanocarriers. Using aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization, we synthesized a well-defined diblock polyelectrolyte system, poly(2-methacryloyloxyethyl phosphorylcholine methacrylate)-block-poly((vinylbenzyl) trimethylammonium) (PMPC-PVBTMA), at various neutral and charged block lengths to compare directly against PCM structure-property relationships centered on poly(ethylene glycol)-block-poly((vinylbenzyl) trimethylammonium) (PEG-PVBTMA) and poly(ethylene glycol)-block-poly(l-lysine) (PEG-PLK). After complexation with a common polyanion, poly(sodium acrylate), the resulting PCMs were characterized by dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). We observed uniform assemblies of spherical micelles with a diameter ~1.5-2× larger when PMPC-PVBTMA was used compared to PEG-PLK and PEG-PVBTMA via SAXS and DLS. In addition, PEG-PLK PCMs proved most resistant to dissolution by both monovalent and divalent salt, followed by PEG-PVBTMA then PMPC-PVBTMA. All micelle systems were serum stable in 100% fetal bovine serum over the course of 8 h by time-resolved DLS, demonstrating minimal interactions with serum proteins and potential as in vivo drug delivery vehicles. This thorough study of the synthesis, assembly, and characterization of zwitterionic polymers in PCMs advances the design space for charge-driven micelle assemblies.

Effect of mixed solvents on polyelectrolyte complexes with salt

Strongly interacting polyelectrolyte complexes (PECs) are a versatile class of materials whose physical states can be driven from solids into liquids and ultimately into homogenous solution upon salt addition. However, many of these materials can display high stability using common monovalent salts, leading to difficulties in accessing the entire PEC spectrum. Here, the model system, composed of two styrenic polyelectrolytes, required exceptionally high salt to drive phase transition. We term the amount of salt required to drive these transitions salt resistance. In water, the PEC transferred from solid into liquid at 2.5 M NaBr and never fully dissociated within the studied salt range. We discovered an unconventional approach of weakening salt resistance by switching the solvent to miscible ethylene glycol/water and ethanol/water, allowing us to systematically introduce more hydrophobic constituents. Employing microscopy to determine physical states qualitatively, we found that higher hydrophobicity lowered salt resistance for phase transition and disassembly.

Langevin Dynamics Simulations of the Exchange of Complex Coacervate Core Micelles: The Role of Nonelectrostatic Attraction and Polyelectrolyte Length

Complex coacervate core micelles (C3Ms) are promising encapsulators for a wide variety of (bio)molecules. To protect and stabilize their cargo, it is essential to control their exchange dynamics. Yet, to date, little is known about the kinetic stability of C3Ms and the dynamic equilibrium of molecular building blocks with micellar species. Here we study the C3M exchange during the initial micellization by using Langevin dynamics simulations. In this way, we show that charge neutral heterocomplexes consisting of multiple building blocks are the primary mediator for exchange. In addition, we show that the kinetic stability of the C3Ms can be tuned not only by the electrostatic interaction but also by the nonelectrostatic attraction between the polyelectrolytes, the polyelectrolyte length ratio, and the overall polyelectrolyte length. These insights offer new rational design guides to aid the development of new C3M encapsulation strategies.

Interparticle Interactions in Dilute Solutions of Polyelectrolyte Complex Micelles

The application of dilute solutions of polyelectrolyte complex (PEC) micelles for delivering therapeutic nucleic acids into disease sites has gained momentum. This Letter reports a detailed characterization of PEC micelles in dilute solutions including their internal structures and the determination of the interparticle interactions. The polymer concentration ranges from 0.1 to 0.5 wt %, a regime where micelle-micelle interactions are infrequent. We employ synchrotron small-angle X-ray scattering (SAXS) to simultaneously probe the morphology, internal structure, and radius of gyration (R_g) of the self-assemblies formed by charged diblock polyelectrolytes and homopolyelectrolytes. The emerging appearance of the structure factor in SAXS profiles with the increasing polymer concentration demonstrates the presence of the repulsive intermicellar correlations, which is further confirmed by the differences between the "reciprocal R_g" estimated by Guinier approximation and the "real space R_g" determined by pair distribution functions. We find that the soft corona chains tethered on the surface of phase-separated complex domains are compressed when micelles come close to the point where a hard-sphere interaction takes over. These findings contribute to the fundamental understanding of the structure and space-filling constraints in the complexation-driven self-assemblies and advance the rational design of cationic polymer-based nonviral gene delivery vectors.

Mapping the phase behavior of coacervate-driven self-assembly in diblock copolyelectrolytes

Oppositely-charged polymers can undergo an associative phase separation process known as complex coacervation, which is driven by the electrostatic attraction between the two polymer species. This driving force for phase separation can be harnessed to drive self-assembly, via pairs of block copolyelectrolytes with opposite charge and thus favorable coulombic interactions. There are few predictions of coacervate self-assembly phase behavior due to the wide variety of molecular and environmental parameters, along with fundamental theoretical challenges. In this paper, we use recent advances in coacervate theory to predict the solution-phase assembly of diblock polyelectrolyte pairs for a number of molecular design parameters (charged block fraction, polymer length). Phase diagrams show that self-assembly occurs at high polymer, low salt concentrations for a range of charge block fractions. We show that we qualitatively obtain limiting results seen in the experimental literature, including the emergence of a high polymer-fraction reentrant transition that gives rise to a self-compatibilized homopolymer coacervate behavior at the limit of high charge block fraction. In intermediate charge block fractions, we draw an analogy between the role of salt concentration in coacervation-driven assembly and the role of temperature in χ-driven assembly. We also explore salt partitioning between microphase separated domains in block copolyelectrolytes, with parallels to homopolyelectrolyte coacervation.

High Throughput Screening of Glycopolymers: Balance between Cytotoxicity and Antibacterial Property

To search for synthetic agents with low cytotoxicity and good antibacterial activity is essential for antimicrobial applications. Here we report a high throughput technique that carried out in multiwell plates via recyclable-catalyst-aided, opened-to-air, and sunlight-photolyzed RAFT (ROS-RAFT) polymerization. By using this method, three key monomers (MAG the sugar unit, DMAPMA the positively charged monomer, and DEMAA the hydrophobic monomer) can be polymerized in a controlled manner to afford glycopolymers. This simple high throughput technology is used to synthesize glycopolymers with variable compositions. The bacterial adhesion/killing ability and cytotoxicity of synthesized polymers have been evaluated, and glycopolymers with certain composition can achieve a balance of low cytotoxic and good antibacterial activity.

Polyelectrolyte Complexation of Oligonucleotides by Charged Hydrophobic-Neutral Hydrophilic Block Copolymers

Polyelectrolyte complex micelles (PCMs, core-shell nanoparticles formed by complexation of a polyelectrolyte with a polyelectrolyte-hydrophilic neutral block copolymer) offer a solution to the critical problem of delivering therapeutic nucleic acids, Despite this, few systematic studies have been conducted on how parameters such as polycation charge density, hydrophobicity, and choice of charged group influence PCM properties, despite evidence that these strongly influence the complexation behavior of polyelectrolyte homopolymers. In this article, we report a comparison of oligonucleotide PCMs and polyelectrolyte complexes formed by poly(lysine) and poly((vinylbenzyl) trimethylammonium) (PVBTMA), a styrenic polycation with comparatively higher charge density, increased hydrophobicity, and a permanent positive charge. All of these differences have been individually suggested to provide increased complex stability, but we find that PVBTMA in fact complexes oligonucleotides more weakly than does poly(lysine), as measured by stability versus added salt. Using small angle X-ray scattering and electron microscopy, we find that PCMs formed from both cationic blocks exhibit very similar structure-property relationships, with PCM radius determined by the cationic block size and shape controlled by the hybridization state of the oligonucleotides. These observations narrow the design space for optimizing therapeutic PCMs and provide new insights into the rich polymer physics of polyelectrolyte self-assembly.

Polyelectrolyte complexation via viscoelastic phase separation results in tough and self-recovering porous hydrogels

Polyelectrolyte complexation is utilized to create bulk, porous hydrogels with high toughness and self-recovery due to reversible ionic associations.

Structural transitions and encapsulation selectivity of thermoresponsive polyelectrolyte complex micelles

Polyelectrolyte complex micelles containing thermoresponsive coronas can exhibit varying morphologies and encapsulate multivalently charged therapeutics for drug delivery applications.

Computer study of the solubilization of polymer chains in polyelectrolyte complex cores of polymeric nanoparticles in aqueous media

The formation and structure of nanoparticles containing non-polar polymer chains solubilized in interpolyelectrolyte complex (IPC) cores and the partitioning of non-polar chains between bulk solvent and IPC cores were studied by coarse-grained computer simulations. The choice of the model system was inspired by experimental results published by van der Burgh et al. (Langmuir, 2004, 20, 1073-1084). The dissipative particle dynamics (DPD) simulations reproduced the structure and basic features of co-assembled nanoparticles described by experimentalists well at the semi-quantitative coarse-grained level and revealed new properties of co-assembled particles. The simulated co-assemblies were used as reference systems for the solubilization studies. Their results show that non-polar polymers (electrically neutral and compatible with core-forming chains) solubilize easily in IPC cores. They intermix with polyelectrolyte blocks in cores and do not hinder, but, on the contrary, they slightly promote the electrostatic co-assembly.