Chemical Feedback in Templated Reaction-Assembly Networks

Growing polyion complex micelles: kinetics and mechanism of electrostatic template polymerization and assembly

HYPOTHESIS

Electrostatic-templated polymerization (ETP) is a recently developed strategy for robust fabrication of stable polyion complex (PIC) micelles with regulated size and morphology, yet the kinetics and mechanism about this one pot process remain elusive.

EXPERIMENTS

In ETP method, ionic monomers are polymerized in the presence of an oppositely charged ionic-neutral diblock copolymer as template. We investigate the monomer conversion and electrostatic assembly as a function of time, under different polymerization conditions of ionic strength, pH, template/monomer molar ratio and the presence of a cross-linker.

FINDINGS

The template copolymer accelerates the monomer conversion and formation of PIC micelles dependent on ionic strength and pH. The process follows the "Pick-up" mechanism, where monomers first convert into oligomers which complex with template to induce further chain growth and electrostatic assembly. Introducing cross-linker hardly impacts the reaction kinetics and "Pick-up" route, while it creates PIC micelles containing cross-linked ionic network assembly with the template. Further removing the template by concentrated salt provides purified ionic nanogels. The disclosed findings not only provide a better understanding of the polymerization-assembly process, but also optimize the controls of electrostatic-templated polymerization for the rational design and fabrication of diverse PIC micelles and polyelectrolyte particles.

Robust Electrostatic-Templated Polymerization for Controllable Synthesis of Stable and Permeable Polyelectrolyte Vesicles

Polymer vesicles are of profound interest for designing delivery vehicles and nanoreactors toward a variety of biomedical and catalytic applications, yet robust synthesis of stable and permeable vesicles remains challenging. Here, we propose an electrostatic-templated polymerization that enables fabrication of polyelectrolyte vesicles with simultaneously regulated stability and permeability. In our design, cationic monomers were copolymerized with cross-linkers in the presence of a polyanionic-neutral diblock copolymer as a template. By properly choosing the block length ratio of the template, we fabricated a type of polyion complex vesicle consisting of a cross-linked cationic membrane, electrostatically assembled with the template copolymer which can be removed by sequential dissociation and separation under concentrated salt. We finally obtained stable polyelectrolyte vesicles of regulated size, membrane permeability, and response properties by tuning the synthesis factors including ionic strength, cross-linker type, and fraction as well as different monomers and concentrations. As a proof-of-concept, lipase was loaded in the designed cationic vesicles, which exhibited enhanced enzyme stability and activity. Our study has developed a novel and robust strategy for controllable synthesis of a new class of stable and permeable polymer (polyelectrolyte) vesicles that feature great potential applications as functional delivery carriers and nanoreactors.

Like-Charge PISA: Polymerization-Induced Like-Charge Electrostatic Self-Assembly

We report the use of l-aspartic acid chiral ionic hydrogen bonds to drive liquid-liquid phase separation (LLPS) and precision two-dimensional electrostatic self-assembly in photo-RAFT aqueous polymerization-induced self-assembly (photo-PISA). Homopolymerization can yield salt-resistant, 3 nm ultrafine fibril-structured 5 nm ultrathin lamellae via LLPS, a left-to-right-handed chirality transition, and a droplets-to-lamellae transition. Like-charge block copolymerization leads to supercharged yet identical fibril-structured ultrathin lamellae, also, via LLPS, the left-to-right chirality transition and the droplets-to-lamellae transition. Ultrafine structures maintain intactness upon the seeded polymerization of the oppositely charged monomer. This work demonstrates that amino acid chiral ionic hydrogen bonds are powerful for the precision synthesis of salt-resistant ultrathin membrane nanomaterials.

Chemical Feedback in Templated Reaction-Assembly of Polyelectrolyte Complex Micelles: A Molecular Simulation Study of the Kinetics and Clustering

The chemical feedback between building blocks in templated polymerization of diblock copolymers and their consecutive micellization was studied for the first time by means of coarse-grained molecular dynamics simulations. Using a stochastic polymerization model, we were able to reproduce the experimental findings on the effect of chemical feedback on the polymerization rates at low and high solution concentrations. The size and shape of micelles were computed using a newly developed software in Python conjugated with graph theory. In full agreement with the experiments, our simulations revealed that micelles formed by the templated micellization are more spherical and have a lower radius of gyration than those formed by the traditional two-step micellization method. The advantage of molecular simulation over the traditional kinetic models is that with the simulation, one studies in detail the heterogeneous polymerization in the presence of the oppositely charged template while also accounting for the incompatibility between reacted species, which significantly influences the reaction process.

Switchable Electrostatically Templated Polymerization

We report a switchable, templated polymerization system where the strength of the templating effect can be modulated by solution pH and/or ionic strength. The responsiveness to these cues is incorporated through a dendritic polyamidoamine-based template of which the charge density depends on pH. The dendrimers act as a template for the polymerization of an oppositely charged monomer, namely sodium styrene sulfonate. We show that the rate of polymerization and maximum achievable monomer conversion are directly related to the charge density of the template, and hence the environmental pH. The polymerization could effectively be switched "ON" and "OFF" on demand, by cycling between acidic and alkaline reaction environments. These findings break ground for a novel concept, namely harnessing co-assembly of a template and growing polymer chains with tunable association strength to create and control coupled polymerization and self-assembly pathways of (charged) macromolecular building blocks.

FRET-Based Determination of the Exchange Dynamics of Complex Coacervate Core Micelles

Complex coacervate core micelles (C3Ms) are nanoscopic structures formed by charge interactions between oppositely charged macroions and used to encapsulate a wide variety of charged (bio)molecules. In most cases, C3Ms are in a dynamic equilibrium with their surroundings. Understanding the dynamics of molecular exchange reactions is essential as this determines the rate at which their cargo is exposed to the environment. Here, we study the molecular exchange in C3Ms by making use of Förster resonance energy transfer (FRET) and derive an analytical model to relate the experimentally observed increase in FRET efficiency to the underlying macromolecular exchange rates. We show that equilibrated C3Ms have a broad distribution of exchange rates. The overall exchange rate can be strongly increased by increasing the salt concentration. In contrast, changing the unlabeled homopolymer length does not affect the exchange of the labeled homopolymers and an increase in the micelle concentration only affects the FRET increase rate at low micelle concentrations. Together, these results suggest that the exchange of these equilibrated C3Ms occurs mainly by expulsion and insertion, where the rate-limiting step is the breaking of ionic bonds to expel the chains from the core. These are important insights to further improve the encapsulation efficiency of C3Ms.