Alexander A. Steinschulte

A nondestructive, statistical method for determination of initiation efficiency: dipentaerythritol-aided synthesis of ternary ABC3 miktoarm stars using a combined “arm-first” and “core-first” approach

The preparation of miktoarm stars, based on poly(ethylene oxide) (PEO), poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and either poly(propylene oxide) (PPO) or poly(ethyl glycidyl ether) (PEGE), is described. Hereby, partly protected dipentaerythritol (dipentaerythritoldiacetonide) is used as a bifunctional alcohol in a polymer-based Williamson ether synthesis to become the core of the star. Mesylated PEO is reacted first with excess dipentaerythritoldiketal. This ensures full modification of the PEO with one telechelic dipentaerythritol moiety. This telechelic PEO with one hydroxyl function is then converted to a diblock copolymer with the dipentaerythritol unit at the junction point between the blocks. To achieve this, two pathways have been developed: (a) by reaction with ready-prepared, mesylated PPO and (b) by ring-opening, anionic polymerization of ethyl glycidyl ether, leading to a narrow dispersed block copolymer. The advantages and disadvantages of both routes are discussed, though both provide perfect scaffolds for further polymer grafting. This is achieved by mild deprotection of both diblocks in order to yield 4 hydroxyl functions at the core of the future star. After attachment of an initiator, atom transfer radical polymerization (ATRP) is used to grow up to 4 arms of PDMAEMA from the center of each diblock copolymer. Thus, (PEO)–(PDMAEMA)k–(PPO) or (PEO)–(PDMAEMA)k–(PEGE) heteroarm stars are prepared by a combined “arm-first” and “core-first” method. The molecular characterization is accompanied by NMR, size exclusion chromatography (SEC), osmometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI ToF MS). The latter method even allows an estimation of the initiation site efficiency during ATRP. In turn, the final molecular formula of the stars can be derived. We illustrate that a decreased initiation site efficiency generates a specific footprint in the molecular weight distribution, which could be partly reproduced by MALDI ToF MS. By comparing the simulated spectra with the real ones, one can draw conclusions on the initiation site efficiency. The obtained initiation site efficiency is found to be comparable to the one obtained by the standard destructive method: determination of molecular weight of cleaved arms, which is tedious and polymer-consuming. Therefore, it is anticipated that both the synthetic procedures as well as the analytics can be easily adapted to other polymers.

Balancing Segregation and Complexation in Amphiphilic Copolymers by Architecture and Confinement

Segregation is a well-known principle for micellization, as solvophobic components try to minimize interactions with other entities (such as solvent) by self-assembly. An opposite principle is based on complexation (or coacervation), leading to the coassembly/association of different components. Most cases in the literature rely on only one of these modes, though the classical micellization scheme (such as spherical micelles, wormlike micelles, and vesicles) can be enriched by a subtle balance of segregation and complexation. Because of their counteraction, micellar constructs with unprecedented structure and behavior could be obtained. In this feature, systems are highlighted, which are between both mechanisms, and we study concentration, architecture, and confinement effects. Systems with inter- and intramolecular interactions are presented, and the effects of polymer topology and monomer sequence on the resulting structures are discussed. It is shown that complexation can lead to altered micellization behavior as the complex of one hydrophobic and one hydrophilic component can have a very low surface tension toward the solvent. Then, the more soluble component is enriched at the surface of the complex and acts as a microsurfactant. Although segregation dominates for amphiphilic copolymers in solution, the effect of the complexation can be enhanced by branching (change of architecture). Another possibility to enhance the complexation is by confining copolymers in a (pseudo-) 2D environment (like the one available at liquid-liquid interfaces). These observations show how new structural features can be achieved by tuning the subtle balance between segregation and complexation/solubilization.

Stimulated Transitions of Directed Nonequilibrium Self-Assemblies

Near-equilibrium stimulus-responsive polymers have been used extensively to introduce morphological variations in dependence of adaptable conditions. Far-less-well studied are triggered transformations at constant conditions. These require the involvement of metastable states, which are either able to approach the equilibrium state after deviation from metastability or can be frozen on returning from nonequilibrium to equilibrium. Such functional nonequilibrium macromolecular systems hold great promise for on-demand transformations, which result in substantial changes in their material properties, as seen for triggered gelations. Herein, a diblock copolymer system consisting of a hydrophilic block and a block that is responsive to both pressure and temperature, is introduced. This species demonstrates various micellar transformations upon leaving equilibrium/nonequilibrium states, which are triggered by a temperature deflection or a temporary application of hydrostatic pressure.