Polymer Nanospheres with Hydrophobic Surface Groups as Supramolecular Building Blocks Produced by Aqueous PISA.

A robust and straightforward synthesis of waterborne polymer nanospheres bearing the supramolecular association unit dialkoxynapthalene at their surface is presented using polymerization-induced self-assembly (PISA). A RAFT agent bearing this unit is first employed to produce poly(acrylic acid) chains, which are then chain-extended with styrene (S) to spontaneously form the nano-objects via RAFT aqueous emulsion polymerization. The particular challenge posed by the dialkoxynapthalene hydrophobicity can be overcome by the use of PISA and the deprotonation of the poly(acrylic acid). At pH = 7, very homogeneous latexes are obtained. The particle diameters can be tuned from 36 to 105 nm (with a narrow particle size distribution) by varying the molar mass of the PS block. The surface accessibility of the dialkoxynapthalene moieties is demonstrated by complexation with the complementary host cyclobis(paraquat-p-phenylene) (CBPQT4+ · Cl- ), highlighting the potential of the nanospheres to act as building blocks for responsive supramolecular structures.

Aromatic Xanthates and Dithiocarbamates for the Polymerization of Ethylene through Reversible Addition-Fragmentation Chain Transfer (RAFT).

Aromatic xanthates and dithiocarbamates were used as chain-transfer agents (CTAs) in reversible addition-fragmentation chain-transfer (RAFT) polymerizations of ethylene under milder conditions (≤80 °C, ≤200 bar). While detrimental side fragmentation of the intermediate radical leading to loss of living chain-ends was observed before with alkyl xanthate CTAs, this was absent for the aromatic CTAs. The loss of living chain-ends was nevertheless detected for the aromatic xanthates via a different mechanism based on cross-termination. Narrow molar-mass distributions with dispersities between 1.2 and 1.3 were still obtained up to number average molar masses Mn of 1000 g mol-1 . The loss of chain-ends was minor for dithiocarbamates, yielding polyethylene up to Mn =3000 g mol-1 with dispersities between 1.4 and 1.8. While systems investigated showed significant rate retardation, the dithiocarbamates are the first CTAs giving polyethylene with a high livingness via RAFT polymerization.

Controlled Radical Polymerization of Ethylene Using Organotellurium Compounds.

The first successfully controlled radical polymerization (CRP) of ethylene is reported using several organotellurium chain-transfer agents (CTAs) under mild conditions (70 °C, 200 bar of ethylene) within the concept of organotellurium-mediated radical polymerization (TERP). In contrast to preceding works on CRPs of ethylene applying reversible addition-fragmentation chain-transfer (RAFT), the TERP system provided a high livingness and chain-end functionalization of polyethylene chains. Molar-mass distributions with dispersities between 1.3 and 2.1 were obtained up to average molar masses of 5000 g mol-1 . As in the RAFT system, the high reactivity of the growing polyethylenyl radical led to an inherent side reaction. For the presented TERP systems, however, this side reaction did not result in dead chains, while it could even be effectively suppressed by a good choice of the CTA.

Gelation Kinetics and Mechanical Properties of Thiol-Tetrazole Methylsulfone Hydrogels Designed for Cell Encapsulation.

Hydrogel precursors that crosslink within minutes are essential for the development of cell encapsulation matrices and their implementation in automated systems. Such timescales allow sufficient mixing of cells and hydrogel precursors under low shear forces and the achievement of homogeneous networks and cell distributions in the 3D cell culture. The previous work showed that the thiol-tetrazole methylsulfone (TzMS) reaction crosslinks star-poly(ethylene glycol) (PEG) hydrogels within minutes at around physiological pH and can be accelerated or slowed down with small pH changes. The resulting hydrogels are cytocompatible and stable in cell culture conditions. Here, the gelation kinetics and mechanical properties of PEG-based hydrogels formed by thiol-TzMS crosslinking as a function of buffer, crosslinker structure and degree of TzMS functionality are reported. Crosslinkers of different architecture, length and chemical nature (PEG versus peptide) are tested, and degree of TzMS functionality is modified by inclusion of RGD cell-adhesive ligand, all at concentration ranges typically used in cell culture. These studies corroborate that thiol/PEG-4TzMS hydrogels show gelation times and stiffnesses that are suitable for 3D cell encapsulation and tunable through changes in hydrogel composition. The results of this study guide formulation of encapsulating hydrogels for manual and automated 3D cell culture.

Spherical Gold-Nanoparticle Assemblies with Tunable Interparticle Distances Mediated by Multifunctional RAFT Polymers.

A strategy for the controlled assembly of gold nanocrystals into dispersed three-dimensional superstructures is presented. A multifunctional RAFT agent was used to prepare multiblock polystyrene (4.4-17.8 kDa) with trithiocarbonate groups as junctions between the individual blocks. Addition of these polymers to two-phase Brust-Schiffrin gold nanoparticles (4.1 nm) resulted in the formation of stable gold-nanoparticle assemblies dispersed in toluene. TEM analysis revealed that the interparticle distances in these superstructures can be tuned over an unprecedented wide range by employing multiblock polymers with an adjusted degree of polymerization and thus tailored trithiocarbonate distances. Cross-linking of the gold nanoparticles in the assemblies by multifunctional trithiocarbonates was proven by AFM showing partly preserved globular shape after deposition on a solid substrate. The reported strategy is expected to prove useful when interparticle distances in nanoparticle assemblies need to be tuned in a liquid phase or on surfaces.