Multiblock Copolymers of Styrene and Butyl Acrylate via Polytrithiocarbonate-Mediated RAFT Polymerization

When linear polytrithiocarbonates as Reversible Addition-Fragmentation chain Transfer (RAFT) agents are employed in a radical polymerization, the resulting macromolecules consist of several homogeneous polymer blocks, interconnected by the functional groups of the respective RAFT agent. Via a second polymerization with another monomer, multiblock copolymers—polymers with alternating segments of both monomers—can be prepared. This strategy was examined mechanistically in detail based on subsequent RAFT polymerizations of styrene and butyl acrylate. Size-exclusion chromatography (SEC) of these polymers showed that the examined method yields low-disperse products. In some cases, resolved peaks for molecules with different numbers of blocks (polymer chains separated by the trithiocarbonate groups) could be observed. Cleavage of the polymers at the trithiocarbonate groups and SEC analysis of the products showed that the blocks in the middle of the polymers are longer than those at the ends and that the number of blocks corresponds to the number of functional groups in the initial RAFT agent. Furthermore, the produced multiblock copolymers were analyzed via differential scanning calorimetry (DSC). This work underlines that the examined methodology is very well suited for the synthesis of well-defined multiblock copolymers

Increasing the Gas Barrier Properties of Polyethylene Foils by Coating with Poly(methyl acrylate)-Grafted Montmorillonite Nanosheets

Low-density polyethylene (LDPE) foils were coated with a thin film of polymer-grafted Montmorillonite (MMT) nanosheets, which form a barrier against gas diffusion due to their unique brick-and-mortar arrangement. The MMT nanosheets were grafted with poly(methyl acrylate) (PMA), a soft and flexible polymer. Already very thin films of this nanocomposite could reduce gas permeability significantly. The impact of the topology of the surface-grafted polymer on gas permeability was also studied. It was found that grafting MMT nanosheets with a mixture of star-shaped and linear PMA and with PMA that is cross-linked via hydrogen bonds further decrease gas permeability. The presented strategy is quick and simple and allows for the easy formation of effective gas barrier coatings for LDPE foils, as used in food packaging

Polyethylene-Grafted Gold and Silver Nanoparticles Using Catalyzed Chain Growth (CCG)

We report an efficient synthesis route for the formation of gold/silver-core–PE-shell nanohybrids in a simple self-assembly approach using PE with strong aurophilicity and argentophilicity, via thiol- and trithiocarbonate terminated moieties. This united the unique properties of polyethylene (PE) with gold and silver nanoparticles, using the well-defined end-group design of PE. These nanocomposites showed a similar solubility as PE, as confirmed by dynamic light scattering, and could be fully incorporated into a polyethylene matrix with different particle contents, as visualized by transmission electron microscopy. Using UV/vis-spectroscopy, we observed reversible, thermoresponsive aggregation/deaggregation properties in the nanohybrids, validating the strong and effective anchoring of PE on gold/silver surfaces

Tailoring Confinement: Nano-Carrier Synthesis via Z-RAFT Star Polymerization

A new pathway to nano-sized hollow-sphere particles from six-arm star polymers with an amphiphilic core-corona structure, synthesized in a four-step-procedure by means of reversible addition-fragmentation chain transfer (RAFT) polymerization is presented, in order to achieve more stable and versatile nano-container systems, which could be applied in the fields of drug delivery or catalyst storage. Star-shaped amphiphilic, diblock copolymers serve as globular platforms for synthesizing uniform hollow structures. By the introduction of monomer units carrying UV-cross-linkable dimethyl maleimido functionalities into the outer sphere of these star polymers, the carrier’s shell could be stabilized under UV-irradiation. After removal of the RAFT-core—constituting the central hub of the star polymer—by aminolysis, the carrier is ready for loading

RAFT-Polymers with Single and Multiple Trithiocarbonate Groups as Uniform Gold-Nanoparticle Coatings

The nanostructures of nanohybrids with gold-nanocrystal cores and shells of <i>N</i>-isopropylacrylamide polymers containing single or multiple trithiocarbonate (TTC) groups were analyzed in detail. The polymers were synthesized in RAFT polymerizations and then grafted to gold nanoparticles (∼14 nm) from citrate reduction. Analysis via TEM revealed self-assembled hexagonal lattices of gold cores. The interparticle distances increased, when polymers with single TTC end groups and growing molar masses were employed as coating agents, allowing for a reference curve to be found. When using polymers containing multiple TTC groups along their backbone for the surface modification, the spacings between the gold cores were unexpectedly low and remained constant, independent of the polymers’ chain lengths and the number of trithiocarbonate groups, which showsbacked up by results from size-exclusion chromatographythat these macromolecules are wrapped around the gold nanoparticles with several gold–TTC junctions. Absolutely no cross-linking was observed for these novel and promising nanohybrids

Termination Kinetics of Surface-Initiated Radical Polymerization Measured by Time-Resolved ESR Spectroscopy after Laser-Pulse Initiation

The single-pulse–pulsed-laser polymerization–electron spin resonance (SP-PLP-ESR) method was used to study the termination reaction in polymerization of <i>n</i>-butyl methacrylate (<i>n</i>-BMA) from silica nanoparticles. The polymerization from the surface was initiated by the surface-attached photoinitiator 2,2-dimethoxy-2-phenyl-1-(2-(2-(trimethoxysilyl)­ethyl)­phenyl)­ethane-1-one (DMPTS). The obtained termination rate coefficient at 335 K of <i>k</i>_t = (5.4 ± 0.7) × 10⁶ L mol^–1 s^–1, which could unambiguously be assigned to the termination reaction between two surface-attached macroradicals, is about 2 orders of magnitude smaller than the value found in homogeneous <i>n</i>-BMA polymerization. Surprisingly, no chain-length dependence of the termination rate coefficient could be observed. This fact and temperature-dependent studies of <i>k</i>_t, which yield an activation energy that is in perfect agreement with the activation energy of propagation of <i>n</i>-BMA, indicate that the termination between surface-attached macroradicals proceeds under reaction diffusion control, as their translational diffusion is suppressed