Catalyst-Controlled Transannular Polyketide Cyclization Cascades: Selective Folding of Macrocyclic Polyketides

The biomimetic synthesis of aromatic polyketides from macrocyclic substrates by means of catalyst-controlled transannular cyclization cascades is described. The macrocyclic substrates, which feature increased stability and fewer conformational states, were thereby transformed into several distinct polyketide scaffolds. The catalyst-controlled transannular cyclizations selectively led to aromatic polyketides with a defined folding and oxygenation pattern, thus emulating β-keto-processing steps of polyketide biosynthesis

Insights into the Bead Fusion Mechanism of Expanded Polybutylene Terephthalate (E-PBT)

Expandable polystyrene (EPS) and expanded polypropylene (EPP) dominate the bead foam market. As the low thermal performance of EPS and EPP limits application at elevated temperatures novel solutions such as expanded polybutylene terephthalate (E-PBT) are gaining importance. To produce parts, individual beads are typically molded by hot steam. While molding of EPP is well-understood and related to two distinct melting temperatures, the mechanisms of E-PBT are different. E-PBT shows only one melting peak and can surprisingly only be molded when adding chain extender (CE). This publication therefore aims to understand the impact of thermal properties of E-PBT on its molding behavior. Detailed differential scanning calorimetry was performed on neat and chain extended E-PBT. The crystallinity of the outer layer and center of the bead was similar. Thus, a former hypothesis that a completely amorphous bead layer enables molding, was discarded. However, the incorporation of CE remarkably reduces the crystallization and re-crystallization rate. As a consequence, the time available for interdiffusion of chains across neighboring beads increases and facilitates crystallization across the bead interface. For E-PBT bead foams, it is concluded that sufficient time for polymer interdiffusion during molding is crucial and requires adjusted crystallization kinetics

Prediction of warping in thermoplastic AFP-manufactured laminates through simulation and experimentation

The thermoplastic automated fibre placement (T-AFP) process is a non-autoclave method for in-situ consolidation of thermoplastic composite material on a piecewise constructed laminate. High thermal gradients and nonlinear material behaviour, especially due to crystallization, make predic-tions of process-induced stress and warping difficult. This paper describes a method for simulating parts manufactured by T-AFP using a detailed material model to capture the dynamic nature of the process. The material model is flexible and can be altered to describe different semi-crystalline matrices, in this study focusing on low-melt polyaryletherketone (LM-PAEK). Two laminate panels are simulated within this work and assess the impact of a heated tooling on overall part warping. Panel warping is validated by performing 3D-scans of T-AFP-manufactured laminates produced using the same parameters as the simulation. The results show a good match between numeric and experimental warping, especially for heated tools, thus providing a useful method for predict-ing laminate warping and reducing the demand on manufacturing experimentation

STEERING LIMITATIONS FOR IN-SITU AUTOMATED FIBRE PLACEMENT MANUFACTURED STRUCTURES

Laser-assisted Automated Fibre Placement (AFP) is a promising manufacturing technology for the cost- and time-efficient manufacture of large-scale carbonfibre-reinforced components. This technology enables in-situ consolidation of thermoplastic prepreg material without subsequent autoclave processing. The manufacture of advanced structures often requires curved layup paths, for example, to implement predetermined ply angles on a double curved geometry. This curvature can lead to defects, which are particularly critical with in-situ consolidation, as they cannot be mitigated by subsequent autoclave or hot press processes. This work aims to quantify the frequency and magnitude of steering-induced defects for different manufacturing parameters

Correction: Weingart et al. Expanded Polycarbonate (EPC)—A New Generation of High-Temperature Engineering Bead Foams. <i>Polymers</i> 2020, <i>12</i>, 2314

In the original publication [...

Catalyst‐Controlled Transannular Polyketide Cyclization Cascades: Selective Folding of Macrocyclic Polyketides

The biomimetic synthesis of aromatic polyketides from macrocyclic substrates by means of catalyst-controlled transannular cyclization cascades is described. The macrocyclic substrates, which feature increased stability and fewer conformational states, were thereby transformed into several distinct polyketide scaffolds. The catalyst-controlled transannular cyclizations selectively led to aromatic polyketides with a defined folding and oxygenation pattern, thus emulating β-keto-processing steps of polyketide biosynthesis