Towards New Robust Zn(II) Complexes for the Ring-Opening Polymerization of Lactide Under Industrially Relevant Conditions

The synthesis of bio-based and biodegradable plastics is a hot topic in research due to growing environmental problems caused by omnipresent plastics. As a result, polylactide, which has been known for years, has seen a tremendous increase in industrial production. Nevertheless, the manufacturing process using the toxic catalyst Sn(Oct)2 is very critical. As an alternative, five zinc acetate complexes have been synthesized with Schiff base-like ligands that exhibit high activity in the ring-opening polymerization of non-purified lactide. The systems bear different side arms in the ligand scaffold. The influence of these substituents has been analyzed. For a detailed description of the catalytic activities, the rate constants kapp and kp were determined using in-situ Raman spectroscopy at a temperature of 150 °C. The polymers produced have molar masses of up to 71 000 g mol−1 and are therefore suitable for a variety of applications. Toxicity measurements carried out for these complexes proved the nontoxicity of the systems. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Tuning a robust system: N,O Zinc Guanidine Catalysts for the ROP of Lactide

Non-toxic, highly-active and robust complexes are the holy grail as ideal green catalysts for the polymerisation of biobased and biodegrable polylactide. Four new zinc guanidine complexes [ZnCl2(TMG4NMe2asme)], [ZnCl2(TMG5Clasme)], [ZnCl2(TMG5Measme)] and [ZnCl2(TMG5NMe2asme)] with different electron-donating and electron-withdrawing groups on aromatic backbone of the ligand have been synthesised. Ligands are derived from low-cost commercially available compounds and have been converted in a three- or four-step synthesis into the desired ligand in good yields. The compounds have been fully characterised and tested in the ROP of rac-LA in the melt under industrially relevant conditions. The complexes are based on the recently published structure [ZnCl2(TMGasme)] which has shown high activity in the polymerisation of lactide at 150 °C. Different substituents in the para-position to the guanidine moiety significantly increase the polymerisation rate whereas positioning substituents in meta-position causes no change in the reaction rate. With molecular weights over 71 000 g mol-1, the best catalyst produces polymer for several industrial applications and its polymerisation rate approaches the one of Sn(Oct)2. The robust systems are able to polymerise non-purified lactide. The initiation of the polymerisation is suggested to occur by impurities in the monomer

Investigation of the formation mechanism of homo- and copolymer microgels for a model-based process design of microgels

Process analytical technology (PAT) is used widely in pharmaceutical and biotechnology industries to monitor and control the parameters of the processes during product manufacturing. For these sectors, PAT is a crucial department to maintain the quality of the product. It can be implemented in-line, where the sample is not removed from the process stream. However, PAT can also be used to monitor and control the reaction progress in order to learn more about the product formation mechanisms. In this work, PAT will be applied to synthesis of microgels – cross-linked polymeric networks that can respond to diverse stimuli. It will be shown how the in line analytical techniques can improve the understanding of microgel synthesis processes. The in-line monitoring techniques focus on reaction calorimetry, turbidity, dynamic light scattering (DLS) measurements and Raman spectroscopy. Moreover, it will be discussed how computational simulation can benefit and support the experimental studies on microgels. Collaborative approach combining experimental findings and simulated predictions can help to generate microgels of different sizes, shapes or properties and study their reaction mechanisms. The study will focus on temperature responsive microgels based on N vinylcaprolactam (VCL) and N-isopropylacrylamide (NIPAM) monomers or their copolymers. The microgel synthesis will focus on batch and semi-batch methods as well as studies on how nitrogen purging approach prior to polymerization initiation influences the microgel formation process and the final product. In-depth study will be discussed on enlarging the microgel diameters as well as generating different shapes of microgels functionalized with hydrophobic polymers. The aim of this work is to undergo fundamental studies on well-known temperature responsive microgel systems in order to better understand their formation mechanisms. Furthermore, the motivation of the project is to gain the ability to predict more complex microgel systems for versatile applications and scale-up synthesis processes

Electrochemical contrast switching between black and white appearance of gelatin-covered zinc

Zinc and its alloys are widely used in the surface protection of metallic structural materials. Thus, zinc is an interesting and relevant candidate material for preparing stimuli-responsive surfaces. In this work, the switching of the optical appearance of zinc between black and white by an applied electrode potential is demonstrated. The zinc surface was covered by gelatin films and subjected to cyclic voltammetry (CV) in a chloride-containing electrolyte which induced pitting corrosion on the zinc surface. Between the different parts of the CV cycles, a reversible change in optical appearance was observed. During the oxidative half-cycles, the surfaces appear white, and during the reductive half-cycles, the surfaces appear brown to black, i.e. dark. Surface characterisation by x-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy shows that the gelatin coating is slightly oxidised during intial stages of the process, but remains intact and present at the surface. Raman spectra prove the presence of ZnO at the interface. Surface analysis shows only minor differences in composition between the black and white surfaces. Based on the available characterisation data, the white appearance associated with anodic currents is attributed to the formation of a non-passivating ZnO. The black appearance associated with cathodic currents is attributed to reduction of surface-confined zinc species, including ZnO and Zn2+. The role of the gelatin is presumably to prevent diffusion of the dissolution products into solution by complex formation and by acting as a diffusion barrier; gelatin will also affect the morphology of the reduction products. A similar switching was observed when gelatin was added to chloride electrolyte; surface analysis showed gelatin adsorption in this case. The black/white switching may, e.g. be useful for surfaces self-indicating corrosion potentials of galvanised steel

Model-Based Optimization of Microgel Synthesis in the μm Size Range

Microgels are functional polymer colloids with diverse applications, for example, in medicine, engineering, or agriculture. Some applications require large microgels in the size range of mu m, which could previously only be synthesized using microfluidics or a synthesis procedure involving a fed-batch reaction with a temperature ramp. We use dynamic optimization to determine a simpler synthesis recipe that allows us to synthesize microgels in the pm range using a batch procedure. First, a sensitivity analysis shows that the microgel size is sensitive to the initial concentration of initiator and the reactor temperature. Second, the dynamic optimization yields a batch recipe that provides microgels of the desired mu m size range. The recipe uses a low initial initiator concentration and a low reactor temperature. The respective recipe is validated experimentally; model simulation results and experimental measurements of the hydrodynamic diameter show good agreement

Dataset to "Model-based Prediction of the Hydrodynamic Radius of Collapsed Microgels and Experimental Validation"

Microgels are functional colloidal polymer networks with diverse applications. Various applications require microgels of different sizes. The microgel size is determined during the synthesis and depends among other conditions on the reactor type, reactor temperature, monomer-to-initiator ratio, cross-linker concentration, and surfactant concentration. While experimental data covering these synthesis conditions are available in literature, a model taking into account all of the above factors has not yet been proposed. We present a mechanistic model considering all of the above conditions that agrees with experimental data from various literature sources. The effect of surfactant type and concentration is included by addition of a term in the coagulation kinetics. The reactor type and Reynolds number in the reactor is accounted for with a semi-empirical parameter describing the kinetics of particle coagulation. This parameter is fitted to the data of one of the available experiments, while all other experiments are used for validation. The model predictions quantitatively match the experimental data for stirred batch reactors. For unstirred batch reactors, the agreement with the experimental data is only qualitative. The mechanistic model enables model-based design of functional microgels for new applications