Stefan Hilf

Adaptable Hetero Diels-Alder Networks for Fast Self-Healing under Mild Conditions

A novel adaptable network based on the reversible hetero Diels-Alder reaction of a cyanodithioester and cyclopentadiene is presented. Reversible between 50-120 °C, the adjustable and self-healing features of the network are evidenced via temperature dependent rheology experiments and repetitive tensile tests whereas the networks chemical structure is explored by temperature dependent (1) H MAS-NMR spectroscopy.

Thermally reversible Diels-Alder-based polymerization: an experimental and theoretical assessment

A pair of monomers capable of undergoing reversible polymerization—based on reversible Diels–Alder (DA) chemistry—as a function of the applied reaction temperature is presented. Specifically, the reaction of isophorone bis(sorbic carbamate), a difunctional diene, with 1,4-phenylenebis(methylene)bis((diethoxyphosphoryl)methanedithioformate), a difunctional dithioester, was studied in detail. Various factors, including the monomer concentration, the type of solvent, and the presence of a Lewis acid, that influence this step-growth polymerization were evaluated. The solvent type was found to have a significant effect on the DA reaction rate. Under the optimized conditions, which are 1.8 g mol−1 of each monomer in acetonitrile with 1.1 equivalents of zinc chloride at 50 °C for 4 h, a polymer with a peak molecular weight of 9600 g mol−1 (relative to poly(styrene) standards) was obtained. The resulting polymer was employed to investigate the correlation between time, temperature, and percentage of debonded monomers achieved during the retro DA (rDA) reaction. In addition, theoretical predictions of the rDA temperature were obtained via ab initio quantum chemical calculations. The monomeric diene and dienophile system was employed for the calculations of the equilibrium constants at various rDA reaction temperatures to correlate the percentage of bonded molecules with the applied temperature. It was calculated that 60% of the polymer becomes debonded at a temperature (Tqc) of around 220 °C, a result that agrees well with that obtained experimentally (Texp = 219 °C).

Fast and catalyst-free hetero-Diels-Alder chemistry for on demand cyclable bonding/debonding materials†

A new dithioester possessing a cyano Z-group (cyano-dithioester (CDTE)) has been synthesized via a 2-step, one-pot reaction. The cyano-substituted dithioester has been found to undergo fast reversible hetero-Diels–Alder (HDA) reactions at ambient temperature, without the need for a catalyst, as demonstrated by ESI-MS and UV-Vis experiments. To apply the bonding/debonding on demand system to materials science, a cyano-dithioester di-linker was synthesized and employed as a di-functional dienophile in a HDA-based polymerization reaction with a bis-cyclopentadiene polymer. The reversible bonding of the polymer systems were demonstrated by on-line UV-Vis spectroscopy, on-line NMR spectroscopy, and on-line high temperature DLS, as well as via GPC in situ trapping experiments and high-level ab initio molecular orbital calculations.