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).