Reversible oligomerization of 3-aryl-2-cyanothioacrylamides via [2_s + 4_s] cycloaddition to substituted 3,4-dihydro-2<i>H</i>-thiopyrans

Novel difunctional monomers are synthesized which are able to dimerize as well as oligomerize via the hetero-Diels–Alder (HDA) mechanism. These monomers are based on 3-aryl-2-cyanothioacrylamides, which are used as functional units for the HDA reaction to form 3,4-dihydro-2<i>H</i>-thiopyrans. Different substituents of the aromatic ring are utilized to shift the equilibrium to the dimeric or oligomeric species. The HDA reaction is reversible and can be influenced by the temperature as well as the concentration. In contrast, the <i>N,N</i>-dimethylthioacrylamide analogues do not dimerize or the dimerization is not reversible. The most promising compound is furthermore investigated in detail by ¹H NMR spectroscopy to elucidate the dimerization kinetics. In addition, a difunctional molecule is able to oligomerize reversibly, as confirmed by means of ¹H NMR, size exclusion chromatography (SEC), and ESI MS measurements. A respective alcohol is employed as initiator for the ring-opening polymerization of l-lactide resulting in an end-functional polymer that undergoes reversible dimerization as confirmed by SEC measurements.