Atom-Economic Synthesis of Phosphorus-Containing Biobased Aromatic Diol for Flame-Retardant Robust Polyurethane

Flame-retardant polyurethanes (PUs) based on renewable raw materials are highly desired because they meet comprehensive requirements in sustainability and fire safety. In this work, considering the structural features of vanillin, a robust and rigid phosphorus-containing biobased diol, 6-(hydroxy(4-hydroxy-3-methoxyphenyl)methyl)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (VAN-DOPO), was successfully designed and prepared via an atom-economic reaction between vanillin (VAN) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The VAN-DOPO monomer was then cured with an HDI trimer to obtain flame-retardant PU thermosets with an increased LOI value of 31.4% and flame-retardant grades reaching the V-0 level, with a phosphorus content of 2.46 wt %. Besides, changes in the glass-transition temperature, network structure, and toughening effect in tensile behaviors were observed as a consequence of the bulky VAN-DOPO moieties remarkably influencing the hydrogen bonds in PU thermosets. These results not only provide a facile strategy to prepare flame-retardant biobased PUs but also provide guidance to modulate their flame retardance and mechanical performance, thus further providing significant insights into the design and preparation of biobased synthetic polymers.