Reducing High-Molecular-Weight Oligomers in Biobased Pentamethylene Diisocyanate Trimers

New biobased pentamethylene diisocyanate (PDI) trimers have received increasing attention because of their high biochar content and excellent performance in coatings. However, their viscosity can affect the curing rate and application of coatings. This study aimed to decrease the viscosity of PDI trimers by reducing the content of high-molecular-weight oligomers. Using 2-hydroxypropyltrimethylisooctanoic acid ammonium salt as a catalyst at a dosage of 0.3 wt ‰, the catalyst was added in two batches at 80 °C, with an interval of 15 min between each addition. The reaction lasted for 90 min until the NCO conversion rate reached 15.71%. The resulting PDI trimer contained a high-molecular-weight oligomer content of 0.53% and exhibited a viscosity of 1425 mPa·s. Furthermore, to address the significant increase in the content of high-molecular-weight oligomers during the later stages of the reaction, a novel reaction pathway was proposed to elucidate the generation of high-molecular-weight oligomers from those with lower molecular weights. Gaussian calculations were employed to determine the reaction energy barriers of different pathways, confirming that the reaction pathway for synthesizing a heptamer from two trimers and one monomer is possible. Moreover, this study provides insights into methods for mitigating the increase in high-molecular-weight oligomers in the later stages of the reaction.