Exploration of the rapid growth of high-molecular-weight oligomers in the later stage of diisocyanate self-polymerization

Diisocyanate trimers are important products in the polyurethane industry. Large amounts of high-molecular-weight oligomers, which change the physical and chemical properties of the final products, are generated in the later stages of the synthesis process. It is important to understand the self-polymerization of high-molecular-weight oligomers. The relationship between this phenomenon and the PDI monomer concentration was first experimentally demonstrated. Then the self-polymerization of pentamethylene diisocyanate (PDI) was investigated via Dmol3 calculations using the linear synchronous transition (LST) and quadratic synchronous transition (QST) methods. Calculations revealed a new synthesis pathway with a reaction energy barrier of 31.548 kcal/mol, leading to the conversion of pentamers into decamers. In the initial stage of the polymerization reaction, the PDI monomer concentration was high, and the reaction followed a step-growth polymerization. However, as the PDI monomer concentration decreased and the PDI pentamer concentration increased in the later stage of the reaction, a new synthesis pathway emerged, leading to the direct synthesis of high-molecular-weight oligomers from the available oligomers. This study further demonstrates the self-polymerization of diisocyanate and provides a comprehensive understanding of its synthesis process. By inhibiting this new pathway can reduce the content of high-molecular-weight oligomers, increase yields, and reduce the product viscosity, all of which are beneficial for chemical process optimization.