Reaction mechanism of diisocyanate solution chain extension on poly(D,L-lactic acid) produced by direct melt polycondensation
Starting from cheaper D,L-lactic acid (D,L-LA), lower molecular weight poly(D,L-lactic acid) (PDLLA) was directly synthesized via melt polycondensation.By using PDLLA as prepolymer, 2,4-toluylene diisocyanate (TDI) as chain extender, and tetrahydrofuran (THF) as solvent, poly(D,L-lactic acid)s biodegradable material was synthesized through diisocyanate chain extension in solution state.When using different precipitators to terminate the solution chain extension, the solubility in chloroform,viscosity-average molecular weight (Mη), structure, and heat properties of chain extension products were different.Before precipitation, the reaction mechanism of chain extension in solution state was similar to that in the melt state.However, different precipitators had different termination mechanisms.When precipitated by metchanol, the reaction between residual NCO group and OH group in metchanol could keep the structure of product basically unchanged.When water was used instead of metchanol, the complex reaction between residual NCO group and water was apt to produce cross-linking products.