Metallic organophosphate catalyzed bulk ring-opening polymerization

Aliphatic polyesters are broadly used in biomedical materials, food packaging and drug delivery. The use of catalytic ring-opening polymerization (ROP) by dual catalysis, combining organocatalysis and metal complex catalysis, is a powerful strategy toward these valuable polyesters. In an endeavor to combine metal catalysis and organocatalysis in polymerization, we suggested the use of metal salts of an organophosphoric acid as bifunctional catalysts in the ROP of cyclic esters and of cyclic carbonates. Four metal organophosphates, viz. lithium, sodium, magnesium, and calcium diphenyl phosphates, were evaluated as catalysts for the ROPs of trimethylene carbonate (TMC), δ-valerolactone and lactide with 3-phenyl-1-propanol (PPA) as an initiator for obtaining aliphatic polyesters. Magnesium diphenyl phosphate (MgDP) showed relatively high catalytic activity and ideal control. The molecular weights of the PTMCs measured using ¹H NMR matched well with the theoretical ones. ¹H NMR, ¹³C NMR, and MALDI-ToF MS measurements demonstrated that the initiator and monomer polymerized quantitatively. Bulk polymerization with various [TMC]₀/[PPA]₀/[MgDP]₀ ratios afforded PTMCs with expected molecular weights (M_{n, NMR} = 3.21-11.7 kg mol^-1) and relatively narrow dispersities (M_w/M_n = 1.16-1.19). Kinetic studies, chain extension experiments and diblock copolymer synthesis were carried out to identify that the MgDP-catalyzed ROP proceeded in a controlled/living manner. A bifunctional catalytic mechanism for the MgDP catalyzed ROP was suggested from ¹H NMR, ¹³C NMR and ³¹P NMR titration experiments. For evaluating the biosafety of the PTMC produced in the bulk ROP under MgDP catalysis, untreated PTMC samples were tested by an MTT assay using the L929 cell line. A result of more than 90% relative cell viabilities demonstrates the excellent cell compatibility of the PTMCs for potential biomedical applications.