Fecha de publicación: 15/11/2024 Fuente: Journal of applied polymer Lugar: RESEARCH ARTICLE In the present study, metal-residue-free PLA with high degree of regularity were synthesized by combining a novel phosphonitrile-based catalyst, HTGCP, with (thio)urea under mild reaction conditions and controlled molecular weight. The reaction is highly active and can achieve 99% conversion in less than 1 h. The P
m can reach 0.91 at −20°C.

ABSTRACT
Despite the extraordinary success has been achieved in metal catalyst-promoted stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA), well-controlled stereoselective rac-LA ROP by organic catalyst still remains a scientific challenge. In this study, we showcase the remarkable ability of the phosphazene base 2,2,4,4,6,6-hexa(tetramethyl-guanidyl)-1,3,5,2λ5,4λ5,6λ5-cyclictri(phosphazene) (HTGCP) to facilitate the stereoselective ROP of rac-LA at low temperatures, yielding isotactic stereoblock poly (lactic acid) (PLA) with a stereoselectivity index (P
m) up to 0.86. Furthermore, by harnessing the synergy between HTGCP and a (thio)urea, both high stereoselectivity (P
m up to 0.85) and remarkable catalytic activity (> 99% monomer conversion within an hour) can be achieved at ambient temperature. Notably, upon decreasing the temperature to −20°C, an even higher degree of regularity is attained (P
m up to 0.91). The direct linear correlation between molecular weights (M
n), monomer conversions, and the narrow dispersity (Đ) of the resulting polymers, coupled with their high end-group fidelity, underscores the meticulous control exerted over the polymerization. Based on these findings, we propose that the chain end control mechanism underlies the synthesis of isotactic stereoblock PLA from rac-LA in the presence of HTGCP, offering a novel and efficient pathway to produce high-performance PLA materials.