Stereoselective ring‐opening polymerization ofrac‐lactide catalyzed by phenyl diphosphazene/urea binary catalytic system

Abstract

Polylactide (PLA) is a fully bio‐derived polyester with great biodegradability, biocompatibility, and mechanical properties. Synthesis of stereoregular PLA by highly stereoselective organocatalyzed ring opening polymerization (ROP) of racemic lactide (rac‐LA) at room temperature is challenging despite some important developments in the past few years. In this contribution, two bulky phenyl diphosphazene bases, p‐PDPB and m‐PDPB, were conveniently synthesized by the Staudinger reaction. In combination with different hydrogen‐bond donors such as ureas and squaramides, they could mediate the stereocontrolled ROP of rac‐LA in THF at room temperature. Semicrystalline PLAs with narrow dispersity and high tacticity (Pm up to 0.84) were successfully synthesized in a well‐controlled manner using phenyl diphosphazene based/urea binary catalytic system. Structural analysis verified the linear structure and high end‐group fidelity of the resulting polymers. Moreover, the minimal transesterification of the polymer backbone proved by the MALDI‐TOF MS analysis indicated the good controllability of the phenyl diphosphazene based binary catalytic system.

Stereoselective ring-opening polymerization of rac-lactide catalyzed by the proton sponge bisphosphazene 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene

“…4A and B), indicating the interaction rather than ion pair formation between HMPN and BnOH. The relatively low downfield value of hydrogens in methylene in the HMPN/BnOH system, compared with that of other phosphazene bases with lower p K a values, 30,31 was due to the kinetic inactivity to deprotonation of the proton sponge. 35 When U-2 was employed with an equal molar ratio, the resonances attributed to PhC H 2 – upshifted from 4.34 ppm to 4.32 ppm (Fig.…”

“…The minor peaks with lower intensity ( M n = 72(2 n − 1) + 108.1 (BnOH) + 39.0 (K + ) g mol −1 ) were also attributed to the linear PLA polymer with BnO/H chain ends, demonstrating the existence of transesterification reactions, which was also observed in other phosphazene/urea binary catalytic systems. 29–31…”

“…28,29 Our group developed a series of phosphazene bases with variable basicity and structure to construct binary organocatalysts for stereoselective ROP of rac -LA. 30,31 C 3 N 3 -Py-P 3 and urea with compatible basicity/acidity afforded PLA with great tacticity ( P m = 0.92, T m = 190 °C) within 10 minutes at 20 °C. 30…”

“…As we all know, the basicity and topological structure of phosphazene bases are of great importance to their catalytic properties, including both the activity and the stereoselectivity. 24,29–33 Proton sponges possessing two basic nitrogen centers able to act as a chelating ligand for a proton, represented by 1,8-bis(dimethylamino)naphthalene (DMAN), were discovered by Alder in 1968 with unexpectedly high basicity compared to nonchelating weakly basic aromatic amines. 34,35 The basicity of proton sponges could be improved via replacing the naphthalene skeleton with other aromatic spacers or changing the basic nitrogen centres or their adjacent environment.…”

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

“…The most active area of research in the synthesis of PLA is the ROP of LA. Advancements have been made in metal-based, ,− organic, − and enzymatic ,, catalysis for the efficient synthesis of both PLLA and other stereoisomers. The most noteworthy catalyst, which is used for the industrial ROP of LA, is tin­(II) octoate, Sn­(Oct) 2 , thanks to its high activity, efficiency, and stability.…”