Phosphazene Bases as Organocatalysts for Ring‐Opening Polymerization of Cyclic Esters

Abstract

Over the past several years, organocatalyzed polymerization reactions have attracted considerable attention, and these efforts have led to major advances. A large number of organic compounds have been proven active for the polymerization of a large variety of monomers. In particular, phosphazene bases (PBs) are a family of extremely strong, non‐nucleophilic, and uncharged auxiliary bases, and have shown their remarkable potential as organocatalysts for the ring‐opening polymerization (ROP) of cyclic monomers. By deprotonation of weak acids or in combination with lithium cation, PBs significantly enhance the nucleophilicity of the initiator/chain‐end, thus allowing fast and usually controlled anionic polymerization. In this feature article, the recent advances in phosphazene‐catalyzed ROP of cyclic esters are summarized. This review is divided into three sections, including general features, design and synthesis, and catalytic applications. It aims to provide a critical analysis of PB‐mediated ROP systems and a useful guide for the further design of organocatalysts applied to polymer synthesis. An outlook is given at the end.

The rise of bio-inspired polymer compartments responding to pathology-related signals

“…This drawback of anionic ROP reactions was recently overcome by metal-free ROPs, where the activation of monomers is performed in the presence of organocatalysts, such as N-heterocyclic carbenes, N-heterocyclic olefins or phosphazene bases. 100,101 Stimuli with relevance for sensing pathological conditions Gradients and fluctuations of pH are the most explored triggers of biosensors since they are associated with a broad variety of diseases, for example cancer or Alzheimer's disease. 126 A slightly acidic extracellular microenvironment (pH 6.5-7.2) and a slightly basic intracellular pH are typical for many solid tumors, while intracellular compartments, such as endosomes and lysosomes have pH values of 5.0-6.0 and 4.5-5.0, respectively.…”

DFT Modeling of Organocatalytic Ring-Opening Polymerization of Cyclic Esters: A Crucial Role of Proton Exchange and Hydrogen Bonding

“…A number of publications were devoted to the use of phosphazenes in ROP of cyclic esters [95,[182][183][184][185][186][187][188][189][190]. For example, phosphazenes demonstrated high catalytic activity in δVL [182] εCL [182,185], l-LA [182] polymerization and block copolymerization of εCL with l-LA [182], being initiated by alcohols [182,185] and amides [185] as O-H and N-H acids, respectively (Scheme 13).…”

High Molecular‐Weight Cyclic Polyesters from Solvent‐Free Ring‐Opening Polymerization of Lactones with a Pyridyl‐Urea/MTBD

“…In addition, a variety of organocatalysts have been reported in the synthesis of polyesters via ROP of lactones in the literature. [ 23–26 ] It has been noticed that some of them exhibited the capacity for synthesis of polyesters with different architectures, forming cyclic or linear polymers in the absence or presence of an initiator, respectively, including 1,8‐diazabicyclo‐[5.4.0]undec‐7‐ene (DBU) and N ‐heterocyclic carbenes (NHCs). [ 25–33 ] Among these catalysts, NHCs were highlighted due to their excellent activity toward various monomers, for example, lactones, lactides, N ‐carboxyanhydrides, and carbosiloxanes.…”