N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 1: Triisobutylaluminum as an efficient controlling agent

Abstract

Ring-opening polymerization (ROP) of propylene oxide (PO) is achieved at 25 °C either in bulk or in solution, using N-heterocyclic carbenes (NHCs) and triisobutylaluminum (i-Bu 3 Al) as a bicomponent catalytic system. Transfer to monomer was not observed and poly(propylene oxide)s with predictable molar masses up to 60 000 g.mol -1 and low dispersities were obtained. In presence/absence of an alcohol as the initiator, the polymerization of PO follows anionic or zwitterionic ROP mechanisms, respectively. The addition of the Lewis acid strongly improves the efficiency of NHCs for the polymerization of substituted epoxides. It is established that i-Bu 3 Al is involved both in the formation of an initiating/propagating complex of moderate basicity/nucleophilicity and in the coordination of PO, enabling the activation of the monomer towards the complexed nucleophilic active species. Block copolyethers are also prepared by PPO chain extension experiments. All (co)polyethers were thoroughly characterized by 1 H NMR spectroscopy, SEC and MALDI-TOF mass spectrometry as means to prove the control and benefit of this NHC approach for epoxides ROP.

N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 2: Toward dihydroxytelechelic polyethers using triethylborane

“…Following the conclusion of a parallel work using NHC's combined with triisobutylaluminum (i-Bu 3 Al) [29], efficient 1,3-bis(isopropyl)-4,5(dimethyl)imidazol-2-ylidene, I i Pr; Figure S1) due its higher nucleophilicity [30] was chosen to be combined with Et B for the ROP of PO in the presence of different alcohols as initiators. Other NHC's, e.g.…”

Borinane Boosted Bifunctional Organocatalysts for Ultrafast Ring-Opening Polymerization of Cyclic Ethers

“…In order to broaden the applications of polyethers and avoid the presence of metal residues resulting from the use of metal complexes, several efforts have been made to prepare polyethers under metal-free conditions. Organocatalysts such as phosphazene bases, N -heterocyclic carbenes, − N -heterocyclic olefins (NHO), ,, triethylborane (TEB) coupled with onium salts, − and 9-borabicyclo[3.3.1]­nonane (9-BBN)-based bifunctional catalysts − have been utilized for the ROP of epoxides. For instance, water was added as a transfer agent in the ROP of PO catalyzed by one of the 9-BBN bifunctional catalysts recently disclosed to obtain PPO telechelics .…”

Borinane Boosted Bifunctional Organocatalysts for Ultrafast Ring-Opening Polymerization of Cyclic Ethers

“…In order to broaden the applications of polyethers and avoid the presence of metal residues resulting from the use of metal complexes, several efforts have been made to prepare polyethers under metal-free conditions. Organocatalysts such as phosphazene base, 24 N-heterocyclic carbenes (NHC), 25,26,27 N-heterocyclic ole ns (NHO) 7,28,29 , triethylborane (TEB) 30,31,32 , and 9-borabicyclo[3.3.1]nonane (9-BBN) based catalysts 33 have been utilized for ROP of epoxides. The TEB system enables the synthesis of well-de ned polyethers, but is not suitable for the preparation of low molar mass polyols.…”