Controlled Oxyanionic Polymerization of Propylene Oxide: Unlocking the Molecular‐Weight Limitation by a Soft Nucleophilic Catalysis

Abstract

The oxyanionic ring-opening polymerization of propylene oxide (PO) from an exogenous alcohol activated with benign (complexed) metal-alkali carboxylates is described. The equimolar mixture of potassium acetate (KOAc) and 18-crown-6 ether (18C6) is demonstrated to be the complex of choice for preparing poly(propylene oxide) (PPO) in a controlled manner. In the presence of 18C6/KOAc, hydrogen-bonded alcohols act as soft nucleophiles promoting the PO S N 2 process at room temperature and in solvent-free conditions while drastically limiting the occurrence of parasitic hydrogen abstraction generally observed during the anionic ROP of PO. The resulting PPO displays predictable and unprecedented molar masses (up to 20 kg mol −1 ) with low dispersities (Ð M < 1.1), rendering the 18C6/KOAc complex the most performing activator for the oxyanionic polymerization of PO reported to date. Preliminary studies on the preparation of block and statistical copolyethers are also reported.

Quasi-alternating copolymerization of oxiranes driven by a benign acetate-based catalyst

“…Initial studies confirmed the importance of this adduct interaction 26 , since this mechanism occurs entirely through hydrogen bonding between the propagating alcohol end-group and the acetate base. Despite the slow initial 18C6/KOAc/BnOH supramolecular organization by monomer molecules, the intermediate species formed plays a key role in increasing polymerization control and while both homopolymerizations are characterized by a long inhibition period, regardless of the nature of the initiator used, the polymerization kinetics exhibit similar profiles.…”

“…Before copolymerizations were undertaken, we first investigated the homopolymerizations of PO and AGE in bulk at room temperature from a benzyl alcohol (BnOH) initiator activated by 18C6/KOAc complex ([BnOH] 0 /[18C6/KOAc] 0 = 2). Our group previously reported that 15 days were required to homopolymerize PO at 21 °C to get a degree of polymerization (DP = [PO] 0 /[OH] 0 ) of 130 26 . Therefore, to reduce the reaction time, low molecular weight PPO and poly(allyl glycidyl ether) (PAGE) were prepared by targeting a degree of polymerization (DP = [M] 0 /[BnOH] 0 , with M, the monomer employed, i.e., PO or AGE) of 25.…”

Unexpected Quasi-Alternating Copolymerization of Oxiranes Driven by a Benign Acetate-based Catalyst

“…Before copolymerizations were undertaken, we first investigated the homopolymerizations of PO and AGE in bulk at room temperature from a benzyl alcohol (BnOH) initiator activated by 18C6/KOAc complex ([BnOH]0/[18C6/KOAc]0 = 2). Our group previously reported that 15 days were required to homopolymerize PO at 21 °C to get a degree of polymerization (DP = [PO]0/[OH]0) of 130 26 .…”

“…To determine whether the nature of the initiator molecule impacts the initiation step, we examined the homopolymerizations using isopropyl alcohol (iPrOH Initial studies confirmed the importance of this adduct interaction 26 , since this mechanism occurs entirely through hydrogen bonding between the propagating alcohol end-group and the acetate base. Despite the slow initial 18C6/KOAc/BnOH supramolecular organization by monomer molecules, the intermediate species formed plays a key role in increasing polymerization control and while both homopolymerizations are characterized by a long inhibition period, regardless of the nature of the initiator used, the polymerization kinetics exhibit similar profiles.…”

“…In this context, alkali metal carboxylate catalysts, eventually complexed by crown ethers 24,25 , have played a crucial role in the development of various (co)polymers due to their ability to be used in bulk, of their low cost and low toxicity. Our group has recently demonstrated the potential of an equimolar mixture of potassium acetate (KOAc) and 18crown-6 ether (18C6) as a robust and benign catalytic system for preparing PPO in a controlled fashion at room temperature and in solvent-free conditions 26 . In the presence of 18C6/KOAc complex, hydrogen-bonded alcohols act as soft nucleophiles promoting the PO polymerization, while drastically limiting the occurrence of parasitic hydrogen abstraction 27,28,29,30,31 .…”