Decoupling Catalysis and Chain-Growth Functions of Mono(μ-alkoxo)bis(alkylaluminums) in Epoxide Polymerization: Emergence of the N–Al Adduct Catalyst

Abstract

A mono(μ-oxo)bis(alkylaluminum) (MOB) catalyst and initiator for epoxide polymerization, [(H 3 C) 2 NCH 2 CH 2 (μ 2 -O)Al(iBu) 2 •Al(iBu) 3 ] (1), produced a ca. 170-fold enhancement in epoxide polymerization rate over previously reported MOB initiators demonstrated with allyl glycidyl ether (AGE). This discovery reduces polymerization times to minutes. 1 exhibited an exponential dependence of polymerization rate on concentration, rather than an expected low integer order relationship. A proposed polymerization intermediate was identified via direct synthesis, isolation, kinetic comparison, and corroborating in situ spectroscopic evidence to be a symmetric bis((μ-alkoxo)dialkylaluminum) (BOD) with a characteristic R 3 N•AlR′ 3 (N−Al) adduct. The N−Al adduct on the BOD intermediate is proposed to act as a catalyst, whereas the aluminoxane ring is proposed to be the site of monomer enchainment on the basis of mass spectrometry and spectroscopic analysis of resultant polymer structure. The distinct catalytic and initiation/propagation functionalities were separated into separate species, and the catalytic activity of the N−Al adduct was demonstrated in the presence of a distinct aluminoxane initiator. Each 1 equiv of N−Al adduct relative to initiator resulted in an abrupt (ca. 5−10fold) increase in the polymerization rate of AGE. The resultant N−Al adduct catalyst represents a versatile tool for rapid functional macromolecular synthesis.

Aluminum-Based Initiators from Thiols for Epoxide Polymerizations

“…10% after 7 days), while just catalyst and catalyst and ligand resulted in no conversion after 7 days. This suggests that both catalyst and initiator are necessary for efficient conversion, which is consistent with previous work with BOD-initiated polymerizations Figures S6–S8).…”

“…Recently developed mono­(μ-alkoxo)­bis­(alkylaluminum) (MOB) compounds are tolerant to monomer functional group while still providing molecular weight control for epoxide polymerizations without any chain transfer reactions present. , Additionally, the MOB compounds are trivial to synthesize and use for epoxide polymerizations and therefore provide access for nonexperts to polyethers with tunable molecular weight, composition, and architecture. ,, The MOB compounds can be split into two separate components that facilitate epoxide polymerization; one consisting of a polymerization catalyst portion (e.g., Et 3 NAlMe 3 ) and one consisting of a bis­((μ-alkoxo)-dialkylaluminum) (BOD) initiator portion (e.g., [Et 2 Al­(μ-OCH 2 CH 2 OMe)] 2 ) . The result of this split was that the polymer end group could be controlled using different initiator chemistries with a limited effect on polymerization kinetics.…”

“…4,24,25 The MOB compounds can be split into two separate components that facilitate epoxide polymerization; one consisting of a polymerization catalyst portion (e.g., Et 3 NAlMe 3 ) and one consisting of a bis((μ-alkoxo)-dialkylaluminum) (BOD) initiator portion (e.g., [Et 2 Al(μ-OCH 2 CH 2 OMe)] 2 ). 26 The result of this split was that the polymer end group could be controlled using different initiator chemistries with a limited effect on polymerization kinetics. Previously, all MOB/BOD initiators were based on alcohol-containing ligands, which limits the potential ligand moieties.…”

The versatile, functional polyether, polyepichlorohydrin: History, synthesis, and applications

“…75 Later iterations improved reaction kinetics and control. Lynd, Ferrier, and co-workers varied the alkyl substitutions on the aluminum 72 and on the amine of the alcohol ligand, 73 to investigate kinetic effects. They found kinetics were enhanced with smaller alkyls on the amine and bulkier alkyls on the aluminum.…”

“…The enhanced kinetics led to polymerizations being completed in minutes at mild temperatures while maintaining M n and dispersity control. Most recently, the catalyst and initiator portions were separated, with the so-called "NAl catalyst" formed from the reaction of triethylamine and trimethyl aluminum and the initiator formed from an alcohol 73 or thiol 40 containing ligand reacted with trimethyl aluminum. Examples of the initiator structures as well as the NAl catalyst structure can be seen in Figure 11(B,C), respectively.…”