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, [(H3C)2NCH2CH2(μ2-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 R3N·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/p...