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

A mono(mu-oxo)bis(alkylaluminum) (MOB) catalyst and initiator for epoxide polymerization, [(H3C)(2)NCH2CH2(mu(2)-O)Al(iBu)(2)center dot 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((mu-alkoxo)-dialkylaluminum) (BOD) with a characteristic R3N center dot AlR3' (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. 5-10 fold) increase in the polymerization rate of AGE. The resultant N-Al adduct catalyst represents a versatile tool for rapid functional macromolecular synthesis.