Catalytic Ring-Opening Polymerization of Propylene Oxide by Organoborane and Aluminum Lewis Acids

Catalytic ring-opening polymerization of propylene oxide (PO) was studied with 12 organoborane and aluminum catalysts in combination with 12 hydroxylic initiators. These catalysts vary in Lewis acidity and ligand steric bulk, whereas the initiators differ in their functional groups and Brønsted acidity. This study examined four aspects of PO polymerization:  degree of polymerization control, effects of catalyst and initiator structure on activity and polymer molecular weight, reactions of catalyst with initiator and catalyst with monomer, and structures of PPOs produced. In the absence of hydroxylic initiators, B(C₆F₅)₃ predominantly catalyzes isomerization of PO to propionaldehyde in hexanes and additionally produces low oligomers in toluene; interestingly, the sterically encumbered perfluorobiphenyl borane B(C₁₂F₉)₃ affords no such isomerization products. With addition of sufficient high concentration of hydroxylic initiators in a [PO]₀:[−OH]₀ ratio of 41.7, however, PPOs with the desired M_n range of a few thousand dalton and low PDI of <1.3 can be produced with the B(C₆F₅)₃/initiator system. The linear dependence of PPO M_n on monomer conversion is observed up to ∼84% conversion. The PO polymerization activity strongly depends on Lewis acidity of the borane catalyst, with B(C₆F₅)₃ being most active; as Lewis acidity of the boranes decreases, the activity declines sharply. The activity is also proportional to Brønsted acidity of hydroxylic initiators, with aromatic carboxylic acids being most effective. However, excess of carboxylic acid and water initiators decomposes the borane catalyst via elimination of pentafluorobenzene, resulting in low catalytic activity and producing low oligomers; such a decomposed catalyst structure has been characterized by X-ray diffraction analysis. On the other hand, the borane catalyst is very stable toward alcohol initiators; strong activation of 1,4-butanediol, a weak Brønsted acid, by B(C₆F₅)₃, is demonstrated by the spectroscopic data and X-ray structural characterization for the borane:diol adducts. In comparison, the Al complexes are much less effective for PO polymerization, especially when used with hydroxylic initiators, due to the instability toward hydroxylic initiators. Nevertheless, in the absence of such initiators, alumoxane substantially free of trialkylaluminum and a three-coordinate cationic aluminum complex produce PPOs of M_n = 4040 and M_n = 10 600, respectively. Analyses using ¹³C NMR spectroscopy indicate that the PPOs produced from this study are atactic and essentially regioirregular, while MALDI−TOF MS spectra reveal the presence of two types of linear PPO structures having the initiator and water molecules as end groups, respectively, plus a small amount of cyclic PPO. Except for diol and triol initiators (B(C₆F₅)₃ as catalyst) which produce PPOs having higher primary hydroxyl contents with a typical [primary OH]/[secondary OH] ratio ≥ 60/40, the remaining PPO samples give about an equal amount of primary and secondary hydroxyl groups.