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 Bronsted 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 Mn range of a ...