Concerning the Mechanism of the Ring Opening of Propylene Oxide in the Copolymerization of Propylene Oxide and Carbon Dioxide To Give Poly(propylene carbonate)

The reactions between (TPP)AlX, where TPP = tetraphenylporphyrin and X = Cl, O(CH₂)₉CH₃, and O₂C(CH₂)₆CH₃, and propylene oxide, PO, have been studied in CDCl₃ and have been shown to give (TPP)AlOCHMeCH₂X and (TPP)AlOCH₂CHMeX compounds. The relative rates of ring opening of PO follow the order Cl > OR > O₂CR, but in the presence of added 4-(dimethylamino)pyridine, DMAP (1 equiv), the order is changed to O₂CR > OR. From studies of kinetics, the ring opening of PO is shown to be first order in [Al]. Carbon dioxide inserts reversibly into the Al−OR bond to give the compound (TPP)AlO₂COR, and this reaction is promoted by the addition of DMAP. The coordination of DMAP to (TPP)AlX is favored in the order O₂C(CH₂)₆CH₃ > O₂CO(CH₂)₉CH₃ ≫ O(CH₂)₉CH₃. The microstructure of the poly(propylene carbonate), PPC, formed in the reactions between (TPP)AlCl/DMAP and (R,R-salen)CrCl and rac-PO/S-PO/R-PO and CO₂, has been investigated by ¹³C {¹H} NMR spectroscopy. The ring opening of PO is shown to proceed via competitive attack on the methine and methylene carbon atoms, and furthermore attack at the methine carbon occurs with both retention and inversion of stereochemistry. On the basis of these results, the reaction pathway leading to ring opening of PO can be traced to an interchange associative mechanism, wherein coordination of PO to the electrophilic aluminum atom occurs within the vicinity of the Al−X bond (X = Cl, OR, O₂CR, or O₂COR). The role of DMAP is two-fold:  (i) to labilize the trans Al−X bond toward heterolytic behavior, and (ii) to promote the insertion of CO₂ into the Al−OR bond.