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)AIX, where TPP = tetraphenylporphyrin and X = Cl, O(CH2)(9)CH3, and O2C(CH2)(6)CH3, and propylene oxide, PO, have been studied in CDCl3 and have been shown to give (TPP)AIOCHMeCH(2)X 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)AIX is favored in the order O2C(CH2)(6)CH3 O2CO(CH2)(9)CH3 much greater than O(CH2)gCH(3). 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 C-13 {H-1} 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.