Mechanistic studies of the copolymerization of epoxides with carbon dioxide and ring-opening polymerization of cyclic esters
Abstract (Summary)The development of new generation polymers is guided by Green Chemistry and sustainability. Biodegradable polymers derived from readily renewable resources, e.g., plants and carbon dioxide, are attractive as environmentally friendly alternatives to polymers derived from fossil fuels, and will play an increasing role in the market place. Polyoxygenates, such as polyesters and polycarbonates, are one important family of such polymers. In this work, the microstructure of poly(propylene carbonate) (PPC) has been studied by C-13 NMR spectroscopy. By analyzing polymer samples and model compounds, we observed tetrad and triad sensitivity for the carbonate and propylene carbon atoms, respectively. 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 and 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. 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. The gas-phase studies in an electrospray tandem mass spectrometer revealed that, for a series of related ions LM+, L = TPP or (R,R)- N,N’-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexenediamino) ((R,R)-salen), the Lewis acidity order follows M = Cr(III) ~ Al(III) > Co(III), and notable differences were observed when PO molecules coordinated to those metal centers. The preparations of LAlOCH2CH(S)MeCl, where L = (R,R) or (S,S)-salen, are reported together with the respective LAlOEt compounds, and their reactivities toward L- and rac-lactides in various solvents reveal the surprising complexity of the stereopreference for the ring-opening event.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/2004