Regio-and Stereo-chemistry Control Of The Alternating Copolymerization Of CO₂ And Propylene Oxide

The physical properties of a polymer are determined mainly by the relative stereochemistry(the spatial arrangement of atoms or groups in a polymeric unit) of adjacent locations in the polymeric chains.As a result,the control of polymer microstructure is one of the most important goals in the area of stereoselective polymerization catalysis.There are much interested in CO₂/aliphatic epoxides(such as propylene oxide) copolymerization,which was first reported by Inoue in 1969.Recently,our group reported that the binary catalyst systems composed of the chiral tetradentate Schiff base cobalt complex(SalenCo(â…¢)X) as the electrophlie and ionic ammonium salt or sterically hindered strong base as the nucleophile can effectively promote the asymmetric copolymerization of CO₂ and racemic propylene oxide(PO) to produce poly(propylene carbonate)(PPC) with excellent activity,relative high enantioselectivity(ee%～70%),＞95%head-to-tail connectivity,and＞99%carbonate linkages. In sharp contrast to the regio-and stereoeslective polymerization catalyzed by chiral SalenCo(â…¢)X complexes,although chromium Salen derivatives were also proved to be an effective catalyst for the coupling of PO and CO₂ to afford linear PPC,the polycarbonate produced is regioirregular,and the catalyst activity and polymer enantioselectivity is also very poor.In this dissertation,it is found that various poly(propylene carbonate)s(PPC)s with different head-to-tail linkages(from～60%to 93%) could be synthesized by the copolymerization of PO and CO₂ with chromium Salen complexes as catalyst.Altering the axial X group and Salen ligand framework of the chromium complexes or/and the addition of cocatalyst significantly affect stereochemistry of the resulting polymers.A SalenCrX complex with a low nucleophilic axial X anion and a conjugated cyclic diamine backbone in the Salen ligand,combined with a sterically hindered strong organic base,is an ideal binary catalyst system for the copolymerization of CO₂ and PO to produce polycarbonate with relative high head-to-tail connectivity and＞99%carbonate linkages.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that the sterically hindered strong organic base predominately plays an initiator of polymer-chain growth in the copolymerization process.Apart from the effect of the ligand set of the chromium Salen complexes on polymer stereochemistry,a "polymer chain-end control" mechanism also significantly affects the stereochemistry of the resulting polycarbonates.Chiral SalanCr(â…¢)X complexes(a saturated version of SalenCr(â…¢)X) are designed,and in conjunction with an ionic quaternary ammonium salt or organic strong base can efficiently catalyze the asymmetric,regio-and stereoselective alternating copolymerization of CO₂ with rac-PO at mild conditions to afford isotactic-enriched polycarbonates with～95%head-to-tail linkages and moderate enantioselectivity.The facts indicate that apart from the asymmetric induction of chiral diamine backbone,sterically hindered substitute groups on the aromatic rings,and the presence of sp³-hydridized amino donors and its N,N'-disubstituted groups in chiral SalanCr(â…¢)X complexes all play important roles in controlling polymer stereochemistry and enantioselectivity.The stereochemical studies of PPCs derived from the copolymerization of CO₂ with chiral PO at various conditions reveal that the polymer enantioselectivity is proportional to its head-to-tail linkages with regard to regioselective ring-opening of the epoxide.These new catalyst systems predominantly exceed the previously much-studied SalenCr(â…¢)X systems in catalytic activity,polymer enantioselectivity and stereochemistry control.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that DMAP predominately plays an initiator of polymer-chain growth in the copolymerization process.Electrospray ionization mass spectrometry(ESI-MS) in combination with FT-IR spectra study suggest that the coordination modes of these two chromium complexes with DMAP and the induction periods of CO₂/PO copolymerization and the rate of polymer-chain growth with these two catalyst systems are very different.Furthermore,a straightforward approach to insight into the details of copolymerization of CO₂ and epoxide by SalanCr(â…¢)X/DMAP catalyst system is provided.