Fundamental Studies towards Transistion Metal Catalysis and Application of Chromium Salen Complexes for the Synthesis of Polymers
?The body of this work spans both fundamental organometallic chemistry and the application of previously studied catalyst systems to produce new polymeric materials. The cone angle of triphenylphosphite was estimated to be 128 degrees by Tolman in the late 70s; however, metal complexes bearing this ligand undergo cis/trans isomerization via a mechanism indicative of greater steric requirements. X-ray crystallographic studies coupled with data compiled from the Cambridge Crystallographic Database, were used to more accurately calculate the steric demand of this wieldy used ligand to be approximately 140 degrees. Additionally, in depth kinetic studies of the interaction of furan ligands with electron deficient manganese and chromium metal centers were performed. Data collected from timescales ranging from minutes to microseconds was utilized to calculate the bond dissociation energy of both 2,3-dihydrofuran (DHF) and furan. The aromatic furan ligand was found to bind to the both metals 7-10 kcal mol-1 weaker than DHF. Additionally, the more electron rich chromium center was found to bind both ligands ?weaker than the manganese center implying a minimization of the M-L pie -back bonding interaction. Solution studies coupled with DFT calculations were utilized to estimate the extent that the furan ligand is dearomatized by approximately 50% upon interaction with the metal center. Application-based studies of the separation of polymer catalyst mixtures were also undertaken. The addition of the 1000 Dalton poly(isobutylene) arms to the salen ligand in (salen)CrCl complexes yielded a catalyst that could be extracted from the reaction mixture containing poly(cyclohexene carbonate) via the addition of heptane. Another approach, not requiring catalyst modification, utilized a secondary amine to facilitate the purification of the polymer product. The reaction of an amine with CO2 to form an ionic liquid resulted in the precipitation of the polymer while the catalyst and byproducts remained in the liquid carbamate phase. Both approaches provided improvements over the long standing method of precipitating the polymer using methanol and strong acid. Lastly, the previous work of the Darensbourg group utilizing (salen)CrCl catalyst to produce polycarbonates from CO2 and epoxides was employed to synthesized sulfur rich poly(thiocarbonate)s. The effects of both CS2 loading and temperature on the copolymerization of CS2 and cyclohexene oxide were studied. Optimal conditions of 1 equivalent of CS2 and 50 degrees C were found to selectively produce the desired polymeric material. The observation of multiple thiocarbonate as well as carbonate functionalities, led to a detailed study of the reaction byproducts to gain insight into the copolymerization process.