Molecular level approach to the origin of the steric control in organozinc catalyst for oxirane polymerization

Abstract

AbstractA new type of single crystals was formed by a reaction between diethylzinc and DL‐1‐methoxy‐2‐propanol. The constituent unit of this single crystal was shown to be a two to two complex of zinc dialkoxide and ethylzinc alkoxide, [MeOCH2 CH(Me)OZnOCH(Me)CH2 OMe]2 · [EtZnOCH(Me)CH2 OMe]2, which has a chair type structure. The two to two complex was found to exhibit higher activity and stereospecificity in the PO polymerization compared with those of the one to six complex which were reported previously. Molecular level elucidation was made on the basis of NMR studies for polymerization system with a partly deuterated two to two complex as initiator. The excellent stereospecificity in isotactic propagation was ascribed to the highly chiral hole around the active site of the chair type complex. Another series of study revealed chemical behavior of cyclohexene oxide (CHO), which is different from that of PO, toward several organozinc compounds. The two to two complex was the only catalyst which showed high activity for both MO and CHO polymerizations. Three samples (I), (II) and (III) of poly‐(CHO) were prepared with Et2Zn, EtZnOR and the two to two complex, respectively. After careful examinations of each of the three samples, it was concluded that most parts of the main chain of polymer (III) molecule were composed of syndiotactic triad, SS‐RR‐SS(or RR‐SS‐RR), while polymer (I) and (II) molecules contained syndiotactic and heterotactic triads in comparative amounts. In the CHO polymerization, the chiral hole of the two to two complex cannot recognize the mode of orientation of the achiral monomer, and the complex seems to serve as a simple bulky group which facilitates syndiotactic addition of CHO monomer molecule to the active site, strong steric effects being exerted from the penultimate unit of the growing chain during the propagation stage.