Synthesis of highly isotactic (mm > 0.70) polyacrylonitrile by anionic polymerization using diethylberyllium as a main initiator

An attempt was made to synthesize steroregular polyacrylonitrile (PAN) with a high triad isotacticity (i.e. the content of mm (m, meso)) exceeding 0.70 by the anionic polymerization method. In the stereospecific polymerization of acrylonitrile (AN) initiated by diethylberyllium (Et₂Be) in xylene at 130°C, the (mm) content as well as the viscosity-average molecular weight (MÞ_v) of PAN increased by addition of diisobutylaluminum hydride (i-C₄H₉)₂AlH) as an additive to the polymerization system. Maximum (mm) content, attained in the molar ratio region of (i-C₄H₉)₂AlH/Et₂Be > 1.0, was about 0.73. The stereospecific polymerization of AN was also initiated using a mixture of Et₂Be and di-n-hexylamagnesium ((Et₂Be/(n-C₆H₁₃)₂ Mg system), where both Et₂Be and (n-C₆H₁₃)₂ Mg can induce the stereospecific polymerization of AN at 130°C. The (mm) content of the PAN sample prepared using the Et₂Be/(n-C₆H₁₃)₂ Mg system ((mm) = 0.64) was higher than that of PAN samples synthesized using Et₂Be alone ((mm) = 0.56) and (n-C₆H₁₃)₂Mg alone ((mm) = 0.51) under the same conditions except initiator. A significant difference in ¹³C chemical shifts of α-carbons between Et₂Be (1.35 ppm) and (n-C₆H₁₃)₂Mg (10.72 ppm) dissolved in hydrocarbon solvent at 110°C leads us to the conclusion that when Et₂Be induces the stereospecific polymerization in the Et₂Be/(n-C₆H₁₃)₂Mg system as initiator, the main role of (n-C₆H₁₃)₂Mg is considered to be the suppression of the association of Et₂Be (active site) itself.