Preparation of water-soluble, syndiotacticity-rich, low molecular weight poly(vinyi alcohol) microfibrils in high yields with the low-temperature polymerization of vinyl pivalate in tetrahydrofuran and saponification

To prepare water-soluble, syndiotacticity-rich poly(vinyl alcohol) (PVA) micro-fibrils for various industrial applications, we synthesized syndiotacticity-rich, low molecular weight PVA by the solution polymerization of vinyl pivalate (VPi) in tetrahydrofuran (THF) at low temperatures with 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of poly(vinyl pivalate) (PVPi). Effects of the initiator and monomer concentrations and the polymerization temperature were investigated in terms of the polymerization behaviors and molecular structures of PVPi and the corresponding syndiotacticity-rich PVA. The polymerization rate of VPi in THF was proportional to the 0.91 power of the ADMVN concentration, indicating the heterogeneous nature of THF polymerization. The low-temperature solution polymerization of VPi in THF with ADMVN proved to be successful in obtaining water-soluble PVA with a number-average degree of polymerization (P_n) of 300-900, a syndiotactic dyad content of 60-63%, and an ultimate conversion of VPi into PVPi of over 75%. Despite the low molecular weight of PVA with P_n = 800, water-soluble PVA microfibrillar fibers were prepared because of the high level of syndiotacticity. In contrast, for PVA with P_n = 330, shapeless and globular morphologies were observed, indicating that molecular weight has an important role in the in situ fibrillation of syndiotacticity-rich PVA.