Effect of syndiotacticity on the morphology of water-soluble low molecular weight poly(vinyl alcohol) by solution copolymerization of vinyl pivalate/vinyl acetate in tetrahydrofuran and saponification

Syndiotacticity-rich low molecular weight (LMW) poly(vinyl alcohol) (PVA) was synthesized by solution copolymerization of vinyl pivalate (VPi) and vinyl acetate (VAc) with various monomer ratios in tetrahydrofuran at low temperature using 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of copoly(VPi/VAc). Solution copolymerization of VPi and VAc by ADMVN and saponification produced water soluble syndiotacticity-rich LMW PVA with number-average degrees of polymerization (P_n)s of 220-520, syndiotactic diad (s-diad) contents of 55.5-61.3%, and with maximum conversions of VPi and VAc into copoly(VPi/VAc) of 70-80%. The effect of stereosequences of PVA was investigated in terms of morphology of water-soluble LMW PVA. Especially, to precisely identify the effect of syndiotacticity of LMW PVA on the change of morphology, we prepared various PVAs with similar P _n of 1200 and with different s-diad contents of 55.5-61.3%, respectively. The PVA with s-diad content of 61.1% revealed a well-defined fibrous morphology. Each fiber was composed of a number of microfibrils. As the s-diad content of PVA decreased, some morphological change was observed. The specimen with s-diad content of 59.9% formed a divided precipitate with microfibrillar structure. In contrast, in the case of PVA with s-diad content of 55.7%, irregular shaped particles were observed.