1. Field of the Invention
The present invention relates to a polyvinyl alcohol having a high stereoregularity, and more specifically to a novel polyvinyl alcohol having markedly high isotacticity.
2. Description of the Prior Art
Polyvinyl alcohol, which is one of a few crystalline water soluble polymers, has excellent interfacial characteristics and mechanical characteristics, and is hence used for processing paper and fibers and stabilizing emulsions, and also as a starting material for polyvinyl alcohol film and polyvinyl alcohol fiber. In recent years, development for its application to new fields has actively been conducted. For example polyvinyl alcohol has been attracting attention as a material for gel.
Commercially available polyvinyl alcohols have nearly atactic stereoregularity, i.e. a triad isotacticity of about 22%. The tacticity herein is determined by .sup.1 H-NMR spectrometry on a polyvinyl alcohol specimen solution in DMSO (dimethyl sulfoxide)-d.sub.6. It is known that the resonance absorption of the hydroxyl groups of a polyvinyl alcohol in DMSO-d.sub.6 is divided into 3 peaks attributable to, when counted from low magnetic field side, isotactic, heterotactic and syndiotactic each by triad expression, respectively. The ratio of the integrated areas of the peaks can determine the tacticity of the polyvinyl alcohol. With respect to the chemical shift of the peaks, while they shift depending on the measuring temperature, the peaks show a chemical shift at 35.degree. C. of isotactic=4.63 ppm, heterotactic=4.45 ppm and syndiotactic=4.22 ppm (J. R. DeMember, H. C. Haas and R. L. MacDonald, J. Polym. Sci., Part B, 10, 385(1972)). The tacticities of various polyvinyl alcohols herein are those determined by this method unless otherwise specified.
The triad isotacticity of an isotactic polyvinyl alcohol as determined by .sup.1 H-NMR spectrometry on its solution in DMSO-d.sub.6 was reported as 55% (J. R. DeMember, H. C. Haas and R. L. MacDonald, J. Polym. Sci., Part B, 10, 385(1972)). The above isotactic polyvinyl alcohol was obtained by a process proposed by Okamura et al (S. Okamura, T. Kodama and T. Higashimura, Macromol. Chem., 53, 180(1962)).
The triad isotacticity of an isotactic polyvinyl alcohol as determined by .sup.1 H-NMR spectrometry on its solution in DMSO-d.sub.6 was reported as 70.2% (T. K. Wu and D. W. Ovenall, Macromolecules, 6, 582(1973)). The above polyvinyl alcohol was obtained by a process proposed by Murahashi et al (S. Murahashi, S. Nozakura and M. Sumi, J. Polym, Sci., Part B, 3, 245(1965)). No polyvinyl alcohol having an isotacticity higher than the above has however been obtained yet.
The polyvinyl alcohol obtained by the above process and having an isotacticity of 55% has lower thermal resistance, crystallinity and melting point than conventional polyvinyl alcohol having an isotacticity of 22% and is soluble in cool water, thus showing reduced water resistance. The above polyvinyl alcohol having an isotacticity of 70.2% has been found to have higher melting point and improved water resistance as compared with the polyvinyl alcohol having an isotacticity of 55%, but it is still unsatisfactory with respect to melting point, crystallinity, water resistance, thermal resistance and the like.