Tacticity of Poly(vinyl alcohol) 577 methyl radicals with ethylene.13 The very small fraction of ethyl branches for which there is ,3C evidence may be introduced via the extension of the Roedel mechanism described by Willbourn;1 this must now be regarded as a reaction of (13) A.
The main purpose of this investigation was to elucidate the mechanism of gelation of poly(vinyl alcohol)-water systems and the structure of junctions in gel networks. Changes of gel rigidities were measured as a function of gelling time at the temperatures of 30-80°C for four samples of different tacticities, and the numbers of junctions in the gel were calculated using the theory of rubber elasticity. The formation of junctions can be treated as the binary association of free cross-linking sequences with the activation energy of 13 kcal/mol. The study of the correlation between gelling temperatures and melting temperatures of gels shows that the junctions formed at a given temperature should be treated as multitudinous in structure and the copolymeric character of atactic poly(vinyl alcohol) chains should be considered. The structures of junctions for various gelling temperatures are discussed, assuming that cross-linking loci in poly(vinyl alcohol) chains are syndiotactic sequences of different sequence lengths.
Partly urethanized polyvinyl alcohol samples, in a range of degree of urethanization up to about 10 mole‐%, were prepared by treatment of polyvinyl alcohol with urea. These were fractionated in a water–n‐propanol system into fractions, each having nearly the same nitrogen content. The degree of polymerization of the partly urethanized polyvinyl alcohol was estimated from the intrinsic viscosity of polyvinyl alcohol obtained by hydrolysis of the corresponding urethanized polymer. The effects of introduced urethane residue on solubility and dissolved state of polymer were investigated in water and theta solvents. Further, some thermodynamic parameters in water were discussed in relation to those of partly acetylated polyvinyl alcohol.
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