Modified polyvinyl alcohol and process for producing the same

1. A modified polyvinyl alcohol containing bond units of formula (1) in its molecular main chain:

wherein each of X1 and X2 is a methyl group, g is an integer of from 0 to 3, h is 0, Y1 is a hydrogen atom; and

0 to 20 mol % of copolymerizable monomers

obtained by copolymerizing at least one monomer having an ethylenically unsaturated double bond of formula (3) and a monomer having a vinyl ester unit by solution polymerization in methanol and saponifying an obtained copolymer

wherein each of X3 and X4 is a methyl group i is 0, Y2 is a hydrogen atom

wherein said monomer having an ethylenically unsaturated double bond and said monomer having a vinyl ester unit are continuously added to a polymerizer and

wherein said modified polyvinyl alcohol has a saponification degree of from 30 to 99.9 mol % and does not contain units from polymerization of any of itaconic acid, maleic acid and maleic anhydride.

2. The modified polyvinyl alcohol according to claim 1 , which shows an absorbance of at least 0.05 at 270 nm in the ultraviolet absorption spectrum of its 0.2 mass % aqueous solution, methanol solution or water/methanol mixed solution.

3. The modified polyvinyl alcohol according to claim 1 , which shows a peak top at 265 to 275 nm in the ultraviolet absorption spectrum attributable to a structure of formula (2):

4. The modified polyvinyl alcohol according to claim 1 , wherein the content of unmodified polyvinyl alcohol residues is at most 25 mass %.

5. A process for producing the modified polyvinyl alcohol as defined in claim 1 , which comprises copolymerizing at least one monomer of formula (3), a monomer having a vinyl ester unit; and 0 to 20 mol % of copolymerizable monomers, and then saponifying the obtained copolymer:

wherein each of X3 and X4 is a methyl group i is 0, Y2 is a hydrogen atom.

6. The process for producing the modified polyvinyl alcohol according to claim 5 , which comprises copolymerizing, in addition to said at least one monomer of formula (3) and said monomer having a vinyl ester unit, other monomers copolymerizable with these monomers, and then saponifying the obtained copolymer.

7. The modified polyvinyl alcohol according to claim 1 , wherein a content of monomer having an ethylenically unsaturated double bond in the modified polyvinyl alcohol is from 0.1 to 50 mol %.

8. The modified polyvinyl alcohol according to claim 1 , wherein a content of monomer having an ethylenically unsaturated double bond in the modified-polyvinyl alcohol is from 0.1 to 10 mol %.

9. The modified polyvinyl alcohol according to claim 1 , having a number average molecular weight of from 1,900 to 66,500.

10. The modified polyvinyl alcohol according to claim 1 , having a number average molecular weight of from 3,800 to 28,500.

11. The modified polyvinyl alcohol according to claim 1 , wherein a 0.2 mass % aqueous, methanol or aqueous methanol solution of said modified polyvinyl alcohol has an absorbance at 270 nm of at least 0.05.

12. The modified polyvinyl alcohol according to claim 1 , wherein a 0.2 mass % aqueous, methanol or aqueous methanol solution of said modified polyvinyl alcohol has an absorbance at 270 nm of at least 0.10.

13. The modified polyvinyl alcohol according to claim 1 , obtained by copolymerizing at least one monomer having an ethylenically unsaturated double bond of formulae (3) and a monomer having a vinyl ester unit.

14. The modified polyvinyl alcohol according to claim 13 , wherein the at least one monomer having an ethylenically unsaturated double bond of formulae (3) is dimethyl maleate.