To obtain a copolymer having a protected ethylenically unsatd. double bond in its side chain by comprising a plurality of specific structural units and having a specific number average degree of polymerization.
This polyvinyl ester-type copolymer comprises 99.999-30 mol% of a structural unit represented by formula I and 0.001-70 mol% of a structural unit represented by formula II, and has a number average degree of polymerization of 10-30,000. This copolymer is saponified by adding a basic catalyst, and then is reacted by further adding an acidic material or a basic material in order to remove the protection and to generate a double bond. Accordingly, a polyvinyl alcohol-type polymer is obtd. which comprises 99.999-30 mol% of a structural unit represented by formula III and 0.001-70 mol% of a structural unit represented by formula IV and has a number average degree of polymerization of 10-30,000. In the formulas, R1 is 1-5C alkyl; R2-R4, Ra and Rc are each hydrogen or 1-5C alkyl; V is R5-CON(Ra) or the like; W is a direct combination, R7-CON(Rc) or the like; P is a protected double bond or the like; S is a polymerizable double bond or the like; R5 is a direct bond, aryl or the like; and R7 is a divalent aliph. group or the like.
ARIMOTO KIKUO
ANDO YOSHINORI
