提供具有降低的水含量的改性乙烯醇（VA），以呈现实际需要的形式稳定性，其制备方法和离子交换膜。 （1）作为结构单元的乙烯醇单体单元（A），亚乙烯基单体单元（B）和具有羟基以外的极性基团的聚合单元（C）的共聚物（P）。 （2）一种共聚物（P）的制造方法，其特征在于，包括：提供包含乙烯醇聚合物单元（A'）和除了羟基以外的极性基团的聚合物单元（C）的共聚物（P'）， 在酸性条件下，将共聚物（P'）进行脱水以通过脱水形成多烯结构，从而引入亚乙烯基单体单元（B）。 （3）以非羟基以外的极性基团的聚合物单元（C）为具有离子性基团的聚合物单元（CA）作为主要成分的共聚物（PA）的离子交换膜。

Vinyl alcohol (VA), modified VA copolymer, that there is provided a manufacturing method and the ion exchange membrane so as to give a viable dimensionally stable by reducing the water content of the copolymer. (1) VA-based monomer unit (A), a copolymer of the polymer component (C) having a polar group other than a vinyl-series monomer unit (B) and the hydroxyl group in the component (P). (2) heating the "copolymer (P containing and polymer component having a polar group other than a hydroxyl group (C)) VA polymer component (A) 'under an acidic condition processing, dehydration poly to Yen, a vinyl-series monomer unit ( characterized in that the introduction of B), the manufacturing method of the copolymer (P). (3) copolymer polymer component polymer component (C) having a polar group other than the hydroxyl groups of (P) has an ionic group (CA) ion-exchange membrane comprising as a main component of a copolymer (PA).