Asymmetric gas separation membrane and process for gas separation

An asymmetric gas separation membrane made of a soluble aromatic polyimide comprising a repeating unit represented by general formula (I):

• wherein,
• B comprises 10 to 70 mol % of a tetravalent unit B1 having a diphenylhexafluoropropane structure represented by general formula (B1), and 90 to 30 mol % of a tetravalent unit B2 having a biphenyl structure represented by general formula (B2);
• and
• A comprises 10 to 50 mol % of a bivalent unit A1 having a hexafluorinated structure selected from the group consisting of a unit represented by general formula (A1a) and a unit represented by (A1b), and 90 to 30 mol % of a bivalent unit A2 having a sulfur-containing heterocyclic structure selected from the group consisting of a bivalent unit represented by general formula (A2a) and a bivalent unit represented by general formula (A2b);
• wherein r is 0 or 1 and a phenyl ring is optionally substituted with an OH group;
• wherein Y represents O or single bond;
• wherein R and R′ are hydrogen or an organic group and n is 0, 1 or 2;
• wherein R and R′ are hydrogen or an organic group, and X is —CH2— or —CO—.