Suitable chain extenders may be selected from polyols such as ethylene glycol, diethylene glycol, butanediol, dipropylene glycol and tripropylene glycol; aliphatic and aromatic amines, e.g., 4,4'-methylene dianilines having a lower alkyl substituent positioned ortho to each N atom; certain imino-functional compounds such as those disclosed in European Patent Applications Nos. 284 253 and 359 456 a