A polymer characterized by comprising repeating units represented by the following formula (1) and having a number-average molecular weight, in terms of polystyrene, of 10.sup.3 to 10.sup.8. ##STR00001## (In the formula, R.sub.1 represents hydrogen, alkyl, alkoxy, alkylthio, aryl, aryloxy, arylthio, arylalkyl, arylalkoxy, arylalkylthio, arylalkenyl, arylalkynyl, amino, substituted amino, silyl, substituted silyl, silyloxy, substituted silyloxy, monovalent heterocyclic group, or halogeno; and rings D and E each represents an optionally substituted aromatic ring.)