Production processes of arylene thioether oligomer and copolymer

A process for the production of an arylene thioether copolymer, which comprises forming a carboxylated arylene thioether oligomer by

• (1) first causing an alkali metal sulfide to act on a poly(arylene thioether) having recurring units represented by the general formula (I):
• ##STR11## wherein Ar is an arylene group, in a water-containing polar organic solvent to depolymerize the poly(arylene thioether), thereby preparing an oligomer having at least one alkali thiolate group;
• and
• (2) second causing a carboxylating agent represented by the general formula (II):
• ##STR12## wherein X is a halogen atom, R.sub.1 denotes a hydrogen atom, an alkyl group having 1-4 carbon atoms, an aryl group or an alkali metal atom, and Ar' is an arylene group, to act on the alkali thiolate-containing oligomer obtained in the first step, thereby forming the carboxylated arylene thioether oligomer;
• and
• reacting (A) a carboxylic acid component consisting of the carboxylated arylene thioether oligomer alone, or a mixture of the carboxylated oligomer in an amount of at least 50 wt. % and another carboxylic compound in an amount less than 50 wt. % with (B) an alkylenediol.