in a process for producing poly from an alkali metal sulfide and a p - dihalobenzene , a product polymer of an advantageously increased molecular weight is obtained by : providing a mixture comprising an alkali metal sulfide , a polar aprotic solvent , a sodium salt of an aliphatic carboxylic acid represented by the general formula : rcoona , the content of water present in said mixture apart from any water of hydration or crystallization which may be contained in said sulfide and sodium salt being in the range of 5 to 20 moles per mole of said sulfide ; thermally dehydrating said sulfide component in said mixture by removing at least part of the water from said mixture ; and then contacting the resulting dehydrated mixture with a p - dihalobenzene .