A method for the manufacture of polyether ketone ketone (PEKK), including: (i) providing a 1,4-bis(4-phenoxybenzoyl)benzene-Lewis acid complex; (ii) purifying said 1,4-bis(4-phenoxybenzoyl)benzene-Lewis acid complex; (iii) reacting said 1,4-bis(4-phenoxybenzoyl)benzene-Lewis acid complex with at least one difunctional aromatic acyl chloride in a reaction solvent and optional additional Lewis acid to obtain a product mixture including a PEKK-Lewis acid complex; and (iv) decomplexing the PEKK-Lewis acid complex to obtain a PEKK polymer. Further, a composition including at least 40 wt. % of 1,4-bis(4-phenoxybenzoyl)benzene-Lewis acid complex and an anhydrous aprotic solvent or solvent mixture, characterized in that it includes less than 1 wt. %, preferably less than 0.5 wt. % and in particular less than 0.1 wt. % of molecules including xanthydrol groups and its use for the manufacture of polyether ketone ketone.