The molecular weight of highly aromatic polymers made by the Friedel- Crafts polymerization of EN monomers of the formula Ar--L--Ar. sup.1 --X, wherein X is selected from SO.sub.2 Y or COY, Y being a radical selected from --OH, halogen, or alkyl, wherein Ar and Ar.sup.1 are independently ##STR1## wherein L is --CO--, --SO.sub.2 --, phenyleneoxy, --NHCO--, a covalent bond, --O--, or --CR.sub.2 --, wherein each R is independently hydrogen, an alkyl or fluoroalkyl group, phenyl or a phenyl group substituted by an electron withdrawing group such as halogen, NO.sub.2 or --CN, is controlled by including in the reaction mixture an acylatable aromatic compound whose rate of acylation under Friedel-Crafts conditions is at least about 150 times that of benzene. The molecular weight of other polymers made from EN type monomers by Friedel-Crafts catalysis can also be controlled in this way.
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