A process for making a phosphoester polymer comprising the recurring monomeric units of formula I: ##STR1## wherein: X is --O-- or --NR"--, where R" is H or alkyl;L is a divalent organic moiety;R' is H, alkyl, alkoxy, aryl, aryloxy, heterocyclic, or heterocycloxy; andn is between about 25 to 2,000,is described. The process comprises the steps of:(a) polymerizing in the presence of a solvent p moles of a di-XH compound having formula II:H--X--L--X--H II wherein X and L are as defined above, with q moles, where p.apprxeq.q, of a phosphorodihalo compound to form a polymer of formula I, wherein n is about 12 to 1000, having a first molecular weight Mw.sub.1, wherein the solvent is present in an amount greater than about 5 ml of solvent per gram of compound of formula II;(b) removing at least about 25% of the solvent to form a more concentrated reaction mixture; and(c) further polymerizing the concentrated reaction mixture for an additional time sufficient to produce a polymer of formula I wherein n is between about 25 and 2,000, the polymer having a second molecular weight Mw.sub.2, which is significantly higher than Mw.sub.1.