the techniques mentioned can be used to control the number average molecular weight the products are mixtures of polymers with several molecular weights characterized by a distribution the order of addition of the reactants is not critical however the preferred order is to add the difunctional electrophile first when acid acceptors which are not buffers are used such as hydroxide it is most preferable to add a portion at the beginning to achieve the desired ph and then add the remainder concurrently with the difunctional electrophile finally it is desirable to conduct these polymerizations at high concentrations this reduces the amount of solvent which must be removed to isolate the product also in certain cases the product precipitates from the reaction solution near the end of the reaction and may be isolated by simply decanting the solvent most of the inorganic salt which results from reaction of the acid acceptor is removed in this process the concentration is not critical and may be from 0 5 to 50 weight percent expressed as weight of diamine to weight of solvent a preferred range is 0 5 to 5 weight percent the crude polydispersed product may be isolated by precipitation of the reaction solution into a solvent which is water miscible but is a poor solvent for the product examples of such solvents are acetone methanol ethanol isopropanol anti hiv achiral anionic oligomers can be used to prevent syncytium formation in cells infected with hiv i virus.