the starting materials are ppe solutions which are produced after the ppe polycondensation is interrupted . these solutions no longer contain catalyst . typically , they are comprised of a ppe , i . e ., a polymer of a phenol wherein the phenol is at least ortho substituted . typically employed phenol monomers are described , for example , in german os no . 32 24 692 , os no . 32 24 691 , and os no . 33 13 864 . the subject of german os no . 33 32 377 is a method of purifying ppes . suitable solvents for the ppe reactions are also described in the german patents referred to in the preceding paragraph . toluene and toluene - methanol mixtures ( with methanol content ≦ 30 vol . %) have proven particularly advantageous . the first flash evaporation stage ( fig2 ) is comprised of an evaporator 2 to which a dilute ppe solution is continuously fed . the temperature in this first stage should be at most 20 ° c ., preferably at most 10 ° c ., greater than the boiling point of the solvent or the solvent mixture , in order not to subject the polymer to unnecessary thermal stress . when toluenemethanol mixtures are used , the set point temperature will be , e . g ., about 115 ° c . the contents of the evaporator are constantly recycled via a loop including the heat exchange w1 and the flash valve v1 . the temperature is kept down to about 10 ° c . above the solvent boiling point by adjusting the recycle quantity . the concentration of ppe is continuously monitored , e . g ., via the density , and is adjusted to the desired value which is between 20 and 35 wt . %. the solution emerging from the first stage is fed to the single - pass evaporator 6 via the heat exchanger w2 and the flash valve v2 . in the second stage the desired final concentration can be accurately adjusted by controlled heating the solution to temperatures less than 100 ° c . in excess of the boiling point of the solvent in exchanger w2 . other features of the invention will become apparent in the course of the following description of an exemplary embodiment which is given for illustration of the invention and is not intended to be limiting thereof . in the installation illustrated in fig1 the first flash container b2 , is fed with 24 . 3 kg / hr . of a 11 . 2 wt . % solution of ppe in toluene , and is concentrated at 112 ° c . to 25 . 6 wt . % by continuous recycle pumping via heat exchanger w1 . the resulting solution is passed through heat exchanger w2 and is then flashed in container b4 at the rate of 10 . 6 kg / hr . a 53 . 7 wt . % solution of ppe is drawn off into container b6 . the container b2 is fed with 24 . 0 kg of a 11 . 2 wt . % solution of ppe in toluene , and this solution is concentrated at 180 ° c . to 54 . 1 wt . % ppe and is then passed through heat exchanger w2 ( here used as a cooler ) and b4 , and into container b6 at 120 ° c . comparison example b : concentrating with two direct fiow through flash stages in series the concentrating of a 11 . 4 wt . % toluene solution of ppe led to a final concentration of 53 . 9 wt . %, while a 10 . 1 wt . % starting solution resulted in a final concentration of 47 . 6 wt . %. these data are evidence that concentrating in two successive single - pass flash evaporation stages leads to different final concentrations for different starting concentrations . it can be seen from a comparison of example 1 of the invention and comparison examples a and b that the product undergoes only minor thermal stressing in the inventive method . the method for the present invention also enables one to prescribe a specific ppe content in the concentrated solution . the process comparison given in table 1 shows that the time load on the polymer , i . e ., the time at elevated temperature , with single - stage concentrating is twice that with two - stage concentrating . table 1__________________________________________________________________________process comparison one - stage concentrating inventive two - stage process according to concentrating process german os 33 15 802__________________________________________________________________________feed rate to b2 , kg / hr : 24 . 3 24 . 0ppe concentration in 11 . 2 11 . 2the feed , wt . % temperature in b2 , ° c . : 112 180mean combined residence time 76 163of the ppe in b2 and w1 , min : combined residence time of 175 375longest - residing 10 % ofthe ppe in b2 and w1 , min : ppe concentration in b2 , wt . % 25 . 6 54 . 1feed rate to b4 , kg / hr : 10 . 6 5 . 0temperature downstream of w2 , 203 120 ° c . : residence time in w2 , min : 7 15temperature in b4 and b6 , 120 120 ° c . input rate to b6 , kg / hr : 5 . 0 5 . 0ppe concentration in b6 , wt . %: 53 . 7 54 . 1__________________________________________________________________________ obviously , numerous modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein .