in the present invention , as the n , n &# 39 ;- dialkylethylenediamine expressed by the formula ( 1 ), n , n &# 39 ;- dimethylethylenediamine , n , n &# 39 ;- diethylethylenediamine , n , n &# 39 ;- dipropylethylenediamine , n , n &# 39 ;- diisopropylethylenediamine , n , n &# 39 ;- dibutylethylenediamine , etc . are enumerated . when urea is reacted with these compounds , the corresponding 1 , 3 - dialkyl - 2 - imidazolidinones expressed by the formula ( 2 ) are obtained . in particular , the process of the present invention is the most suitable process for commercially producing dmi from n , n &# 39 ;- dimethylethylenediamine . the n , n &# 39 ;- dialkylethylenediamines of the formula ( 1 ) may be easily prepared from the corresponding lower monoamines according to various known methods . for example , n , n &# 39 ;- dimethylethylenediamine may be obtained by reacting ethylene dichloride with an excess quantity of methylamine in water solvent . the process of the present invention wherein these n , n &# 39 ;- dialkylethylenediamines and urea are used , is carried out in the presence of a polar solvent at 180 ° c . or higher , and in the initial period reaction of the present invention is a reaction for forming a 1 , 1 &# 39 ;- dialkyl - 1 , 1 &# 39 ;- dimethylenebisurea as an intermediate . thus , in the initial period reaction , in order to inhibit the reaction of urea itself and thereby quantitatively form the 1 , 1 &# 39 ;- dialkyl - 1 , 1 &# 39 ;- dimethylenebisurea as an intermediate , it is preferred to carry out the reaction at 180 ° c . or lower , preferably at 140 ° c . or lower . when the initial period reaction is carried out at such a temperature or lower , reaction proceeds quantitatively between the n , n &# 39 ;- dialkylethylenediamine and urea , and at that time the pressure in the reactor is gradually raised by nh 3 gas released with the advance of the reaction and becomes a constant pressure before long , whereby it is possible to confirm the end point of the initial period reaction . when the temperature is successively raised up to 180 ° c . or higher , preferably 200 ° c .- 300 ° c ., to subject the intermediate to thermal decomposition reaction , the corresponding 1 , 3 - dialkyl - 2 - imidazolidinone is obtained with a high yield . if the temperature is lower than 180 ° c ., the reaction rate is low , while if it exceeds 300 ° c ., a problem is raised with respect of heating method . as the solvent used in the process of the present invention , hydrocarbons and halogenated hydrocarbons are unsuitable and polar solvents are used . examples of preferred solvents are polar , non - protonic solvents such as n , n &# 39 ;- dimethylformamide , n , n &# 39 ;- dimethylacetamide , tetramethylurea , dimethylsulfoxide , hexamethylphosphoroamide , sulfolane , methyl isobutyl ketone , nitrobenzene , tetrahydrofuran , dioxane , etc . further , since low boiling temperatures require an excessively pressure - resistant apparatus , solvents having a boiling point of 180 ° c . or higher are preferred . in particular , with respect of the yield and in order to avoid the troublesome solvent separation , it is preferred to use as the solvent , the desired 1 , 3 - dialkyl - 2 - imidazolidinone itself , which has been produced by the reaction . in the process of the present invention , by using such polar solvents , it is possible to carry out a mild reaction at a relatively low temperature to obtain the desired objective product with a high yield . the ratio of the quantity of urea fed , to that of n , n &# 39 ;- dialkylethylenediamine may be usually in the range of 0 . 5 to 2 . 5 in mol ratio . however , if urea of more than 2 mols relative to n , n &# 39 ;- dialkylethylenediamine are fed and used , i . e . if the theoretical quantity of more is used , then when the latter period higher temperature reaction is carried out at 180 ° c . or higher , cyanuric acid is byproduced in a large quantity to thereby reduce the yield of the objective product and result in a troublesome operation of removing it . thus , in order to avoid cyanuric acid formation , it is necessary to use urea in a quantity less than its theoretical equivalent , that is , less than twice by mol , preferably in the range of 0 . 6 to 1 . 2 times by mol based on n , n &# 39 ;- dialkylethylenediamine at the time of their feed and cause n , n &# 39 ;- dialkylethylenediamine to remain in the latter period reaction system , but in such a case , if unreacted n , n &# 39 ;- alkylethylenediamine remains in excess , it is impossible to raise the temperature up to a desired one under the atmospheric pressure since the boiling point of n , n &# 39 ;- alkylethylenediamine is lower than that of the latter period decomposition reaction ; hence it is necessary to carry out the reaction under pressure . in order to avoid this matter , when the initial period reaction is carried out using urea in a quantity of about 2 mols in the vicinity of equivalent based on n , n &# 39 ;- dialkylethylenediamine when it is fed , and successively the temperature is raised and the latter period decomposition reaction is carried out at 180 ° c . or higher , then if n , n &# 39 ;- dialkylethylenediamine is reacted while it is added , it is also possible to carry out the reaction under the atmospheric pressure . at that time , the quantity of n , n &# 39 ;- dimethylethylenediamine added freshly is added so that the quantity may be nearly equimolar to that of 1 , 1 &# 39 ;- dimethyl1 , 1 &# 39 ;- dimethylenebisurea . namely it is preferred that the ratio of the quantities of n , n &# 39 ;- dimethylethylenediamine and urea used in the whole reaction be chosen so as to be nearly equimolar , and n , n &# 39 ;- dimethylethylenediamine be used in nearly two divided portions in the initial period reaction and the decomposition reaction step . a concrete preferred embodiment wherein the process of the present invention is carried out under the atmospheric pressure is as follows : into a reactor equipped with a reflux condenser , a thermometer , a dropping funnel and a mechanical stirrer are added n , n &# 39 ;- dialkylethylenediamine , urea in an about equivalent quantity thereto , and as solvent a portion of the objective product itself obtained by distilling the reaction fluid under reduced pressure , each in definite quantities . the temperature is raised and reaction is carried out at 140 ° c . or lower . as the reaction proceeds , nh 3 gas is generated , and the end point of the reaction can be judged by termination of the gas generation . the temperature is successively raised to 180 ° c . or higher . when the temperature reached a temperature in the vicinity of 180 ° c ., reaction is carried out while a definite quantity of n , n &# 39 ;- dialkylethylenediamine is freshly dropwise added in small portions . after completion of the reaction , it is possible to take out the objective product from the reaction fluid only by direct distillation . when the objective product itself is used as the solvent for the reaction , it is unnecessary to separate it from the solvent at the time of distillation to afford a very simplified process , and after completion of the reaction , it is possible to use the distillate at the time of distillation , as it is , without purification , as the solvent for the next time reaction . the present invention will be described in more detail by way of examples . into a 500 ml stainless autoclave were fed n , n &# 39 ;- dimethylethylenediamine ( 88 . 1 g , 1 . 0 mol ), urea ( 60 . 1 g , 1 . 0 mol ) and dmi ( 100 . 0 g ). the reaction temperature was raised up to 210 ° c . over about 30 minutes and reaction was carried out at the temperature for 3 hours . the pressure inside the system reached 14 . 5 kg / cm 2 g as the highest pressure . after completion of the reaction , the reaction fluid was distilled and after about 2 / 3 of the quantity of dmi was distilled off , crystals deposited in the still residue were filtered off , followed by successively distilling the filtrate to obtain a dmi fraction ( 192 . 7 g ) having a purity of 99 . 5 % according to gas chromatography ( yield : 80 . 8 %). the still residue consisted of cyanuric acid partly containing dmi . in addition , using various kinds of solvents in place of dmi , reaction and treatment were carried out in all the same manner as in example 1 , to obtain results shown in the following table : table______________________________________use of various kinds of solventsno . solvent dmi yield (%) ______________________________________2 toluene 5 . 03 1 , 2 - dichloroethane 25 . 44 ethylene glycol 55 . 55 isopropyl alcohol 70 . 06 methyl isobutyl ketone 83 . 07 n -- methyl - 2 - pyrrolidone 80 . 58 n , n &# 39 ;-- dimethylacetamide 82 . 2______________________________________ into a 500 ml stainless autoclave were fed n , n &# 39 ;- diethylethylenediamine ( 116 . 2 g , 1 . 0 mol ), urea ( 60 . 1 g , 1 . 0 mol ) and 1 , 3 - diethyl - 2 - imidazolidinone ( 100 . 0 g ). the temperature was raised up to a reaction temperature of 210 ° c . in about 30 minutes and reaction was carried out at the temperature for 3 hours . the pressure in the system reached 14 kg / cm 2 g as the highest pressure . after completion of the reaction , the reaction fluid was distilled , and after about 2 / 3 of the quantity of 1 , 3 - diethyl - 2 - imidazolidinone was distilled off ( boiling point : 120 °- 123 ° c ./ 20 - 23 mmhg ), crystals deposited in the still residue was filtered , followed by successively distilling the filtrate to obtain a fraction of 1 , 3 - diethyl - 2 - imidazolidinone ( 214 . 8 g ) having a purity of 99 . 5 % according to gas chromatography ( yield : 80 . 0 %). the still residue after the distillation consisted of cyanuricacid partly containing 1 , 3 - diethyl - 2 - imidazolidinone . reaction and treatment were carried out in the same manner as in example 2 except that n , n &# 39 ;- diethylethylenediamine was replaced by n , n &# 39 ;- dipropylethylenediamine ( 144 . 3 g , 1 . 0 mol ) and reaction was carried out in 1 , 3 - dipropyl - 2 - imidazolidinone ( 100 . 0 g ), to obtain 1 , 3 - dipropyl - 2 - imidazolidinone having a purity of 99 . 7 % according to gas chromatography ( boiling point : 146 °- 148 ° c ./ 20 - 24 mmhg , yield : 81 . 5 %). into a 500 ml stainless autoclave were fed n , n &# 39 ;- dimethylethylenediamine ( 88 . 1 g , 1 . 0 mol ), urea ( 60 . 1 g , 1 . 0 mol ) and dmi ( 100 g ). the temperature was raised and reaction was carried out at a reaction temperature of 120 ° c . for 8 hours . after start of the reaction , the pressure in the system rose gradually and became constant at 5 . 5 kg / cm 2 g ; thus the temperature was raised up to 210 ° c . in about 30 minutes , and reaction was carried out at the temperature for 3 hours . the pressure in the system reached 14 . 5 kg / cm 2 g at the highest temperature . after completion of the reaction , the reaction fluid was distilled under reduced pressure to obtain a dmi fraction having a purity of 99 . 5 % according to gas chromatography ( 208 . 6 g , yield : 94 . 7 %). the still residue after the distillation consisted of cyanuric acid partly containing dmi . into a 500 ml stainless autoclave were fed n , n &# 39 ;- diethylethylenediamine ( 116 . 2 g , 1 . 0 mol ), urea ( 60 . 1 g , 1 . 0 mol ) and 1 , 3 - diethyl - 2 - imidazolidinone ( 100 . 0 g ). the temperature was raised . reaction was carried out at a reaction temperature of 120 ° c . for 8 hours . after start of the reaction , the pressure in the system rose gradually and became almost constant in the vicinity of 5 . 5 kg / cm 2 g ; thus the temperature was successively raised up to 220 ° c . in about 30 minutes and reaction was carried out at the temperature for 3 hours . the pressure in the system reached 14 . 0 kg / cm 2 g as the highest pressure . after completion of the reaction , the reaction fluid was distilled under reduced pressure to obtain a 1 , 3 - diethyl - 2 - imidazolidinone fraction having a purity of 99 . 5 % according to gas chromatography ( 237 . 0 g , yield : 95 . 5 %). the still residue after the distillation consisted of cyanuric acid partly containing 1 , 3 - diethyl - 2 - imidazolidinone . into a 300 ml glass flask equipped with a reflux condenser , a thermometer , a dropping funnel and a stirrer were fed n , n &# 39 ;- dimethylethylenediamine ( 44 . 1 g , 0 . 5 mol ), urea ( 60 . 1 g , 1 . 0 mol ) and dmi ( 100 . 0 g ) as solvent . into the dropping funnel was put n , n &# 39 ;- dimethylethylenediamine ( 44 . 1 g ). the temperature was raised up to 120 ° c . at which reaction was carried out . as the reaction proceeds , nh 3 gas was generated and crystals separated during the reaction . after about 12 hours , nh 3 gas generation ceased . at that time , the temperature was further raised up to 210 ° c . above a temperature in the vicinity of 200 ° c ., n , n &# 39 ;- dimethylethylenediamine was dropwise added through the dropping funnel over about 2 hours . reaction was then carried out at the same temperature for one hour . after completion of the reaction , the reaction fluid was distilled under reduced pressure to obtain a dmi fraction having a purity of 99 . 5 % according to gas chromatography ( 211 . 3 g , yield : 97 . 0 %).