the olefinically unsaturated compound having two conjugated carbon - carbon double bonds may be an unsubstituted or a substituted alkadiene or cycloalkadiene , preferably having 4 to 30 and in particular 4 to 20 carbon atoms per molecule . the alkadiene or cycloalkadiene may be substituted with , for instance , one or more halogen atoms or cyano , ester , alkoxy , carboxy or aryl groups . if the substituents are not inert under the reaction conditions , the carbonylation reaction may be accompanied with other reactions . examples of suitable olefinically unsaturated compounds are 1 , 3 - alkadienes , for example , 1 , 3 - tetradecadiene and 1 , 3 - hexadecadiene . other examples are 1 , 3 - butadienylcyclohexane , 5 - chloro - 1 , 3 - pentadiene , 1 , 3 - hexadiene , 6 - chloro - 1 , 3 - hexadiene , 3 , 5 - pentadienylbenzene and 4 -( 2 , 4 - butadienyl ) toluene . very good results have been obtained with 1 , 3 - butadiene , which is usually quantitatively converted with high selectivity to carboxylic di - esters . the selectivity to a certain compound expressed in a percentage is defined as ## equ1 ## in which &# 34 ; a &# 34 ; is the amount of olefinically unsaturated compound that has been converted into that certain compound and &# 34 ; b &# 34 ; is the total amount of olefinically unsaturated compound that has been converted . 1 , 3 - butadiene is partly converted into dimethyl adipate , dimethyl 2 - methylglutarate and dimethyl 2 - ethylsuccinate . it is an advantage of the process according to the present invention that the carboxylic di - esters are formed in one step . dimethyl adipate can easily be converted into adipic acid , which is a starting material in the manufacture of fibers or engineering plastics . dimethyl 2 - methylgutarate and dimethyl 2 - ethylsuccinate can be used for the preparation of surface active ester mixtures ( german auslegeschrift no . 2 , 517 , 354 ) and pharmaceutical products ( u . s . pat . no . 4 , 105 , 789 and no . 4 , 154 , 937 ). dimethyl 2 - methylglutarate may be used for the preparation of flavor imparting agents ( u . s . pat . no . 4 , 168 , 280 ) and curing agents for epoxy resins ( european patent application no . 88 , 047 ). the balance of the 1 , 3 - butadiene is converted into methyl 3 - pentenoate . this mono - ester can be isolated from the reaction mixture and subjected to a second carbonylation step for conversion into carboxylic di - esters . modifying the process according to the invention by replacing one , two or three aryl groups in the phosphine of the general formula i with an alkyl or a cycloalkyl group or by using one or more aryl groups carrying an electron - donating substituent , such as a p - alkoxy group ( para with respect to the c - p bond ) or a dialkylamino group results in a low conversion of the starting olefinically unsaturated compound and a low selectivity to carboxylic di - esters , if any . each of the aryl groups r 1 , r 2 and r 3 preferably contains not more than 18 , in particular 6 - 14 , carbon atoms . examples of suitable r 1 , r 2 and r 3 groups are the naphthyl group and , in particular , the phenyl group . the presence of an electron - withdrawing substituent on the aryl groups r 1 , r 2 and r 3 usually allows a higher selectivity to carboxylic di - esters . examples of electron - withdrawing substituents are chlorine and fluorine atoms and nitro , trihalomethyl , m - alkoxy and m - aryloxy groups ( meta with respect to the c - p bond ) and &# 34 ; halo &# 34 ; representing iodo , bromo , chloro or fluoro . good results have been obtained with trifluoro groups . other examples are monohalomethyl and dihalomethyl groups . according to a preferred embodiment of the present invention , a considerably enhanced selectivity to carboxylic di - esters in which the two carbonyl groups are located in a 1 , 4 - position , with the same conversion of the starting olefinically unsaturated compound , is obtained when an organic phosphine of the general formula ## str2 ## is also combined with the catalytic system . in formula ii r 4 , r 5 , r 6 and r 7 each individually representing an aryl group , optionally substituted with one or more electron - withdrawing substituents , and q representing an alkylene group having 1 to 6 carbon atoms in a straight chain between the two phosphorus atoms . in this manner , 1 , 3 - butadiene can be converted with enhanced selectivity into dimethyl adipate . this effect is very surprising , because the presence of an organic phosphine of the general formula ii as the sole organic phosphine results in very little , if any carbonylation . preferably , r 4 , r 5 , r 6 and r 7 are equal and , in particular , represent phenyl groups . the alkylene group q preferably has 2 to 6 carbon atoms in the said straight chain and , more preferably , consists of 2 to 6 methylene groups . very good results have been obtained with 1 , 4 - di ( diphenylphosphino ) butane . it was found that when the present process is modified by using less than 5 gram atom of trivalent phosphorus ( if a phosphine of formula ii is also present , calculated on the sum of the phosphines of formula i and ii ) per gram atom of palladium , the conversion of the starting olefinically unsaturated compound is low with a high selectivity to carboxylic mono - esters . suitably , not more than 150 and usually not more than 50 gram atom of trivalent phosphorus are used per gram atom of divalent palladium . ( a ) hydrogen fluoride did not lead to any conversion of the starting olefinically unsaturated compound at all , ( b ) hydrogen iodide or p - toluenesulphonic acid gave an extremely low conversion of the starting olefinically unsaturated compound , ( c ) hydrogen bromide gave a low conversion of the starting compound . modifying the present process by using less than one mol of the hydrogen chloride per atom of trivalent phosphorus present in the catalytic system gives reduced formation of carboxylic di - esters . preferably , not more than 10 mol of hydrogen chloride are used per gram atom of trivalent phosphorus present in the catalytic system , but this ratio may be higher than 10 , for example up to 50 , if desired . the alcohols used in the process according to the invention may be aliphatic , cycloaliphatic or aromatic and may be substituted with one or more substituents , such as mentioned hereinbefore in connection with the olefinically unsaturated compounds to be used as starting material . the alcohol may therefore also be a phenol . the alcohols preferably contain not more than 20 carbon atoms per molecule . examples of suitable alcohols are methanol , ethanol , propanol , 2 - propanol , butanol , isobutanol , tert - butyl alcohol , 2 - ethylhexanol , n - decanol , stearyl alcohol , benzyl alcohol , cyclohexanol , allyl alcohol , chlorocapryl alcohol , ethylene glycol , 1 , 2 - propanediol , 1 , 4 - butanediol , glycerol , polyethylene glycol , 1 , 6 - hexanediol , phenol , the cresols and the xylenols . special preference is given to alkanols having 1 to 10 carbon atoms per molecule . very good results have been obtained with methanol . if the alcohol has more than one hydroxy group per molecule , different carboxylic di - esters may be formed , depending on the molar ratio existing between the reagents . for instance , depending on the quantity of olefinically unsaturated compound used , either a glycerol mono - ester or a glycerol di - ester may be produced from glycerol . the presence of water in the reaction mixture allows formation of dicarboxylic acids . mixtures of an alcohol and water may be used , in which case a mixture of caboxylic di - esters and dicarboxylic acids is formed . suitable divalent palladium compounds are the salts of palladium with , for instance , hydrogen chloride , nitric acid , sulfuric acid or alkanoic acids having not more than 12 carbon atoms per molecule . a catalyst used by preference is palladium acetate . palladium complexes may be used , for example , palladium acetylacetonate or bis - triphenylphosphinepalladium sulfate . the quantity of palladium catalyst is not critical . preference is given to the use of quantities in the range between 10 - 5 and 10 - 1 gram atom palladium per mol of olefinically unsaturated compound . in the process according to the invention , the carbon monoxide may be used pure or diluted with an inert gas , such as nitrogen , noble gases or carbon dioxide . generally , the presence of more than 10 % by volume of hydrogen is undesirable , since under the reaction conditions it may cause hydrogenation of the olefinic compound . generally , preference is given to the use of carbon monoxide or a carbon monoxide - containing gas which contains less than 5 % by volume of hydrogen . the present process is preferably carried out at a temperature in the range between 50 ° c . and 200 ° c . the total pressure preferably lies between 1 and 100 , in particular 20 and 75 bar . the molar ratio of the olefinically unsaturated compound to the alcohol is not critical . the molar ratio between hydroxy groups and olefinic double bonds may lie , for instance , between 0 . 1 : 1 and 10 : 1 ; preference is given to the use of the minimum amount of one equivalent for each carbon - carbon double bond . the process according to the present invention is carried out in the presence of an aprotic solvent . it has been found that replacing the aprotic solvent with a protic solvent , such as , for example , methanol , gives very little conversion of the starting olefinically unsaturated compound . good results have been obtained with ethers , in particular with anisole , 2 , 5 , 8 - trioxanonane ( also referred to as &# 34 ; diglyme &# 34 ;) and diphenyl ether . another example of a suitable ether is diisopropyl ether . good results have also been obtained with aromatic hydrocarbons , in particular with toluene ; other examples are benzene and the xylenes . further examples of suitable aprotic solvents are sulfoxides , such as , for example , dimethyl sulfoxide , and sulfones such as , for example , diisopropyl sulfone and tetrahydrothiophene 1 , 1 - dioxide ( also referred to as &# 34 ; sulfolane &# 34 ;). a reaction product of the carbonylation reaction may also be used as a solvent . the process according to the invention may be carried out batchwise , continuously or semi - continuously . the invention is further illustrated by means of the following examples which are not to be construed as limiting the invention . examples 1 - 12 and comparative experiments a - k were carried out in the following manner . a 250 - ml magnetically stirred hastelloy c autoclave (&# 34 ; hastelloy &# 34 ; is a trade mark ) was charged with methanol ( 10 ml ) and a solvent ( 40 ml ). 1 , 3 - butadiene was conducted through the liquid until the volume was increased by 10 ml . subsequently , palladium acetate ( 1 mmol ), a phosphine and an acid were added . the table hereinafter states which solvent , phosphine and acid were used and the quantities of the phosphine and the acid . the autoclave was flushed with carbon monoxide , and pressurized with carbon monoxide till a partial pressure of the value given in the table was reached , sealed and heated to the temperature stated in the table . after a reaction time of 5 hours , the contents of the autoclave were analyzed by means of gas - liquid chromatography . the conversion of 1 , 3 - butadiene and the selectivities to methyl 3 - pentenoate and to methyl esters of alkanoic acids having 6 carbon atoms per molecule are presented in the table . this table also presents the mol % of dimethyl adipate in the methyl esters of alkanoic acids having 6 carbon atoms per molecule . comparative experiments a , b , c and e show that the presence of p - toluenesulfonic acid , hydrogen iodide and hydrogen fluoride does not lead to dicarbonylation . comparative experiment d shows that the presence of hydrogen bromide leads to very little dicarbonylation . comparison of example 1 where a molar ratio phosphine to palladium of 6 was used with comparative experiment f where this ratio was 3 , shows more dicarbonylation in example 1 with a higher fraction of dimethyl adipate . comparison of example 2 where a molar ratio hydrogen chloride to phosphine of 22 / 3 was used with comparative experiment g where this ratio was 2 / 3 shows more dicarbonylation in example 2 . examples 2 , 3 and 4 show that diglyme , anisole and diphenyl ether respectively , are good solvents . comparative experiment h shows no reaction in methanol as a solvent . comparative experiment i shows no dicarbonylation in the presence of diethylphenylphosphine . comparative example j shows no dicarbonylation in the presence of a phosphine having an electron - donating substituent on the phenyl groups . comparison of examples 6 and 3 shows slightly more dicarbonylation and selectively to dimethyl adipate in the presence of anisole and a chlorine substituent on the phenyl group . comparison of examples 7 and 4 shows more dicarbonylation in the presence of diphenyl ether and a chlorine substituent on the phenyl group . comparison of examples 5 and 8 shows that the presence of a fluorine substituent on the phenyl group gives more dicarbonylation than the presence of a chlorine substituent . comparison of examples 9 and 3 shows that the presence of a trifluoromethyl group gives more dicarbonylation with less formation of dimethyl adipate . comparison of examples 10 and 11 shows that the presence of 1 , 4 - di ( diphenylphosphino ) butane in combination with tri ( m - methoxyphenyl ) phosphine results in a considerably enhanced selectivity to dimethyl adipate . example 12 shows that also very good results are obtained by combining the former phosphine with tri ( m - chlorophenyl ) phosphine . comparative experiment k shows that no reaction takes place in the presence of 1 , 4 - di ( diphenylphosphino ) butane as the sole phosphine . example 1 was modified by replacing the 1 , 3 - butadiene with 1 , 3 - cyclohexadiene ( 10 ml ) and by using a temperature of 135 ° c . instead of 150 ° c . the conversion of 1 , 3 - cyclohexadiene was 95 % and the selectivities to cyclohexenemonocarboxylic acids and cyclohexanedicarboxylic acids were 26 % and 74 %, respectively . table__________________________________________________________________________ex - tempera - partial conversion selectivity , am - comp . catalytic system ture , pressure butadiene , %, tople exp . phosphine mmol acid mmol solvent ° c . co , bar % a . sup . 3 b . sup . 4 c . sup . 5__________________________________________________________________________1 triphenyl - 6 hcl 8 toluene 150 50 100 18 82 25 phosphine a triphenyl - 6 p - toluene 8 &# 34 ; 150 50 trace -- -- -- phosphine sulphonic acid b triphenyl - 6 hcl 4 &# 34 ; 150 50 30 100 0 -- phosphine p - toluene 4 sulphonic acid c triphenyl - 6 hi 8 diglyme 150 50 5 100 0 -- phosphine d triphenyl - 6 hbr 8 toluene 155 50 40 90 10 not de - phosphine termined e triphenyl - 6 hf 8 &# 34 ; 150 50 0 -- -- -- phosphine2 triphenyl - 6 hcl 16 diglyme 135 50 100 33 67 30 phosphine f triphenyl - 3 hcl 4 toluene 150 50 60 48 52 20 phosphine g triphenyl - 6 hcl 4 diglyme 155 50 80 75 25 20 phosphine3 triphenyl - 6 hcl 8 anisole 155 50 100 22 78 28 phosphine h triphenyl - 6 hcl 8 methanol 150 50 trace -- -- -- phosphine4 triphenyl - 6 hcl 8 diphenyl 155 . sup . 50 . sup . 1 90 25 75 36 phosphine ether i diethyl - 6 hcl 8 toluene 150 50 20 100 0 -- phenylphosphine j tri ( p - methoxy - 6 hcl 8 diglyme 155 50 60 100 0 -- phenyl ) phosphine5 tri ( p - chloro - 6 hcl 8 &# 34 ; 150 50 100 40 60 32 phenyl ) phosphine6 tri ( p - chloro - 6 hcl 8 anisole 155 50 100 20 80 31 phenyl ) phosphine7 tri ( p - chloro - 6 hcl 8 diphenyl 155 . sup . 50 . sup . 2 100 20 80 35 phenyl ( phosphine ether8 tri ( p - fluoro - 6 hcl 8 diglyme 150 50 100 32 68 32 phenyl ) phosphine9 tri ( m - tri - 6 hcl 8 anisole 155 50 100 17 83 22 fluoromethyl - phenyl ) phosphine10 tri ( m - methoxy - 6 hcl 8 diphenyl 155 . sup . 50 . sup . 2 100 25 75 42 phenyl ) phosphine ether11 1 , 4 - di ( di - 2 hcl 16 diphenyl 155 . sup . 60 . sup . 2 100 25 75 53 phenylphosphino )- ether butane tri ( m - methoxy - 8 phenyl ) phosphine12 1 , 4 - di ( di - 5 hcl 16 diphenyl 155 . sup . 60 . sup . 1 100 30 70 57 phenylphosphino )- ether butane tri ( m - chloro - 2 phenyl ) phosphine k 1 , 4 - di ( di - 4 hcl 10 diphenyl 155 50 trace -- -- -- phenylphosphino )- ether butane__________________________________________________________________________ . sup . 1 after 2 . 5 h of reaction the total pressure was released to 40 bar . sup . 2 after 2 . 0 h of reaction the total pressure was released to 40 bar . sup . 3 a = methyl 3pentenoate . sup . 4 b = methyl esters of c . sub . 6 -- alkanedioic acids . sup . 5 c = fraction of dimethyl adipate , mol %, in methyl esters of c . sub . 6 -- alkanedioic acids