the polymerization aid for use in the process of the present invention is selected from among sodium salts of aliphatic carboxylic acids represented by the general formula rcoona ( r is an aliphatic hydrocarbon group having 1 - 20 carbon atoms ). several examples of these sodium salts of aliphatic carboxylic acids include : sodium acetate , sodium propionate , sodium isobutyrate , sodium n - lactate , sodium n - valerate , sodium iso - valerate , sodium hexanoate , sodium heptanoate , sodium octanoate , sodium n - nonanoate , sodium 2 - methyloctnoate , sodium n - decanoate , sodium undecylate , sodium dodecanoate , sodium octadecanoate , sodium nonadecanoate , sodium heneicosanoate , and mixtures of these salts . the sodium salts of aliphatic carboxylic acids for use in the present invention may be either anhydrous or hydrous . these salts are added in amounts that generally range from 0 . 05 to 4 moles , preferably from 0 . 1 to 2 moles , per mole of p - dihalobenzene . if the addition of the sodium salt of aliphatic carboxylic acid is less than 0 . 05 moles per mole of p - dihalobenzene , the salt is not sufficiently effective to increase the molecular weight of the end polymer . if the addition of the sodium salt of aliphatic carboxylic acid is more than 4 moles per mole of p - dihalobenzene , troubles will occur such as difficulty in agitating the contents of the reactor . it is essential for the purpose of the present invention that the sodium salt of aliphatic carboxylic acid be added to the reaction system before the alkali metal sulfide is dehydrated . in the method of the present invention , water is added to the reaction system before dehydration of the alkali metal sulfide and the water to be added should be free water which is clearly distinguished from the water of crystallization in alkali metal sulfides or salts of aliphatic carboxylic acids . it is important for the purpose of the present invention that such &# 34 ; free water &# 34 ; be added in an amount of 5 - 20 moles per mole of the alkali metal sulfide to the reaction system before dehydration . if such water is added in an amount of less than 5 moles per mole of the alkali metal sulfide , it is insufficient to enhance of ability of the sodium salt of an aliphatic carboxylic acid to increase the molecular weight of the end polymer . if more than 20 moles of free water is added per mole of the alkali metal sulfide , more energy is required to achieve dehydration of the alkali metal sulfide , which is disadvantageous from an economic viewpoint . in order to enhance the ability of the sodium salt of an aliphatic carboxylic acid to increase the molecular weight of the end polymer , the alkali metal sulfide , polar aprotic solvent , sodium salt of an aliphatic caboxylic acid and the above - specified amount of water must be present simultaneously in the reaction system prior to dehydration of the alkali metal sulfide . the object of the present invention is achieved only when the alkali metal sulfide is dehydrated after these four components are incorporated in the reaction system . exemplary alkali metal sulfides for use in the present invention include lithium sulfide , sodium sulfide , potassium sulfide , rubidium sulfide , cesium sulfide and mixtures thereof . these sulfides or mixtures thereof may be used in the form of a hydrate . these alkali metal sulfides are prepared by reacting alkali metal hydrosulfides with alkali metal bases , or hydrogen sulfide with alkali metal bases . they may be prepared either in situ or outside of the reaction system before they are added to the system for polymerization of p - dihalobenzene . among the alkali metal sulfides listed above , sodium sulfide is particularly preferred for use in the present invention . before adding p - dihalobenzene for polymerization , water is preferably removed from the reaction system by distillation or some other suitable method so that its content will be no more than about 4 moles per mole of the alkali metal sulfide . it is also possible to adjust the amount of water in the reaction system during the process of polymerization . illustrative p - dihalobenzenes that can be used in the present invention include p - dichlorobenzene , p - dibromobenzene , p - diiodobenzene and mixtures thereof , with p - dichlorobenzene being particularly advantageous . other monomers may be copolymerized with p - dihalobenzene as long as their content is less than 30 mol % of p - dihalobenzene , and examples of such copolymerizable monomers include m - dihalobenzenes such as m - dichlorobenzene , o - dihalobenzenes such as o - dichlorobenzene , and dihalo aromatic compounds such as dichloronaphthalene , dibromonaphthalene , dichlorodiphenylsulfone , dichlorobenzophenone , dichlorodiphenylether , dichlorodiphenyl sulfide , dichlorodiphenyl , dibromodiphenyl and dichlorodiphenyl sulfoxide . if desired , polyhalo aromatic compounds containing at least 3 halogens in one molecule may also be employed in small enough amounts not to impair the linearity of the end polymer , and illustrative polyhalo aromatic compounds include trichlorobenzene , tribromobenzene , triiodobenzene , tetrachlorobenzene , trichloronaphthalene and tetrachloronaphthalene . polar solvents are preferably used as solvents for polymerization in the process of the present invention , and particularly preferred solvents are those which are aprotic and which are stable against alkalies at elevated temperatures . exemplary solvents for polymerization include n , n - dimethylacetamide , n , n - dimethylformamide , hexamethyl phosphorylamide , n - methyl - ε - caprolactam , n - ethyl - 2 - pyrrolidone , n - methyl - 2 - pyrrolidone , 1 , 3 - dimethylimidazolidinone , dimethyl sulfoxide , sulfolane , tetramethylurea , and mixtures thereof . polymerization of p - dihalobenzene is performed at 200 °- 300 ° c ., preferably at 220 °- 280 ° c ., for a period of 0 . 5 - 30 hours , preferably 1 - 15 hours , under agitation . the molar ratio of the alkali metal sulfide to p - dihalobenzene which are to be used in the process of the present invention is preferably in the range of from 1 : 0 . 9 to 1 : 1 . 1 . polar aprotic solvents may be used in such amounts that 3 - 60 wt %, preferably 7 - 40 wt %, of the polymer will be present in the mixture after polymerization . the resulting poly ( p - phenylenesulfide ) may be recovered from the reaction mixture by any ordinary method , such as a method consisting of removing the solvent by distillation , flashing or some other suitable means , washing the polymer with water , and recovering it , or a method consisting of removing the solvent by filtering the reaction mixture , washing the polymer with water and recovering it . the second method is preferred since it imparts a minimum degree of thermal history to the polymer , thereby preventing coloration or gelation of the polymer . the poly ( p - phenylenesulfide ) which is the end product of the process of the present invention must contain at least 70 mol % of ## str1 ## as the constituent units . it may also contain copolymerizable units if their content is less than 30 mol % of the polymer , and examples of such copolymerizable units include : m - phenylene sulfide unit ## str2 ## o - phenylene sulfide unit ## str3 ## phenylene sulfide sulfone unit ## str4 ## phenylene sulfide ketone unit ## str5 ## phenylene sulfide ether unit ## str6 ## diphenylene sulfide unit ## str7 ## etc . the poly ( p - phenylenesulfide ) thus produced by the process of the present invention has its molecular weight increased in a linear form , so it is suitable for use as a material that is to be extrusion - molded into shaped articles such as fibers , films and tubes . if necessary , various additives may be incorporated in the polymer and illustrative additives include : ceramic fibers such as glass fibers , carbon fibers and alumina fibers ; reinforcing fillers such as aramid fibers , totally aromatic polyester fibers , metal fibers and potassium titanate whiskers ; inorganic fillers such as calcium carbonate , mica , talc , silica , barium sulfate , calcium sulfate , kaolin , clay , pyroferrite , bentonite , sericite , zeolite , nepheline syenite , attapulgite , wollastonite , ferrite , calcium silicate , magnesium carbonate , dolomite , antimony trioxide , zinc oxide , titanium oxide , magnesium oxide , iron oxide , molybdenum disulfide , graphite , gypsum , glass beads , glass powder , glass baloons , quartz , and silica glass ; and organic or inorganic pigments . other additives that may be incorporated as required include mold release agents such as aromatic hydroxy derivatives , silane or titanate based coupling agents , lubricants , heat stabilizers , weather - proofing agents , nucleating agents , foaming agents , corrosion inhibitors , ion trapping agents , flame retardants and flame retarding aids . if necessary , homopolymers , random copolymers , block copolymers and graft copolymers , either on their own or as admixtures , may be mixed with the poly ( p - phenylenesulfide ), and they include : polyethylene ; polybutadiene ; polyisoprene ; polychloroprene ; polystyrene ; polybutene ; poly - α - methylstyrene ; polyvinyl acetate ; polyvinyl chloride ; polyacrylate esters ; polymethacrylate esters ; polyacrylo - nitrile ; polyamides such as nylon 6 , nylon 66 , nylon 610 , nylon 12 , nylon 11 , and nylon 46 ; polyesters such as polyethylene terephthalate , polybutylene terephthalate and polyarylate ; polyurethane ; polyacetal ; polycarbonate ; polyphenylene oxide ; polysulfone ; polyether sulfone ; polyaryl sulfone ; polyether ketone ; polyether ether ketone ; polyimide ; polyamideimide ; silicone resins ; phenoxy resins ; and fluorine resins . the following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting . in the examples and comparative examples shown below , the melt viscosities of the samples of poly ( p - phenylenesulfide ) prepared were measured with a kohka - type flow tester ( die , 0 . 5 mm . sup . φ × 2 mm l ) at 300 ° c . under a load of 10 kg . a 500 - ml autoclave was charged with 0 . 5 moles of sodium sulfide na 2 s · 2 . 9 h 2 o , 125 ml of n - methyl - 2 - pyrrolidone ( hereinafter abbreviated as nmp ), 0 . 15 moles of sodium acetate and 8 . 3 moles of distilled water . the temperature in the autoclave was raised to 200 ° c . with stirring under a nitrogen stream so as to distil off 165 . 4 g of water accompanied with 14 . 8 g of nmp . after cooling the system to 170 ° c ., 0 . 49 moles of p - dichlorobenzene ( hereinafter abbreviated as p - dcb ) was added together with 42 ml of nmp and the system was closed under a nitrogen stream , followed by polymerization at 245 ° c . for 5 hours . after completion of the polymerization , the system was cooled and the contents were thrown into water . following repeated cycles of washing with about 5 l warm water and filtration , the residual cake was washed once with methanol and vacuum - dried with heating overnight to obtain small white granules of poly ( p - phenylenesulfide ). the polymer yield was 95 % and its melt viscosity was 215 pa . s ( see table 1 ). polymerization was performed as in example 1 except that the molar ratio of na 2 s to p - dcb charged , the amount of water added ( molar ratio of added water to na 2 s ), the type of the sodium salt of aliphatic carboxylic acid , the amount of its addition ( molar ratio of rcoona to na 2 s ), the polymerization temperature and time were changed as shown in table 1 . the results are also shown in table 1 . polymerization was performed as in example 1 except that water was not added and that polymerization consisted of two stages , the first stage being continued for 2 hours at 230 ° c . and the second stage continued for 2 hours at 265 ° c . the resulting polymer was in the form of small granules ; its yield was 95 % and it had a melt viscosity of 75 pa . s . this result shows that sodium acetate used in the absence of water was not highly effective in increasing the molecular weight of the end polymer ( see table 1 ). polymerization was performed as in example 1 except that water was added in an amount of 3 moles per mole of sodium sulfide and that the molar ratio of na 2 s to p - dcb charged was adjusted to 1 . 00 . the resulting polymer was in the form of small granules ; its yield was 92 % and it had a melt viscosity of 116 pa . s . this result shows that the ability of sodium sulfide to increase the molecular weight of the end polymer was not satisfactorily high when the amount of water added was less than 5 moles per mole of sodium sulfide ( see table 1 ). polymerization was performed as in example 1 except that na 2 s . 2 . 9 h 2 o was replaced by na 2 s . 9h 2 o , the molar ratio of na 2 s to p - dcb was adjusted to 1 . 00 , no water was added and that sodium acetate was replaced by sodium acetate trihydrate . the resulting polymer was in the form of small granules ; its yield was 93 % and it had a melt viscosity of 102 pa . s . this result shows that the water of crystallization present in the polymerization system did not contribute much to the increase in the molecular weight of the end polymer ( see table 1 ). polymerization was performed as in example 1 except that a mixture of sodium sulfide , sodium acetate and water was heated at 200 ° c . to achieve partial dehydration and that thereafter , nmp was added to the system which was re - heated at 200 ° c . to achieve complete dehydration . the resulting polymer was in powder form ; its yield was 93 % and it had a melt viscosity of 41 pa . s , which was much lower than the values attained in examples 1 - 8 ( see table 1 ). polymerization was performed as in example 1 except that a mixture of two moles of water per mole of sodium sulfide to be used and sodium acetate was dehydrated by heating at 200 ° c . in nmp , and that thereafter , sodium sulfide and three moles of water per mole of sodium sulfide were added to the mixture , which was re - heated at 200 ° c . to achieve complete dehydration . the resulting polymer was in the form of small granules ; its yield was 92 % and it had a melt viscosity of 94 pa . s , which was much lower than the values attained in examples 1 - 8 ( see table 1 ). polymerization was performed as in example 1 except that sodium acetate was added together with p - dcb and nmp after completion of dehydration . the resulting polymer was in the form of small granules ; its yield was 92 % and it had a melt viscosity of 83 pa . s , which was much lower than the values attained in examples 1 - 8 ( see table 1 ). as comparative examples 1 - 6 show , the sodium salt of an aliphatic carboxylic acid is not highly effective in increasing the molecular weight of the end polymer if it is not present together with nmp , sodium sulfide , sodium salt of an aliphatic carboxylic acid and a specified amount of water in the reaction mixture to be dehydrated . the advantages of the present invention can be achieved only when the mixture containing the above - specified components is dehydrated . table 1__________________________________________________________________________ h . sub . 2 o / na . sub . 2 s polymer - na . sub . 2 s / added molar rcoona / ization polymer - p - dcb water ratio na . sub . 2 s tempera - ization melt ( molar ( molar after ( molar ture time yield viscosity na . sub . 2 s ratio ) ratio ) dehydration rcoona ratio ) (° c .) ( hr ) (%) ( pa . s ) __________________________________________________________________________ex . 1 na . sub . 2 s . 2 . 9h . sub . 2 o 1 . 02 16 . 6 1 . 1 sodium 0 . 30 245 5 95 215 acetate ( anhydrous ) 2 &# 34 ; 1 . 00 &# 34 ; &# 34 ; sodium &# 34 ; &# 34 ; &# 34 ; 92 304 acetate ( anhydrous ) 3 &# 34 ; &# 34 ; &# 34 ; 1 . 2 sodium &# 34 ; &# 34 ; 10 94 240 acetate ( anhydrous ) 4 &# 34 ; &# 34 ; &# 34 ; 1 . 3 sodium &# 34 ; 250 &# 34 ; &# 34 ; 216 acetate ( anhydrous ) 5 &# 34 ; &# 34 ; 10 . 0 1 . 0 sodium &# 34 ; 245 8 93 200 acetate ( anhydrous ) 6 &# 34 ; &# 34 ; 5 . 0 1 . 2 sodium &# 34 ; &# 34 ; &# 34 ; 92 183 acetate ( anhydrous ) 7 &# 34 ; &# 34 ; 16 . 6 1 . 3 sodium 0 . 50 &# 34 ; 10 93 247 acetate ( anhydrous ) 8 &# 34 ; &# 34 ; &# 34 ; 1 . 1 sodium 0 . 30 &# 34 ; &# 34 ; 94 172 propionate ( anhydrous ) comp . ex . 1 &# 34 ; 1 . 02 0 1 . 2 sodium &# 34 ; 230 2 95 75 acetate 265 2 ( anhydrous ) 2 &# 34 ; 1 . 00 3 . 0 &# 34 ; sodium &# 34 ; 245 5 92 116 acetate ( anhydrous ) 3 na . sub . 2 s . 9h . sub . 2 o &# 34 ; 0 1 . 3 sodium &# 34 ; &# 34 ; &# 34 ; 93 102 acetate ( 3h . sub . 2 o ) 4 *. sup . 1 na . sub . 2 s . 2 . 9h . sub . 2 o 1 . 02 16 . 6 0 . 5 sodium &# 34 ; &# 34 ; &# 34 ; &# 34 ; 41 acetate ( anhydrous ) 5 &# 34 ; &# 34 ; 5 . 0 *. sup . 2 1 . 3 sodium &# 34 ; &# 34 ; 8 92 94 acetate ( anhydrous ) 6 *. sup . 3 &# 34 ; &# 34 ; 16 . 6 1 . 2 sodium &# 34 ; &# 34 ; 5 &# 34 ; 83 acetate ( anhydrous ) __________________________________________________________________________ *. sup . 1 a mixture of na . sub . 2 s , sodium acetate and h . sub . 2 o was dehydrated by heating , followed by addition of nmp and reheating for dehydration . *. sup . 2 a mixture of sodium acetate and 2 moles of h . sub . 2 o per mole of na . sub . 2 s to be added was dehydrated by heating in nmp , and thereafter , na . sub . 2 s and 3 moles of h . sub . 2 o per mole of na . sub . 2 s were added to the mixture , which was reheated for comple dehydration . *. sup . 3 after dehydration of na . sub . 2 s , sodium acetate was added togethe with pdcb and nmp . as will be understood from the foregoing description , the process of the present invention allows a poly ( p - phenylenesulfide ) of increased molecular weight to be produced even if a sodium salt of an aliphatic carboxylic acid is used as a polymerization aid . the resulting poly ( p - phenylenesulfide ) is suitable for use not only in injection - molding applications but also in producing films , fibers and other shaped articles by extrusion molding .