Patent Publication Number: US-11649400-B2

Title: Polymer dispersion by controlled radical polymerization

Description:
The present application is a national stage of PCT Application No. PCT/CN2018/109879 filed on Oct. 11, 2018. 
     FIELD OF INVENTION 
     The present invention relates to a process of preparing a polymer dispersion by controlled radical polymerization and the polymer dispersion thereof. The present invention also relates to use of the polymer dispersion, notably as friction reducer for oil and gas field fracturing operation in subterranean treatments. 
     BACKGROUND ART 
     During the past decades, the use of slickwater fracturing has been widely utilized in well stimulation operation, which led to the burgeoning exploitation of unconventional shale oil and gas and thus bringing about significant impacts on global energy landscape and consumption. In slickwater fracturing, fracturing fluid is usually comprised of a friction reducer as main ingredient together with certain functional additives. Friction reducers (FRs) used in slickwater fracturing are typically based on high molecular weight linear polymers such as guar, polyacrylamide and their derivatives, often in powder, slurry and inverse emulsion forms. In contrast to powder FRs which requires additional pre-dissolution process with special equipment, emulsion based FRs is almost exclusively used in North American market mainly due to its fast hydration, operational simplicity and efficiency to use at extremely low dosage. 
     To manufacture inversion emulsion FRs, generally aqueous phase containing acrylamide and co-monomers is emulsified and dispersed by a hydrocarbon solvent (e.g. paraffin oil) as continuous phase in the aid of low HLB emulsifier. After triggered by free radical polymerization, a water-in-oil system is formed, consisting of polymer particles trapped in aqueous phase stabilized by oil phase. U.S. Pat. Nos. 6,686,417 and 8,640,774 disclose the synthesis of copolymer of acrylamide with various anionic and cationic monomers made from aforementioned technology and its usage in friction reducing treatment. While inversion emulsion FRs were successfully employed in oil field industry, the oil phase in the final product still poses environmental concern due to the potential safety hazards associated with the hydrocarbon solvent. A greener approach, such as purely aqueous based dispersion, to produce such liquid based FRs is preferred and desired from long-term standpoint. 
     U.S. Pat. No. 7,271,134 discloses an aqueous polymer dispersion comprising an anionic salt, a water-based acrylamide-acrylic acid copolymer, a polyol, an anionic surfactant, and a nonionic surfactant to provide friction reduction without forming a gel into a subterranean formation. 
     U.S. Pat. No. 5,837,775 discloses the preparation of nonionic and anionically charged acrylamide-acrylic acid copolymer via conventional radical polymerization in aqueous media. The process is a two-step synthesis, including firstly polymeric dispersant synthesis followed by emulsion polymerization by using the above synthesized polymeric stabilizer from first step to control the particle formation in aqueous phase. The polymeric stabilizer served as a molecular barrier, separate from the intended acrylamide-acrylic acid copolymer, to stabilize intended copolymer in the aqueous phase. The final polymer products from above processes produce a mixture of two families of the polymers, the polymeric stabilizer and the acrylamide-acrylic acid copolymer. Such obtained polymer is useful as flocculant, drainage aids in paper manufacturing, etc. 
     U.S. Pat. No. 6,787,506 discloses the usage of one or more dispersion polymers to reduce the friction resulting from turbulent flow in an aqueous fracturing fluid in an oil field fracturing process. The polymer is composed of one or more nonionic monomers and one or more cationic or anionic monomers, respectively, typically are acrylamide/acrylic acid copolymer, acrylamide/dimethylaminoethyl acrylate methyl chloride quaternary salt/dimethylaminoethyl acrylate benzyl chloride quaternary salt terpolymer and so on. In both U.S. Pat. Nos. 5,837,775 and 6,787,506, the polymer dispersant made from first step was produced through traditional free radical polymerization, hence it virtually stabilized the intended polymer particle generated from second step through physical adsorption and interactions, such as van der waals force and electrostatic interaction. The final polymer product is the physical mixture of two families of the polymer, the polymeric stabilizer and the acrylamide copolymer. This may cause issues to stability of the mixture and problems when they are used in oil and gas applications. 
     There is a need to provide a polymer dispersion which has good stability and is easy to prepare. There is a need to provide a polymer dispersion which can provide excellent friction reducing capacity. 
     SUMMARY OF INVENTION 
     In one aspect, the present invention relates to a process for preparing a copolymer via controlled living polymerization. The process typically comprises two steps. In a first step (A), an ionic charged polymeric dispersant, either anionic, cationic or amphoteric, is prepared from at least an ionic (either anionic, cationic or amphoteric) monomer in the presence of a radical polymerization control agent. The resulting ionic polymeric dispersant is a living polymer which may be copolymerized with additional monomers when added. In a second step (B), said living polymer is charged into an aqueous solution comprising a nonionic monomer, and optionally an ionic monomer, the mixture is then subject to continuous polymerization. The resulting copolymer of this second step is a copolymer which comprises a block of the ionic charged polymeric dispersant at one terminal of the copolymer and a block of the monomers that are added in the second step. The copolymer obtained is a single polymer since the living polymer obtained in the first step reacts and forms covalent bond with subsequently loaded monomers and becomes part of the polymeric chain in the copolymer obtained at the second step. 
     In another aspect, the present invention provides a polymer dispersion comprising:
         a) 2 to 30 weight percentage of a water soluble copolymer;   b) 10 to 50 weight percentage of a water soluble salt;   c) 0 to 5 weight percentage of a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer is obtained by the above mentioned process.
       

     The present invention also provides a polymer dispersion comprising:
         a) 2 to 30 weight percentage of a water soluble copolymer;   b) 10 to 50 weight percentage of a water soluble salt;   c) 0 to 5 weight percentage of a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer comprises (i) a block of an ionic charged polymeric dispersant at one terminal; and (ii) a block of a nonionic monomer, optionally containing an ionic monomer.
       

     In some embodiments, said ionic charged polymeric dispersant is an anionic charged polymeric dispersant prepared from at least an anionic monomer according to general formula (II): 
                         
wherein:
 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4+ , or NR 4+  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl. 
     In some embodiments, said nonionic monomer is according to general formula (III): 
                         
wherein:
 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 3  and R 4 , independently, is hydrogen or methyl. 
     In some embodiments, said copolymer comprises: 
     (i) a block of an anionic charged polymeric dispersant prepared from at least an anionic monomer according to general formula (II): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4   + , or NR 4   +  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl; and 
     (ii) a block of a nonionic monomer, optionally containing an ionic monomer; wherein said nonionic monomer is according to general formula (III): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 4  and R 5 , independently, is hydrogen or methyl. 
     The polymer dispersion described herein can be used for various applications, in particular as a friction reducer in oil and gas processes. Accordingly, in another aspect of the present invention, there is provided a method for reducing friction resulting from turbulent flow in an aqueous fracturing fluid in an oil field fracturing process comprising adding to the aqueous fracturing fluid an effective friction-reducing amount of the polymer dispersion described herein. It has been found that the inventive polymer dispersion provides excellent friction reducing capacity and good stability. 
     The present invention also provides use of the polymer dispersion described herein as a friction reducer in an oil field fracturing process. 
    
    
     DETAILED DESCRIPTION 
     Throughout the description, including the claims, the term “comprising one” or “comprising a” should be understood as being synonymous with the term “comprising at least one”, unless otherwise specified, and “between” should be understood as being inclusive of the limits. 
     It should be noted that in specifying any range of concentration, weight ratio or amount, any particular upper concentration, weight ratio or amount can be associated with any particular lower concentration, weight ratio or amount. 
     As used herein, the term “alkyl” means a saturated hydrocarbon radical, which may be straight, branched or cyclic, such as, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, t-butyl, pentyl, n-hexyl, cyclohexyl. 
     As used herein, the term “hydroxyalkyl” means an alkyl radical, which is substituted with a hydroxyl groups, such as hydroxymethyl, hydroxyethyl, hydroxypropyl, and hydroxydecyl. 
     As used herein, the terminology “(C n -C m )” in reference to an organic group, wherein n and m are each integers, indicates that the group may contain from n carbon atoms to m carbon atoms per group. 
     As used herein, the term “ionic monomer” means a monomer possessing an anionic or cationic charge, or a monomer possessing an amphoteric charge. Typical anionic monomers include 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof, (meth)acrylic acid sodium salt and vinyl phosphate. Typical cationic monomers include 2-acryloxyethyl trimethyl ammonium chloride, methacryloxyethyl trimethyl ammonium chloride, and diallyldimethylammonium chloride. Typical amphoteric monomers include betaine group-containing (meth) acrylic ester and betaine group-containing (meth) acrylamide. 
     As used herein, the term “nonionic monomer” means a monomer without charge, typically such as acrylamide and N,N-dimethylacrylamide. 
     As used herein, the term “polymeric dispersant” means a hydrophilic polymer soluble and dissolvable in water, which may typically function as a dispersant for following polymerization reactions. The term “polymer dispersion” means a dispersion in which discrete polymer particles containing one or more nonionic and/or ionic monomers are dispersed in aqueous phase. 
     As used herein, the term “radical polymerization control agent” means a compound that is capable of extending the lifetime of the growing polymer chains in a polymerization reaction and of giving the polymerization a living or controlled nature. This control agent is typically a reversible transfer agent as used in controlled radical polymerizations known under the terminology RAFT or MADIX, which typically use a process of reversible addition-fragmentation transfer, such as those described, for example, in U.S. Pat. No. 9,580,535, PCT international publication nos. WO96/30421, WO 98/01478, WO 99135178, and WO 98/58974. 
     In one aspect, the present invention relates to a process for preparing a copolymer via controlled living polymerization. The process typically comprises two steps. 
     First Step 
     In a first step (A), an ionic charged polymeric dispersant is prepared from at least an ionic monomer in the presence of a radical polymerization control agent. The resulting ionic charged polymeric dispersant is a living polymer which may be copolymerized with additional monomers when added. 
     The ionic monomer may be anionic, cationic or amphoteric. A nonionic monomer may also be added on top of said ionic monomer, in such case, the resulting ionic charged polymeric dispersant comprises the ionic monomer and the nonionic monomer. It is also appreciated that the ionic monomer used may be a single monomer or a mixture of two or more monomers. 
     Preferably, the ionic monomer is an anionic monomer and the ionic charged polymeric dispersant is an anionic charged polymeric dispersant. 
     Water, one or more inorganic salts, any polymerization additives such as chelants, pH buffers or chain transfer agents may be added. In addition, further processing, structure modifying and/or stabilizing agents may be added to the mixture. 
     Radical Polymerization Control Agent 
     The radical polymerization control agent used in the first step or, where appropriate, in the process of this invention is advantageously a compound bearing a thiocarbonylthio group —S(C═S)—. According to some embodiments, the radical polymerization control agent may bear several thiocarbonylthio groups. It may optionally be a polymer chain bearing such a group. Thus, this radical polymerization control agent may, for example, correspond to general formula (I) below: 
     
       
         
         
             
             
         
       
     
     in which: Z represents:
         a hydrogen atom,   a chlorine atom,   an optionally substituted alkyl or optionally substituted aryl radical,   an optionally substituted heterocycle,   an optionally substituted alkylthio radical,   an optionally substituted arylthio radical,   an optionally substituted alkoxy radical,   an optionally substituted aryloxy radical,   an optionally substituted amino radical,   an optionally substituted hydrazine radical,   an optionally substituted alkoxycarbonyl radical,   an optionally substituted aryloxycarbonyl radical,   an optionally substituted carboxyl or acyloxyl radical,   an optionally substituted aryloxy radical,   an optionally substituted carbamoyl radical,   a cyano radical,   a dialkyl- or diaryl-phosphonato radical,   a dialkyl-phosphinato or diaryl-phosphinato radical, or   a polymer chain,       

     and R 1  represents:
         an optionally substituted alkyl, acyl, aryl, aralkyl, alkene or alkyne group,   a saturated or unsaturated, aromatic, optionally substituted carbocycle or heterocycle, or   a polymer chain, which is preferably hydrophilic or water-dispersible when the agent is used in the first step.       

     The groups R 1  or Z, when they are substituted, may be substituted with optionally substituted phenyl groups, optionally substituted aromatic groups, saturated or unsaturated carbocycles, saturated or unsaturated heterocycles, or groups chosen from the following: alkoxycarbonyl or aryloxycarbonyl (—COOR—), carboxyl (—COOH), acyloxy (—O 2 OR′), carbamoyl (—CONR′), cyano (—CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (—OH), amino (—NR′), halogen, allyl, epoxy, alkoxy (—OR′), S-alkyl, S-aryl, groups of hydrophilic or ionic nature such as alkali metal salts of carboxylic acids, alkali metal salts of sulfonic acid, polyalkylene oxide chains (PEO, PPO), cationic substituents (quaternary ammonium salts), R′ representing an alkyl or aryl group, or a polymer chain. 
     For the radical polymerization control agent of general formula (I), it is generally preferred for the group R 1  to be of hydrophilic nature. Advantageously, it is a water-soluble or water-dispersible polymer chain. 
     The group R 1  may alternatively be amphiphilic, i.e. it may have both hydrophilic and lipophilic nature. It is preferable for R 1  not to be hydrophobic. 
     As regards the radical polymerization control agent of general formula (I), R 1  may typically be a substituted or unsubstituted, preferably substituted, alkyl group. It may nevertheless comprise other types of group R 1 , especially a ring or a polymer chain. 
     The optionally substituted alkyl, acyl, aryl, aralkyl or alkyne groups generally contain 1 to 20 carbon atoms, preferably 1 to 12 and more preferentially 1 to 9 carbon atoms. They may be linear or branched. They may also be substituted with oxygen atoms, especially in the form of esters or sulfur or nitrogen atoms. 
     Among the alkyl radicals, mention may be made especially of methyl, ethyl, propyl, butyl, pentyl, isopropyl, tert-butyl, pentyl, hexyl, octyl, decyl or dodecyl radicals. 
     The alkyne groups are radicals generally containing from 2 to 10 carbon atoms, and contain at least one acetylenic unsaturation, such as the acetylenyl radical. 
     The acyl group is a radical generally containing from 1 to 20 carbon atoms with a carbonyl group. 
     Among the aryl radicals, mention may be made especially of the phenyl radical, optionally substituted especially with a nitro or hydroxyl function. 
     Among the aralkyl radicals, mention may be made especially of the benzyl or phenethyl radical, optionally substituted especially with a nitro or hydroxyl function. 
     When R 1  or Z is a polymer chain, this polymer chain may be derived from a radical or ionic polymerization or derived from a polycondensation. 
     Preferably, the radical polymerization control agent is a compound which comprises a thiocarbonylthio group —S(C═S)—. Thus, for example, it may be a compound which comprises a xanthate group (bearing —S(C═S)—O— functions), for example a xanthate. 
     Preferably, the radical polymerization control agent is a compound chosen from xanthates, trithiocarbonates, dithiocarbamates and dithiocarbazates. Especially, xanthates bearing an O-ethyl xanthate function —S(C═S)—OCH 2 CH 3 , such as O-ethyl-S-(1-methoxycarbonylethyl) xanthate (CH 3 CH(CO 2 CH 3 ))S(C═S)—OEt. Another suitable radical polymerization control agent is dibenzyltrithiocarbonate of formula PhCH 2 S(C═S)SCH 2 Ph, in which Ph refers to phenyl. 
     Anionic Monomer 
     Examples of the anionic monomer suitable for the present invention include and not limited to: ethylenically unsaturated carboxylic acids, sulfonic acids and phosphonic acids, and/or derivatives thereof such as acrylic acid (AA), methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, fumaric acid, vinylsulfonic acid, (meth)allylsulfonic acid, sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid, 2-hydroxy-3-methacryl oyloxypropylsulfonic acid, styrenesulfonic acids, vinylphosphonic acid, α-methylvinyl phosphonic acid, allylphosphonic acid, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) or a salt thereof. 
     According to every one of the invention embodiments, the anionic monomer is preferably according to general formula (II): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4+ , or NR 4   +  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl. 
     In a preferred embodiment, 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof, or its combination with vinylsulfonic acid and a salt thereof, styrene sulfonic acid and a salt thereof, is used. 
     Cationic Monomer 
     Examples of the cationic monomers suitable for the present invention include and not limited to: dialkylaminoalkyl acrylates and methacrylates and their quaternary or acid salts, including, but not limited to, dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl acrylate methyl sulfate quaternary salt, dimethyaminoethyl acrylate benzyl chloride quaternary salt, dimethylaminoethyl acrylate sulfuric acid salt, dimethylaminoethyl acrylate hydrochloric acid salt, diethylaminoethyl acrylate, methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl sulfate quaternary salt, dimethylaminoethyl methacrylate benzyl chloride quaternary salt, dimethylaminoethyl methacrylate sulfuric acid salt, dialkylaminoalkylacrylamides or methacrylamides and their quaternary or acid salts such as acrylamidopropyltrimethylammonium chloride, dimethylaminopropyl acrylamide methyl sulfate quaternary salt, dimethylaminopropyl acrylamide sulfuric acid salt, dimethylaminopropyl methacrylamide methyl sulfate quaternary salt, methacrylamidopropyltrimethylammonium chloride, dimethylaminopropyl methacrylamide sulfuric acid salt, diethylaminoethylacrylate, diethylaminoethylmethacrylate and diallyldialkylammonium halides such as diallyldiethylammonium chloride and diallyldimethyl ammonium chloride. In a preferred embodiment, 2-acryloxyethyl trimethyl ammonium chloride, methacryloxyethyl trimethyl ammonium chloride, dimethyl diallyl ammonium chloride, acrylamidopropyl trimethyl ammonium chloride, methacrylamidopropyl trimethyl ammonium chloride was used solely or in combinations thereof. 
     Amphoteric Monomer 
     Examples of the amphoteric monomer suitable for the invention include and not limited to: betaine type, for instance, betaine group-containing (meth) acrylic ester, betaine group-containing (meth) acrylamide; sulfobetaine type, for instance sulfopropyl dimethylammonium propyl acrylamide, sulfopropyl dimethylammonium propyl methacrylamide (SPP), sulfohydroxypropyldimethylammoniumpropylmethacrylamido (SHPP), 2-vinyl(3-sulfopropyl)pyridinium betaine, 4-vinyl(3-sulfopropyl)pyridinium betaine, 1-vinyl-3-(3-sulfopropyl) imidazolium betaine, or sulfopropylmethyldiallylammoniumbetaine. 
     Nonionic Monomer 
     Examples of the nonionic monomer suitable for the invention include and not limited to: any water soluble monomer having neither anionic or cationc charge such as acrylamide, methacrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N,N-dimethyl(meth) acrylamide, N,N-diethylacrylamide, N-vinylformamide, N-vinyl-N-methyl formamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinyipropionamide, N-vinyl-N-methylpropionamide and N-vinylbutyramide, but not limited to other kinds of nonionic monomers, for example N-vinylformamide, N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinylpyrrolidone, N-vinylpiperidone; N-vinyipropionamide, N-vinyl-N-methylpropionamide and N-vinylbutyramide. 
     According to every one of the invention embodiments, the nonionic monomer is preferably according to the general formula (III): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 3  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 4  and R 5 , independently, is hydrogen or methyl. 
     The living polymeric dispersant obtained in the first step is then subject to the second step of the process. 
     Second Step 
     In the second step (B), the living polymeric dispersant from the first step is charged into an aqueous solution comprising at least a nonionic monomer, optionally with an ionic monomer, followed by continuous polymerization to obtain a copolymer which comprises (i) a block of the ionic polymeric dispersant, notably that formed in the first step; and (ii) a block of nonionic monomer, notably formed by the monomers added in the second step. 
     Examples of the nonionic monomer suitable for the second step of the process include and not limited to: any water soluble monomer having neither anionic or cationc charge such as acrylamide, methacrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N,N-dimethyl(meth) acrylamide, N,N-diethylacrylamide, N-vinylformamide, N-vinyl-N-methyl formamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinyipropionamide, N-vinyl-N-methylpropionamide and N-vinylbutyramide, but not limited to other kinds of nonionic monomers, for example N-vinylformamide, N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinylpyrrolidone, N-vinylpiperidone; N-vinyipropionamide, N-vinyl-N-methylpropionamide and N-vinylbutyramide. 
     According to every one of the invention embodiments, the nonionic monomer used in the second step is preferably according to the general formula (Ill): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 3  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 4  and R 5 , independently, is hydrogen or methyl. 
     An ionic monomer may also be loaded in the second step, on top of the nonionic monomer. The ionic monomer may be selected from those cationic, anionic and amphoteric monomers described above. 
     The copolymer obtained in the second step is a single polymer since the living ionic charged polymeric dispersant reacts and forms covalent bond with subsequently loaded monomers, and becomes part of the copolymer chain. 
     The copolymer preferably has an average molecular weight of 750,000 to 10,000,000 Daltons, more preferably 1,000,000 to 5,000,000 Daltons, even more preferably from 2,500,000 to 5,000,000 Daltons. 
     Initiators for Radical Generation 
     An initiator for radical generation is also used in the process. Numerous free radical forming compounds may be used as the water-soluble initiator utilized as catalysts for the polymerization reactions. Typical compounds used as catalysts may be those that form free radicals via thermal decomposition, referred to in the art as “thermal initiators” or combinations of compounds that fond free radicals via oxidation/reduction reactions. Such catalysts are combinations of an oxidizing agent and a reducing agent and are commonly referred to in the art as “redox initiators”. Either thermal or redox catalysts may be used for the present invention. 
     Typical catalysts utilized as the thermal initiators include, for example, persulfates, specifically potassium persulfate, sodium persulfate, ammonium persulfate, 2,2′-Azobis(2-methylpropionamide)dihydrochloride and the like. Typical redox initiators include, for example, combinations of oxidizing agents or initiators such as peroxides, specifically benzoyl peroxide, t-butyl hydroperoxide, lauryl peroxide, hydrogen peroxide, 2,2′-diazobisisobutyronitrile, and the like. Typical reducing agents include sodium bisulfite, sodium formaldehyde sulfoxylate, sodium hydrosulfite, and ascorbic and isoascorbic acid. The water-soluble initiator may be employed in an amount of from 0.1 to 3 weight percent of the total monomer weight, and more preferably from about 0.1 to 1 weight percent of the total monomer charge. 
     The process of the invention may be conducted using the conditions for MADIX process as known to a skilled person, notably those according to U.S. Pat. No. 9,580,535. 
     Polymer Dispersion 
     In another aspect of the invention, there is provided a polymer dispersion comprising a copolymer obtained by the process described herein. The polymer dispersion further comprises a water soluble salt and optionally a polyol. 
     Advantageously, the present invention provides a polymer dispersion comprises:
         a) 2 to 30 weight percentage of a water soluble copolymer;   b) 10 to 50 weight percentage of a water soluble salt;   c) 0 to 5 weight percentage of a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer is obtained by the process described herein.
       

     The present invention also provides a polymer dispersion comprising:
         a) a water soluble copolymer;   b) a water soluble salt;   c) optionally a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer comprises (i) a block of an ionic charged polymeric dispersant at one terminal; and (ii) a block of a nonionic monomer, optionally containing an ionic monomer.
       

     Advantageously, the polymer dispersion comprises:
         a) 2 to 30 weight percentage of a water soluble copolymer;   b) 10 to 50 weight percentage of a water soluble salt;   c) 0 to 5 weight percentage of a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer comprises (i) a block of an ionic charged polymeric dispersant at one terminal; and (ii) a block of a nonionic monomer, optionally containing an ionic monomer.
       

     Said ionic charged polymeric dispersant may also comprises a nonionic monomer, where a nonionic monomer is added in the first step of the process described herein. 
     The copolymer preferably comprises from 10% to 80% of the block (i) and from 20% to 90% of the block (ii), more preferably from 10% to 40% of the block (i) and from 40% to 90% of the block (ii), even more preferably, from 15% to 25% of the block (i) and from 70% to 85% of the block (ii). 
     The component (a) may preferably be present in an amount of 5 to 30% more preferably from 15 to 25%. 
     The component (b) may preferably be present in an amount of 10 to 30% more preferably from 20 to 30%. 
     In some embodiments, the component (c) is present in an amount of 1 to 5%. In some embodiments, the component (c) is absent in the polymer dispersion. 
     In a preferred embodiment, said polymer dispersion comprises a water soluble copolymer which comprises (i) a block of an ionic charged polymeric dispersant at one terminal; and (ii) a block of a nonionic monomer, optionally containing an ionic monomer; wherein said ionic charged polymeric dispersant is an anionic charged polymeric dispersant prepared from at least an anionic monomer according to general formula (II): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4+ , or NR 4   +  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl; and/or said nonionic monomer is according to general formula (III): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 3  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 4  and R 5 , independently, is hydrogen or methyl. 
     Water soluble Salt 
     The water soluble salts suitable for the invention include and not limited to: ammonium sulfate, ammonium chloride, potassium chloride, calcium chloride, sodium bromide, calcium bromide, sodium sulfate, magnesium sulfate, aluminum sulfate, sodium nitrate, magnesium nitrate, potassium nitrate, and combinations thereof, and it is not limited to alkali metal and alkaline earth metal acetate, benzoate and formate. 
     Polyol 
     Polyols suitable for the invention include and not limited to: dialcohols, polyhydric alcohols, monosaccharides and disaccharides. Preferred polyols include propylene glycol, isoprene glycol, 1,3-butanediol, dipropylene glycol, glycerol, diglycerol, triglycerol, polyglycerol, trimethylolpropane, erythritol, pentaerythritol, sorbitan, sorbitol, glucose, maltitol, saccharose, trehalose, polyethylene glycol. 
     Preferred polyols may be selected from ethylene glycol, glycerol, propylene glycol, and pentaerythritol. 
     Other additives or components which are known to those skilled in the art may also use in accordance with the present invention. These include chain transfer agents, which are used to control molecular weight, additives to adjust pH, and compounds utilized as protective colloids which provide additional stability to the polymer particles. 
     The polymer dispersion is notably an aqueous emulsion, in particular a water-in-water emulsion, wherein the copolymer is dispersed in aqueous phase in the emulsion. 
     The polymer dispersion described herein is particular useful as a friction reducer. Accordingly, the present invention provides a method for reducing friction resulting from turbulent flow in an aqueous fracturing fluid in an oil field fracturing process comprising adding to the aqueous fracturing fluid an effective friction-reducing amount of the polymer dispersion described herein. 
     In particular, there is provided a method for reducing friction resulting from turbulent flow in an aqueous fracturing fluid in an oil field fracturing process comprising adding to the aqueous fracturing fluid an effective friction-reducing amount of a polymer dispersion, wherein said polymer dispersion comprises:
         a) 2 to 30 weight percentage of a water soluble copolymer;   b) 10 to 50 weight percentage of a water soluble salt;   c) 0 to 5 weight percentage of a polyol; and   d) water;
 
weight percentages are based on total weight of the polymer dispersion; wherein said copolymer comprises (i) a block of an ionic charged polymeric dispersant at one terminal; and (ii) a block of a nonionic monomer, optionally containing an ionic monomer.
       

     Preferably, said ionic charged polymeric dispersant is an anionic charged polymeric dispersant prepared from at least an anionic monomer according to general formula (II): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4   + , or NR 4   +  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl. 
     Preferably, said ionic charged polymeric dispersant is an anionic charged polymeric dispersant prepared from at least an anionic monomer according to general formula (II): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 2  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 2  is hydrogen or methyl; 
     M is hydrogen, an alkali metal ion such as Na + , K +  and Ca 2+ , or NH 4   + , or NR 4   +  wherein R is a C 1  to C 5  alkyl or hydroxyalkyl; 
     and/or said nonionic monomer is according to general formula (Ill): 
     
       
         
         
             
             
         
       
     
     wherein: 
     R 3  is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 3  is hydrogen or methyl; 
     R 4  and R 5 , independently, is a hydrogen or a C 1  to C 5  alkyl or hydroxyalkyl, preferably, R 4  and R 5 , independently, is hydrogen or methyl. 
     When used as friction reducer, the polymer dispersion described herein may be added to an aqueous fracturing fluid to reduce the friction resulting from turbulent flow in the aqueous fracturing fluid in an oil field fracturing process where the fracturing fluid is pumped at high velocity into a subterranean formation. Typical velocities for fracturing fluids, which generally are pumped through a 2-5 inch pipe, range from about 5 to about 80 or more feet per second and most commonly are about 10 feet per second. 
     The aqueous fracturing fluids may include fresh water, brines containing polyvalent cations, including sodium chloride, potassium chloride, ammonium chloride and calcium chloride, and acidic fracturing fluids. 
     Various additives may be added to the fracturing fluid to change the physical properties of the fluid or to serve a certain beneficial function. Generally a propping agent such as sand or other hard material is added which serves to keep the fractures open after the fracturing operation. Also, fluid loss agents may be added to partially seal off the more porous sections of the formation so that the fracturing occurs in the less porous strata. Viscosifying agents may also be added so that the propping agent may be dispersed within the fluid during injection and more easily carried. 
     Other oilfield additives that may also be added to the fracturing fluid include emulsion breakers, antifoams, scale inhibitors, H 2 S and O 2  scavengers, biocides, surface tension reducers, shale and clay stabilizers, paraffin/asphaltene inhibitors and corrosion inhibitors. 
     The composition of the fracturing fluid may be adjusted depending on the particular well or formation to be fractured. For example, in fracturing certain formations it may be desirable to use a high concentration of the propping agent, while in other formations, little or no propping agent may be used. 
     In general, the polymer dispersion is added to the fracturing fluid, preferably continuously, in an amount of from about 100 to about 2500 ppm, preferably from about from 200 to 2000 ppm and more preferably from about 500 to about 1000 ppm based on the aqueous fracturing fluid. 
     The polymer dispersion is preferably applied on site by a blender metering product into the fracturing fluid. The polymer dispersion may be added by simply pouring from the container into the fluid stream or very accurately by using a positive displacement pump tied to a feedback from the flowmeter on the blender. 
     EXAMPLE 1 
     Preparation of Ionic Polymeric Dispersant 
     Polymeric dispersant was prepared in accordance with the following procedure: 
     250.03 g of 2-acrylamide-2-methyl propane sulfonate sodium salt (50 wt %), 0.05 g of Rhodixan® A1 (radical polymerization control agent according to the invention, from Solvay), and 250.11 g of deionized water were charged into a 1-litre reactor equipped with mechanical stirrer, the mixture was agitated under 200 rpm for 30 mins to form a homogenous solution. The solution was sparged with 800 cc/min of nitrogen gas for 40 mins and then was heated to 35° followed by addition of 0.5 g of 5 wt % aqueous solution of sodium persulfate (SPS) and 1.3 g of 5 wt % aqueous solution of sodium formaldehyde sulfoxylate (NaFS). After every one hour, same amount of SPS and NaFS were added. After 3 hours, the formed viscous solution was heated to 45-50° C., and 0.5 g of 10 wt % solution of 2,2′-Azobis(2-methylpropionamide)dihydrochloride was added. After aging for 1 hour, the mixture was cooled down to room temperature and the polymeric dispersant solution was recovered. 
     EXAMPLE 2 
     Preparation of Polymer Dispersion 
     71.66 g of ammonium sulphate, 12.58 g of sodium sulphate, 0.05 g of EDTA and 213.2 g of deionized were charged into a 1-litre resin reactor and stirred for 15 mins. Subsequently, 95.12 g of acrylamide (50 wt %), 8.6 g of glycerol, 53.09 g of 2-acrylamide-2-methyl propane sulfonate sodium (50 wt %) and 44.6 g of the polymeric dispersant prepared in Example 1 were added and agitated to form a homogenous solution. The solution was sparged with 800 cc/min of nitrogen gas for 40 mins and then was heated to 40° C. followed by addition of 0.75 g of 10 wt % aqueous solution of 2,2′-Azobis(2-methylpropionamide)dihydrochloride. After every 30 mins, same amount of 2,2′-Azobis(2-methylpropionamide) dihydrochloride was added. During the polymerization process, the entire mixture became milky. After 3 hours, 3 g of 10 wt % solution of 2,2′-Azobis(2-methyl propionamide)dihydrochloride was added and aged for 1 hour. The final polymer dispersion was obtained and tested for friction reducing performance. 
     EXAMPLE 3 
     Preparation of Polymer Dispersion 
     71.65 g of ammonium sulphate, 12.61 g of sodium sulphate, 0.05 g of EDTA and 213.0 g of deionized were charged into a 1-litre resin reactor and stirred for 15 mins. Subsequently, 118.45 g of acrylamide (50 wt %), 4.3 g of glycerol, 29.59 g of 2-acrylamide-2-methyl propane sulfonate sodium (50 wt %) and 44.63 g of the polymeric dispersant prepared in Example 1 were added and agitated to form a homogenous solution. The solution was sparged with 800 cc/min of nitrogen gas for 40 mins and then was heated to 40° C. followed by addition of 0.75 g of 10 wt % aqueous solution of 2,2′-Azobis(2-methylpropionamide)dihydrochloride. After every 30 mins, same amount of 2,2′-Azobis(2-methylpropionamide) dihydrochloride was added. During the polymerization process, the entire mixture became milky. After 3 hours, 3 g of 10 wt % solution of 2,2′-Azobis(2-methylpropionamide)dihydrochloride was added and aged for 1 hour. The final polymer dispersion was obtained. 
     EXAMPLE 4 
     Preparation of Polymer Dispersion 
     286.72 g of ammonium sulphate, 50.45 g of sodium sulphate, 0.2 g of EDTA and 816.1 g of deionized were charged into a 2.5-litre resin reactor and stirred for 15 mins. Subsequently, 380.4 g of acrylamide (50 wt %), 31.98 g of glycerol, 212.41 g of 2-acrylamide-2-methyl propane sulfonate sodium (50 wt %) and 213.42 g of the polymeric dispersant prepared in Example 1 were added and agitated to form a homogenous solution. The solution was sparged with 800 cc/min of nitrogen gas for 40 mins and then was heated to 45-50° C. followed by addition of 3 g of 10 wt % aqueous solution of 2,2′-Azobis(2-methylpropionamide)dihydrochloride. After every 30 mins, same amount of 2,2′-Azobis(2-methylpropionamide) dihydrochloride was added. During the polymerization process, the entire mixture became milky. After 3 hours, 12 g of 10 wt % solution of 2,2′-Azobis(2-methyl propionamide) dihydrochloride was added and aged for 1 hour. The final polymer dispersion was obtained. 
     EXAMPLE 5 
     Friction Reduction Tests 
     Friction reduction efficiency of the polymer dispersion was determined by using flow loop tester designed to measure the pressure drop during the recirculation process. Prior to the test, opened the water in valve above the reservoir bucket and rinse out the bucket. Next, filled the reservoir bucket with water and started the pump to allow the water to circulate and rinse the flow loop for 5 mins. After 5 mins, turned off the pump and closed the water in valve. Wiped the inside of the reservoir. 
     For each test, filled the reservoir with 4 gallons of the desired test water. Started the pump. The pump was set to 24.77 hz. (5 gallons/min) Recorded the initial pressure (Po) when the pressure stabilizes. Inject the sample into the injection port and start the timer. Record the pressure at 10, 20, 30 seconds, 1, 2, and 3 minutes. After 3 minutes, stop the pump and open the waste valve, draining the reservoir. 
     Two types of test water were used for FR performance evaluation: fresh tap water and 10% of salt water. Temperature was 25° C. Friction reduction efficiency (FR %) is calculated based on the pressure drop after polymer dispersion was introduced vs. the initial water pressure within the pipeline. Typical 1 gpt (0.1% v/v) loading of the polymeric dispersion was used for friction reduction testing. Friction reduction effects of the different polymeric dispersions are shown in Tables 1 and 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 FR % of polymer dispersions in tap water 
               
            
           
           
               
               
               
               
            
               
                 Time 
                   
                   
                   
               
               
                 (seconds) 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 0 
                 0 
                 0 
                 0 
               
               
                 10 
                 68.0 
                 67.2 
                 66.6 
               
               
                 20 
                 68.0 
                 67.2 
                 66.8 
               
               
                 30 
                 67.5 
                 67.2 
                 66.4 
               
               
                 60 
                 67.1 
                 67.0 
                 65.3 
               
               
                 120 
                 66.7 
                 66.9 
                 64.1 
               
               
                 180 
                 67.9 
                 66.9 
                 66.5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 FR % of polymer dispersions  
               
               
                 in 10% salt water 
               
            
           
           
               
               
               
               
            
               
                 Time 
                   
                   
                   
               
               
                 (seconds) 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 0 
                 0 
                 0 
                 0 
               
               
                 10 
                 62.6 
                 61.4 
                 60.3 
               
               
                 20 
                 62.7 
                 61.6 
                 60.7 
               
               
                 30 
                 61.9 
                 61.2 
                 60.0 
               
               
                 60 
                 60.3 
                 60.1 
                 59.9 
               
               
                 120 
                 59.5 
                 58.8 
                 59.2 
               
               
                 180 
                 58.7 
                 58.5 
                 57.8 
               
               
                   
               
            
           
         
       
     
     As shown in Tables 1 and 2, the inventive polymer dispersions exhibited fast hydration rate to achieve the maximum FR % and remarkable salt-tolerance. The inventive polymer dispersions demonstrated good friction reducing capacity.