Abstract:
This invention provides an efficient method for synthesizing trithiocarbonates, RSC(S)SR′ that can be used, for example, as RAFT agents in the living polymerization of styrenics, acrylates, and other olefinic monomers.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention provides an efficient method for synthesizing trithiocarbonates, RSC(S)SR′. Such compounds can be used, for example, as RAFT agents in the living polymerization of styrenics, acrylates, and other olefinic monomers.  
       BACKGROUND  
       [0002]     RAFT (reversible addition fragmentation chain transfer) processes have been disclosed for the preparation of low-polydispersity polymers from styrenics, acrylates, methacrylates, and copolymers thereof, see e.g., WO 99/05099, WO 99/31144 and EP0910587.  
         [0003]     Trithiocarbonates have been identified as suitable RAFT agents, but commercially attractive methods for their preparation have been lacking. For example, methods that convert a thiol to the corresponding sodium salt frequently involve the use of sodium hydride, a potentially hazardous chemical when handled at large scales. Previous methods have used multistep reactions involving isolation and purification steps difficult to perform at larger scale. Previous methods have also reported low yields.  
         [0004]     A continuing need exists for an efficient and scalable process for RAFT agents.  
       SUMMARY  
       [0005]     One embodiment of this invention provides a process for preparing a trithiocarbonate having the formula RSC(S)SR′, wherein R is a substituted or unsubstituted C 1 -C 20  linear or branched alkyl group, a substituted or unsubstituted C 3 -C 6  cyclic alkyl group, a substituted or unsubstituted aryl group, or a heterocyclic group and wherein R′ is a substituted or unsubstituted C 1 -C 10  linear or branched alkyl group or a substituted or unsubstituted C 3 -C 6  cyclic alkyl group, the process comprising reacting a thiol having the formula RSH, sequentially with an aqueous base, carbon disulfide and an alkylating agent having the formula R′X, wherein X is chlorine, bromine or iodine, in a water/alcohol solvent medium, whereby a mixture comprising the product RSC(S)SR′ and a by-product salt of chlorine, bromine or iodine are formed in the solvent medium. 
     
    
     DETAILED DESCRIPTION  
       [0006]     The process of this invention provides a practical, single step, high yielding procedure for the preparation of a wide variety of useful trithiocarbonates having the formula RSC(S)SR′. In one embodiment of this invention, a thiol, RSH, is reacted sequentially with an aqueous base and carbon disulfide and an alkylating agent having the formula R′X in a solvent mixture comprising about 1-20 vol % water and about 80-99 vol % alcohol.  
         [0007]     Suitable alkyl thiols, RSH, are those in which R is a substituted or unsubstituted C 1 -C 20  linear or branched alkyl group, a substituted or unsubstituted C 3 -C 6  cyclic alkyl group, a substituted or unsubstituted aryl group, or a heterocyclic group. Suitable substituents include alkyl, halo, cyano, aryl, and alkoxy groups. Suitable R′ groups include substituted or unsubstituted C 1 -C 10  linear or branched alkyl groups and substituted or unsubstituted C 3 -C 6  cyclic alkyl groups. Suitable substituents include alkyl, halo, cyano, aryl, alkoxy, and carboxylate groups. X is chlorine, bromine or iodine.  
         [0008]     During the course of the reaction, the halide from R′X is converted to a halide salt as a by-product. In one embodiment, the trithiocarbonate product can then be isolated by filtering the reaction mixture to remove the halide by-product salt and recovering the product from the filtrate to provide pure, e.g. &gt;98% purity, crystals of the product.  
         [0009]     Solubility of the product, RSC(S)SR′, is influenced by the selection of R and R′, and by the temperature and composition of the aqueous alcohol medium. In some embodiments of this invention, it may be convenient to select R and R′ such that the product is soluble in the initial aqueous alcohol medium to allow separation from the by-product salt by filtration, but insoluble in an altered solvent medium. As illustrated in the Example, crystallization of the desired product from the aqueous alcohol can be induced by increasing the water content of the solvent and decreasing the temperature.  
         [0010]     The aqueous base can be any alkali metal hydroxide, but sodium hydroxide and potassium hydroxide are preferred.  
         [0011]     In one preferred embodiment of the invention, the aqueous base is sodium hydroxide, RSH is dodecanethiol and R′X is chloroacetonitrile as shown in the reaction below. The preferred alcohol for this reaction is isopropanol. The reaction provides the intermediate RSC(S)SNa which is then treated with chloroacetonitrile to form the trithiocarbonate RAFT agent, as shown:
 
C 12 H 25 SH+NaOH+CS 2 →[C 12 H 25 SCS 2 Na]+ClCH 2 CN→C 12 H 25 SC(S)SCH 2 CN+NaCl
 
 Following the reaction, the halide salt, NaCl, is removed by filtration. The product C 12 H 25 SC(S)SCH 2 CN is recovered at reduced temperatures from the filtrate medium on dilution with water as a crystalline material. One advantage of this embodiment is the easy removal of the halide salt by filtration and the crystallization of the product from the same medium after a modest increase in water content, giving high yields and highly pure product. In the event the trithiocarbonate product with a different combination of R and R′ is not crystalline; the product can be recovered by a phase separation upon water addition. 
 
         [0012]     The reaction can be performed at a temperature of from about −10° C. to about 40° C. Preferably the temperature is about −5° C. to about 15° C. Most preferably, the temperature is about 0° C. to about 5° C.  
         [0013]     In one embodiment, the aqueous alcohol is a water/alcohol mixture with a ratio of 1-20 vol % water and 80-99 vol % alcohol. Preferably the range is 3-10 vol % water and 90-97 vol % alcohol. More preferably, the ratio is 4-6 vol % water and 94-96 vol % alcohol.  
         [0014]     The alcohol is a water-soluble alcohol or a mixture of water-soluble alcohols. In one embodiment, the alcohol is isopropanol.  
         [0015]     Many thiols, RSH, and halo compounds, R′X, useful in this invention are commercially available. Others can be readily synthesized by techniques known in the art.  
       EXAMPLE  
       [0016]     This example demonstrates the preparation of S-cyanomethyl S-dodecyl trithiocarbonate RAFT agent: A 1000 mL 3-neck round bottom flask (fitted with mechanical stirrer, septum, thermocouple well, and reflux condenser with N 2  bubbler) was charged with sodium hydroxide (12.18 g, 304.5 mmol) and water (30 mL). Isopropanol (500 mL) was added to the solution of sodium hydroxide, and the reaction mixture was cooled to about 5° C. The cooled reaction mixture was treated dropwise with dodecanethiol (60.6 g, 300 mmol).  
         [0017]     The reaction mixture was stirred for 30 min at 5° C., cooled to 0° C., and treated with carbon disulfide (24.0 g, 315 mmol) by syringe over a ca. 10 min period to produce a yellow solution which was stirred at ca. 0°-5° C. for 0.5 h.  
         [0018]     The reaction mixture was treated with chloroacetonitrile (23.8 g, 315 mmol) dropwise by syringe over a ca. 20 min period while maintaining the temperature between 0 and 5° C. The reaction mixture was stirred at 0° C. for 2 h.  
         [0019]     The reaction mixture was warmed to ca. 30° C. and filtered to remove sodium chloride. The solid residue was washed with a 5 mL portion of isopropanol. The combined filtrate was treated with water (50 mL). Product crystals of S-cyanomethyl S-dodecyl trithiocarbonate were formed as the temperature decreased. After the bulk of crystals had formed at room temperature, the mixture was cooled to 0° C. for 1.5 h and the first crop of crystals was collected. There was obtained 82.3 g of bright yellow flakes after drying. The filtrate was treated with another 50 mL water and chilled to provide 8.5 g of the product in the second crop of crystals.  1 H NMR analyses showed both crops of ca. 98% purity (93% isolated yield).