Polymerization of chloroprene, emulsion

The only other diene that has been used extensively for commercial emulsion polymerization is chloroprene (2-chloro-1,3-butadiene) (Hofmann, 1989 Johnson, 1976 Stewart et al., 1985 Blackley, 1983 Musch and Magg, 1996). The chlorine substituent apparently imparts a marked reactivity to this monomer, since it polymerizes much more rapidly than butadiene, isoprene, or any other 

Various recipes (Morton et al., 1956 Morton and Piirma, 1956) can be used for emulsion polymerization of chloroprene, with potassium persulfate as a popular initiator. A basic recipe (Neal and Mayo, 1954) which illustrates several interesting features about this monomer is shown in Table 2.9. Two 

Sulfur copolymerizes (Neal and Mayo, 1954 Mochel and Peterson, 1949) with the chloroprene, forming di- and polysulfide linkages in the chain (as illustrated in Eq. (2.29)). The latex is 

Another feature of the emulsion polymerization of chloroprene that distinguishes it from that of the other dienes is the fact that it leads to a predominantly trans-lA chain microstructure. Thus, even at ambient polymerization temperature, the poly chloroprene contains over 90% trans-1,4 units, as shown in 

TABLE 2.10 Effect of Polymerization Temperature on Polychloroprene Chain Microstructure  

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Over 5.5 billion pounds of synthetic rubber is produced annually in the United States. The principle elastomer is the copolymer of butadiene (75%) and styrene (25) (SBR) produced at an annual rate of over 1 million tons by the emulsion polymerization of butadiene and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by the emulsion polymerization of chloroprene at an annual rate of over 125,000 t. Butyl rubber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene (10%) at an annual rate of about 150,000 t. Polybutadiene, polyisoprene, and EPDM are produced by the anionic polymerization of about 600,000, 100,000, and 350,000 t, respectively. Many other elastomers are also produced. 

A method for the controlled emulsion polymerization of chloroprene using dithiocarbamic esters as sulfur-based chain transfer agents is described. The method provides industrially relevant molar masses with Mn s> 50,000 daltons with good yields in acceptable times. It was further determined that when pKa values for the dithiocarbamic acid precursors were less than 12, the thioester was ineffective as a regulator. 

Radical-Initiated Homopolymerization. When this homopolymerization is carried out with benzoyl peroxides or other radical formers in a manner analogous to emulsion polymerization of chloroprene, highly crosslinked polymers are formed. They are insoluble in organic solvents such as toluene, benzene, or chloroform. Radical polymerization in toluene, benzene, or hexane leads only to insoluble products. 

Polychloroprene, developed and sold under the trade name Neoprene by DuPont, was the first commercially successful synthetic elastomer. It is produced by free-radical emulsion polymerization of chloroprene (2-chloro-l,3-butadiene). The commercial material is mainly /raw5-l,4-polychloroprene, which is crystallizable. 

Various recipes [79, 80] can be used for emulsion polymerization of chloroprene, with potassium persulfate as a popular initiator. A basic recipe [81] which illustrates several interesting features about this monomer is shown in Table IX. Two aspects of this recipe are especially noteworthy the use of a rosin soap, and the presence of elemental sulfur. Rosin soaps are notorious as retarders in emulsion polymerization, as are most polyunsaturated fatty acids. Yet complete conversion can be attained within a few hours. With saturated fatty acid soaps, the reaction is almost completed [79] within one hour at 40°C ... 

Cochet F, Claverie I.P., GraiUat C., Sauterey F., Guyot A., Emulsion polymerization of chloroprene in the presence of a maleic polymerizable surfactant Control of gel formation at low conversion. Macromolecules, 37(11), 2004, 4081-4086. 

Under similar conditions, the emulsion polymerization of chloroprene proceeds about 700 times faster than that of isoprene. The resulting heat of polymerization therefore must be dissipated as quickly as possible by effective external cooling or by using flow techniques. Alternatively, the reaction rate can be moderated by adding an inhibitor. Sulfur is used commercially for this purpose with chloroprene, although this inhibitor cannot be used in the polymerization of butadiene or isoprene. Coagulation of emulsion is finally brought about by the addition of acids or multivalent salts. The emulsion polymerizates possess only about 1.5% 1,2 structures. 

Figure 4.26 Polychloroprene produced from the emulsion polymerization of chloroprene...

Polychloroprene elastomers are produced by free radical emulsion polymerization of 2-chloro-l,3-butadiene monomer. The emulsion polymerization of chloroprene involves the dispersing of monomer droplets in an aqueous phase by means of suitable surface active agents, generally at a pH of 10-12. Polymerization is initiated by addition of free radical catalyst at 20-50° C. To obtain a bigb conversion in the polymerization reaction and processable polymer, the addition of sulfur, thiuram disulfide, or mercaptans is necessary (Whitehouse 1986). 

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