Chloroprene, polymerization

Commercial chloroprene polymerization is most often carried out in aqueous emulsion using an anionic soap system. This technique provides a relatively concentrated polymerization mass having low viscosity and good transfer of the heat of polymerization. A water-soluble redox catalyst is normally used to provide high reaction rate at relatively low polymerization temperatures. 

The mechanism of chloroprene polymerization is summarized in Scheme 4.11. Coleman et ai9iM have applied l3C NMR in a detailed investigation of the microstructure of poly(chloroprene) also known as neoprene. They report a substantial dependence of the microstructure on temperature and perhaps on reaction conditions (Table 4.3). The polymer prepared at -150 °C essentially has a homogeneous 1,4-tra/rv-niicrostructure. The polymerization is less specific at higher temperatures. Note that different polymerization conditions were employed as well as different temperatures and the influence of these has not been considered separately. 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

Fig, 2l, Polymerization of methyl methacrylate and chloroprene by rubber mastication (69). 1 23.8% methyl methacrylate and 24.2% chloroprene added initially. 2 24.2% chloroprene polymerized, then 23.8% methyl methacrylate. 3 23,8% methyl methacrylate polymerized,... 

During 1973, at a chloroprene polymerization plant in the United States, airborne concentrations of chloroprene were found to range from 14 to 1420 ppm [50-5140 mg/m ] in the make-up area, from 130 to 6760 ppm [470-24 470 mg/m in the reactor area, from 6 to 440 ppm [22-1660 mg/m ] in the monomer recovery area and from 113 to 252 ppm [409-912 mg/ni l in the latex area (Infante et al., 1977). Concentrations in the air inside a Russian polychloroprene rubber plant were 14.5-53.4 mg/m (Mnatsakayan et al., 1972). In a Russian chloroprene latex manufacturing facility, chloroprene concentrations varied from 1 to 8 mg/m (Volkova et al., 1976). 

The mechanism of chloroprene polymerization is summarized in Scheme 4.11, Coleman et have applied NMR in a detailed investigation of the... 

Table 5.9 lists the structures of polychloroprenes that form by free-radical polymerization at different temperatures. Chloroprene polymerizes by cationic polymerization with the aid of Lewis acids in chlorinated solvents. When aluminum chloride is used in a mixture of ethyl chloride-methylene chloride solvent mixture at -80 °C, the polymer has 50% 1,4 units. If it is polymerized with boron trifluoride, the product consists of 50-70% 1,4-adducts. A veiy high trans-1,4-poly(2-chloro-1,3-butadiene) forms by X-ray radiation polymerization of large crystals of... 

Unsymmetrical diene monomers such as chloroprene polymerize by four reaction pathways 1,4-head-to-head, 1,4-head-to-tail, 1,2-and 3,4-polymerization (Fig. 1). The concentrations of microstructure vary with polymerization temperature (51) (Table 2). The (Z)- or trans-configuration predominates at conventional polymerizations carried out in the range of 10-45°C. 

The rubber industries use alkanolamines as secondary vulcanization accelerators to give improved mechanical properties and better heat resistance. Alkanolamines help stabilize rubber dispersions and may also act as an antioxidant for some rubbers. Alkanolamines are activators for the dielectric heating and microwave vulcanization processes. The various alkanolamines are used in the cold cure of certain rubbers and can act as chain termination agents in the chloroprene polymerization. 

Experiments were also performed with the aim of polymerizing a mixture of two monomers [88]. The reaction rate and the composition of the graft copolymer conform to the rules of mechanochemical synthesis and radical copolymerization. If the two monomers have almost equal reactivities, the composition of the copolymer is approximately that of the initial monomer mixture (Fig. 5.20). The reaction rate is also intermediate between those of the separate monomers. If the two mechanically activated monomers have different reactivities, the two monomers largely polymerize separately. The reaction rate depends on the polymer properties for the monomer that polymerizes first. As an example, in the system chloroprene-methyl methacrylate [88], the more active chloroprene polymerizes at first the reaction rate is low as polychloroprene is a flexible chain and the second monomer behaves as an inert solvent, reducing the viscosity. The reaction rate increases in the second part of the reaction when an appreciable amount of poly-(methyl methacrylate) is produced (see Fig. 5.21). Two monomers added... 

Chloroprene polymerizes thermally to yield a true rubbery solid with a characteristic liquid-like structure. Stretching produces crystallinity and the expected X-ray pattern for an oriented fiber. A detailed mechanistic analysis yielded the multiple structures that appear during the reaction. Another synthetic pathway leads to polychloroprene latex. Characterization of these particles with the ultracentrifuge yielded a highly peaked distribution with a mean radius near 0.06 micron. Because of the very small particle size compared with natural mbber latex, this material can be used for many applications requiring minute particles. The detailed analysis and keen structural insight set a standard that is rarely met, even today. 

---