Polymerization, emulsion chloroprene

Polymerization temperature principally affects three characteristics of the polymer obtained from chloroprene emulsion polymerizations gel content and molar mass distribution, stereoregularity, and the tendency of the polymer to crystallize. Reducing the polymerization temperature decreases the tendency for polymer gel to form. This is illustrated by the results reported by Mochel [22], own in Figure 15.12, for polychloroprenes produced by emulsion polymerization at 10 °C and 40 °C in the presence of 0.6 parts of sulfur per 100 parts by mass of chloroprene, and before peptization with a thiuram disulfide. The onset of the formation of polymer gel is retarded by reducing the polymerization temperature whereas a substantial proportion of the polymer is gelled at only 10% conversion when polymerization is carried out at 40 C, the polymer is essentially gel-fipee up... 

Pettelkau and Ehrig [36] also described an up-flow reactor for chloroprene emulsion polymerization. Their reactor is operated full with a bottom propeller-type agitator that is mounted at an angle to achieve good mixing without internal baffles. Non-geometric scale-up with large H/D ratios are used to increase heat transfer area in the jacket. 

Synthetic. The main types of elastomeric polymers commercially available in latex form from emulsion polymerization are butadiene—styrene, butadiene—acrylonitrile, and chloroprene (neoprene). There are also a number of specialty latices that contain polymers that are basically variations of the above polymers, eg, those to which a third monomer has been added to provide a polymer that performs a specific function. The most important of these are products that contain either a basic, eg, vinylpyridine, or an acidic monomer, eg, methacrylic acid. These latices are specifically designed for tire cord solutioning, papercoating, and carpet back-sizing. 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

Chloroprene mbber is usually manufactured by either batch or continuous emulsion polymerization and isolated either by freeze coagulation or dmm drying of a polymer film. Figure 1 is a schematic flow sheet of this process. 

Fig. 2. Effect of polymerization temperature on the crystalline melting point of chloroprene mbbers produced by emulsion polymerization ...

Chemistry of polychloroprene rubber. Polychloroprene elastomers are produced by free-radical emulsion polymerization of the 2-chloro-1,3-butadiene monomer. The monomer is prepared by either addition of hydrogen chloride to monovinyl acetylene or by the vapour phase chlorination of butadiene at 290-300°C. This latter process was developed in 1960 and produces a mixture of 3,4-dichlorobut-l-ene and 1,4-dichlorobut-2-ene, which has to be dehydrochlorinated with alkali to produce chloroprene. 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

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... 

Although not a telomerization, it is mentioned here, that syndiotactic 1,2-polybutadienes were prepared in aqueous emulsions with a 7t-allyl-cobalt catalyst [33]. Similarly, chloroprenes were polymerized using aqueous solutions of [PdCl2(TPPMS)2] and [RhCl(TPPMS)3] as catalysts at 40 °C in the presence of an emulsifier and a chain growth regulator (R-SH, R=Cio-Cis) [35]. Despite the usual low reactivity of chlorinated dienes, these reactions proceeded surprisingly fast, leading to quantitative conversion of 10 g chloroprene in 2 hours with only 50 mg of catalyst (approximate TOP = 3500 h- ). 

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. 

Emulsion polymerization was first employed during World War II for producing synthetic rubbers from 1,3-butadiene and styrene. This was the start of the synthetic rubber industry in the United States. It was a dramatic development because the Japanese naval forces threatened access to the southeast Asian natural-rubber (NR) sources, which were necessary for the war effort. Synthetic mbber has advanced significantly from the first days of balloon tires, which had a useful life of 5000 mi to present-day tires, which are good for 40,000 mi or more. Emulsion polymerization is presently the predominant process for the commercial polymerizations of vinyl acetate, chloroprene, various acrylate copolymerizations, and copolymerizations of butadiene with styrene and acrylonitrile. It is also used for methacrylates, vinyl chloride, acrylamide, and some fluorinated ethylenes. 

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. 

Although there are numerous references to the emulsion polymerization of vinyl ferrocene, they all appear to emanate from a single source (j4). These workers polymerized vinyl ferrocene alone, and with styrene, methyl methacrylate, and chloroprene. No characterization was reported other than elemental analysis. The molding temperatures reported (150 - 200 C) correspond to the Tg range indicated by Pittman ( ) for similar copolymers. The initiation system was preferably azobisisobutyronltrile, although potassium persulfate was also used. Organic peroxides were contraindicated, due to oxidation problems with the ferrocene moiety. 

Emulsion polymerization is the basis of many industrial processes, and the production volume of latex technologies is continually expanding—a consequence of the many environmental, economic, health, and safety benefits the process has over solvent-based processes. A wide range of products are synthesized by emulsion polymerization, including commodity polymers, such as polystyrene, poly(acrylates), poly (methyl methacrylate), neoprene or poly(chloroprene), poly(tetrafluoroethylene), and styrene-butadiene rubber (SBR). The applications include manufacture of coatings, paints, adhesives, synthetic leather, paper coatings, wet suits, natural rubber substitutes, supports for latex-based antibody diagnostic kits, etc. ... 

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. 

A recent paper by a Rusian team [18] describe tte use of a few new surfiners, one being cationic, namely JV-decylaceto-2-methyl-5 vinylpyridinium bromide (V), and the others being anioic, namely decyl (or dodecyl), sodium ethyl sulfonate, methacrylamides (VI), decyl (or dodecyl)-phenyl (Na or K sulfonate) acrylate (VII), and decyl ester of sodium (or K or NH4) sulphocin-namic acid (VIII). These surfmers were used for emulsion polymerization of styrene, butylacrylate or chloroprene, in the presence of KPS or AIBN without any other surfactants. It should be noted that the consumption of these surfactants take place early in the polymerization process which is faster than in... 

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. 

The polymer which is obtained when chloroprene is emulsion-polymerized in the absence of sulfur or a chain-transfer agent is a tough, insoluble non-plastic material. If sulfur is dissolved in the chloroprene prior to polymerization (at a level of 0.5-1.5 parts by mass per 100 parts by mass of monomer), the product is still a tough insoluble material which is unsuitable for use as an elastomer. However, by heating the latex obtained (still in an alkaline condition) with a thiophilic reagent, such as tetraethylthiuram disulphide (Vlll), the polymer becomes plasticized or peptized to a soft, plastic, soluble polychloroprene which is suitable for use as an elastomer. 

Table 15.5 shows the results of Mochel [22] for the effect of conversion upon the gel content of polychloroprene rubbers prepared by emulsion polymerization at 40 °C. In section (a) of this table are shown results for polymers produced in the absence of added sulfur section (b) shows results for polymers produced with the addition of 0.6 parts of sulfur per 100 parts by mass of chloroprene, before chemical peptization of the polymer. In both types of reaction system, polymer gel begins to form quite early in the reaction. However, these results indicate that sulfur has a slight tendency to act as a modifier during the polymerization, in that the onset of gel-formation is delayed when sulfur is present. Also delayed is the pdnt at which the polymer is virtually entirely gel. Mochel et al. [23] have reported results for the molar mass distribution of a thiuiam-modified polychloroprene rubber produced by emulsion polymerization at 40 °C,... 

The effect of polymerization temperature upon the microstmcture of poly-chloroprenes produced by emulsion polymerization is illustrated by the results, reported by I ynard and Mochel [25], shown in Table 15.6. The chloroprene units are present mainly as trans- A structures, irrespective of the polymerization temperature. However, the distribution of the microstructures does depend somewhat upon polymerization temperature. The ratio cw-l,4/tra 5-l,4 units decreases as the polymerization temperature is reduced, but the overall content of 1,4 units increases. The balance comprises 1,2 and 3,4 units in approximately equal proportions, except for polymers produced at very low temperatures, where the 1,2 units predominate over the 3,4 units. A consequence of the overall content of 1,4 units increasing with decreasing polymerization temperature is that the sum of the contents of 1,2 and 3,4 units decreases. As will be seen in Section 15.4.4, although 1,2 units are present in relatively low concentration, their presence is very important for the technology of polychloroprene rubbers. 

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... 

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... 

Chloroprene is polymerized commercially by free-radical emulsion polymerization. The reaction is carried out at 40 °C to a 90% conversion. A typical recipe for such an emulsion polymerization is as follows ... 

Alkane sulfonates are applied in a widespread manner in emulsion polymerization. They are used as processing aids, in particular in the emulsion polymerization of vinyl chloride, vinyl acetate, styrene and acrylonitrile. Because they possess no double bonds, alkane sulfonates do not act as radical chain stoppers. Well-known lattices derived from emulsion polymerization are poly(vinyl chloride), ethylene-vinylacetate copolymers, polyacrylates, and butadiene and chloroprene rubbers. Alkane sulfonates also offer good stabilizing effects in lattices against coagulation by fillers. 

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