Chloroprene stereoregular polymerization

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... 

Geometric isomerism. When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. As illustrated in Fig. 14.14, conjugated dienes such as isoprene and chloroprene can be polymerized to give either 1,2-, 3,4, or 1,4-polymer. In the case of 1,4-polymers, both cis and trans configurations are possible. Also, stereoregular (i.e., isotactic and syndiotactic) polybutadienes can be produced in case of 1,2- and 3,4-polymerization. 

The tendency for a polychloroprene rubber to crystallize increases as the polymerization temperature is reduced. This tendency is illustrated by results of Maynard and Mochel [26], given Figure 15.13, for the effect of polymerization temperature upon the melting temperature of the pol3mien as the polymetizadon temperature is reduced, so the temperature at which crystallization is able to occur increases progressively. This is believed to be primarily a consequence of die effect of temperature upon the stereoregularity of the polymer. The results shown in Table 15.6 indicate that it is probably the enhanc concentration of ds-1,4 units in polychloroprenes produced at higher temperatures which is responsible for the retardation of crystallization, rather than the presence of the 1,2 and 3,4 units. It appears that, for chloroprene homopolymers, other variables, such as molar mass and crosslink density, have little effect upon tendency to crystallize. 

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-1,4 chain microstructure. Thus, even at ambient polymerization temperature, the polychloroprene contains over 90% trans-1,4 units, as shown in Table X, which illustrates the effect of polymerization temperature on stereoregularity of the chain [87]. As expected, lower polymeriza-... 

The free-radical polymerization of chloroprene (2-chloro-l,3-butadiene) is a stereoselective polymerization that has been known for many years. However, this stereochemical fact has been largely ignored or superficially treated in some organic texts. One will note that six stereoregular polymers are possible. A 1,4 addition can give either cis-1,4- or tra%s-l,4-polychloroprene. The 1,2 and 3,4-additions account for the other four possibilities since each addition can be either isotactic or syndiotactic. The macromolecule, which is produced by free-radical synthesis, is practically all trans-l,4-polychloroprene. 

A so-called all-trans stereoregular polymer has also been prepared. The technique used was that of radiation polymerization of large crystals of chloroprene that had been formed by cooling to temperatures in the range -130 to -180 C (Garrett et ai, 1970). 

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