Polychloroprene, microstructure

Microstructure. Whereas the predominate stmcture of polychloroprene is the head to tail /n7 j -l,4-chloroprene unit (1), other stmctural units (2,3,4) are also present. The effects of these various stmctural units on the chemical and physical properties of the polymer have been determined. The high concentration of stmcture (1) is responsible for crystallization of polychloroprene and for the abiUty of the material to crystallize under stress. Stmcture (3) is quite important in providing a cure site for vulcanization, but on the other hand reduces the thermal stabiUty of the polymer. Stmctures (3),(4), and especially (2) limit crystallization of the polymer. 

Summarizing, the variations in the microstructure are responsible for significant changes in the polychloroprene properties. The main modifications produced in the polychloroprene chains affect its properties as follows ... 

Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

Polymer liquid crystals, 75 107-111 Polymer matrices, 26 761-765 Polymer-matrix composites, 73 502 26 751, 755-756 fabrication of, 26 765 Polymer melts, 75 108-109 27 730-731 chain fluctuations in, 27 714 viscosity of, 20 99 21 712-714 Polymer metal composites, smart, 22 718 Polymer microspheres, 9 73-75 Polymer microstructure, polychloroprene, 79 836-838... 

The microstructure of polychloroprene, polymerised at +12 to +70 °C, was analysed using H- and 13C-NMR [32], Signal assignments were made for head-and-tail arrangements of trans-1,4 units, which was the major component of polychloroprene, and for other isomeric units. Polymerisation at high temperature resulted in a slight increase in head-to-head and tail-to-tail linkage of trans-1,4 units as well as the increase in cis-1,4 units. 

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. 

Table 15.6 Effect of polymerization temperature upon microstructure of polychloroprenes produced by emulsion polymerization (Maynard and Mochel [26])...

TABLE 2.10 Effect of Polymerization Temperature on Polychloroprene Chain Microstructure ... 

Ebdon, J.R., 1978. Microstructure of polychloroprene determined by C-13 nuclear magnetic resonance. Polymer 19 (10), 1232-1233. 

Petiaud, R., Pham, Q.T., 1985. Polychloroprenes 1. Study of the microstructure using H-1 nuclear magnetic resonance. J. Polym. Sci. Part A Polym. Chem. 23 (5), 1333-1342. 

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-... 

Table 12 Microstructure of polychloroprene as related to polymerization temperature. 

The polychloroprenes exhibit a higher degree of crystallinity than most elastomers and this has a significant effect on the rubbery properties, particularly tensile strength. Since this crystallinity is dependent on the polymer microstructure it is particularly important that the latter be characterized and made more controllable. 

Variations in microstructure are responsible for significant changes in polymer properties. Figure 1.5 shows the main modifications of the polychloroprene chain... 

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