Polychloroprene, isomers

Chloroprene Elastomers. Polychloroprene is a polymer of 2-chloro-l,3-butadiene. The elastomer is largely composed of the trans isomer. There are two basic polymer types the W-type and the G-type. G-types are made by using a sulfur-modified process W-types use no sulfur modification. As a result, G-types possess excellent processing and dynamic properties, and tend to be used in V-belts. However, they have poorer aging properties than W-types. The W-types tend to be used in appHcations requiring better aging, such as roUs and mechanical goods (see Elastomers, SYNTHETIC-POLYCm.OROPRENE). 

Besides butadiene, another important monomer for the synthetic elastomer industry is chloroprene, which is polymerized to the chemically resistant polychloroprene. It is made by chlorination of butadiene follow by dehydrochlorination. As with most conjugated dienes, addition occurs either 1,2 or 1,4 because the intermediate allyl carbocation is delocalized. The 1,4-isomer can be isomerized to the 1,2-isomer by heating with cuprous chloride. 

The results of stereochemical interest which came out of this work may be indicated (Bunn, 1942 a-c). It paved the way to a solution of the crystal structure of rubber itself (the cis isomer of poly-isoprene) and of the synthetic rubber-like substance polychloroprene... 

Wallace Carothers will be the subject of one of our Polymer Milestones when we discuss nylon in Chapter 3. Among his many accomplishments in the late 1920s and early 1930s, Carothers and his coworkers made a major contribution to the discovery and eventual production of the synthetic rubber, polychloroprene. It was synthesized from the diene monomer, chloroprene, CH2=CCI-CH=CHr Chloroprene, which is a very reactive monomer—it spontaneously polymerizes in the absence of inhibitors— was a product of some classic studies on acetylene chemistry performed by Carothers and coworkers at that time. In common with butadiene and iso-prene, in free radical polymerization chloroprene is incorporated into the growing chain as a number of different structural isomers. Elastomeric materials having very different physical and mechanical properties can be made by simply varying the polym-... 

PVDF is just one polymer where sequence isomerism has been obseved using NMR spectroscopy. The 13C NMR spectrum of polychloroprene, for example, has also been analyzed by one of your authors (in collaboration with his coworkers) and the olefinic region is shown in Figure 7-28. It proved possible to assigu lines to triad sequence isomers of the trans-1,4 units (TH and HT) and also triad sequences containing the cis-lA... 

What kind of tacticity do you expect in poly-1-butene Which form is likely to be less crystalline What kind of isomerism do you expect in polychloroprene Which isomer should be most ciystalline ... 

Chloroprene Elastomers. Polychloroprene is made by polymerizing 2-chloro-1,3-butadiene (eq. 7). The elastomer is largely composed of the trans isomer. There are three major polymer types the G-type, the W-type, and the T-type. 

Each of these structural isomers may have atactic, isotactic, or syndiotactic stereoisomers (making for six more possible isomers). Thus, in principle at least, there are eight different isomers of polychloroprene possible. 

Figure 1.3 compares the structural units of commercially available polychloroprene. In this polymer the 1,4 addition [1], in particular the l,4-polymer structures are combined in sequential isomers derived from head to tail (IV), head to head (VT), and tail to tail (V) addition [9]. 

This technique was first applied to the isolation of the vibrational bands of the crystalline isomer of rra s-l,4-polychloroprene [81]. The spectrum of a cast film of predominately (>90%) rrans-l,4-polychloroprene polymerized at —20 C was compared with the spectrum of the same sample heated to 80°C (above the melting point) for 15 min. 

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