Chloroprene chain structure

Polychloroprene. In contrast to PBD and PIP, the radiation cross-linking of polychloroprene does not appear to proceed by a chain-reaction mechanism (see Chloroprene Polymers). The yields of cross-links [G(X)] were measured by a number of methods and determined to be 3.9 (NMR), 4.8 (swelling ratio), and 3.2 (sol. fraction) (351). This close correlation, and the observation of a narrow resonance in the NMR spectrum assigned to cross-link structures, indicated that cross-linking in this material occiu-s randomly throughout the material. Peaks are observed in the NMR spectra because of new chain structures formed on irradiation. 

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-... 

Some s)mthetic rubbers are superior to natural rubber in some ways. Neoprene is a s)mthetic elastomer (an elastic polymer) with properties quite similar to those of natural rubber. The basic structural unit is 2-chloro-l,3-butadiene, commonly called chloroprene, which differs from isoprene in having a chlorine atom rather than a methyl group at carbon 2 of the 1,3-butadiene chain. 

The polymerization of butadiene monomer proceeds with chain propagation via 1,2-,, A-trans- or 1,4-cw-additions. If the polymerization is controlled to form mostly the 1,2-addition product, the polymer has a — CH2— chain with a terminal vinyl, — CH=CH2, substituent, at alternating carbon atoms. However, if 1,4-addition dominates the polymerization proceeds to form a polymer chain with a molecular structure of — (CH2 —CH=CH—CH2) —, normally with a trans configuration at the double bond. 2-Chloro-1,3-butadiene (CH2=CC1—CH=CH2 chloroprene) and 2-methyl-1,3-butadiene (isoprene) are polymerized in a similar manner. With these compounds, the asymmetry of the carbon atoms at positions 1 and 4 produces a variety of addition products with 1,2-, 1,4-cw,, A-trans, and 3,4-configurations. In the case of polyisoprene, which in nature occurs as natural rubber, the 1,4-cis configuration is the dominant structure. A summary of the polymerization products of butadiene, isoprene, and chloroprene is provided in Fig. 31. 

It becomes evident that with a reasonably simple monomer (chloroprene being a substituted butadiene), one must consider four structures for the incorporation of monomer units, two types of interunit connections, two potential comonomers, long-chain branching, and cross-linking, as well as molecular weight distribution. The sum of all of these structural variables determines, for the most part, the end-use properties of the polymer and aU are strong functions of polymerization conditions. 

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