Unsaturated Polyhydrocarbons

A number of molecules with two conjugated double bonds, such as 1,3-butadiene or 2-methyl-1,3-butadiene (isoprene), can undergo a 1-4 polymerization with the migration of the doubie bond in the 2 position. The resulting polymer contains isolated double bonds, but their presence leads to the possibility of having cis- and trans- forms, as shown below  

Polyisoprene (R = CH3) with a c/s-1,4 configuration is common in nature in different species of piants and is known as natural rubber. Trans-polyisoprene is found in two naturai resins known as gutta-percha and balata. Natural or synthetic polyisoprenes, as well as polybutadiene, are among the most common elastomers with many practical uses. Other elastomers with extensive practical applications are copolymers, many of them using butadiene or isoprene in the starting monomer mixture. 

A different subclass of unsaturated hydrocarbon type polymers is formed by polyacetylenes. This type of polymer contains conjugated double bonds in a linear structure, and due to their special electrical properties they have been the subjects of numerous studies including some on thermal stability. 

Various other polymers have double bonds in their carbon backbone. Some are polyhydrocarbons such as poly-a-pinene (see Section 6.8), while others are polymers including aromatic cycles in the backbone or possibly heteroatoms. These polymers are typically classified under different groups (see Chapters 8 and 15). 

Polymers with unsaturated carbon chain backbone 

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Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

Vulcanization is an industrial process applied to various polymers from the class of unsaturated polyhydrocarbons. The major practical use of vulcanized elastomers is the tire industry. Tires are made from various polymer blends, including natural rubber, typically between 20 and 50%. The other polymers used in various blends that can be vulcanized include copolymers such as poly(styrene-co-1,3-butadiene) or SBR, poly(acrylonitrile-co-1,3-butadiene-co-styrene) or ABS, poly(isobutylene-co-isoprene), poly(ethylene-co-propylene-co-1,4-hexadiene, etc. 

Poly(2-chloro-1,3-butadiene) and other halogenated unsaturated polyhydrocarbons... 

Results for some other halogenated unsaturated polyhydrocarbons were reported in literature. For example, poly(perfluoro-4-chloro-1,6-heptadiene) upon heating between 320° C and 400° C is reported to completely volatilize [12]. Also, reports on chlorinated rubber show that by heating from ambient to 500° C, it generates almost quantitatively HCI, and in the later stages of reaction CH4, C2H4, and H2 [13],... 

Pyrolysis of chlorinated unsaturated polyhydrocarbons is in some respects similar to that of the parent polyhydrocarbon, and in some others similar to halogenated saturated hydrocarbon type polymers. The elimination of the hydrohalogenated acid takes place relatively easily, and the polymers from this class are not resistant to heating. In practice, additives that enhance the resistance to heating frequently are added. Since the elimination of the acid seems to accelerate thermal decomposition, similarly to the case of PVC, metallic oxides that scavenge the acid increase the resistance to heating. The main pyrolysis products of each polymer are not modified by the addition of these additives. 

Proof of the high reactivity of the unsaturated units present in the elastomers we used was obtained by polymerizing vinyl chloride in the presence of different amounts of two hydrocarbons, n-hexane and 2-methyl-2-butene these may be taken as models of the saturated polyhydrocarbon and of the allyl system present in the EPDM terpolymer. AIBN (0.13%) was used as initiator. 

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