Diene rubbers cyclization

Cyclization can be also achieved in a separate reaction step after polymerization. Cyclization of Nd-based diene rubbers has been investigated since the... 

Chlorine treatment of natural rubber gives chlorinated rubber III. Since the products contain up to 65% chlorine, substitution obviously occurs along with addition across the double bonds (since the latter would only lead to a theoretical maximum of 51%C1). As indicated by spectroscopic studies, some cyclization to cyclohexane structures occurs also. Chlorinated rubber solutions are Used as adhesives for diene rubber-metal laminates. 

Cyclized natural rubber has been of some use in providing stiffness to diene rubber vulcanizates without materially increasing the density. For this purpose it is in some competition with the high styrene resins. Furthermore there are today a number of elastomers available, alternative to the diene polymers blended with cyclized rubber or high styrene resins, such as certain block copolymers and polyurethanes which possess this property without the need to add such stiffening fillers. 

This chapter has considered the application of several well-known reactions of alkenes to diene polymers. Whilst the basic reactions are generally predictable from a knowledge of alkene reactivity they are influenced by the fact that the double bond is part of a very long chain molecule consisting of double bond-containing repeat units. In particular the tendency to cyclize has been noted, for example in the case of chlorinated natural rubber and with rubber hydrochloride whilst in other cases degradation or cross-linking has occurred. It may be noted here that the ability to produce cyclized natural rubber is a direct consequence of the affinity of a carbonium ion for a double bond when activated in a polymeric environment. 

Natural rubber and other 1,4-poly-1,3-dienes are cyclized by treatment with strong protonic acids or Lewis acids [Golub, 1969 Subramaniam, 1988]. The reaction involves protonation of the double bond (Eq. 9-40) followed by cyclization via attack of the carhocation on the double bond of an adjacent monomer unit (Eq. 9-41). Some bicyclic and polycyclic... 

Cyclization reactions of natural rubber and other polymers from conjugated dienes have been known for a long time. The reactions occur in the presence of Lewis and strong protonic acids. They result in loss of elastomeric properties and some unsaturation. Carbon cations form in the intermediate step and subsequent formation of polycyclic structures ... 

Halogenation and hydrohalogenation of elastomers have been reported extensively in the literature [26]. The main problems with these reactions are the cyclization and chain scission that occur parallel to the halogenation reaction. These introduce difficult problems in the characterization of the resulting products. Despite these problems, several products have been prepared and commercialized. Chlorination of poly(l, 4-butadiene) to prepare a product similar to poly(vinyl chloride) has been reported by several workers [27]. This process had extensive side reactions and chain degradation. The chlorination of butyl rubber and conjugated diene-butyl rubbers gives end products that are used in the tire industry as inner liners for air retention. 

There is some evidence to indicate that the cyclization of natural rubber was the first man-made chemical modification of a diene polymer since it is reported (Bedford and Winkelmann, 1923) that in 1791 a certain Leonhardi observed that natural rubber treated with sulphuric acid became hard and brittle. 

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