Cyclized Natural Rubber

As natural rubber is a product of nature, its properties are determined by the biochemical pathway by which the polymer is synthesized in the plant. In the case of natural rubber the polymerization process cannot be tailored like that of synthetic rubbers. The only option to modify natural rubber is after it has been harvested from the tree. The important modified forms of natural rubber include hydrogenated natural rubber, chlorinated natural rubber, hydro-halogenated natural rubber, cyclized natural rubber, depolymerised liquid natural rubber, resin modified natural rubber, poly(methyl methacrylate) grafted natural rubber, poly(styrene) grafted natural rubber, and epoxidized natural rubber [33,34]. Thermoplastic natural rubber prepared by blending natural rubber and PP is considered as a physically modified form of natural rubber. 

Chlorination of natural rubber, involving both addition and substitution (with some cyclization), yields a product with improved chemical and corrosion resistance. Chlorination of polyethylene in the presence of sulfur dioxide results in substituting both chloride and sulfonyl chloride groups into the polymer. A commercially useful material is one which contains about 12 chlorides and one sulfonyl chloride per 40-45 repeating units. This extensive substitution converts the polyethylene, a plastic, into an elastomer by destroying crystallinity. 

Chlorination of natural rubber (NR) is carried out with chlorine in carbon tetrachloride solution at 60-90°C to yield a chlorinated rubber containing about 65% chlorine, which corresponds to 3.5 chlorine atoms per repeat unit. The process is complex and includes chlorine addition to the double bond, substitution at allylic positions, and cyclization. Chlorinated rubber has high moisture resistance and is resistant to most aqueous reagents (including mineral acids and bases). It is used in chemical- and corrosion-resistant paints, printing inks, and textile coatings. Bromination of butyl rubber is also practiced [Parent et al., 2002]. 

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... 

Surface treatments consist of washing with solvent, abrading, or, in the most demanding applications, cyclizing with acid. The most common elastomers to be bonded in this way include nitrile, neoprene, urethane, natural rubber, SBR, and butyl rubber. It is more difficult to achieve good bonds with silicones, fluorocarbons, chlorosulfonated polyethylene, and polyacrylate. 

Synthetic polyisoprenes are superior to natural rubber in terms of consistency of properties. The are also freer of contaminants, and are preferred for applications that require lighter color, for personal care items and for derivatization to chlorinated and cyclized rubber products that are used in the adhesives and coatings industries. 

Chlorination of natural rubber in solution, in latex, or in solid form, results in substitution, addition, and cyclization reactions [34]. Kraus and Reynolds [36] have identified the following three distinct stages in the reaction of gaseous chlorine with natural rubber dissolved in carbon tetra chloride at 80°C ... 

Hydrochlorination of natural rubber is often accompanied by cyclization ... 

Additive-type chlorination of natural rubber can also be carried out with phenyl iododichloride or with sulfuryl chloride. Traces of peroxides must be present to initiate the reactions. This suggests a free-radical mechanism. Some cyclization accompanies this reaction as well. In (XU, for the first 25 chlorine atoms that add per each 100 isoprene units, 23 double bonds disappear and only a small quantity of HCl forms. The subsequent 105 chlorinations, however, cause a loss of only 53 double bonds. 

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 ... 

Other examples include the free radical polymerization of vinyl isocyanate with subsequent anionic polymerization of the isocyanate side groups, as well as the corresponding polymerization of acrylonitrile or poly(acrylonitrile). Free radical and anionic polymerizations are to be preferred over those initiated by cations, since cationic polymerization frequently tends to give rise to transfer reactions which interrupt the transannulation. For example, in the cyclization of natural rubber with the aid of concentrated acids or Lewis acids, an average of only three transannular rings is obtained [see Equation (25-9)]. The intermolecular elimination of water from poly(vinyl methyl ketone) only produces single, double, and triple rings ... 

The molar mass is additionally fixed by these regulators so that mastication is no longer necessary. The cold polymerization is more favorable than the warm polymerization, since more /rnns-rich structures are produced. Ci5-rich polymers, of course, tend more to cyclization, which produces "stringiness,"" that is, an undesirable increase in viscosity, during subsequent processing. Buna S can be mixed directly with natural rubber. It is primarily used for the running surfaces of car tires. 

Heating natural rubber to over 250° C in the presence of protons causes the molar mass to decrease from about 300000 to 3000-10,000 with simultaneous cyclization. Depending on the reaction conditions and the yield, mono-, di-, and tricyclic structures separated by CHa groups or noncyclized isoprene units are produced ... 

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. 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene [42]. The free-radical mechanisms, on the other hand, cause cross-linking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene, however, with addition of as much as 30% by weight of chlorine without cyclization [39, 40]. Also, chlorination of polyalkenamers both cis and trans yields soluble polymers. X-rays show that the products are partly crystalline [43, 44]. The crystalline segments obtained from... 

Solvent washing and abrading are common treatments for most elastomers, but chemical treatment is required for maximum properties. Many synthetic and natural rubbers require cyclizing with concentrated sulfuric acid until hairline fractures are evident on the surface. 

Cyclized rubber n. A thermoplastic resin produced by reacting natural rubber with stannic chloride or chlorostannic acid. This causes a reduction of the unsaturation and formation of condensed ring structures, typically with two or three rings being... 

Polymers related to natural rubber include ebonite, chlorinated rubber, oxidized rubber, cyclized rubber, and gutta-percha and balata. 

The modification of polyisoprene by treatment with strong acid dates, in the case of natural rubber, to at least the late 18th century. 1 It has been accepted for some time now that the resinification and loss of rmsaturation acconpanying such treatment are due, inmost instances, to acid-catalyzed cyclization. The earlier work on sudi cyclization has been reviewed thoroij lyl and only a brief overview of the main aspects will be given here. 

In Table IV are shewn the results of our NMR analyses of 20 sanples of cyclized polyisoprene, prepared as noted in Table I. Because of the relevance bo the block copolymersl0 12 mentioned above, the bulk of the data are for cycliz lithivm polyisoprene. Ihere are, however, sene results for cyclized natural rubber, both hevea and gu yule. 

Patterson, D. J. and Koenig, J. L. (1987). A Fourier-transform infrared and nuclear magnetic resonance study of cyclized natural rubber. Mak-romol Chem. 188(10), 2325-2337. 

The cyclization of natural rubber is generally agreed to involve protonation and addition of the resulting carbonium ion to an adjacent double bond to form a 6-membered ring. This reaction can proceed along the chain to give a polycyclic structure ... 

Cyclized natural rubber is extremely resistant to aqueous acids and alkalis and finds use in surface coatings and adhesives. 

Over the years many chemical derivatives of rubbers have been introduced. These include chlorinated rubber, rubber hydrochloride and cyclized rubber. Whilst little work was done in this area in the period 1955-75 the renewed appreciation that natural rubber, unlike coal and oil, is a replaceable raw material has stimulated further study of the products to be derived from it. 

Hydrochlorination of natural rubber usually yields a polymer with slightly less than the theoretical chlorine content for complete reaction (e.g. 33 5 cf. 33-9%). NMR studies (Golub and Heller, 1964) have established that some cyclization occurs during hydrohalogenation and this can account for the discrepancy. 

---