Diene rubbers hydrogenation

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

The simplest diene rubber for hydrogenation is BR. The structure of the hydrogenated BR (HBR) depends on the different microstructures present in BR. Hydrogenation of BR with high 1,4 structure converts this elastomeric polymer into a tough semicrystalline... 

Rubbers, often based on poly diene rubbers or else copolymers of dienes like 1,3-butadiene, were the first successful toughening additives, and they are effective partly because they have a low modulus, 100 to 500 times lower than that of most thermoplastic polymers. Unfortunately polydienes introduce chemical double bonds which are susceptible to UV, thermal and oxidative degradation. Hydrogenation removes some of them. Acrylic compounds and ethylene copolymers are also popular impact modifiers, and they do not necessarily introduce double bonds. 

In principle the hydrogenation of a diene rubber is one of the simplest reactions. Complete hydrogenation of polybutadiene would give a polyethylene structure whilst hydrogenation of polyisoprene that of an alternating ethylene-propylene copolymer. 

In the case of olefins and unsaturated polymers such as the diene rubbers the above formal schemes may require some modification. For example the hydrogen atom in the formalized structure RH usually refers specifically to an a-methylene hydrogen. It is also to be noted that as the a-methylenic activity of the olefin decreases so does the reaction become modified. Instead of the chain propagation proceeding simply by abstraction of labile hydrogen atoms (reaction (2b)) the peroxy radicals also add to the double bond and this in turn leads to cross-linking. 

Yamada and co-workers [59] pointed out that spectroscopic methods such as IR spectroscopy and NMR spectroscopy previously used in sequencing studies on ethylene-propylene-diene and hydrogenated acrylonitrile butadiene rubbers often encountered the same difficulties experienced with the analysis of vulcanised rubbers, i.e., their insolubility. 

Scheme 1 Outline reaction scheme for the sulfur vulcanization of diene rubbers. R represents the rubber chain and H is normally an allylic hydrogen atom X = accelerator residue...

There exist a considerable number of compounds containing labile chlorine which bring about sulfurless vulcanisation at levels of approximately 3 phr. Basic chemicals such as lead oxides and amines are needed. It may be assumed that diene rubbers are crosslinked by such systems through the formation of C-C links this would mean, initially, hydrogen chloride is split off and later neutralised by the base. Examples of chemicals that act in this maimer are ... 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

The hydrogenation of unsaturated polymers and copolymers in the presence of a catalyst offers a potentially useful method for improving and optimizing the mechanical and chemical resistance properties of diene type polymers and copolymers. Several studies have been published describing results of physical and chemical testing of saturated diene polymers such as polybutadiene and nitrile-butadiene rubber (1-5). These reports indicate that one of the ways to overcome the weaknesses of diene polymers, especially nitrile-butadiene rubber vulcanizate, is by the hydrogenation of carbon-carbon double bonds without the transformation of other functional unsaturation such as nitrile or styrene. 

The unsaturated structure of the diene hydrocarbon rubbers makes them susceptible to attack by both oxygen and ozone. Oxidative degradation of all rubbers, irrespective of their structures, is inevitable as the energy associated with incident natural light is approximately three times that of a typical carbon-carbon or carbon-hydrogen bond. 

The stream from the cryogenic unit which is rich in C /C-olefins can be fractionated and selectively hydrogenated (to remov traces of dienes) to yield the pure olefins. Common uses of propene are the production of polypropylene, acrylonitrile, cumene etc. Butene can be catalytically dehydrogenated to butadiene which is used in the production of synthetic rubbers. 

An outstanding property of these polymers is their shear stability. The sonic shear stability testsfci indicate that these polymers are superior to some of the currently used polymers of ethylene-propylene or methacrylate type. The excellent stability of the hydrogenated diene-styrene polymers is attributed to their relatively low molecular weight and narrow distribution consistent with the established theory of shear degradation of polymers. The most recent developments in this field are block polymer VI improvers with dispersancy properties, built into the molecule by chemical modification of the rubber block. 2... 

One of the butadiene dimerization products, COD, is commercially manufactured and used as an intermediate in a process called FEAST to produce linear a,CO-dienes (153). COD or cyclooctene [931-87-5], obtained from partial hydrogenation, is metathesized with ethylene to produce 1,5-hexadiene [592-42-7] or 1,9-decadiene [1647-16-1], respectively. Many variations to make other diolefins have been demonstrated. Huls AG also metathesized cyclooctene with itself to produce an elastomer useful in rubber blending (154). The cyclic cis,trans,trans-tnene described above can be hydrogenated and oxidized to manufacture dodecanedioic acid [693-23-2]. The product was used in the past for the production of the specialty nylon-6,12, Qiana (155,156). 

Catalytic conversions were experimentally studied in Russia toward the end of the nineteenth century, and especially in the twentieth century, and regularities were empirically established in a number of cases. The work of A. M. Butlerov (1878) on polymerization of olefins with sulfuric acid and boron trifluoride, hydration of acetylene to acetaldehyde over mercury salts by M. G. Kucherov (1881) and a number of catalytic reactions described by V. N. Ipatieff beginning with the turn of the century (139b) are widely known examples. S. V. Lebedev studied hydrogenation of olefins and polymerization of diolefins during the period 1908-13. Soon after World War I he developed a process for the conversion of ethanol to butadiene which is commercially used in Russia. This process has been cited as the first example of commercial application of a double catalyst. Lebedev also developed a method for the polymerization of butadiene to synthetic rubber over sodium as a catalyst. Other Russian chemists (I. A. Kondakov I. Ostromyslenskif) were previously or simultaneously active in rubber synthesis. Lebedev s students are now continuing research on catalytic formation of dienes. 

During the last 15 years several companies have developed hydrogenated grades of nitrile rubber to both improve its thermal stability and solvent resistance. Although the hydrogenation of a poly diene backbone was done as early as the 1920s, real commercial products with acrylonitrile were not introduced until the mid-1980s.59... 

---