Diene polymers, hydrogenation

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. 

Table 19.1 Outline of major patents with respect to catalysts for diene-based polymer hydrogenation. 

RhCl(PPh3)3 has been used for the homogeneous hydrogenation of various diene-based polymers, and its catalytic mechanism is understood to a considerable extent. Parent et al. [81] proposed a mechanism which has been found to be consistent with the kinetic data for various diene-based polymer hydrogenation systems and an understanding of the coordination chemistry of RhCl(PPh3)3 in solution. The main points comprising the mechanism are outlined as follows ... 

Hergenrother, W.L. Doshak, J.M. Process and Isomerizing Agent for Isomerizing Partially Hydrogenated Diene Polymers. U.S. Patent 5,314,967, May 24, 1994 Bridgestone/Firestone. 

Since oxidation is a chemical process, the established carbon-hydrogen bond strengths can be used to estimate ease of hydrogen abstraction. Table 5.3 [75]. These suggest that PMA backbones should be most stable to oxidative attack OCP polymers would be expected to be less stable whilst the stability of styrene-diene polymers could vary depending on the extent of 1,2 vs. 1,4 structures present. The benzylic hydrogens from styrene are potential sites for attack, and it is obviously critical that hydrogenation of the olefinic unsaturation is as complete as possible. 

Table IV. Effect of Hydrogenation and Block Structure in 25% Styrene-Diene Polymers...

In the past few years, the hydrogenation of a variety of diene polymers and copolymers made with anionic initiators and d-orbital transition metal catalysts has been studied extensively. It is of interest to investigate the hydrogenated... 

Standard alkene reactions such as hydrogenation, halogenation, hydrohalogenation and isomerization as adapted to a diene polymer situation. 

Since 1,4-polyisoprene has a secondary carbon atom at the double bond it follows that it is generally more reactive to both free radicals and to carbonium ions than 1,4-poly butadiene. The typical addition reactions associated with the double bond suggest that the ultimate hydrogenated, halogenated, hydrohalogenated and isomerized diene polymers would have the same structure irrespective of the initial cis-ltrans- ratio. 

In recent years there has been increased use of the transitory species, diimide, for hydrogenation of alkenes and this has been applied to diene polymer systems (Harwood et a/., 1973 Mango and Lenz, 19731. 

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... 

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