Unsaturated hydrocarbons polymerization

In Group 14 (IV), carbon serves as a Lewis base in a few of its compounds. In general, saturated ahphatic and aromatic hydrocarbons are stable in the presence of BF, whereas unsaturated ahphatic hydrocarbons, such as propjdene or acetylene, are polymerized. However, some hydrocarbons and their derivatives have been reported to form adducts with BF. Typical examples of adducts with unsaturated hydrocarbons are 1 1 adducts with tetracene and 3,4-benzopyrene (39), and 1 2 BF adducts with a-carotene and lycopene (40). 

Fig. 1. Slurry butyl mbber polymerization C2, C, and are unsaturated hydrocarbon streams.

The earliest theory, advanced by Fischer and Tropsch in 1926 (84), proposed that the reaction proceeded via formation of intermediate metal carbides which react on the catalyst surface to form methylene groups. It was then suggested that these methylene groups polymerize on the surface to form hydrocarbon chains, which desorb as saturated and unsaturated hydrocarbons. In 1939 Craxford and Rideal expanded the carbide theory, proposing (85), for cobalt-based catalysts, the following reaction sequence ... 

As another case study a process synthesis of an emulsion polymerization process is given (Hurme and Heikkila, 1998). In emulsion polymerization unsaturated monomers or their solutions are dispersed in a continuous phase with the aid of an emulsifier and polymerized. The product is a dispersion of polymers and called a latex. The raw materials are highly flammable unsaturated hydrocarbons and the reaction is exothermic which both cause a risk. The main phases and systems in an emulsion polymerization plant are listed in Table 31. 

The polymerization products of propylene have been observed to be saturated hydrocarbon polymers and terpenelike unsaturated hydrocarbons (Kuhn, 64). The condensation of formaldehyde with phenols and cyclohexanols by means of aqueous hydrogen fluoride has also been observed (Badertscher el al., 65). 

The role of gas phase initiation processes was further explored by Tibbitt et al. . These authors proposed that the polymerization of unsaturated hydrocarbons in a 13.56 MHz plasma is initiated by free radicals formed in the gas by electron-monomer collisions, the initiation reactions listed in Table 6. Moreover, it was assumed that the formation of free radicals on the polymer surface due to the impact of charged particles could be neglected. This assumption is supported by the fact that at 13.56 MHz and pressures near one torr the discharge frequency is significantly greater than either f, or f and that as a result the fluxes of charged particles to the electrode surfaces are quite small. 

The Mechanism and Kinetics of Plasma Polymerization Table 6. Reaction Mechanism for Plasma-Polymerization of Unsaturated Hydrocarbons... 

Table 7. Fitted Rate Coefficients for the Plasma Polymerization of Unsaturated Hydrocarbon Monomers...

Before 1925, there were a few compounded oils made for special purposes, such as lubrication of marine engines and steam cylinders, but additives were not used in automotive crankcase oils. In the 1930 s, chemical compounds made by condensation of chlorinated paraffin wax with naphthalene were found to lower the pour points of oils. Pour depressants (9) apparently are adsorbed on small wax crystals which separate from oils when they are chilled. The protective adsorbed layer of additive prevents the normal interlacing of larger wax crystals which forms a gel. In 1934 polymerized unsaturated hydrocarbons first came into large scale commercial use to lower the temperature coefficient of viscosity of oils. Other compounds for increasing the viscosity index of oils have since become common. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

Because of its chemical and practical significance, polymerization is exhaustively reviewed and excellent monographs,114-129 a multivolume encyclopedia,130 as well as the series Advances in Polymer Science,131 cover all aspects of the field. Consequently, our aim is not a comprehensive treatment, but to discuss the most important general aspects of the major types of polymerization of unsaturated hydrocarbons and to give selected examples of practical significance. 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

Silver ions form similar alkene complexes which are soluble in aqueous solution and may be used to effect the separation of unsaturated hydrocarbons from alkanes. Catalysis for the polymerization of alkenes also form metal-alkene complexes which lead to polymerized product. 

A similar mechanism has been proposed for photonitrosylations, with the difference that the reaction of the alkyl radical with NOC1 (Eq. 5) is not competitive (absence of chain reaction) and that the rate of reaction 6 cannot prevent dismutation (Eq. 4) and subsequent radical polymerization of unsaturated hydrocarbons (e.g., cyclohexene in the case of caprolactam synthesis). 

Radical generating systems of this sort may be used for the initiation of many addition polymerization reactions including those of acrylonitrile and unsaturated hydrocarbons. The information on systems other than those derived from hydrogen peroxide is very meager. 

The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. Mol and J. A. Moulijn One-Component Catalysts for Polymerization of Olefins Yu. Yermakov and V. Zakharov The Economics of Catalytic Processes J. Dewing and D. S. Davies Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases... 

Higher hydrocarbons are formed by the polymerization of ethylene. Any higher unsaturated hydrocarbons present are converted to the corresponding alcohols by hydration. 

The silyl fragmentation in superacids initiated by a controlled temperature increase is a method to generate persistent carbocations, such as the vinyl cation 378, which are not accessible by direct protonation of unsaturated hydrocarbons because of excessive formation of oligomeric and polymeric products. 

Saturated hydrocarbons have lower electron affinity than unsaturated hydrocarbons. In dry argon, the decalin anion radicals are formed, if at all, in extremely low concentration. It suggests that their further reactions are insignificant. In the presence of decalin, no wear occurs. In dry or wet air, decalin works worse, but still effectively. The anion radicals of oxygen, which are formed in greater concentration than that of the anion radicals of decalin, can initiate decalin oxidation. Oxidation products were capable of accepting exoelectrons and were involved in further reactions, with the formation of polymeric or organometallic lubricants. 

To combine olefinic gases by polymerization to form heavier fractions, the combining fractions must be unsaturated. Hydrocarbon gases, particularly olefins, from cracking reactors are the major feedstock of polymerization. 

---