This invention relates to the production of cyclodimers of dienes, particularly to the inhibition of polymerization of the dienes during production of the cyclodimers (cyclodienes).
Such soluble iron complex catalysts as dicarbonyldinitrosyl iron Fe(NO)2(CO)2] are known to facilitate the dimerization of 1,3-butadiene to produce 4-vinylcyclohexene (VCH). This process is taught in such references as "The Catalytic Dimerization of Dienes by Nitrosylcarbonyl Transition -metal Compounds", J. P. Candlin and W. H. Janes, J. Chem. Soc. (C)., p. 1856 (1968), and "Catalytic Reactions Involving Butadiene. I. Selective Dimerization to 4-vinylcyclohexene With Polymetallic Precursors", 1. Tkatchenko, J. Organometallic Chem., vol 124. pp. c39-c42, (1977).
A. Mortreux and F. Petit ("A New Route to Coordination Catalysis by Electrogeneration of Organometal Transition Reactive Species - A Review", Applied Catalysis vol 24, pp. 1-15, (1986)) have taught that the zero valent dinitrosyliron fragment, "Fe(NO).sub.2 " is central to the activity of such catalysis. They also discuss a proposed mechanism for the catalysis.
In such a dimerization processes, however, it is noted that there is also some polymerization of the diene to polydiene, e.g. butadiene to polybutadiene. The polymerization and resulting polymer not only consume starting material but also produce by-products which contaminate the vinylcyclohexene and must be removed from it. Additionally, the polymer fouls equipment. Several known inhibitors of butadiene polymerization also can react with the iron complex dimerization catalysts. Such inhibitors which are commonly used include 4-tertiary-butyl catechol (TBC) and N,N-diethylhydroxylamine (DEHA) often in combination with phenylenediamine. (see "Butadiene Polymers", chapter by W. M. Saltman in "Encyclopedia of Polymer Science and Technology", vol 2, p 683 (1965): "Butadiene, General", chapter by T. Reilly in "Encyclopedia of Chemical Processing and Design", vol 5, p 156, (1986): and Japan Patent 61,130,242 (Chemical Abstracts 105:173217). These inhibitors possess active hydrogens capable of being reduced by the low valent iron complexes. Such an oxidation of the iron would render the complex incapable of performing its catalytic task. In addition, the weak acid nature of such inhibitors will leave equilibrium controlled amounts of anions capable of coordinating to the iron and blocking sites of catalytic activity.
It is known to use certain hindered phenols to inhibit certain polymerizations but not polymerization of butadiene.
It would be desirable to have an inhibitor which would not react with the complex, but would inhibit the formation of polybutadiene.
Such polymer inhibition generally involves radical scavenging and chain termination, thought to involve traces of oxygen. Thus, certain compounds commonly referred to as antioxidants can function as polymerization inhibitors. One instance of such inhibitors is the use of 3,5-di-t-butyl-4-hydroxyanisole to inhibit polymerization of acrylamides. (Chemical Abstracts 86:107210)