Thermal reactions of chloroprene

Ebdon, John Russell (1968). Thermal reactions of chloroprene. University of Birmingham. Ph.D.

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Abstract

Dilatometric studies of the overall dimerization of chloroprene in systems containing DPPH as a polymerization inhibitor have shown that it is essentially a second order process. However, there appears to be a small contribution to the rate from a third order dimerization reaction. The Arrhenius parameters for the overall process are similar to those of other diene dimerizations. The kinetics for the formation of the individual dimer isomers (cyclobutane and cyclohexene derivatives) have been studied using a high- temperature gas-liquid chromatographic technique. The two types of isomer have been shown to have similar energies and entropies of activation.

Evidence is presented which suggests that the mechanism of dimerization involves the three possible types of one- centre attack of one diene molecule upon another (1,1; 1,4; 4,4) to give intermediates which are probably diradical in character and which possess similar free energy. The third order reactions may involve no more than a transfer of energy between some of the dimer intermediates and additional monomer.

Studies of the slow polymerization, which occurs spontaneously in purified chloroprene stored under vacuum, have shown this to be a complex process. A previous study of the polymerization suggested that polymer is generally formed from the products of the dimerization reaction. The present work indicates that different mechanisms of polymerization are possible under different conditions. Unless the monomer is very carefully purified, an irreproducible rate of polymerization is observed which generally corresponds to the polymerization of chloroprene monomer. This is not a true thermal process and is probably initiated by traces of chloroprene peroxide. Occasionally, polymerization behaviour is observed which is more consistent with a polymerization of chloroprene dimers. It has been suggested that the initiator of this polymerization also may be an impurity present in the monomer, and that initiation may occur by a redox process. If chloroprene is extensively purified under a vacuum of 10\(^{-5}\) mm. Hg or better, an extremely low reproducible rate of polymerization is obtained. This reaction has been interpreted in terms of a mechanism involving the polymerization of dimer intermediates, and it would appear to be a true thermal process. The relationship between this polymerization and the type of dimer polymerization observed earlier is discussed.

Chloroprene pretreated with oxygen subsequently polymerizes at a rate much faster than those observed in the purified monomer. This polymerization has been shown to be a direct polymerization of monomer, and may be interpreted in terms of two parallel reactions, both initiated by chloroprene peroxides (probably 1,2- and 1,4-polyperoxides). Rate constants for the decomposition of one of the initiators have been evaluated from an analysis of the polymerization conversion/time curves (using a method developed by Tobolsky).

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):