Cyanoprene

There are three processes described in the literature for synthesizing cyanoprene (1-5). 

Besides the 1,4- and 3,4-adducts, cyclic structures are also present. Four representative radical polymerizations of cyanoprene are compared in Table I. Differential thermal anlysis shows, for the material made from toluene, a glass stage of about 10°C and a melting range above 90°C. The material has medium crystallinity while the material made in THF is largely amorphous. 

Anionically Initiated Polymerization. The disadvantages of radical polymerization of cyanoprene result from the operating conditions (temperatures) too many side reactions, chain-terminating reactions, and consecutive reactions occur. Because of this and the dimerization tendency of cyanoprene, catalysts had to be found that could fulfill two contradictory requirements. They should be so reactive that it would be possible to work at temperatures that exclude dimerizations as com-... 

In view of these preconditions, it was obvious to use anionic initiators. With the lithium alkyls used by Wei—for example, butyllithium or other organic alkali compounds—only insoluble thermoplastic products are obtained, even at — 80 °C in both THF and toluene because these initiators are too reactive. The reactions that take place include attack by the metallo-organic compound and the already metallized cyanoprene (or the metallized polymer) on the nitrile groups of both monomer and polymer. 

Table II. Cyanoprene Homopolymers Produced by Different Catalyst...

Regarding the structure of the cyanoprene homopolymers (Table II), a trend becomes apparent when using the different catalyst systems. In a sample polymerized with diphenylphosphine lithium, 70% 1,2-linkages and about 30% 1,4-linkages were found, but the presence of traces of 3,4-adducts could not be excluded. There was no indication of cyclic structures although with stronger diphenylphosphine-potassium cyclic compounds were found. About 62% were 1,2-linkages while the remainder consisted of about equal parts of 1,4- and 3,4-adducts, the cyclic proportions included. 

Cyanoprene can also be polymerized with tributylphosphine or other trialkylphosphines but only in toluene, benzene, etc., and, not in ethers. The mechanism is probably similar to that with amides or macroions in a living polymer mechanism, or, again, transfers would have to be assumed. The former may be assumed this way ... 

Figure 1. Infrared spectrum of anionically polymerized cyanoprene...

Coordinate Homopolymerization. When Ziegler-Natta catalysts of the type TiCl4/alkylaluminum compounds are used, no polymerization occurred because the cyanoprene (like acrylonitrile for instance) reacts with the catalyst and destroys it. Polymerization occurs, however, when metal acetyl acetonates and organoaluminum compounds are used. For example, coordinate polymerization with a mixture of cobalt acetyl acetonate and ethylaluminum dichloride results in a polymer that corresponds mainly to the radical-produced polymer. 

Figure 3. Emulsion copolymer of chloro-prene/cyanoprene (14% cyanoprene polymer) shear modulus plotted over temperature.

Radical Copolymerization. We carried out an emulsion copolymerization with chloroprene at a chloroprene cyanoprene ratio of 9 1. At 60% conversion, an insoluble copolymer with cyanoprene lumped... 

Anionic Copolymerization. For normal mixing-type copolymerization, the cyanoprene was added to the solvent containing the catalyst and the comonomer. Depending on the initial proportions, of monomers, products were obtained with varying cyanoprene proportions and correspondingly different properties. 

For example, in one experiment, toluene, chloroprene, and tribu-tylphosphine were placed in a vessel, and a cyanoprene-toluene solution was aded dropwise at 0°C. The result was a 60% yield of a thermoplastic product with 14.2% nitrogen, corresponding to a chloroprene proportion of 17%. Analogous experiments were carried out with monomers such as styrene and isoprene, and with other catalysts. 

A further possibility consists of subjecting the catalyst to incipient polymerization with small quantities of styrene, isoprene, or butadiene (organic alkali compounds also can be used for this purpose), then adding dropwise a mixture of cyanoprene and selected monomers at low temperatures. If no incipient polymerization takes place, metallo-organic catalysts will only give a homopolymerization of the cyanoprene. 

When block copolymerization was done, isoprene, styrene, or butadiene was prepolymerized at room temperature, and cyanoprene polymerized at low temperatures. Styrene gave thermoplastic polymers butadiene, depending on the quantity ratio selected, gave thermoplastic to rubberlike polymer soluble in polar solvents such as DMF, but largely resistent to organic solvent such as toluene. 

The expressions are an outcome of the terminal model theory with several steady-state assumptions related to free-radical fiux (14,23). Based on copolymerization studies and reactivity ratios, chloroprene monomer is much more reactive than most vinyl and diene monomers (Table 1). 2,3-Dichloro-l,3-butadiene is the only commercially important monomer that is competitive with chloroprene in the free-radical copolymerization rate. 2,3-Dichlorobutadiene or ACR is used commercially to give crystallization resistance to the finished raw polymer or polymer vulcanizates. a-Cyanoprene (1-cyano-l,3-butadiene) and /3-cyanoprene (2-cyano-1,3-butadiene) are also effective in copolymerization with chloroprene but are difficult to manage safely on a commercial scale. Acrylonitrile and methacrylic acid comonomers have been used in limited commercial quantities. Chloroprene-isoprene and chloroprene-styrene copolymers were marketed in low volumes during the 1950s and 1960s. Methyl methacrylate has been utilized in graft polymerization particularly for vinyl adhesive applications. A myriad of other comonomers have been studied in chloroprene copolymerizations but those copolymers have not been used with much commercial success. 

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