Reversibility cyanoacrylate

Since amines initiate cyanoacrylate polymerization, the monomer cannot be isolated directly, because a polymer is generated immediately after formation of the monomer. An acid is then added to the polymer, and heat (140-180°C) is applied to the reaction mixture. Because of the relatively low ceiling temperature of the polymer, the pure monomer can be isolated, in greater than 80% yield, by the thermal reversion of the polymer back to the free monomer [4,5]. 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

Reversion is usually much faster in flexible materials because water permeates them more easily. Hydrolysis has been seen in certain epoxy, polyurethane, and cyanoacrylate adhesives. The reversion rate also depends on the type and amount of catalyst used in the formulations and the degree of crosslinking. Best hydrolytic properties are obtained when the proper stoichiometric ratio of base material to catalyst is used. 

These same researchers [317] reported the anionic polymerization of n-butyl cyanoacrylate in macroemulsion and miniemulsion. Dodecylbenzenesulfonic acid (DBSA) was used as the surfactant. The DBSA slows the rate of interfacial anionic polymerization through reversible termination, preventing an undesirably high degree of polymerization. Polymerization in macroemulsion resulted in a much higher degree of polymerization, perhaps due to droplet polymerization where the interface is less significant. 

Two experimental results lead the authors to conclude that reaction between pyridine and cyanoacrylate is reversible. 

If monomer and a comparatively high concentration of pyridine are mixed at room temperature, monomer equivalent to the pyridine concentration does not immediately react. (Acid, a fraction of the pyridine concentration, had to be added to suppress propagation.) At -95 °C there is little difference between the rates of phosphine and pyridine initiated polymerizations. Phosphine reacts rapidly with cyanoacrylate, so must pyridine. That an equivalent of monomer does not react very rapidly at room temperature suggests that this exothermic reaction is reversible. 

Johnston and Pepper35 have found similar behaviour in another anionic polymerization. When dichloromethane was substituted for tetrahydrofuran the rate of amine initiated cyanoacrylate polymerizations fell markedly. The dielectric constants of these two solvents are similar. However, THF is a good donor, weak charge acceptor solvent, methylene chloride the reverse. 

Chemical properties Polyether-based materials have higher resistance to hydrolysis but are more vulnerable to photo-oxidation than polyester types. Good chemical resistance. Solvents cause reversible swelling but not solution. PUs can be adhered with polyurethanes, nitriles, epoxies and cyanoacrylates. 

Ethyl cyanoacrylate is a little less polar than methyl cyanoacrylate, and has the ability to wet plastic surfaces more readily, and is a better solvent for plastics. With this added ability to make intimate contact with the surface, the bonds on plastic are stronger with ethyl cyanoacrylate than with the methyl ester. This difference in performance gives rise to the adage that methyl is for metal and ethyl is for everything else. Sometimes this difference can be utilized in reverse to good advantage to avoid stress cracking on such sensitive plastics as polycarbonate and polyacrylate. 

Fig. 7.9a-c. Previous cystectomy for transitional cell carcinoma of the bladder subsequent left nephrectomy for carcinoma of the renal pelvis, a Leak from left ureter (ileal conduit), b Cyanoacrylate glue plugging ureter (arrow), c Contrast study next day carried out with the patient in a prone position, resulting in reversal of the image no leak shown... 

Co-encapsulated doxorubicin and curcumin in chitosan/polyCbutyl cyanoacrylate) NPs Reversion of multidrug resistance [277]... 

Duan J, et al. Reversion of multidrug resistance by co-encapsulation of doxorubicin and curcumin in chitosan/poly(butyl cyanoacrylate) nanoparticles. Int J Pharm. 

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