Properties and application of 2-Cyanoacrylates

ISSN 1811-2382, Polymer Science, Ser. C, 2007, Vol. 49, No. 3, pp. 240–244. © Pleiades Publishing, Ltd., 2007. Original Russian Text © Yu.G. Gololobov, 2007, published in Klei. Germetiki. Tekhnologii, 2004, No. 5, pp. 4–8. Properties and Application of 2-Cyanoacrylates Yu. G. Gololobov Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia e-mail: yugol@ineos.ac.ru; zavin@ineos.ac.ru Received December 12, 2003 Abstract—The properties of 2-cyanoacrylates are considered. The geometric structure of 2-cyanoacrylic acid and its 1-adamantylmethyl ester is established by X-ray diffraction methods. Two methods for expanding the temperature range of the service lives of adhesives based on crosslinked 2-cyanoacrylates are considered. The first method is based on the incorporation of unsaturated carbon–carbon bonds into ester fragments of alkyl 2cyanoacrylates followed by crosslinking via these bonds. The second method consists in the copolymerization of methyl or ethyl cyanoacrylates with different unsaturated compounds. The examples of the application of 2-cyanoacrylates in medicine for pulmonary, cardiac, renal, osseous, and dental surgery are presented. DOI: 10.1134/S1811238207030071 INTRODUCTION Alkyl 2-cyanoacrylates (ACAs) were obtained as early as in 1949 [1] and, already in 1955, it was proposed that they be employed as one-component, coldcuring adhesives [2]. Then the commercial production of ACA-based fast-curing adhesives was organized [3, 4]. Alkyl 2-cyanoacrylates are unique adhesives, primarily because of two specific features. First, they are matchless in the rate of bonding. At room temperature, bonded surfaces are fastened over a few seconds and a high strength rapidly develops in adhesive joints. Second, ACAs efficiently bond both living tissues (they are authorized for application in medicine) and organic materials (plastics, rubbers, and wood), as well as inorganic materials (stones, metals, glass, porcelain, and ceramics) in different combinations. The exceptions seem to be only Teflon and polyethylene, which need surface treatment with special activators (amines, phosphines, etc.) before bonding [5, 6]. Because ACAs are capable of instantaneous polymerization (and, hence, of efficient bonding) under the effect of trace amounts of water and other diverse compounds, the polymer chemistry of ACAs was practically the only field thoroughly studied before the beginning of the 1990s. For example, the polymer chemistry of ACAs was mainly considered in monographs [3, 4] and reviews [7–9] published in the 1980s. Among the monomeric derivatives of 2-cyanoacrylic acid, only its esters were described. A new stage in the development of ACA chemistry is associated with the synthesis of cyanoacrylic acid (CAA) [10] and the use of this acid and its derivatives in organic synthesis [11, 12]. PHYSICAL PROPERTIES OF 2-CYANOACRYLATES During 50 years elapsed from the preparation of the first alkyl 2-cyanoacrylate, approximately 100 compounds of this type have been synthesized. A large group consists of ACAs comprising different alcohol fragments [13–25]. Among these compounds are esters containing double and triple bonds; hinged oxygen bridges; carbonyl and carboxyl groups; and chlorine, fluorine, and silicon atoms. Esters of di-, tri-, and polyhydric alcohols [15, 26, 27] compose a special group. Numerous cyanoacrylates based on mercaptans [16] or phenols [15] should be noted. Whereas cyanoacrylic acid and many of bis(cyanoacrylates) are crystalline compounds, the majority of CAA esters are liquids with boiling points of 50–150°C at 0.2–2.0 mmHg. The IR spectra of various ACAs and bis(cyanoacrylates) are similar, as they display four characteristic absorption bands at 1600–1640 (C=C), 1720–1750 (C=O), 2260 (CN), and 3100 cm–1 (CH2 groups in cyanoacrylate fragments) [28]. Signals due to olefin protons of CH2=C(CN) groups (6.9 and 7.1 ppm in deuterated acetone and 5.5 and 6.3 ppm in deuterated benzene) are the most typical of the 1H NMR spectra. The 13C NMR spectra of 2-cyanoacrylic acid (deuterated acetone) demonstrate signals due to carbon atoms of CN (114.9), C–CN (116.7), CH2 (144.1), and COO (161.3 ppm) groups. According to the X-ray diffraction analysis, 2-cyanoacrylic acid molecules have a flattened structure [29] (see figure). The maximum deviation of nonhydrogen atoms from the middle plane of a molecule amounts to 0.150 Å. The length of C(1)–C(2) bonds (1.492 Å) only slightly 240 PROPERTIES AND APPLICATION OF 2-CYANOACRYLATES exceeds the standard C–C distance (1.475 Å), thus indicating that there is no marked conjugation of C(1)=O and C(2)=C(3) double bonds, with the length of the latter also coinciding with the standard value of 1.321 Å for C=C bonds. The lengths of C(2)–C(4) and C(4)–N bonds are close to the standard values in compounds containing C=C–C=N groups in which the conjugation is probably realized. Strong hydrogen bonds OH…N (1.88 Å) connect molecules in a head-to-tail manner to form zigzag chains. The mutual location of C=C bonds in adjacent molecules of the acid is unfavorable for topochemical reactions in crystals, which is consistent with the resistance of CAA crystals to illumination and prolonged X-ray exposure. 1-Adamantylmethyl ester of CAA and bis(cyanoacrylate) prepared by the transesterification of methyl cyanoacrylate with 1,10-decanediol were also investigated by X-ray diffraction analysis [18]. It was shown that the geometry of 1-adamantylmethyl ester of CAA and bis(cyanoacrylate) is similar to that of CAA. However, the strong electron acceptor groups impart a markedly acidic character to methylene hydrogen atoms of ACAs, which explains the presence of short intermolecular contacts involving CH2 groups in the crystals of examined compounds. It is possible that it is the acidic character of methylene hydrogen atoms that results in the easy polymerization of ACAs induced by the nucleophilic and basic agents through the coordination of the latter with protons of ACA methylene groups. PROSPECTS OF THE APPLICATIONS OF 2-CYANOACRYLATES The intensive (albeit quite short) development of the monomer chemistry of ACAs has led to two important results. On one hand, the chemistry of ACAs stimulated the study of the chemical properties of zwitterion 7 formed from ACAs and trialkylphosphines, which resulted in the discovery of a new reaction of the incorporation of carbamide fragments via C–C bonds. This, in turn, stimulated the development of a catalytic method, namely, the intramolecular electrophilic catalysis involving phosphonium sites. On the other hand, even the first studies of the chemical properties of CAA and its esters made it possible to rely on the synthesis of new ACAs, which enabled us to substantially expand the range of the service characteristics of cold-curing adhesives and to develop new routes for their application in the industry, medicine, and organic synthesis. Ways for Expanding the Temperature Range of the Service of Adhesives Based on 2-Cyanoacrylates The strength of ACA-based adhesive joints depends on at least two factors, all other conditions being equal; these factors are the adhesiveness of a used cyanoacryPOLYMER SCIENCE Series C Vol. 49 No. 3 2007 241 C(3) O(2) 1.325 1.492 C(1) C(2) 1.436 O(1) C(4) 1.140 N(1) Molecular structure of 2-cyanoacrylic acid late per se and the stability of an adhesive joint under the conditions of item service (temperature, humidity, and aggressive medium). At room temperature, commercial methyl and ethyl cyanoacrylates form very strong adhesive joints; however, their stability (especially in aggressive and humid media) at elevated (above 80–100°C) and decreased (below –100°C) temperatures is low. At the same time, according to the numerous experimental data, the application limits of cyanoacrylate adhesives have been noticeably expanded and their quality parameters have been improved. A relatively low stability of ACA-based polymers with respect to severe service conditions can be explained by the presence of quaternary carbon atoms in polymer backbones. It is known that polymers with such groups have a low thermal stability [30]. Therefore, the heat resistance of ACA-based adhesive joints may be enhanced via the modification of polymer backbones by incorporating fragments that would increase the resistance of polymer chains to temperature and aggressive media (mainly aqueous media with different pH values). Available theoretical calculations [4, 31, 32] give only general recommendations concerning the improvement of the quality of ACA-based adhesives. One of the ways to solve this problem is the formation of crosslinked structures [33]. Two approaches to the formation of ACA-based crosslinked structures are described in the literature. One of these consists of the incorporation of unsaturated carbon–carbon bonds into the ester fragments of ACAs followed by the crosslinking of the structures through these bonds. Allyl, propargyl, and more bulky unsaturated CAA esters have a low viscosity, which is a necessary condition for the development of an adhesive–substrate interfacial contact at the first stage of adhesive joint forma- 242 GOLOLOBOV tion [20, 28, 34–36]. Under the effect of trace amounts of water and active ionic groups present on bonded surfaces, acrylate C=C bonds are opened [34, 37]. The thermodynamic investigation of allyl and ethoxyallyl cyanoacrylates [38] has evidenced that at room temperature, these monomers are completely transformed into corresponding polymers via the anionic mechanism and then, at temperatures above 100°C, allyl and propargyl bonds are opened to form crosslinked structures [20, 34, 35, 39]. According to [40], the substrate–adhesive interfacial interaction results from the action of the van der Waals and dipole–dipole forces. The aforementioned bonding forces are supplemented with chemical bonds of different natures (adsorption theory of adhesion) [41]. Note that while cyanoacrylate C=C bonds are opened through the anionic mechanism, the multiple bonds in ester groups are opened only with the involvement of free radicals [34]. As a result, adhesive joints based on unsaturated cyanoacrylates demonstrate higher service characteristics than corresponding joints based on saturated analogs [20, 42]. The main drawback (from the viewpoint of practical applications) of the crosslinked structures based on unsaturated 2-cyanoacrylates is the fact that the adhesive joints are strengthened only at elevated temperatures (>100°C), whereas in many cases, the strengthening must be performed at lower temperatures. Moreover, this procedure may give rise to the formation of a rigid crosslinked polymer, thus imparting brittleness to an adhesive layer [35]. When elastomers are bonded with the adhesives, they can be partly dissolved in ACAs to yield mutually penetrating networks [43] (this process corresponds to the diffusion theory of adhesion). Another approach to the formation of crosslinked adhesive layers implies the copolymerization of methyl or ethyl cyanoacrylates with different unsaturated compounds. It is obvious that the selection of copolymers makes it possible to form adhesive joints with a broad set of properties. This approach is more fruitful because it enables us, in some cases, to form “crosslinked” adhesive layers at lower temperatures. Unsaturated compounds with electronegative polar groups were used to form such copolymers. On one hand, such substituents cause the monomers to copolymerize with ACAs, while on the other hand, they also generate additional forces of bonding with substrates. Much progress has been attained in this way [3, 4, 43–51]. Apparently, CAA esters with dihydric [52] and tribasic [53] alcohols have an almost ideal structure because adhesive joints are, in this case, formed at a high rate under conditions similar to those of the main monomer polymerization. Unfortunately, bis- and tris(cyanoacrylates) are difficult to produce in large amounts at present. The methods proposed in [15, 52] are labor-consuming and expensive and the direct esterification of CAA [17, 18, 26] or its chloride and the transesterification of methyl cyanoacrylate with dihydric alcohols [54] are also only of significance in the laboratory. According to [33, 55–61], the possibility of ACA copolymerization with esters of 2-cyano-2,4- pentadienoic acid is now being extensively studied. Derivatives of ethylene glycol and 2-cyano-2,4-pentadienoic acid are especially promising [7]. At room temperature, these derivatives polymerize under the action of the same catalysts that induce ACA polymerization to form crosslinked structures. The service characteristics of ACAs are substantially improved when they are applied in combination with butadiene derivatives [33, 55]. The synthesis of the crosslinking agents under consideration is based on the Knoevenagel reaction carried out using the corresponding esters of cyanoacetic acid and aldehydes [56, 57]. CH2(CN)COOEt + R'H=CHC(O)H ZnCl2 –H2O R'CH=CHCH=C(CN)COOEt R' = H, Me. This reaction is peculiar in the fact that dehydrated zinc chloride dissolved in dioxane or THF is employed as a catalyst. The synthesis of bis(2-cyanopentadienoates), i.e., dihydric alcohol derivatives with disiloxane units in backbones, is described in [60]. The Use of 2-Cyanoacrylates In Medicine The ability of ACAs to polymerize over a few seconds under extremely mild conditions on the surface of living tissues without any special initiation enables us to consider them to be promising materials for surgery [62]. It is of essential importance that ACA-based polymers degrade quite rapidly under the conditions of a living organism [63]. Isobutyl and isoamyl cyanoacrylates are characterized by a high biocompatibility, low toxicity, and antimicrobial properties. 1,2-Isopropylideneglyceryl cyanoacrylates [64] are very promising. Monitoring of the recovery of tissue cells and other body characteristics in the course of laboratory and clinical studies demonstrated that ACAs provide a strong and elastic bonding of tissues with an antiseptic effect and no harmful consequences [63]. Biological adhesives of this type are used in pulmonary, cerebral, cardiac, renal, hepatic, gastrointestinal, ophthalmologic, respiratory, osseous, and dental surgery [3, 4, 65–68]. Russian, Ukrainian, and Azeri chemists and physicians of the former Soviet Union developed efficient biological adhesives of the MK series and the CO-9 m, CO-9t, and CO-57 grades in which fluorinated methacrylates are used as comonomers. These compositions are nontoxic and possess bacteriostatic and bactericidal properties [69]; they are resistant to disinfectants and their biodegradation is not accompanied by the formaPOLYMER SCIENCE Series C Vol. 49 No. 3 2007 PROPERTIES AND APPLICATION OF 2-CYANOACRYLATES tion of toxic metabolites. They induce no immunological reactions [70]. In addition, ACAs are applied in medicine for the preparation of controlled-release drugs. A method was developed for the incorporation of drugs into ACAbased polymer matrices. When ACAs are added to a drug solution under vigorous stirring, they polymerize with a concomitant sorption of drug molecules in polymer matrix particles with sizes of 170–350 nm. Apomorphine [71] and oxytocin [72] were obtained in this manner. The procedure is general and can obviously be realized in different variants. Other Applications of Cyanoacrylates ACA-based polymers are employed to produce photo- and electronoresists. A photoresist with a sensitivity of 0.2 J cm–1 was obtained by the chemical deposition of perfluoroethyl cyanoacrylate vapor on a substrate [73]. Positive electronoresists were prepared on the basis of ACAs and their copolymers with functionalized monomers [67]. The lengthening of hydrocarbon chains in ACA ester groups markedly deteriorates the adhesive properties of these compounds. ACAs with ester chain lengths of more than six methylene units are used for the formation of monomolecular layers by the Langmuir–Blodgett method [74]. When forming the monomolecular layers, CAA ethers (from hexyl to dodecyl) are polymerized directly on the water surface. Since the polymer cyano group–water interaction energy is low (14.6 kJ), the monolayers thus formed are easy to transfer onto solid substrates. At the same time, the monomolecular films of poly(cyanoacrylates) have a certain adhesion to both hydrophilic and hydrophobic surfaces. The above method of Langmuir–Blodgett film preparation may be applied in the submicron technology for microinstrument production. Cyanoacrylates are readily soluble in liquid carbon dioxide [75] and, as they are packed into aerosol flasks, they may be used when it is required to obtain ACA vapor with high concentrations. 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