Polymerization of Cyanoacrylate Monomers

Cyanoacrylate esters readily polymerize by several different mechanisms. Anionic polymerization is the most common and the most important method with respect to their use as adhesives. Typically this process is initiated by the nucleophilic contaminants found on the surfaces being bonded. The other routes by which cyanoacrylates polymerize or copolymerize involve radically initiated and photochemically initiated reactions. Radical polymerization or copolymerization of cyanoacrylates usually does 

Perhaps the one feature which distinguishes cyanoacrylate esters from all other olefinic monomers is their ability to undergo rapid anionic polymerization initiated by mild nucleophiles, even in the presence of contaminants like water and oxygen which efficiently inhibit most other anionic polymerizations. This is the reason that cyanoacrylates appear to cure spontaneously when spread in thin films between adherends. The structural features responsible for the reactivity of cyanoacrylates are the two strongly electron-withdrawing groups attached to the a-carbon atom (4). These 

The overall conversion of cyanoacrylate monomer to cyanoacrylate polymer can be considered in several stages initiation, propagation, chain transfer, and chain termination. The first stage, initiation, occurs when a nucleophile attacks the carbon atom of a monomer molecule (4) to generate the stabilized carbanion (8, Eq. 4). Depending on whether the 

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These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

Care must also be taken in the choice of rubber to insure that the rubber, or one of its additives, does not initiate the premature polymerization of the monomer. Even very low concentrations of a basic or nucleophilic material in the rubber or elastomer will cause the premature polymerization of an alkyl cyanoacrylate adhesive formulation. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Nanoparticles of synthetic polymers are usually manufactured by dispersion of preformed polymers. Although many methods can be used, they may be classified as monomer polymerization, nanoprecipitation, emulsion diffusion/solvent evaporation, and salting out. An appropriate method is selected mainly depending on polymer and drug natures. Polymerization of polymer monomers has been developed usually using poly(alkyl cyanoacrylate) [96,97]. Organic solvents are usually used in polymerization. A detailed description of this method is not provided here. 

This effect has an interesting consequence for the reactivity of the cyanoacrylate esters. Ethyl betaine will not initiate polymerization of butyl monomer. During addition of BCA to an ECA betaine not only would the stabilising influence of the adjacent ammonium ion be lost, but a thermodynamically less stable anion would have to be formed. 

The exceptionally fast rate of anionic polymerization of cyanoacrylates in the presence of a base, including water, made this class of monomers unique among all acrylic and vinyl monomers. Of the alkyl cyanoacrylate family of monomers, fhe mefhyl- and ethyl-esters are used extensively in industrial and consumer-type adhesives. Meanwhile, the isobutyl, fi-butyl, and n-octyl cyanoacrylate esters are used clinically as blocking agents, sealants, and/or tissue adhesives in different parts of the world due to their much lower toxicity as compared to their more reactive methyl coxmterpart. 

Cyanoacrylates are marketed as contact adhesives. Often popularly known as superglue, they have found numerous applications. In dry air and in the presence of polymerization inhibitors, methyl- and ethyl-2-cyanoacrylates have a storage life of many months. As with many acrylic monomers, air can inhibit or severely retard polymerization of cyanoacrylates. These monomers are, however, prone to anionic polymerization, and even a very weak base such as water can bring about rapid polymerization. 

Figure 5.82(b) represents capsule wall formation by direct polymerization of a monomer (A) such as n-alkyl cyanoacrylate at the water-solvent interface. In this case, water is dispersed in a water-immiscible solvent with the aid of an emulsifier and -alkyl cyanoacrylate is added to the solvent phase from where it difiiises to the solvent-water interface and polymerizes to poly( -alkyl cyanoacrylate), forming capsule wall membrane. 

The use of one-component organic systems for impregnating structures is attended by considerable difficulties for example, the cost of cyanoacrylate monomers is high they can be used only for the impregnation of absolutely dry structures they are quickly polymerized and hence cannot penetrate deeply into materials. The use of radiation for acrylate polymerization is hazardous, expensive, and technicsdly complicated. When polyisocyanates cure by interaction with water, carbon dioxide is liberated and comes out to the siuface where it forms continuous pores ... 

Kutal et reviewed the chemistry of several iron (II) metallocenes that are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1-dibenzoyl -ferrocenes in solutions of ethyl-a-cyanoacrylate produces anionic species that initiate the polymerization of electrophilic monomers. Irradiation of CsHs-Fe (t] - arene) in epoxide containing media generates several cationic species capable of initiating ringopening polymerizations. It was concluded that iron(II) metallocenes exhibit a diversity of photoinitiation mechanisms. 

Most of the investigations into cyanoacrylate polymerization have involved the study of dilute solutions using covalent bases such as amines or phosphines, so this information is not directly relevant to the polymerization of neat monomers between adherends. However, many of the prin-... 

Figure 4c illustrates interfacial polymerization encapsulation processes in which the reactant(s) that polymerizes to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase-continuous phase interface where polymerization occurs and a capsule shell is produced. This type of encapsulation process has been carried out at liquid-liquid and solid-liquid interfaces. An example of the liquid-liquid case is the spontaneous polymerization reaction of cyanoacrylate monomers at the water-solvent interface formed by dispersing water in a continuous solvent phase (10). The polyCalkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the solid-liquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controlled amoimt of surfactant. A water-immiscible monomer that polymerizes by free-radical polsrmerization is added to the system and free-radical pol5mierization localized at the core material-aqueous phase interface is initiated, thereby generating a capsule shell (11). 

Fig. 5. Base (B)-catalyzed anionic polymerization mechanism of cyanoacrylate monomer (M) (a) initiation, (b) propagation, and (c) transfer/termination by acid (HA).

The thermodynamic properties of several cyanoacrylate polymers have been determined using precision adiabatic and isothermal calorimetry (52-55). The Gibbs free energy AGq estimated from the enthalpy Aifo and entropy ASq of the bulk polymerization of various monomers showed that polymerization is thermodynamically feasible over the temperature range -270 to - -160°C at standard pressure. Ceiling temperatures Tc for polymerization were derived from the thermodynamic data and represent the upper temperature limit of polymerization. 

Volatile cyanoacrylate monomers, in particular methyl and ethyl cyanoacrylates, have characteristic acrid odors. Exposure to these materials at levels in the range of 1-5 ppm results in irritation to the eyes, nose, and throat (3). It is recommended that these monomers be used in a well-ventilated area, that skin and eye contact be avoided, and that the user be familiar with the relevant safety data supplied by product manufacturers. A recent epidemiological study in workers with occupational exposure to monomer vapors concluded that there was no increased risk of pulmonary obstruction (eg, asthma) on exposure to average short-time concentrations of less than 0.5 ppm (60). Polymerization of cyanoacrylates is rapid and exothermic and particular care should be taken to avoid bums, which can result from the unexpected bulk polymerization of inadequately stabilized or contaminated monomer samples. 

There are, however, several highly reactive vinyl monomers such as 2-(trifluoromethyl) acrylates and 2-cyanoacrylates that undergo anionic polymerizations in the presence of even weak bases. The photoinitiated anionic polymerizations of these monomers have been achieved using a number of photosensitive metal complexes. For example, the irradiation of alkali salts containing the trans-[Cr(NH3)2(NCS)4] anion at wavelengths in the range of 350-532 nm releases the thiocyanate anion (SCN"). As depicted in Scheme 34, the thiocyanate anion is capable of initiating the anionic chain polymerization of ethyl 2-cyanoacrylate. ... 

As described by the Kutal research group, another approach to the photoinitiated anionic polymerization of ethyl 2-cyanoacrylate involves the photolysis of the platinum bis(acetylacetonate) complex, 117. The irradiation of 117 at wavelengths above 300 nm results in the liberation of the acetylacetonate anion (118, Scheme 35) that is capable of initiating the polymerization of the monomer in a manner similar to the thiocycanate anion as shown in Scheme 34. 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

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