Cyanoacrylates anionic polymerization

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. 

Cyanoacrylate adhesives cure by anionic polymerization. This reaction is catalyzed by weak bases (such as water), so the adhesives are generally stabilized by the inclusion of a weak acid in the formulation. While adhesion of cyanoacrylates to bare metals and many polymers is excellent, bonding to polyolefins requires a surface modifying primer. Solutions of chlorinated polyolefin oligomers, fran-sition metal complexes, and organic bases such as tertiary amines can greatly enhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a critical component of these primers, as solvent swelling of the surface facilitates inter-... 

Polymers are also used as sutures. Fighters and other athletes have used poly(alpha-cyanoacrylates), super glues, to quickly stop blood flow in surface cuts. Today, super glue is also used for, in place of or along with, more traditional polymeric suture threads for selected surface wounds, internal surgery, and retinal and corneal surgery. The alpha-cyanoacrylate polymers (structure 19.22) undergo anionic polymerization in the presence of water. More about sutures is explained in Section 19.6. 

Fig. 2.13 The three basic steps in producing a polymer from the cyanoacrylate monomer via anionic polymerization (moisture curing)...

The value of Fn was determined by 1H NMR spectroscopy and found to be close to unity. By essentially the same method, bifunctional and trifunctional cyanoacrylate functional PIBs have also been prepared. Anionic polymerization of CA-PIB with N,N-dimethyl-p-toluidine as initiator in solution resulted in high MW product (Mn 35,000 gmol ) [107]. Anionic copolymerization of difunctional and trifunctional PIB yielded clear flexible films with low sol fraction. The... 

Similarly, zwitterionic tetramethylenes as initiators of anionic polymerization were also observed. For example, methyl a-cyanoacrylate polymerizes via an anionic mechanism in the presence of n-butyl vinyl ether [90]. A Diels-Alder adduct is also formed. In another example, the reaction of isobutyl vinyl ether and nitroethylene leads to an unstable adduct [91], which is capable of initiating the anionic polymerization of excess nitroethylene, and also the cationic polymerization of added VCZ. 

Although the anionic polymerization mechanism is die predominant one for die cyanoacrylic esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend die usable shelf life, free-radical stabilizers such as quinones or liindered phenols are a necessary part of the adhesive formulation. 

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. 

An interesting characteristic of mthenocene is that it is a photoinitiator for the anionic polymerization of ethyl 2-cyanoacrylate. Solutions with millimolar concentrations of the complex in neat cyanoacrylate polymerize to a hard, plastic solid within seconds upon exposure to light. The low concentration of mthenocene required for the polymerization makes it desirable in applications such as coating and adhesives that require solvent-free solutions of monomer. ... 

In a review of anionic polymerization in 1960s), Swarc notes that bubbling isobutene through methyl cyanoacrylate yields a 1 1 copolymer. He suggests that a highly polar... 

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. 

Cyanoacrylates undergo easily anionic polymerization in the presence of weak bases, including H2O. They are used in commercial products (Super glue) mixed with traces of acidic stabilizers, which are neutralized in contact with various surfaces during the use. 

Alkyl cyanoacrylates are used extensively as instant adhesives and are grouped with vinylidene cyanide, methylene malonates, and a-cyanosorbates at the top of the anionic reactivity scale. As well as readily polymerizing in the presence of conventional anionic initiations such as Bu"Li, they also react readily with tertiary amines and phosphines. Pepper and Johnstonhave examined the catalytic action of triethylphosphine and pyridine at —78 °C in tetrahydrofuran. The former initiator appears to add rapidly and irreversibly to monomer to yield a conventional living anionic polymerization. With pyridine, however,... 

The loss of the addition ability of an ion by delocalization of the electron defect in a conjugated polyene chain is documented by the observation of Subramanian et al. They polymerized monomers with C=C and C=N bonds by electroinitiation. Anionic polymerization of phenylacetylene started very easily but it rapidly died out without any external cause [109]. The polymerization of -alkyl-2-cyanoacrylate in the presence of tetracyanoethy-lene yields only low-molecular-weight products. This is due to the formation... 

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. 

Acrylic resins used as adhesives are formed through radical or anionic polymerization [6]. Radical polymerization can be initiated by UV radiation as well as heat. The two reaction schemes are identieal in principle (Fig. 4). Cyanoacrylates are of special interest for systems with very high reaction rates. Their reaction follows an anionic polymerization mechanism. Since the polarity of the cyanoacrylates is very high, water is able to act as an initiator (Fig. 5). 

In the context of the anionic polymerization of CA derivatives, as considered in Section 10.3.2.2.1, it is notable that the polymerization of cyanoacrylates is also photoinitiated by substituted pyridine pentacarbonyl complexes of tungsten or chromium, i.e. M(CO)5L with M = Cr or W, and L = 2- or 4-vinylpyridine [60]. Photo-released pyridine adds to CA, and the resulting zwitterion initiates the anionic chain polymerization (see Scheme 10.16). 

It was argued that the MLCT excited state has a change in hapticity (from if to rf), and this facilitates solvent attack on the Fe center, leading to loss of a CpR ligand. The anion thus produced efficiently initiates the anionic polymerization of ethyl a-cyanoacrylate. It was likewise shown that cationic polymerizations could be initiated by LF excitation of CpFe( 7 -arene) complexes. 

Kutal has reported that MLCT excitation of benzoyl-substituted ruthenocenes leads to anionic polymerization of ethyl 2-cyanoacrylate (Equation (31)). These complexes do not undergo ligand loss as a result of MLCT excitation, and it was suggested that the charge redistribution in the excited state allowed the complex to act as an anionic initiator. 

When a carbonyl and a cyano group are attached to the same carbon as in methyl 2-cyanoacrylate, the monomer that constitutes Super Glue, anionic polymerization can be initiated by even weak bases such as atmospheric moisture or normal skin dampness. 

In subsequent papers Kutal et al reported that they found that ferrocene and ruthenocene (FeCp2 and RuCp2, where Cp is rj -CsHs) complexes will also photoinitiate anionic polymerizations of a-cyanoacrylate. They suggest that the mechanism of initiation by ferrocene is an attack on the monomer and formation of a radical anion through electron transfer... 

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