Adhesive polymerization scheme

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

Recently, Endo and coworkers have developed novel sulfonium type initiators that can initiate polymerization either upon irradiation or upon thermal treatment. In addition, these photoinitiators are shown to be functional for both cationic and radical polymerizations. This dual activity is particularly important in hybrid curing systems for coatings and adhesions [28]. Scheme 11.6 contains the general steps for the synthesis of these photoinitiators from sulfides. 

Based on the design criteria noted above, the evolution of the absorbable cyanoacrylate-based system commenced with methoxypropyl cyanoacrylate (MFC) and was followed by formulation of MFC with polyether oxalate or polyester carbonate. Figure 5.1 outlines a polymerization scheme and designation of the final polymer. Both V-100 and V-150 tissue adhesives contain the same components but in different proportions. 

When used as adhesives, polymerization or curing is brought about by nucleophihc species present on the substrate. The nucleophile can be an ion or a neutral molecule (amine). If the initiating species is an ion, the polymerization proceeds by an anionic mechanism. If a neutral molecule is involved, the reaction is referred to as a zwitterionic polymerization (Scheme 3). 

Polymerization inhibitors are key additives which prevent premature gelation of the adhesive. The foimulator must carefully balance shelf stability and the required cure on demand. Due to its high propagation rate, MMA is difficult to inhibit. Some comments on specific inhibitors follow. The most common inhibitor to be found in component monomers is 4-methoxyphenol, which is also called the methyl ether of hydroquinone. This inhibitor is effective only in the presence of oxygen. A mechanism has been proposed, and is illustrated in Scheme 13 [128]. 

In contrast, tertiary amines do not possess a proton to transfer, and the reaction of the Michael-type addition adduct with ECA can only initiate polymerization to form high molecular weight adhesive polymer, as shown earlier in Scheme 1. 

Scheme 2.9 General deformulation for paints, plastics, adhesives and inks. After Gooch [4]. Reproduced from J.W. Gooch, Analysis and Deformulation of Polymeric Materials, Paints, Plastics, Adhesives and Intes, Plenum Press, New York, NY (1997), by permission of Kluwer Academic/Plenum Publishers...

Condensation polymers, which are also known as step growth polymers, are historically the oldest class of common synthetic polymers. Although superseded in terms of gross output by addition polymers, condensation polymers are still commonly used in a wide variety of applications examples include polyamides (nylons), polycarbonates, polyurethanes, and epoxy adhesives. Figure 1.9 outlines the basic reaction scheme for condensation polymerization. One or more different monomers can be incorporated into a condensation polymer. 

Manson (72,) expanded the concept to the solid state by observing that the strength of composite materials also depended upon the acid-base interaction between continuous and dispersed phases. More directly, Vanderhoff et al. (21) addressed the issue of adhesion of polymeric materials to corroded steel. They synthesized eight corrosion products of iron, and used the interaction scheme developed by Fowkes and Manson first to characterize the iron corrosion products as Lewis acids or bases and then to select polymer vehicles for practical coating systems. Such results were employed to enhance the adhesion of epoxy systems to substrates which were predominantly iron oxide in nature. A good overview of these Issues was presented by Fowkes in 1983 (74). ... 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

Fig. 5. Model scheme for the immobilization process by radiation polymerization in a supercooled state. (Adhesion method) c biofunctional component...

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

The catalytic capabilities of the Tp ML complexes for this transformation have also been applied to the functionalization of macromolecules such as polyolefins. Thus polybutadienes (Scheme 6a) or styrene-butadiene rubbers (Scheme 6b) have been modified upon addition of carbene units from EDA that were incorporated into the unsaturated C=C bonds of the polymeric chain, providing interesting features to the isolated materials the incorporation of polar groups provided distinct properties regarding their potential use as adhesives, but maintaining the structure of the parent polymer. 

More recently, Schubert and coworkers has made tailor made macromonomers bearing oxetane moiety which is cross-linkable in the presence of cationic species [151]. Recently developed ATRP has been recognized as a useful method to polymerize monomers bearing different functionalities such as epoxides in the side chain that could find potential applications in coatings and adhesives. Scheme 11.47 illustrates the preperation of polymers with oxetanes as pendant groups that are preserved through ATRR The resulting polymer is notably cross-linkable when small amount of cationic initiator is supplied. 

Consequently, one should banish the simplistic scheme that nodules in a polymer necessarily induce damage in a medium that otherwise would exhibit isochoric deformation by plastic shear. On the contrary, (i) damage processes provide an important contribution to the plastic deformation of most polymers and (ii) dispersed particles often decrease the original damage rate of polymeric matrices, except those that are coarse and poorly adhesive such as glass beads. 

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). 

A number of researchers have explored chemical s)mthesis of adhesive protein polypeptide and polymer analog. The synthetic approaches typically involve polymerization of catechol or DOPA-containing monomers, as well as post-polymerization derivatization of polymer endgroups or side chains with DOPA, DOPA peptides or DOPA mimetic molecules. A sampling of chemical stmctures is provided in Schemes 2-4 and described below. 

Cell Adhesion and Detachment, Fig. 2 Chemical reaction scheme for the attachment of a biomolecule to the surface of polymeric microfluidic devices made of... 

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