Acetal type-linkers

Cleavage reagents Benzyl- type linkers Ketal/ acetal linkers Esters/ amide linkers Silyl linkers Triazene linkers Selenium/ sulfur/ stannane linkers... 

The addition of further o-methoxy groups to the p-alkoxybenzyl alcohol framework naturally results in linkers with enhanced acid sensitivities. One of the first linkers of this type was 2-(4-hydroxy-3-methoxyphenoxy)acetic acid (11) developed by Sheppard and colleagues for the Fmoc solid phase synthesis of protected peptide acids for fragment condensation [41]. Treatment of peptidyl resins with... 

Based on a domino Knoevenagel/cne reaction, Tietze and Steinmetz developed a stereoselective solid-phase synthesis of cyclopentane and cyclohexane derivatives of type 326 and 327 using a Merrifield resin modiiled with a propandiol linker 320 as shown in Scheme 4.6.3. Subsequent reaction with monomethyl malonoyl chloride 321 afforded the polymer-bound malonate 322, which, in a two-component domino reaction was treated with unsaturated aldehydes 323 in the presence of a catalytic amount of piperidinium acetate and zinc chloride. Except for a-substituted aldehydes, the initial Knoevenagel condensation occurred without addition of dehydrating agents and the subsequent intramolecular ene reaction gave cyclopentane and cyclohexane derivatives 325 after cleavage firom the resin either by reduction or transesterification. 

Certain types of reactions, such as polymerization, in the confined nanoscale channels/space may have different pathways compared with those in open space. Kitagawa and coworkers have utilized size-tunable MOFs as reactions hosts for radical polymerization of activated monomers, such as styrene, divinylbenzene, substituted acetylenes, methyl methacrylate, and vinyl acetate (Fig. 11) [32-35]. Fara-dicarboxylate linkers of different sizes, a-d, were used to link Cu " and Zn " centers to form 2D sheets which were further linked by triethylenediamine to form 3D frameworks 9a-d (for Cu " ) and lOb-d (for Zn " ) that possess 1-D channels (Fig. 12). 

Originally the adhesive system consisted of an aqueous poly(vinyl alcohol) solution with an isocyanate cross-linker. At this stage the common name was Aqueous Polymer Isocyanate (API) [3]. Further developments including use of different types of polymer emulsions, like poly(vinyl alcohol) (PVA), ethyl(vinyl acetate) (EVAc), styrene butadiene rubber (SBR) or acrylic-styrene (AcSt) emulsion, led to adhesives systems with improved performance, hence today the common abbreviation is EPI [4, 8]. The isocyanate cross-linking agent has also been further developed to improve the compatibility and the reactivity with the water-based component, thus several different types are now available for use in EPI adhesives. 

Different types of water-based emulsions are used in EPI adhesives. The most common are poly(vinyl acetate) (PVAc) emulsion, ethylene vinyl acetate (EVAc) emulsion, vinyl acetate-acrylate copolymerized (VAAC) emulsion, acrylic-styrene (AcSt) emulsion or styrene-butadiene rubber (SBR) latex or modified versions of these emulsion types [1, 8, 9], It has also been reported that tri- or ter-polymer emulsions like vinyl acetate-butyl acrylate-hydroxypropyl methacrylate or emulsions with different combinations of block copolymers can be used [4], Emulsion polymers containing cross-linking functional groups are especially well suited [4,6, 9]. The choice of emulsion(s) will, to a large extent, influence the adhesive properties such as setting time, bond quality, heat resistance, and moisture resistance. EPI adhesive systems are, however, very complex and the total composition (including the choice of cross-linker) and the interaction between the different components will determine the properties of the adhesive. Due to this it is difficult to describe in detail the effect of choosing one type of emulsion over the other. 

This resin will be partly neutralized with acetic or lactic acid, and co-emulsified with a blocked isocyanate cross-linker (p. 225). This type of resin is now the standard protective primer coating on the majority of the world s motor cars. Pigmentation will include anti-corrosive pigments. 

Vinyl acetal polymers can be formulated with other thermoplastic polymers and with a variety of multifunctional cross-linkers. Examples of polymers that are at least partially compatible with PVF or PVB resins include some types of polyurethanes, some types of celluloses, epoxies, isocyanates, phenolics, silicones, unsaturated polyesters, and melamine- and urea-formaldehyde polymers (19-21). Although vinyl acetal polymers are thermoplastic, hydroxyl groups permit cross-linking reactions with a variety of thermosetting resins, for example. 

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