Crosslinking mechanisms condensation reactions

Resole resins are generally crosslinked under neutral conditions between 130 and 200° C or in the presence of an acid catalyst such as hydrochloric acid, phosphoric acid, p-toluenesulfonic acid, and phenolsulfonic acid under ambient conditions.3 The mechanisms for crosslinking under acidic conditions are similar to acid-catalyzed novolac formation. Quinone methides are the key reaction intermediates. Further condensation reactions in resole resin syntheses under basic conditions at elevated temperatures also lead to crosslinking. 

Scheme 5.4 Mechanism of hydrolysis and crosslink formation through the condensation reaction of silanol groups...

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

The reality with all such systems, independent of the specific experimental conditions (strength and concentration of the acid, temperature, type of solvent used, if any), is qualitatively different to such a degree that it has puzzled chemists for decades. In fact, although the expected self-condensation reaction shown in Scheme 6.19 does indeed represent the basic growth mechanism, other irt5)ortant events intervene to modify drastically the polymer structure, to the point that the actual product is crosslinked and deeply coloured [4c, 4d]. 

One of the physically linked networks was crosslinked by a condensation reaction, the other by a free radical addition mechanism. The IPN s were made by heat curing of films of an aqueous emulsion made by mixing the individual emulsions in various (generally equal)proportions. In two cases, the networks were cleanly separated by hydrolysis of one of the component networks to demonstrate that there was effectively no direct chemical bonding... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Figure 6 Mechanism of intermolecular crosslink formation in lysine-containing polypeptides via condensation with bis(AA-hydroxysuccinimidyl)suberates (R = Hm DMSO reactions R = S03 Na in aqueous reactions). 

The simple self-crosslinking treatment also crosslinks GAG chains to collagen [30]. The reaction kinetics are outlined in Fig. 2. The mechanism probably involves condensation of amino groups of collagen with carboxylic groups of glucuronic acid residues on the repeat unit of chondroitin 6-sulfate. Dehydra-... 

The mechanical stability and ion exchange capacity of these condensation resins were modest. A better approach is to prepare a suitable crosslinked base membrane, which can then be converted to a charged form in a subsequent reaction. Ionics is believed to use this type of membrane in many of their systems. In a typical preparation procedure, a 60 40 mixture of styrene and divinyl benzene is cast onto a fabric web, sandwiched between two plates and heated in an oven to form the membrane matrix. The membrane is then sulfonated with 98 % sulfuric acid or a concentrated sulfur trioxide solution. The degree of swelling in the final membrane is controlled by varying the divinyl benzene concentration in the initial mix to control crosslinking density. The degree of sulfonation can also be varied. The chemistry of the process is ... 

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