Acid-Catalyzed Phenol-Formaldehyde Condensation

Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. 

Caution Because of the formation of gaseous formaldehyde all reactions must be carried out in a closed hood. 

5 g (0.345 mol) of phenol, 23 g of a 37% aqueous formaldehyde solution (0.285 mol), 3.75 ml of water, and 0.5 g of oxalic acid dihydrate are placed in a 250 ml three-necked flask,fitted with stirrer and reflux condenser, and heated under reflux on an oil bath for 1.5 h. 75 ml of water are then added, stirred briefly, and allowed to cool, whereby the condensation polymer settles outThe aqueous layer is separated and the residual water distilled off at 50-100 torr while slowly raising the temperature to 150 °C.This temperature is maintained (at most for 1 h) until test samples solidify on cooling.The resin is poured out while still warm and solidifies to a colorless, brittle mass, soluble in alcohol. 

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Acid-Catalyzed Phenol-Formaldehyde Condensation (Novolaks)... 

In a positive-tone resist, the areas that are exposed to the radiation develop away into the solvent faster than unexposed areas, resulting in a positive-tone image of the mask. The majority of commercial, positive-tone, nonchemically amplified resists used today are variations of the well-known, two-component, diazonaphthoquinone-novolac resist [5]. Novolac is a name generally given to acid-catalyzed phenol formaldehyde condensation polymers of the type shown in Fig. 2. 

Figure 1. Reaction scheme of the acid-catalyzed two-step condensation of phenol with formaldehyde (a) and isobutanal (b), and dehydration and cyclization of the primary phenol/isobutanal product (c).

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

Epoxy NovolaC Resins. Epoxy novolacs are multifunctional epoxies based on phenolic formaldehyde novolacs. Both epoxy phenol novolac resins (EPN) and epoxy cresol novolac resins (ECN) have attained commercial importance (53). The former is made by epoxidation of the phenol-formaldehyde condensates (novolacs) obtained from acid-catalyzed condensation of phenol and formaldehyde (see Phenolic Resins). This produces random ortho- and para-methylene bridges. 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

A Friedel-Crafts-type reaction of phenols under basic conditions is also possible. Aqueous alkaline phenol-aldehyde condensation is the reaction for generating phenol-formaldehyde resin.34 The condensation of phenol with glyoxylic acid in alkaline solution by using aqueous glyoxylic acid generates 4-hydroxyphenylacetic acid. The use of tetraalkylammonium hydroxide instead of sodium hydroxide increases the para-selectivity of the condensation.35 Base-catalyzed formation of benzo[b]furano[60]- and -[70]fullerenes occurred via the reaction of C60CI6 with phenol in the presence of aqueous KOH and under nitrogen.36... 

Condensations with carbonyl compounds phenol-formaldehyde resins. Acid or base catalyzes electrophilic substitution of carbonyl compounds in ortho and para positions of phenols to form phenol alcohols (Lederer-Manasse reaction). 

Acid Catalyzed Condensation Polymerizations. The strong protonic acids produced by the photolysis of onium salts I-III can also be employed to catalyze the condensation of phenolic, melamine, and urea formaldehyde resins. Very durable photoresists based on these inexpensive and readily available resins can be made. Such resists generally require a postbake prior to development to complete the condensation and to enhance image formation. 

Acid-Catalyzed Reactions. When an acid catalyst is used and the pH of the phenol/formaldehyde mixture is lowered to 0.5-1.5, somewhat less complicated products are formed. The initial reaction of addition of formaldehyde to the aromatic ring results in an unstable intermediate that rapidly condenses to three possible dihydroxydiarylmethanes. 

Formation of novolak involves an acid-catalyzed reaction of formaldehyde with excess phenol (i.e., formaldehyde-to-phenol mole ratio less than 1). The initial methylol phenols condense with the excess phenol to form dihydroxydiphenyl methane, which undergoes further condensation yielding low-molecular-weight prepolymer or novolak. Unlike resoles, novolaks do not contain residual methylol groups. They are fusible and insoluble. 

The acid-catalyzed condensation of phenols with formaldehyde proceeds in two steps. In the first step, a hydroxymethyl cation attacks a phenolic ring at a position ortho or para... 

This reaction was initially studied by von Baeyer in 1872. It is an acid-catalyzed condensation of aromatic compounds with formaldehyde or formaldehyde derivatives. In normal conditions, the reactive benzene derivatives such as phenols and arylamines are applied to condense with formaldehyde however, a small number of less-reactive aromatics have also been used in this reaction, including benzene, toluene, benzyl chloride,biphenyl, iodobenzene, naphthalene, and mesitylene. Although no yields were given in the early studies, it is reasonable to obtain 70-80% yield for this type of reaction. Many other reactions have been developed to synthesize diarylmethanes, including Katritzky s benzotriazole method, Kochi s dealkylative coupling," Fukuzaw s 1,3-propandiol method, and the reduction method. In general, the condensation occurs at the /tflra-position of substituted aromatics. 

The phenolic resins that form in acid-catalyzed condensations of phenols with formaldehyde are different from resols. At pH below 7, protonation of the carbonyl group of formaldehyde takes place first and is followed by electrophilic aromatic substitution at the ortho and para positions of the phenol. The initial steps of the reactions also take place in water. Here, however, a molar excess of phenol (1.25 1) must be used, because reactions on equimolar basis under acidic conditions form crosslinked resins. At a ratio of 8 moles of formaldehyde to 10 moles of phenol, novolacs of approximate molecular weight 850 form. When the ratio of formaldehyde to phenol is 9 10, a molecular weight of approximately 1000 is reached. This appears to be near the limit, beyond which crosslinking results. The reaction is as follows ... 

The phenolic resins are condensation products of phenol and formaldehyde [144-146, 148]. These materials were among the earliest commercial synthetic plastics. Two different methods [144-146] are used to prepare them. In the first one, the condensations are base catalyzed, while in the second one, they are acid-catalyzed. The products formed with basic catalysts are called resols and with acidic ones novolacs. Phenolic resins are used widely in coatings and laminates. The pure resins are too friable for use as structural materials by themselves. They become useful plastics, however, when filled with various fillers. 

Other relevant reactions of resin acids are those involving their addition to formaldehyde and/or phenols. Formaldehyde adds to abietic acid to form complex mixtures containing mono and dihydroxymethyl derivatives [28-31]. Under alkaline conditions, the formation of the mono derivative is favoured, whereas in acetic acid, the dihomologue (isolated as acetate) is the major product (Fig. 4.10). More recently, studies of the acid-catalyzed condensation of abietic acid with formaldehyde showed that, under such conditions the reactions mixtures were predominantly composed of trimeric stmctures [26, 32]. 

In the phenol/formaldehyde reaction, the rates for the addition and condensation reactions are in the ratio of 1 42. The overall activation energy is 84-100 kJ/mol. The p site in this acid-catalyzed reaction is about 2.4 times more reactive than the o site of the phenol. In general, then, p-methylol phenols are produced, but these are not as attractive commercially as the o,o -methylol-rich novolaks. The curing of novolak should, of course, occur as quickly as possible, which assumes that the reactive p sites are available for reaction (see below). 

The ENR was prepared by two step reactions. The first step was involved in the formation of novolac resin through condensation of phenol and formaldehyde under oxalic acid catalyzed with the molar ratio of phenol to formaldehyde was 1 0.8. In the second step, the epoxidation of previously prepared novolac resin was carried out by reacting novolac with excess of epichlorohydrin with 1 8 molar ratio under 40 % sodium hydroxide catalyst (Saw et al. 2011b, 2012). The schematics of synthesis are shown in Fig. 1. 

The use of silicon-containing phenolic polymers with NDS compounds present as the photoactive polarity switch has been investigated. Formaldehyde condensation polymers with m-trimethyl-silylphenol (19) have been prepared. It is necessary to incorporate phenol as a comonomer in order to decrease the hydrophobicity of these resins so that they are soluble in aqueous base. The maximum silicon content obtainable is around 9% due to the acidolysis of the aromatic carbon-silicon bond during the acid-catalyzed polymerization process. Nevertheless, high resolution images with good oxygen etch resistance are obtained when these resins are formulated into resists with NDS derivatives present. 

Furan no-bake resins are two-component, r.t.-cur-ing, acid-catalyzed systems of furfiiryl alcohol prepolymer with additional monomer. Hot and warm box resins are mainly based on urea-modified fur-ftiryl alcohol-formaldehyde condensates that are cured at 100-170 °C. Also phenol-formaldehyde resins, modified with furfiiryl alcohol, are used. In cold box binder systems, the sand is mixed with a low-viscosity fiiran resin and a peroxide. The core is formed, and SO2 is blown into or generated in situ in the sand to cure the resin rapidly. 

Acid catalysts, such as metal oxides and sulfonic acids, generally catalyze condensation polymerizations. However, some condensation polymers form under alkaline conditions. For example, the reaction of formaldehyde with phenol under alkaline conditions produces methy-lolphenols, which further condense to a thermosetting polymer. 

The high-molecular-weight products formed by the condensation of phenols with carbonyl compounds (especially with formaldehyde) are known as phenolic resins. They are mixtures of structurally nonuniform compounds that are initially soluble and fusible but which can become crosslinked (cured) by subsequent reactions. One distinguishes between acid- and base-catalyzed condensations, since they lead to different end products the properties of the condensation polymer are also affected by the mole ratio of phenol to formaldehyde. 

An NaOH-catalyzed resol (molar ratio NaOH/phenol = 0.03) was synthesized using a formaldehyde/phenol molar ratio of 1.5. After 3 h reaction at 70°C, oxalic acid was added, carrying the pH to 6-6.5, and water distilled under vacuum until the viscosity (at 20°C) increased to 1.5-2 kg m-1 s-1. The condensation of the resol takes place through the free CH2OH groups located in the o, o, and p positions of the phenolic ring (Chapter 2). 

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