Methylolation Methylene bridges

PF Substituted monomer Methylol Methylene bridges Condensed Phenolic Free... 

The novolak resins themselves contain no reactive methylol groups and do not form cross-linked structures on heating. If, however, they are mixed with compounds capable of forming methylene bridges, e.g. hexamethylenetetramine or paraformaldehyde, they cross-link on heating to form infusible, thermoset structures. 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

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. 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

Since a small amount of water is always present in novolac resins, it has also been suggested that some decomposition of HMTA proceeds by hydrolysis, leading to the elimination of formaldehyde and amino-methylol compounds (Fig. 7.15).42 Phenols can react with the formaldehyde elimination product to extend the novolac chain or form methylene-bridged crosslinks. Alternatively, phenol can react with amino-methylol intermediates in combination with formaldehyde to produce ortho-or para-hydroxybenzylamines (i.e., Mannich-type reactions). 

PF resin Methylols ortho/para Methylene bridges o-p Vp-p Methylene bridges/methylols... 

Methylol Substituted monomer group Methylene bridges ... 

Condensed Phenolic Free Methylen bridges Methylols... 

Methylene bridges between amido nitrogens by the reaction of methylol and amino groups on reacting molecules (Figure 19.3a)... 

Figure 15.1 Reactions of formaldehyde with peptides and amino acids. Shown are the four types of reaction products seen when peptides or amino acids are treated with formaldehyde in aqueous solution. These reaction products are methylol (hydroxymethyl) adduct (reaction 15.1), Schiff-base (reaction 15.2), 4-imidazolidinone adduct (reaction 15.3), and one type of methylene bridge [cross-link] (reaction 15.4).

Formaldehyde reacts with proteins to form adducts and cross-links.31516 Metz et al.3 have identified three types of chemical modifications after treatment of proteins with formaldehyde (a) methylol (hydroxymethyl) adducts, (b) Schiff bases, and (c) methylene bridges. The reaction of formaldehyde with proteins is summarized in Figure 19.1, but briefly, formaldehyde reacts primarily with lysine and cysteine to form methylol adducts. The methylol adduct can subsequently undergo a dehydration reaction to form a Schiff base. Adducted primary amine and thiol groups can undergo a second reaction with arginine,... 

Figure 19.1 A schematic view of the common formaldehyde-induced modifications in proteins. Reactive methylol adducts are formed in the initial reaction between formaldehyde and cysteine or the amino groups of basic amino acid residues. The methylol adduct can subsequently undergo a dehydration reaction to form a Schiff s base. Adducted residues can undergo a second reaction to form methylene bridges or can convert to the ethoxymethyl adduct after the ethanol dehydration step.

In reactions at pH 11.0, no phloroglucinol A-ring can be detected after only 1 hour at 100 °C, and there is little change in the amount of methylol or formation of methylene bridge after 6 hours of heating. The methylene signals that are... 

Polymerization under alkaline conditions, using about 1% sodium hydroxide catalyst based on the weight of the phenol, proceeds via a somewhat different mechanism. Methylol phenols, and oligomers consisting of 5 or 6 phenol units connected by methylene bridges, result in the so-called resole resin prepolymer (Eq. 21.33). 

The reaction of urea and melamine with formaldehyde is a condensation reaction that allows firstly the formation of methylol compounds, then the reaction of these to form oligomers and finally the formation of a crosslinked network (Braun and Ritzert, 1989). The first stage of the reaction may be either acid- or base-catalysed, and the subsequent oligomerization is acid-catalysed if condensation to give methylene bridges rather than dimethylene ether linkages is desired (Scheme 1.16). 

These intermediates are water-soluble and may be used to impregnate cellulosic fillers prior to the subsequent crosslinking reaction. The crosslinking reaction (Scheme 1.17) involves firstly the reaction of methylol groups and urea to give a methylene bridge (1). 

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