Methylene linkages

Scheme 4a shows the condensation of a benzyl alcohol group with a phenolic ring position occupied by hydrogen to produce a methylene linkage between two phenolic rings and producing one mole of water as a by-product. This type of condensation occurs at both high and low pH. It is the type most commonly seen in both resoles and novolacs. 

Scheme 4b depicts condensation between a hydroxymethyl group and a phenolic ring where the hydroxybenzyl attacks at a ring position that is already hydroxymethylated. In this case, a methylene linkage is produced between the rings with concurrent loss of one mole each of formaldehyde and water. Both Jones and Grenier-Loustalot et al. demonstrated the occurrence of this reaction pathway beyond doubt under basic conditions. 

Sprung and Gladstone were the first to show the formation of dibenzyl ether linkages during the condensation of hydroxymethyl phenols, as depicted in Scheme 4c [152], These results have been corroborated by a number of researchers since [128,144,147,148], This condensation was reportedly second-order and takes place at pH conditions near neutral and at temperatures less than 130°C [128,147,153], At temperatures of 160°C the dibenzyl ethers are converted to something else, most likely to methylene linkages and formaldehyde [132],... 

In addition to the normal methylene linkage formation involved in polymerization with both resoles and novolaes, other, usually less desirable, eondensation by-products are also seen in novolac synthesis. Among these are benzodioxanes and dibenzyl ethers. The reaction pH has significant effect on the relative amounts produced. Fig. 15 shows typical structures for these by-products. When such byproducts are present, the meaning of the molar ratio changes and variability with respect to molecular weight development, glass transition point, and solubility may be seen. They also lead to poor raw material utilization. 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

Small amounts of various phenolic side products that incorporate groups such as imines, amides, ethers, and ethanes into the networks also form. A number of these side products undergo further reactions which eventually lead to methylene linkages. Some side products generally remain in the networks even after heating at 205°C. 

Condensation reactions between two hydroxymethyl substituents eliminate water to form ether linkages (Fig. 7.23a) or eliminate both water and formaldehyde to form methylene linkages (Fig. 7.23b). Ether formation is favored under neutral or acidic conditions and up to 130°C above which formaldehyde departs and methylene linkages are generated. The methylene linkage formation reaction, which eliminates water and fortualdehyde, is more prevalent under basic conditions. Condensation reactions between hydroxymethyl groups and reactive... 

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

Morterra and Low109,110 proposed that thermal crosslinking may occur between 300°C and 500°C where phenolic hydroxyl groups react with methylene linkages to eliminate water (Fig. 7.43). Evidence for this mechanism is provided by IR spectra which show decreased OH stretches and bending absorptions as well as increased complexity of the aliphatic CH stretch patterns in this temperature range. 

Figure 7.43 Thermal crosslinking of phenolic hydroxyl and methylene linkages.

Ortho-hydroxymethyl substituents, 401 Ortho-ortho methylene linkages, 393,... 

PAR. See Polyarylate (PAR) para-Cresol (p-cresol), 384-385 para-Cresol novolac resin, 425 Paraformaldehyde, 377, 398 Para-para methylene linkages, 393, 395,... 

FIGURE 7.5 13C-NMR spectrum of the pentamer showing four methylene linkages and a methyl terminus. 

Methylene linkages from methylene ether linkages by the splitting out of formaldehyde... 

Methylene linkages by the reaction of methylol groups splitting out water and formaldehyde... 

Figure 3 shows the 60 MHz 1H-NMR spectra of reaction mixtures obtained by heating 2,4-dimethylol-o-cresol in pyridine solution at 100°C for various periods of time. The o-methylol resonance at 5.25 ppm is seen to decrease in intensity, relative to the p-methylol resonance at A.9 ppm, as the reaction proceeds and a new peak at 4.8-4.9 ppm, which is due to methylene ether groups, increases steadily in intensity. Only in the spectrum of the reaction mixture that was heated for 132 hr. is a signal due to methylene linkages evident. 

In the second stage of the reaction, there occurs the condensation between the methylol groups and the amido hydrogen in acidic medium to yield the methylene linkage as given below ... 

---