Methylol units

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

The postulation that the structure of one dimer is 2,4 -dihydroxy-3,3 -dimethoxy-5-ethylbibenzyl is, of course, based on NMR data, on the proposal that phenylcoumarane structures are common in lignin [see (/)], and on evidence that they are cleaved to stilbenes by alkali (14) and eliminate 7-methylol units (2) as formaldehyde by mechanisms quite analogous to those above. Although there are some modest surprises in the structure of dimers isolated so far, it is gratifying how well these results fit into our long-standing concepts of lignin structure and reactions. 

Zinc acetate catalyst produces essentially 100% o-methylol phenol (8) in the first step. The second step gives an approximately equal quantity of 2,2 -(5, 45%) and 2,4 -diphenyhnethylene (6, 45%) bridges, indicating Htde chelate-directing influence. In addition, a small quantity (10%) of methylene ether units (9) (diben2yl ether) is observed at moderate reaction temperature. 

In some resole appHcations, such as foam and foundry binders, a rapid cure of a Hquid resin is obtained at RT with strong acid. The reactions proceed in the same manner as those of novolak resin formation. Methylol groups react at ortho and para phenoHc hydrogen to give diphenyknethane units (41). 

Other crosslinking reactions may be triggered by a catalyzed reaction between different units of a copolymerized functional monomer, such as A -methylol acrylamide or a copolymerized silane compound [86]. 

Extraction tests are used primarily in Japan and Europe, a release test is used in the United States, and standard tests have been compared based on the sources of formaldehyde present in a finished fabric (76,78—80). Finished fabric may contain free formaldehyde, or formaldehyde released from unreacted IV-methylol moieties. 

In alkaline media, phenolic units may react with formaldehyde, forming methylol derivatives that condense with themselves or with another phenol (J, Fig. 1.4). This formaldehyde condensation reaction forms the basis for using technical lignins in the production of adhesives. 

These benzaldehydes could then be directly used as a feedstock for various polymeric products or reduced to form phenolic benzylic alcohol derivatives (i.e., p-methylol groups). The p-methylol groups would thus be active sites, whereas in unmodified lignins, the C-l site is blocked and unreactive. In addition, the oxidative-cleavage step will hydrolyze a portion of the lignin interunit ether bonds, and thus increase the total fraction of free phenolic units to further enhance the reactivity. Other possible benefits are that the lignin would be extensively depolymerized and would form a more uniform feedstock material both conditions would give a product that is easier to handle. 

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). 

By polyaddition of 3-ethyl-3-methylol-oxetane to the hydroxyl group of TMP a dendritic polyol is obtained, with a hyperbranched and much more symmetrical structure compared to glycidol. In principle, the structure is very similar to the structure presented in the Figure 20.2 with the difference that the repeating unit is different (Figure 19.5) ... 

Figure 19.5 Repeating unit in the dendritic polyols based on 3-ethyl-3-methylol...

In a similar approach, CuAAC click couphng reactions were combined with esterification chemistry in a divergent, accelerated AB2 — CD2 strategy for the preparation of dendrimers derived from the monomer unit 2,2-bis(methylol)propionic acid, commonly known as bis-MPA [132], Again, this approach allowed for the preparation of a fourth-generation dendrimer in only four reactions steps, by... 

---