Methylene bridge mechanisms

Between 10 and 15 parts of hexa are used in typical moulding compositions. The mechanism by which it cross-links novolak resins is not fully understood but it appears capable of supplying the requisite methylene bridges required for cross-linking. It also functions as a promoter for the hardening reaction. 

Due to the presence of reactive CH2OH groups, resol oligomers may be converted into highly crosslinked products without the addition of hardeners. Heat curing is conducted at T 130 200°C. The polycondensation mechanisms are complex and different bridges are possible CH2-0-CH2-and CH2. The latter is thermodynamically the most stable. Therefore the methylene bridges are the prevalent crosslinks in cured resols. 

Although none of the applications of the CB[n] family described in Section 4.3 were known in 1998 when we began our work in the area, they provide post-facto support for our assertion that a firm knowledge of the mechanism of CB[n] formation would be important. This section describes our work with S- and C-shaped methylene-bridged glycoluril dimers that provided insight into the early steps of the mechanism of CB[n] formation. 

Fluorouracil is widely used in cancer chemotherapy. It is a precursor of 5-fluoro-dUMP, which is a mechanism-dependent inhibitor of thymidylate synthetase. It forms a stable methylene-bridged complex with methylene-tetrahydrofolate on the enzyme catalytic site that cannot undergo reductive cleavage. 

In aqueous alkaline solution, phenol reacts with formaldehyde (methanal) at low temperatures to form a mixture of 2- and 4-hydroxy-benzyl alcohols. This hederer-Manasse reaction is another example of electrophilic attack which results in the formation of a new C-C bond. The mechanism is illustrated in Scheme 4.14. These products readily lose water to form quinomethanes (methylenecyclohexadienones), which react with more phenoxide. This process is repeated over and over again to produce a cross-linked polymer or phenol-formaldehyde resin (e.g. Bakelite) in which the aromatic rings are linked to methylene bridges. 

The reaction is second order, first order in phenol and first order in the methylene-bridged aromatic (27), Presumably the mechanism involves ipso protonation of the aromatic followed by attack of phenol on the cationic intermediate (27). When applied to coals, it renders them almost completely extractable into pyridine, and the number average molecular weights of the extractable material generally range between 300 and 750 (24). 

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). 

Hence, the initial major product is a mono-substituted phenol [Eq. (1)]. Because the reaction is done under aqueous acidic conditions, the products shown in Eq. (1) are not isolated. Instead, a methylene bridge is formed between the phenyl rings [Eq. (2)]. In both Eqs. (1) and (2) the mechanism is an electrophilic aromatic substitution. Heating the system so as to promote removal of water and polymerization results in thermoplastic material known as novolac (1). This thermoplastic resin can be mixed with hexamethylenetetramine (formed from ammonia and formaldehyde) and stored until cure. Heating this system produces an excess of formaldehyde and ammonia. A cross-linked polymer results from the cure. The linkages are mostly methylene and amino groups. 

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