Methylene-bridged Oligomers

However it has been recognised in recent years that macrocyclic products, the calixarenes, (ref.56) also accompany the linear polymers under basic conditions, albeit in low yield but that also they can be formed in low yields in acidic media (ref.57). The four membered parent phenolic member, calix[4]arene was first noted, in alkafine reaction conditions, many yeas ago (ref.58). Although subsequently 4-t-octylphenol was found to give two products (ref.59) at first thought to be stereoisomeric cyclic tetramers, it was shown in later work that 4-t-butyiphenol afforded not only cyclic compounds with methylene, but also dimethylene ether bridges (ref. 60). 

In fact the composition of the products appears to depend on the reaction conditions. Nevertheless, from 4-tert-butytphenol well-defined preparative details now enable calixarenes with 4,5,6,7,8 phenolic units to be obtained. For the 

A variety of functional groups such as halogen, nitro and azophenyl substituents at the 4-position in the phenol can be introduced into the calixarene by this specific method and it gives also the opportunity for the synthesis of asymmetric compounds (ref.72,73). The structural substituent requirement is the presence ofthe four phenolic units (A,B,C, D) arranged ABCC, ABCD or simply a single 

3-grouping (ref.74). Other structural variations can be incorporated such as a methylenic chain connecting two opposite 4-positions (ref.75) giving structures reminiscent of football crowns. The dimethylene ether bridging unit has been another alternative to the methylene group (ref.76). 

---

Novolacs are obtained by the reaction of phenol and formaldehyde in acidic conditions. Novolac oligomers are linear or slightly branched addition products linked by methylene bridges (molar masses in the range 500 5000 g mol-1). The reaction is usually carried out using a molar ratio CH20/ PhOH close to 0.8, to avoid gelation in the reactor (see Chapter 3) ... 

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. 

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 name cahx[n]arenes was coined by C. D. Gutsche originally to describe cyclic oligomers built up by (4-substituted) phenolic units linked in 2- and 6-position via methylene bridges (I). It is deduced from the calix or cup-like conformation assumed especially by the tetra- and pentamer, which resembles an ancient Greek vase, known as cahx crater , while arene refers to the aromatic units, the number of which is indicated by [n]. AU hydroxy groups in the general formula I are found in cndo-position at the narrow rim of the macrocycle. 

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). 

In 1905, Behrend reported the condensation of glycoluril (Ir) with formaldehyde under strongly acidic conditions to yield an insoluble material now known as Behrend s polymer. To obtain a tractable product, this methylene bridged glycoluril oligomer was recrystallized from hot sulfuric acid yielding a well-defined compound whose cocrystallization with a variety of metal salts was studied extensively... 

A newer development in alkylphenol—formaldehyde resias has been the application of this condensation reaction to produce calixarenes, cydic oligomers of methylene-bridged alkylphenols (20). The smallest cycHc oligomer that has been isolated to date contains four alkylphenol groups. 

The failure of the interfaee between the resin and the wood surfaee (replacement of physieal bondings between resin and reaetive wood surfaee sites by water or other nonresin ehemieals). The adhesion of UF resins to eellulose is sensitive to water not only due to the already mentioned lability to hydrolysis of the methylene bridge and of its partial reversibility, but also beeause theoretical calculations have shown that on most eellulose sites the average adhesion of water to eellulose is stronger than that of UF oligomers [8,48]. Thus, water can displace hardened UF resins from the surface of a wood joint. The inverse effect is valid for PF resins [8,49]. 

The wall formation is a chemical and physical process.Under acidic condition, the amino resin prepolymer, which is methylol compound and water soluble, becomes more hydrophobic because of the etherification and methylene bridge formation. The hydrophilic and hydrophobic interactions drive the oligomer concentrated on the interface and create a primary wall on the oil droplet. And finally with the further polycondensation, the wall is cross-linked and the wall thickness is built up. The related chemistry is presented in Equations 16.1 through 16.4. ... 

In the presence of alcohols, the reactive species is a carbenium ion (4), formed from the hemiacetal (5) (Eq. 7). Efforts to synthesize model compounds continue the work started by Bender and coworkers. A series of new dimeric and trimeric compounds, derived from o-cresol, have been synthesized and their structures determined by IR, NMR, and elemental analysis. Additional homologues of p-cresol have been prepared by Kammerer and Niemann, to study the melting behavior of a wide range of methylene-bridged phenol oligomers. Prepolymer formation, namely novolac production, has been studied in batch and continuous flow reac-... 

---