Fully alkylated amino resin

Fully alkylated amino resins require strong acid catalysis for fast and/or low-temperature cross-linking. Their catalysis mechanism is different from that of partially alkylated resins which respond to weak acid catalysts or general acid catalysis. A fully alkylated melamine resin catalyzed by a strong acid catalyst is a faster curing (cross-linking) agent than a partially butylated amino resin. 

Monomeric partially and fully alkylated amino resins are prepared in two separate reaction steps. Hydroxymethylation is carried out under basic conditions to minimize self-condensation of the amino resin. The pH is then lowered by addition of a mineral acid and alkylation with methanol is carried out. To obtain monomeric amino resins, an excess of formaldehyde has to be used to assure a high level of hydroxymethylation and a low level of residual amide groups that would otherwise lead to polymer formation during the alkylation step. Since water removal by azeotropic distillation is not possible with methanol, a large excess of alcohol is required to achieve complete alkylation. After completion of alkylation the resin solution is neutralized water, alcohol, and residual formaldehyde are removed by vacuum distillation. The salt formed during neutralization is removed by filtration [2.154]. 

The fully alkylated amino resin is characterized by its degree of alkylation and polymerization, and its residual hydroxymethyl and amino content, all of which can significantly affect physical and chemical properties. 

Properties. Alkylated amino resins can be classified into two general classes (1) polymeric, partially alkylated resins which have a lower solids content and (2) the more monomeric, fully and partially alkylated products which have a higher solids content. 

The catalysts which are predominately used are p-toluenesulfonic acid, dodecyl-benzenesulfonic acid, dinonylnaphthalenedisulfonic acid, and their amine salts [2.152]. Compared to partially alkylated amino resins, fully alkylated resins have a lower tendency to undergo self-condensation and produce films which are hard and still more flexible. 

Fully alkylated MF resins may contain <0.25% free formaldehyde. Partially alkylated resins usually have a higher content (< 3%). The free formaldehyde content of an amino resin is largely responsible for the formaldehyde released during the application process. This is only a small fraction of the total formaldehyde released. Hydrolysis of the amino resin and subsequent dehydroxymethylation during curing account for > 80% of the total formaldehyde release (see also p. 84). 

Camarero et al. [108] used the hydrazine safety-catch linker to prepare peptide thioesters. After assembling the peptide using standard Fmoc protocols, the fully protected peptide resin was activated by mild oxidation with N-bromosuc-cinimide (NB S) in the presence of pyridine, forming a reactive acyl diazene that was then deaved with an a-amino add S-alkyl thioester such as H-AA-SEt, where AA is Gly or Ala. After TFA deprotection, peptide thioesters were obtained in good yields. Although the oxidation step did produce racemization, and other sensitive amino acids such as Tyr(tBu) and Trp(Boc) were not affected, Met and Cys presented some problems. Met was completely oxidized, and a reductive cleavage was required. For Cys, the Cys(Trt) derivative should be avoided and use of Cys(Npys) or Cys(S-StBu) is recommended instead. 

The first amino resins used in coatings were partially butylated, polymeric urea-formaldehyde (UF) resins introduced in the late 1920s. Melamine-formaldehyde (MF) resins followed in 1935. In 1960 a range of fully alkylated, predominantly... 

Although resins with a high amino content have a higher content of free formaldehyde than fully alkylated resins, they release lower amounts of formaldehyde during cure. The lowest amount of formaldehyde during application and cure is released by alkylated glycoluril-formaldehyde resins [2.155]. 

Completion of the synthesis of hydroxy-1,4-benzodiazepines was achieved analogously as before acylation of 31 with Fmoc amino acid fluorides, removal of the Fmoc protecting group, cyclisation and alkylation afforded fully functionalised polymer-supported derivatives 34 that were cleaved from the resin by treatment with TFA/Me S/HjO (85 10 5) at r.t. to give products 35 in purities > 80% and in good overall yields. 

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