Mannich bases alkoxylation

Mannich polyols is a very important group of aromatic polyols obtained by the alkoxylation with propylene oxide (PO) [(and/or ethylene oxide (EO)] of the Mannich bases obtained by classical Mannich reaction between phenols (for example phenol, p-nonylphenol), formaldehyde and alkanolamines (diethanolamine, diisopropanolamine, monoethanolamine, monoisopropanolamine and so on). Synthesis of Mannich polyols is divided into two important steps ... 

The anhydrised Mannich base is heated under nitrogen at 80-125 °C (preferably at 80-90 °C to avoid viscosity increase) and PO (or a mixture of PO-EO or EO) is added stepwise within 4-6 hours. The reaction does not need a catalyst. The alkoxylation reaction is catalysed just by the tertiary amino nitrogen formed as a consequence of the Mannich reaction. 

The last traces of alkylene oxides are removed by vacuum distillation at 100-110 °C. After the phenolic group alkoxylation, that is the first group which is alkoxylated, the resulting structure becomes much more stable and it is possible to develop degassing at higher temperature, without the risk of viscosity increase. The resulting Mannich polyols are used in polyurethane foam fabrication without any other supplementary purification. The reactions involved in the alkoxylation of Mannich bases to Mannich polyols are presented in reaction 15.10 [9]. 

For Mannich polyols from phenol, the alkoxylation reaction of the corresponding Mannich base is presented in reaction 15.11. Of course, the Mannich base can have one or two aminomethyl groups or a mixture of these structures. 

The alkoxylation of the Mannich base with PO (or PO-EO mixtures), takes place by the stepwise addition of the oxiranic monomers, at 80-95 °C, in an inert nitrogen atmosphere [5, 9]. Figure 15.4 shows that the Mannich polyols obtained by the oxazolidine technology have lower viscosities than the corresponding Mannich polyols obtained by classical Mannich reactions. This effect is explained by the low viscosity of the intermediate Mannich bases used as starters. 

Addition of EO, together with PO (15-20% EO in the mixture with PO [16-18]), leads to Mannich polyols with lower viscosities than the polyols based exclusively on PO. By using a mixture of diethanolamine and diisopropanolamine (1 1 molar), Mannich polyols with lower viscosities than the Mannich polyols based exclusively on diethanolamine are obtained [11]. As mentioned previously, lower alkoxylation temperatures of 80-90 °C (maximum 95 °C), are preferred because polyols with lower final viscosities are obtained and the alkoxylation rate is higher at lower temperatures than at higher ones (see chapter 13). 

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