Polyether monols

The allyl alcohol then yields vinyl-terminated polyether monols and the presence of monols results in many problems. Hence, the maximum molecular weight available as commercial products is limited to less than 5,000. 

The resulting potassium allylate (the potassium alcoholate of allylic alcohol), initiates the anionic polymerisation of PO, with the formation of undesired polyether monols... 

The presence and quantitative determinations of polyether monols and polyether diols in polyether triols was realised by thin layer chromatography (TLC) [97], column chromatography [73, 94], gel permeation chromatography [4] and NMR spectroscopy [88]. 

Thus, in Figure 4.12 the thin layer chromatograms of PO homopolymers, triols and diols are presented. One observes that the polyether diols are mixtures of polyether diols and monols and the polyether triols are mixtures of polyether triols, diols and monols. The polyether monol was obtained by propoxylation of the allyl alcohol. 

As an initial level, at a polymerisation temperature range of 110-120 °C, the excess of PO needed is around 10-15% for a MW of 3000-3600 daltons, 20-30% for a MW of 4700-5000 daltons and 35-50% for a MW of 6000-6500 daltons. To obtain low unsaturation in conditions of convenient reaction rates, one considers that 105-110 °C is an optimum polymerisation temperature. Sometimes, for high MW polyethers (MW = 6000-6500 daltons), the final quantity of PO (around 35-40% from the total quantity of PO needed) is added at lower polymerisation temperatures of 90-105 °C or even 90-95 °C. It is a sacrifice for the polymerisation rate but results in an increase in the quality of the synthesised polyether, which will have a low unsaturation, corresponding to a low quantity of polyether monol in a polyether triol. 

Reaction 11.4 is used to decrease the unsaturation of polyether polyols simultaneously with the functionality increase. Thus, by introducing a polyether polyol with high unsaturation (0.07-0.09 mequiv/g), a low molecular weight polysiloxane compound, having 2-3 Si-H groups/mol, together with a platinum catalyst, the polyether monol present in the polyether (in fact allyl ether based polyethers) is added to the polysiloxane compound and the monol is transformed into a diol or into a triol (reaction 11.5). 

A very subtle effect of the oligo-polyols functionality is seen in the case of propoxylated polyether polyols, due to the presence in oligo-polyol composition of a polyether monol as a consequence of the rearrangement of PO to allyl alcohol during anionic PO polymerisation (see section 4.1), with this structure ... 

The presence of the polyether monol leads to very modest properties of polyurethanes based on polypropylene glycols obtained by anionic catalysis. In polyaddition reactions involved in polyurethane synthesis a monofunctional polyether is a chain stopper and... 

A spectacular increase in all properties was observed in elastic polyurethanes (especially in polyurethane elastomers, but in flexible foams too), by using polyethers obtained with dimetallic catalysts (DMC) instead of potassium hydroxide. There are obtained directly from synthesis, polyethers with a very low unsaturation, in essence polyethers with a very low content of polyether monols. 

This acidic treatment decreases markedly the PO-based polyether unsaturation, and the undesired monols are transformed into polyether diols. 

The polyethers obtained by the anionic polymerisation of PO initiated by glycerol are not trifunctional, having a lower functionality than 3, but usually in the range 2 < f < 3. The diols and monols decrease the functionality. The functionality is lower for high MW polyethers and for the polyethers obtained at higher polymerisation temperatures. In Figure 4.6, one observes the strong polyether triol functionality decrease as the polyether MW increases. 

In a polyether triol composition the following are present nj = mols of monols, of functionality = 1 n2 = mols of diols, of functionality f2 = 2 n3 = mols of triols, of functionality f3 = 3... 

The anionic polymerisation of PO is in fact a competition of two simultaneous reactions the propagation reaction (R ) and the transfer reaction (Rtr). An interesting way to obtain it directly from the synthesis of low unsaturated polyether polyols, is to accelerate selectively the propagation reaction while the transfer reaction remains unchanged or lower. It is well known that low unsaturation of polyether polyols represents a low monol content and leads to better physico-mechanical properties in the resulting PU, because in the PU chemistry the monol (a monofunctional compound) is a chain stopper, (i.e., it stops the MW increase). 

By using the optical density measurements of thin layer chromatograms, it was possible to obtain a clearer picture of the polyether triol composition, including the determination of the ratio between triols, diols and monols (Figure 4.13). 

DMC catalysts are considered to be the ones that perform best at this time for PO polymerisation initiated by hydroxyl groups. Bayer developed the first continuous process, with a very high productivity, for the synthesis of polyether polyols with DMC catalysts (IMPACT Catalyst Technology). In a short and simple production cycle, a large variety of polyether diols of very low unsaturation for elastomers, sealants, coatings and low monol content polyether triols destined for flexible polyurethane foams are obtained. This is one of the best developments in the last few years in the field of polyether polyol synthesis [2],... 

Ultra-Low Monol PPG High-Performance Polyether Polyols for Polyurethanes... 

When comparing ultra-low monol PPG polyols with PTMEG it is essential to keep in mind the fundamental differences between these two high-performance polyether polyols in order to obtain the maximum benefit in urethane applications. Table 9.13 summarises these differences in terms of their chemical structure, hydroxyl type (reactivity), MWD and crystallisability. 

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